NL7907391A - METHODS AND DEVICES FOR MEASURING THERMAL NEUTRON CHARACTERISTICS - Google Patents

Description

-1- 20949 / JF / jl *

Applicant: Société de Prospection Electrique Schlumberger, Paris, France. Short designation: Methods and devices for measuring thermal neutron characteristics.

The invention relates to a method for detecting the decay over time of thermal neutrons in an earth formation for use in measuring the thermal neutron decay characteristics of the formation, comprising: irradiating an earth formation with a discrete burst of fast neutrons during each of a sequence of irradiation intervals, detecting indications of the concentration of the thermal neutrons in the formation following each neutron burst, and generating signals in response thereto and transmitting the signals during an contiguous sequence of discrete time gates during an irradiation interval, which sequence begins following a discrete time delay after termination of the neutron burst in the irradiation interval and extends over a portion of the remainder of the irradiation interval, a method of detecting decay over time of thematic neutrons in one earth formation, comprising irradiating an earth formation with a discrete burst of fast neutrons, detecting indications of the concentration of thermal neutrons in the formation subsequent to the neutron burst, and generating signals in response to a method of detecting of the decay over time of thermal neutrons in an earth formation for use in measuring the thermal neutron decay characteristics of the formation comprising the steps of: irradiating the earth formation with a discrete burst of fast electrons of duration T during each of a sequence of irradiation intervals and detecting indications of the concentration of thermal neutrons in the formation following each neutron burst and generating signals in response thereto and a method of measuring a thermal neutron decay characteristic of an earth formation, comprising the step of irradiating a formation with a discrete burst of fast neutrons, as well as a thermal neutron decay detector in an earth formation for use in measuring the thermal neutron decay characteristics of the formation, comprising means for irradiating an earth formation 35 with a discrete burst of fast 'neutrons during each of a sequence 790 7 3 91 -2- 2'0949 / JP / jl -1 * Τ · Α of irradiation intervals, a detector means for detecting indications of the concentration of thermal neutrons in the formation following a neutron burst and for generating signals in response thereto, and a. signal gate means responsive to the control signals and to the signals generated by the detector 5 for transmitting signals from the detector means during an contiguous sequence of discrete time gates during an irradiation interval, which sequence follows a discrete time delay after termination of the neutron burst in each irradiation interval and extending over a portion of the remainder of the irradiation interval, an apparatus for detecting the decay of thermal neutrons in an earth formation over time, comprising: means for irradiating the earth formation with a diset burst of fast neutrons, a detector means for detecting indications of the concentration of thermal neutrons in the formation following the neutron burst and for generating signals 15 in response thereto, a device for detecting the decay of thermal neutrons in an earth formation over time for use in the m eating thermal neutron decay characteristics of the formation, comprising: a means for irradiating an earth formation with a burst burst of fast neutrons of time T during each of a succession of irradiation intervals, a detector means for detecting concentration indications of the thermal nuons in the formation following each neutron burst and before generating signals in response thereto and a device for measuring thermal neutron decay characteristic of the earth formation, comprising a first means for irradiating the formation 25 with a discrete burst of fast neutrons .

In general, the invention relates to nuclear well scans, and more particularly to new and improved methods and devices for detecting the decay, or trapping, of thermal neutrons in earth formations in a manner that provides more accurate and reliable measurements. eliminates thermal neutron capture characteristics of the formation.

To date, pulsed neutron capture probes have provided a measure of thermal neutron capture characteristics of the earth formations, for example, the thermal neutron decay constant ('f) and the correlated macroscopic traverse cross section (£), which have been found to be useful for oil or gas bearing distinguish formations from hydrous formations 790 7 3 91 »-3- 20949 / JF / jl * Such scans are particularly useful in recognizing the presence of hydrocarbons in contained formations and detecting changes in water saturation during the production life of a well or well.

Thermal neutron characteristic - measurements are typically made by irradiating a formation with bursts of fast (e.g. 14 MeV) neutrons and following the decay of the thermal neutron concentration in the formation by counting the gamma rays emitted by the formation nuclei after the capture the thermal neutrons during the 10 discrete time intervals, or gates, following each neutron burst. In a prior art tool disclosed in U.S. Pat. No. 3,379,882, the capture gamma rays are measured over two gates which are fixed in both post-burst occurrence and duration. Although useful information is provided in the medium decay formations 15, the solid gate system of the above patent tends to yield unreliable measurements where the decay time of the formation is either very long or very short. In addition, the gamma ray velocity measurement during the second fixed gate is somewhat subject to excessive static variations, especially in formations with a short decay time. A major improvement over this fixed gate system is U.S. Patent 3,266,116 (now Re. 28 477) which describes a sliding gate system using three measurement ports and the time of occurrence after the burst and the duration of all gates are automatically varied by a negative feedback loop action, in accordance with the validly measured value of the decay time constant. The first two gates are timed to detect the capture gamma rays of the formation, and the third gate is timed to detect background gamma rays. This system works to position the gates for an optimal background corrected measurement over a wide range of decay times' t and $ 0 cross sections 1, encountering the inconveniences related to the unreliability and statistical variation in the case of the fixed gate system. extreme decay rates are overcome. For even better results, the latter patent proposes also varying the duration and repetition rate of the neutron bursts as a function of the 55 valid measurement expiration time values. This provides the further advantage of maximizing the neutron generator duty cycle in a manner that is eonsisitent with accurate measurement of the formation decay value being scanned. Later embodiments of the sliding gate concept as described in the latter patent are disclosed in U.S. Pat. Nos. 3,662,179 and 3,890,501. Thermal neutron decay time scans in accordance with U.S. Patent 3,566,116 as described in the latter listed as an important

However, it is desirable to further improve this service. It is particularly desirable to provide even greater static accuracy in the measurement of t, 1 and background by improving the method of detecting the rate of decay of the thermal neutron concentration. Likewise, the infinitely variable negative feedback effect used for this in the sliding gate system is sometimes subject to whine ("jitter") when low count rates are encountered. That is, the variations in the setting of the measurement ports and the neutron bursts sometimes result from Statistical variation in the gamma ray velocities rather than due to the change in the decay time of the information being examined When the decay time falls sharply, as with the bed boundaries, again the feedback loop of the sliding gate tool sometimes falls, although not often. changing the gate time count so fast that this trend keeps up with the debris in the gamma ray velocity This may result in the tool detecting insufficient velocities for the negative feedback loop to operate properly, while the situation in turn keep the gates and salvos (locked) in position s later after the bursts than would be optimal for the new expiration time. While this situation can be easily adjusted by hand and the gates return quickly to the correct positions, it is desirable to avoid such detrimental locking of the calculation circuits. In addition, it is desirable to provide a measurement for all decay time values over the full bereik range that is normal

Occurs, for example, from less than 50; is up to larger than 100 coat, with no discontinuities. It is therefore an object of the invention to provide improved methods and apparatus for measuring the thermal neutron decay time constants and related capture cross-sections of earth formations through a well borehole.

790 7 3 91 * $ -5- 20949 / JF / jl

The object of the invention is to achieve the above object and other objects and to that end provides, according to an aspect of the invention, a method for detecting the decay over time of thermal neutrons in an earth formation for use in measuring the thermal neutron decay characteristics of the formation, comprising: irradiating an earth formation with a discrete burst of fast neutrons during each of a succession of irradiation intervals, detecting indications of the concentration of the thermal neutrons in the formation following each neutron burst, and generating signals in response thereto and transmitting the signals during an adjacent sequence of discrete time gates during an irradiation interval, which sequence begins following a discrete time delay after the termination of the neutron burst in the irradiation interval and extends over a portion of the remainder of the irradiation g interval, characterized in that it further comprises controlling the signal transmitting step in response to an instruction signal related to the measurement of a thermal neutron decay characteristic of the formation based on indications of the thermal neutron concentration detected during one or more preceding irradiation intervals in the sequence. of irradiation intervals, in order to adjust the duration of each discrete time gate 20 occurring in a subsequent irradiation interval by a common discrete scale factor selected from a finite number.

Another aspect of the invention provides a method for detecting the decay over time of thermal neutrons in an earth formation, comprising: irradiating an earth formation with a discrete burst of fast neutrons, detecting indications of the concentration of thermal neutrons in the formation following the neutron burst and generating signals in response thereto, characterized in that it further comprises transmitting signals from the detector member during a time gate sequence beginning after a discrete time delay after termination of the neutron burst and which includes a plurality of contiguous groups of time gates, each group per se comprising a plurality of contiguous discrete time gates, the duration of the time gates being substantially equal within each individual gate group and progressively increasing from group to group in the sequence.

Yet another aspect of the invention provides a method for 790 7 3 91-6 20949 / JF / jl.

detecting the time decay of thermal neutrons in an earth formation, for use in measuring the thermal neutron decay characteristics of the formation, comprising the steps of; irradiating the earth formation with a discrete burst of fast electrons of duration 5 T during each of a succession of irradiation intervals and detecting indications of the concentration of thermal neutrons in the formation following each neutron burst and generating signals in response thereon, characterized in that it further comprises transmitting the signals during an contiguous sequence of discrete time gates 10 during each irradiation interval, which sequence begins after a discrete time after termination of the neutron burst in each irradiation interval and extends over substantially the remainder of the irradiation interval, wherein at least some of the discrete time gates in the sequence have respective durations, which progressively increase with time ending in that of the neutron burst from a shortest duration of less than T to a longest duration of. at least as big as T.

Yet another aspect of the invention provides a method of measuring a thermal neutron decay characteristic of an earth formation, comprising the first step of irradiating a formation with a discrete burst of fast neutrons, characterized by further comprising the following steps include: selecting as a function of a known value the thermal neutron decay characteristic of the formation to be measured, and a given set of two different measurement intervals following each neutron burst from a number of such a set of two measurement intervals, where at each such set of measurement intervals corresponds to a certain value range of the decay characteristic, the determined set of measurement intervals selected is that which corresponds to the decay characteristic value range surrounding the known value of the decay characteristic, measuring indicia of the thermal neutron concept ntration into the formation during each of the measurement intervals in the selected set following each neutron burst, forming a function R of the measurements made in the step described immediately above, determining a new value of the decay characteristic as a function of the value of the function R formed in the immediately preceding step and repeating all the previously described steps using the new values of the decay characteristic determined by the 790 7 3 91 -7- 20949 / JF / jl directly thereto previous step as the known value of the decay characteristic in the new second step.

A further aspect of the invention is accomplished by an apparatus for detecting thermal neutron decay over time, in an earth formation for use in measuring the formation's thermal neutron decay characteristics, comprising a means for irradiating an earth formation with a discrete burst of fast neutrons during each of a sequence of radiation intervals, a detector means for detecting indications of the concentration of thermal neutrons 10 in the formation following a neutron saovo and generating signals in response thereto, and a signal gate means responsive to the control signals and the detector generated signals for transmitting signals from the detector member during an contiguous sequence of discrete time gates during an irradiation interval, which sequence begins following a discrete time delay after termination of the neutron burst in each irradiation interval and extending over a portion of the remainder of the irradiation interval, characterized in that it further comprises a controller responsive to an instruction signal related to a measurement of a thermal neutron capture characteristic of the format based on indications of thermal neutron concentration detected during one or more before radiating intervals in the sequence of irradiation intervals for generating control signals for controlling the generation of the signal gate member so as to adjust the duration of each discrete time gate occurring in a subsequent irradiation interval with a common 25. discrete scale factor value selected from a finite number of discrete scale factor values.

Another aspect of the invention provides an apparatus for detecting the decay of thermal neutrons in an earth formation over time, comprising: means for irradiating the earth formation with a discrete burst of fast neutrons, a detector means for detecting indications of the concentration of thermal neutrons in the formation following the neutron burst and for generating signals in response thereto, characterized in that it further comprises a signal gate means for transmitting signals through the detector means during a time gate sequence starting next on a discrete time delay after termination of 790 73 91 -8- 20949 / JF / jl the neutron burst and which includes a number of contiguous groups of time gates, each group comprising a number of contiguous discrete time gates per se, the duration of the time gates being substantially equal is within each individual gate group and in the sequence progressively increases from group 5 to group.

Yet another aspect of the invention is accomplished by a time decay detector for thermal neutrons in an earth formation for use in measuring formation thermal neutron decay characteristics, comprising: a means for irradiating 10 of an earth formation with a discrete burst of fast neutrons of duration I during each of a succession of irradiation intervals, a detector means for detecting indications of the concentration of the thermal neutrons in the formation following each neutron burst and for generating signals in response thereto, characterized in that it further comprises a signal gate member responsive to the control signals and to the signals generated by the detector for transmitting signals from the detector members during contiguous sequence of the second time gates during each irradiation interval, which sequence starts following a discrete one time delay after termination of the neutron burst in each irradiation interval extends over substantially the remainder of the irradiation interval with at least some of the discrete time gates in the sequence having respective durations that progressively increase with time following the termination of the neutron burst from a shortest duration of less than T to a longest duration of at least as large as T.

A further aspect of the invention provides a device for measuring a spike thermal neutron decay characteristic of a spike formation, comprising a first means for irradiating the formation with a discrete burst of fast neutrons, characterized by further second means for selecting as a function of a known value the thermal neutron decay characteristic of the formation to be measured from a given set of two different measurement intervals following each neutron burst from a number of such sets of two measurement intervals where each of such sets of the measurement intervals correspond to a certain range of the value of the decay characteristic, which particular set of measurement intervals selected is that which 790 7 3 91 t -9- 20949 / JF / jl corresponds to the decay characteristic 'value range surrounding the known value of the decay characteristic, a third o for measuring indications of the thermal neutron concentration in the formation during each of the measurement intervals in the selected set and for generating signals representative thereof, a fourth means responsive to the signals following the nytron burst to generate a signal related to a function R of the measurements and a Fifth member responsive to the function signal for determining a new value of the decay characteristic and generating a signal representative thereof.

Yet another aspect of the invention is accomplished by a method of providing a background-compensated measurement of the induced irradiation level in an earth formation, comprising: irradiating an earth formation with a discrete burst of neutrons during each of a sequence of irradiation intervals, detecting indications of the radiation level in the formation during at least a first detection interval occurring at a first time within each irradiation interval, detecting indications of the background radiation level during a second detection interval occurring at a second time within each irradiation interval, characterized by further comprising the steps of: measuring the average level of the first detected indications over a first number of radiation intervals, measuring the average level of the second detected indications over a second larger> aa Numerous irradiation intervals and combining the first and second measurements to provide a first background-compensated measurement of the average level of induced radiation in the formation.

790 7 3 91 I · * -ν ’’, t - -10- 20949 / JF / jl

For a better understanding of the invention, reference may be made to the following description of possible embodiments thereof in conjunction with the accompanying drawing, in which: Figure j is a schematic drawing of a representative well scan tool construction in accordance with the invention; FIG. 2 is a graphical representation of an illustrative thermal neutron concentration decay curve in three different formations, further showing a preferred neutron generator and detector gate regime in accordance with the invention; Fig. 3 is a block diagram of control, gate, memory, and telemetry circuits of Fig. 1 downhole;

Fig. 4 is a schematic drawing of the instruction signal decoder of FIG. 3; FIG. 5 is a schematic drawing of an embodiment of the instruction signal generator of FIG. 3; FIG. 6 shows details of the time counting generator of FIG. 3; FIG. 7 shows in detail the nearby (or distant) detector pulse adjustment circuit of FIG. 3; FIG. 8 is a schematic drawing of the memory address generator of FIG. 3; FIG. 9 shows an embodiment of the nearby (or distant) detector memory circuit of FIG. 3; FIG. 10 shows an illustrated telemetry window for transferring data to the surface; Fig. 11 is a schematic drawing of the sync / status circuit of Fig. 3; FIG. 12 is a schematic drawing of the telemetry coupling circuit of FIG. 3; Figures 13A and 13B show two generalized time gates for measuring thermal neutron concentrations; and FIG. 14 is a graphical comparison of a possible empirical relationship for calculating in accordance with the invention, portraying the true relationship for the example.

Fig. 1 shows an exemplary embodiment of the invention; a well sensing tool constructed in accordance with the invention comprises a fluid, pressure, temperature and resistant probe or tool 10 which is> (e) adapted to be hung and to be hung moved through a well borehole 12 by a reinforced cable 14. The well borehole 12 is shown as containing a borehole fluid 16 as well as a steel housing 18 and a surrounding cement sleeve 20. Although no housing is shown in the borehole, the tool 10 may have dimensions which suitable for use in housing.

The downhole tool 10 includes a pulsed neutron generator 22 and two radiation detectors 24 and 26, which are spaced differently from the neutron generator 22. The detector 24 located closest to the neutron generator is referred to as the "nearby" detector 1, and the detector 26 located farther away from the neutron source is referred to as the "distant" detector. For the purpose of the invention, the neutron generator 22 is preferably of the type that discretely generates pulses of fast neutrons, for example, 14 MeV, and may be, for example, of the type described in detail in U.S. Patent 2,991,364 and U.S. Patent 3,546,512. The operation of the neutron generator 22 is controlled in part by a neutron generator driver 30, and this circuit may also be of the type described in the aforementioned U.S. Patents. Detectors 24 and 26 can be of any construction suitable for detecting the thermal neutron concentrations in the surrounding earth formation and for this purpose can be of the thermal neutron-sensitive type, for example helium 3-filled proportional counter or of the gamma-sensitive type, as thallium is activated, sodium iodide detectors. In the preferred embodiment, detectors 34 and 26 preferably include sodium iodide scintillation detectors and, for this purpose, may also include conventional photo-multiplier tubes as well as high-voltage photo-multiplier power sources and amplifier discriminators (not shown). It is also to be understood that other power sources (not shown) are provided downhole as required to control the neutron generator 22 and other circuitry is fed downhole power to the source tool 10 via the cable 14 through a power source (not shown) at the surface as usual.

Output pulses from the nearby detector 24 and the distant detector 26, characteristic of the concentration of the thermal neutrons in the irradiated formation, are applied to the signal gate circuits 32. The signal 790 7 3 91 \.

* AW '-12- 20949 / JF / jl signal gate circuits 32 are controlled by gate time counting circuits 33, which also control the operation of the neutron generator driver circuit 30. Of the signal gate circuits 32, the counted detector signals are stored in memory circuits 35 and thereafter, under the control of telemetry logic circuits 37, they are applied to the telemetry circuits 34 downhole for transmission to the surface via cable 14. The joint operation of the neutron generator driving circuit 30, the signal gate circuits 32, the gate time counting circuits 33, the memory circuits 35 and the telemetry circuits 34 are described in detail below with reference to FIGS. 3 to 12.

The downhole telemetry circuits 34 may be of any known construction for encoding, time division multiplexing or otherwise preparing data carrying signals supplied thereto by the telemetry logic circuit 37 and for imprinting such data. on the cable 14. At the surface of the earth, the data carrying signals from the near and far detectors 24 and 26, respectively, are amplified, coded, demultiplexed and otherwise processed as necessary in the surface telemetry circuits 36, which may also be conventional . The telemetry circuits 32 and 36 also include circuitry for receiving and transmitting instructional messages from the surface in view of the selection of the scale factor value F to be used and as will be described more fully below. To this end, circuitry 34 and 36 suitably comprise a two-way data telemetry system suitable for this purpose and with a ten-kilobit per second upwardly directed data rate.

Following circuits 36, the near and far detector signals are counted separately in signal counter circuits 38 to obtain thermal neutron decay curve data over a desired accumulation, interval & T. After termination of the data accumulation time interval AT, which can be selected, for example, to correspond to a desired interval of depth according to the tool countdown rate, the count rate data accumulated in the signal counting circuits 38 is transferred to buffers 40 and the signal counting circuits 38 are reset to zero.

From storage 40, the count rate data is processed in a computer 42, which may suitably include a microprocessor. As will be described more fully below, the computer 42 processes the count rate data of the 790 7 3 91 t *

-T3- 20949 / JF / jS

respective detectors for developing various desired outputs, including, for example, the decay time constants and ^ for the near and distant detectors, the corresponding macroscopic capture cross-section portions, and several other selected outputs such as a ratio (N / F) of the counting rates of the near and far detectors, background counting rates (B ^ and B ^) of the respective detectors and the net counting speeds of certain time gates, for example Ng and Fg for both detectors. All the 2nd outputs can be recorded in a conventional manner as a function of tool depth in a sensor 44.

The usual cable following mechanical connection, indicated in Fig. 1 by 46, is provided for this purpose. As shown by line 48 in Figure 1, the computer 42 sends an appropriate scale factor instruction signal to the surface telemetry circuits 36 for transmission to the gate timing counters 33 downhole for timely setting of the counts and durations of the detection gates for the near and far detectors 24 and 26 and, if desired, for the duration and repetition rate of the neutron bursts. In the graphical representation of Figure 4, the three decay curves 50, 52 and 54 respectively represent the variation with time of the logarithmic. thermal neutron count rate of gamma rays following irradiation of earth formations with short, medium and long decay times (decay rates) of the thermal neutron concentration. For comparison purposes, curves 50, 52 and 54 are normalized at approximately the same peak count rate, although as will be appreciated, this is normally not true in practice. is.

The variable tail portion 56 of each curve represents background and this is also shown as approximately a constant intensity level for the purpose of the illustration.

As is known, the slope of the thermal neutron decay curve of a formation is characteristic of the formation's thermal neutron decay constant, and it is a feature of the invention that the decay curve can be detected or measured more accurately than hitherto possible. In order to realize this goal, 16 discrete time intervals or gates G1 through G16 are provided between successful neutron bursts. As shown in Figure 2, the gates to form a sequence of discrete time gates, which sequence begins after a finite time delay following the determination of the preceding neutron burst and extends throughout 790 7 3 91 «» ·· ►: * -14- 20949 / JF / jl virtually the entire remaining interval between neutron bursts. Advantageously yet not necessary, the gates are contiguous in time.

The purpose of the time delay between the successive neutron bursts and the start of the gate sequence is to allow gamma rays from the immediate borehole environment, for example, borehole fluid, casing, cement sleeves, tool tubes, etc. to die out for count rate data detection. of the formation is started. As shown in Fig. 2, the discrete time gates G, j through G ^ g · are divided into four groups I, II, III and IV of four gates 11, i.e. the time gates G ^ through 10 'form gate group I, time gates G ^ through Gg form gate group II, time gates Gg through G | 2 form gate group III, and time gates G ^ through G ^ g form gate group IV. Within each gate group, the discrete time gates have the same duration. Thus, each of the gates G1 through G1 has the same duration, for example, 25 microsecorides (ps) and the like, the 15 separate time gates of each of the gate groups II, III and IV have the same duration. However, the duration of the time gates progressively increases from gate group to gate group in the sequence. The increment is incremental, i.e., with a finite factor, and preferably the degree of increment is a multiple of the duration of the time gates of the immediately following gate groups. A multiple of two has proven advantageous. Thus, the duration of the discrete time gates Gg through Gg in gate group 2 is preferably twice the duration of discrete time gates G ^ through G ^ in gate group I, i.e. 50 ps.

The duration of the individual time gates Gg to G ^ in gate group III is then twice the duration of the individual time gates Gg to Gg in gate-25 group ill, i.e. 100 ps and the duration of the individual time gates in gate group IV is twice that of the group III gates, i.e. 200 ps. It will be appreciated that whether the specific durations of the gates are binding. Each group pf 'the amount increases in gate times between groups or both can be varied if desired with respect to the values shown above. Also, both the number of gate groups and the number of discrete time gates within each group can be varied correspondingly from the four-four scheme shown in Fig. 2.

Thus, by using narrow gates in the gate stage initial stage and wider gates later in the gate sequence, the narrowest 35 gates are concentrated in the initial region of the thermal neutron case curves 790 7 3 91 I * * / / -15- 20949 / JF / jl 50, 52 and 54, where the rate of change of the counting speed is greatest.

Furthermore, not all time gates need be included in the calculation, but only those containing a significant decay signal.

Thus, not only is a degree of given compactness achieved through the use of variable time detection time gates after the neutron burst, but greater accuracy is provided by excluding the "f" calculation rates of time gates subject to abnormal statistical variations, such as the later gates in short formations As will be described more fully below, the particular gates to be used in calculating K are chosen based on prior measurements of 1 during the same scan run and those gates are empirically determined that they give the minimal statistical variation in the measured value of it over a finite range which spans the value measured thereby.

In accordance with the invention, it has been found that in addition to providing time gates up to Gg which increase the width as a function of time after the neutron burst, even better results are obtained for providing selected incremental setting of the durations of the discrete time gates G ^ through G16 and, if desired, also the durations and repetition periods of the neutron bursts and the duration of the discrete time delay between the end of the burst and the start of the gate sequence as a function of the previously measured value of t. The goal is to position the gates G ^ -G ^ g, the neutron burst, etc. so that the counting speeds in each of the gates used in the counting calculation, i.e. the earlier gates, the mid-range gates and the later gates as may be the case, will be large enough for reliable statistical accuracy in the measurement. Therefore, it is desirable to actually displace the time displacement of the detection time gates, the neutron burst, etc., as the formation's f and thus the decay rate of the thermal neutron concentration curve varies.

Such a time interval setting is accomplished by multiplying each time interval of which the time duration is to be set, by a jointly chosen value of a finite number of discrete scale factor values F. This is shown in Fig. 2, where the duration of the neutron burst is shown as F x 200 ps, the duration of the delay as F x 200 ps, the duration of each of the gates G ^ through G ^ as F x 25 ps, etc. As also shown in FIG. directly succeeding neutron burst in time at the end 790 7 3 91 Δ, *, '1} 1 ν »/ -16- 20949 / JF / jl of the time gate G ^ g, where not only the duration of the neutron burst can be set by the scale factor value F but also the repetition period between successive neutron bursts, as indicated by the value F x 1900 ps. Therefore, it will be recognized that by changing the value of the scale factor F the count rate times for the v several gates can be uniformly changed with respect to the neutron burst in such a way that the detection of the thermal neutron concentration concentration is optimized in the particular formation in which it is employed. That wants to. say that by selecting the scale factor F, the individual gates G1 through G ^ g can be increased or decreased along the time base of the thermal neutron case curve as the rate of change of the curve and thus varies.

By providing a finite number of incremental changes in the width of the detection time gates, it is possible to properly locate the time gates with respect to the portion of a thermal, neutron decay curve that is most characteristic of the decay of the thermal neutron con-. center in the earth formation, namely the straight-line portion of the curve occurring on a semi-scan head like that of Fig. 2, in order to maximize count rates within the gates and thereby improve the accuracy of the measurement of Ί while at the same time reducing the need is avoided for the infinitely variable electronic gates used in prior art scanning tools. For example, it has been found that when three or fewer F values are used, the later gates used for calculating X contain too much background. This results in greater statistical uncertainty, necessitates background measurement over usably long accumulation periods, and requires a separate background calculation for nearly all calculations.

These drawbacks can largely be overcome, if not entirely, by using a larger number of, for example, 5 or 6 scale factor values F. In general, it can be stated that the more incremental values of the scale factor F are applied, the greater the statistical reliability achieved. is. On the other hand, the complexity of the tool increases when a larger number of scale factor values are applied. Accordingly, in accordance with the invention, it has been determined that four incremental values of F are sufficient to obtain improved statistical performance over the entire K range of interest, coinciding with a minimum of tool complexity.

35 The amount of incremental change between F values should be chosen 790 7 3 91 Λ -Ί7- 20494 / JF / jl in conjunction with the specific durations assigned to the gates in gate groups if II, m and IV, for accurate detection ^ The ranges of the decay curve over the entire range of * C are expected to be encountered in the earth formations, for example, <50 ps through J 600 ps.

5 For the gate widths of 25 ps, 50 ps, 100 ps, 200 ps. shown in Figure 2, it is preferable in accordance with the invention to change F with incidences of VT. Therefore, the preferred values F are 1 / §, 1, V "and 3. Also, as will be described hereinafter, stepping F" with the factor V "provides circuit advantages in implementing the F-factor procedure in the scan. 10 tool.

In order to avoid that F changes primarily due to statistical variation in the measured value of X, the criteria for determining whether the F value should be changed based on the current measurement of T is established so that there is an overlap is between adjacent 15 ranges for which one of the two F values associated with those ranges is suitable, In the case of the example of FS 1 / V ~ $, 1, \ T% and 3, a ge. arranges criterion for changing F based on a new measurement;

Table 1 F, <i new change F to old 20 t / Vr> 120 US. 1

1> 210 gravy. VT

VI 7 365 ps. 3

3 <285 ps. V ~ T

25 \ Γ5 * <165 gravy. 1 1 <95 ps. \ / Tf

These criteria allow use of <'s within the 95 ps range. up to 110 ps. of F values of either 1 / or 1, within the range of 165 ps. up to 210 ps. of F values of 1 or Vf and within the range 30 from 285 ps to 365 ps. of F- * values of either V3 or 3. Excessive overlapping regions are thereby prevented within which F need not be changed from the previous values. This prevents the whine (jitter) in Anglo-Saxon countries which is sometimes encountered in the gate control in the infinitely variable gate tool of the prior art.

35 At the beginning of each scan run or where a pre-measured value of 790 7 3 91 -18- 20949 / JF / jl; 6.

) / / · Otherwise unavailable, the initial scaling factor F is set as based on a fictitious' i, e.g. F = 1 for 'J' = 200 gravyThe tool will then automatically change according to the appropriate criteria i.e. that of Table 1 as the valid measured value of 'i changes in; the course of the loop. Before the procedures for calculating and then using a more Ύ value to determine whether or not a change in the scaling factor F is required, reference will first be made to Figures 3 to 12 of the drawing, in which the manner in which the change in the F-value and the associated changes in the time duration for the time gates to G, g, neutron bursts, etc., are implemented in the tool 10 in the hole, are shown.

Fig. 3 is a block diagram showing the interconnections between the neutron generator driver 30, the gate timing counter 16 which, as shown, the signal gate circuit 32, the gate timing counter 33, 15, the memory circuit 35, and the telemetry. logic circuits 37. The coupling between these circuits and the telemetry circuit 34 is also shown. Now that the implement is supplied with power to start a loop, inputs H and L to the gate timing control circuit 60-are automatically set to the logic 0 level. The time counting circuit 60 generates four 20 discrete clock frequencies, each corresponding to one of the scale factor values F = 1 / \ fT, 1, and 3. By changing the logic state of the H and L inputs, a choice is made of the determined frequency and therefore the F-value to be used. Here, it is assumed that the logic state 0 corresponds to F s 1. As will be fully described hereinafter in connection with Fig. 6, the output frequency signal is then sent to a series of scale means, ie counter · / divider circuits and decoders within circuits 60 and is used to generate all time count signals to control the operation of the neutron generator 22 and gate the nearby and distant detectors 24 and 26.

Thus, the time counting circuit 60 provides a neutron burst signal NB which is applied to an instruction signal circuit 64 which in response generates two ion source pulses (ISP1 and ISP1) which drive the neutron generator driving circuit 30 (see also FIG. 1), thus resulting in 35 the neutron generator 22 generates neutron bursts of the desired length of time and repetition rates 790 73 91 -19- 20949 / <IF / µl η '. Detectors 24 and 26 are preferably suppressed during and immediately following each neutron burst to isolate the downstream electronics from excessively high instantaneous count rates. To this end, the time counter generator 60 also generates a suitable blanking pulse for blocking the detector outputs for a specific period of time, for example, twice the duration of the burst. This is described in more detail in connection with Fig. 6.

The time count gate signal generated by time count generator 60 is denoted DCLK and the signal is used to gate the detectors in accordance with the regime of Fig. 2. Generally, this is done by supplying four frequencies related by a factor of two from a multiplexer under the control of a divide by four circuit. The multiplexer output is applied to the counter so that four pulses of the highest frequency are passed through the counter before it switches to pass four pulses of a frequency which is half of the highest frequency, until the entire time-gate signal DCLK is generated. The timing gate signal DCLK in turn creates memory control pulses WT, SET and RDY for use elsewhere in the system. The manner in which the DCLK, WT, SET and RDY signals are generated is explained in detail with reference to Fig. 6.

The gated nearby and distant detector signals NG and FG are applied to the nearby detector pulse counter circuit 68 and remote detector pulse counter circuit 70. In these circuits, the pulses of each detector for each gate period G ^ through G ^ are counted and a binary 25 number representing these values is transferred in parallel to the near and far memory circuits 62 and 74. Each pulse counter circuit actually has two counters. While one of the counters counts the pulses in one time gate, the other transfers the results of the previous count to the memory. Two counters are necessary because the time gates 30 are contiguous and there is not enough time for one counter to perform both operations simultaneously. A characteristic detector pulse counter circuit is shown in more detail in Fig. 7.

Each of the memory circuits 72 and 74 stores 16 8-bit words which represent the accumulated count for each time gate G ^ through G ^ g over a number of 35 neutron irradiation intervals. This is done by addressing the memories via an address generator circuit 76 so that the predetermined value of the accumulated counts for the time gate in question is represented at the input of an adder circuit. The memory output is then added to the current count for that port and the result 5 is again stored in the memory at the address for that time port. The manner in which the address generator 76 controls the memory circuit is described in more detail in connection with Figures 8 and 9.

At a predetermined time, a signal FCLK from the telemetry circuits 34 informs the tool to send the accumulated count for the six-ten ports G through up. The FCLK signal is received in the telemetry coupler 78 which generates an E signal and a TCLK signal which are applied to the memory address generator 76 for use in generating signal LDD in the address generator circuit 76 which are in turn supplied. to the memory circuits 15, 72 and 74 for effecting the transfer of the contents of each memory location along with the generated parity bits, in the parallel to series shift registers (not shown in Fig. 3). The TCLK signal then causes the data to be serially shifted from nearby memory circuit 72 to remote memory circuit 74 so that the remote detector count rate data for each source is placed in front of the near gate detector data. This data string ("string" *) is then passed through sync / status circuit 80 which a four-bit sync encode and status information is positioned at the beginning of the information to form the data signal contained in the following DATA signal. will be called. The DATA signal is in the form of a binary signal, arranged according to FIG. 10, which is sent to the telemetry coupling circuit 78 and then as signal SIO to the telemetry circuit 34 for transmission upwards. In this regard, the coupling circuit 78 mainly operates to ensure that the telemetry circuit 34 and are ready to receive the data before it is sent. The operation of the sync / status circuit 80 and telemetry coupling circuit 78 are described in more detail with reference to Figures 11 and 12.

As will be described below, the computer ^ calculates when the. velocity data is received at the surface and when it falls outside the limits for F = 1 according to Table 1, a two-bit binary code 35 is generated to change the scale factor. There are four instructions which 790 7 3 91 <1 ..

-21- 20949 / JF / jl specify the scale factor and three other constructions, bringing the total to seven. The scale factor instructions result in the generation of H and L signals in the instruction signal circuit 64 which, as described above, are sent to the time counter generator 60 to change the scale factor. The available instructions are shown in Table 2.

Table II

Instruction H.L.code F Comments 01 00 1 · Always 05 01 f? Always 10 q6 10 1 / Y3 * If the previous F is 1 (05) +06 11 3 If the previous F is Fri? 02 - - Positive plateau test 03 - - Negative plateau test o 04 - - Calibration test 15 01-07-06-01 - - Neutrons on

Referring again to FIG. 3, the binary coded instruction signals CMCD are received from the telemetry circuits 34 by the instruction decoder circuits 84 (FIG. 4). As indicated in Table II, the tool can be instructed to perform certain tests, for example, plateau test or calibration test, in addition to changing the F values. It can also be instructed to bring the neutron generator into an equilibrium state, in which only the NB signal is required to initiate a burst. In the decoder circuit 82, the instruction signal is analyzed and the correct output, ie 2 to 7 or CCLR, is activated. The instruction lines all lead to the instruction signal circuits 64, in which the signals are to execute the instructions generated. This is described in detail in conjunction with Fig. 5.

In order to demonstrate the detailed operation of the various circuits shown in Fig. 3, it will be assumed that the results of the first X calculation have resulted in a value of X which is less than 95 µs. From Table 1 it can be determined that the surface computer 42 will require a • change in scale factor from F = 1 to F = 1 / V ^ · The computer will thus generate the instruction 06 (binary bits) corresponding to HL code 10 , as shown in Table II. It should be noted that in Figures 4 to 9 790 73 91 -22- 20949 / JF / ji. , ·· * · i 'and 11 to 12 the numbers in the circuit blocks represent the TV numbers of the CMOS integrated circuits which can perform the indicated functions. These integrated circuits are available from a variety of manufacturers, including Motorola, Fairchild, National Semi-conductor and 5 others.

Fig. 4 shows the details of the instruction decoder 82. In FIG. 4, the F-instruction signal of the telemetry circuits 34 is received by a mono-stable multivibrator 84, which has a time delay of 250 ps and which operates to generate a clock signal from the CMCD signals at the bottom of the hole. For this purpose, the CMCD signal is in the form of a pulse width coded signal with positively going transitions at the boundary between each bit, e.g. every 400 ps. This positively going transition clocks the mono-stable multivibrator 84 to generate output clock signals with positive going transitions in the middle of each bit period. The monostable transition clocks a shift register 86 which receives the CMCD signal applied to its input. So when the CMCD signal is low. in the bit period, a 0 is input to the shift register 86 and a 1 is input when it is high. After six clock pulses from the monostable multivibrator 84, the serial data in the CMCD signal is available in parallel form at the outputs of shift register 86. The three most significant bits from the output of register 86 are applied to an OR gate 88. When contains one of these three outputs 1, it will be passed to the input D of a binary to decimal decoder 90 through an OR gate 92. It inserts an 8 into the decoder 90 to prevent its output being less than 8 Thus, a bit in one of the higher places of the CMCD signal indicates an invalid code, which is above the negative instruction numbers 01 to 07. Accordingly, the instruction code should not be applied to a decoder 90 so that all CMCD signals are input to the shift register 86. For this purpose, the CMCD signal is also applied to a retriggerable monostable multivibrator 94 with a nominal pulse duration of 750 µm, whose output goes high when the first CMCD pulse arrives and remains there after 750 jis. after the last arrival. As shown in Fig. 4, this output is also applied to the D input of the decoder 90 through the OR gate 92 and acts to block the decoder output so that the instruction code is fully loaded into the shift register 86. When the output of the monostable multivibrator 94 790 7 3 91 t -23- 20949 / JF / jl \ ι "\ eventually goes low, decoder 90 decodes the instruction signal and activates one of its output lines depending on the instruction code contained in the CMCD signal In this case, the output will be 6 for the instruction 06 (see Table II). The instruction remains in the shift register 86 until it receives 5 a reset RST pulse from the telemetry couplers 78.

This RST signal is also applied to a scale or divider circuit 96. Divider circuit 96 provides an instruction clearing CCLR signal through an OR gate 98 after receiving eight RST signals without being reset by the output of the monostable multivibrator 94. Thus, when the 10 CMCD pulses stop for some reason, the CCLR signal is still generated. The CCLR signal is also generated through the OR gate 98 by "1" output from the decoder 90, i.e., an 01 instruction resets the circuits to generate F = 1.

The six outputs of a decoder 90 and the CCLR signal are applied to set six latches 100A through 100F in the instruction signal circuit 64 shown in detail in FIG. 5. The CCLR signal can be all latches or directly reset as in the case of the latches 100A and 100D or via OR gates 102A to 102B and 104A to 104B, as in the case of the latches 10QB-100C and 100E and 1Q0F, respectively. Latches 100E and 100F send a plateau test- When the instruction 02 is decoded in the circuit 82 (see Table II), it imprints a main level on the bore input of latch circuit 100E and resets latch 100F through OR gate 104B. As a result, the output of this latch circuit drives the output of amplifier 106 to about + 15 volts, which voltage is applied to the detector power sources through resistors to perform the positive plateau test. received, latch circuit 100E resets through OR gate 104A and latch circuit 100F is set, so that amplifier delivers 106 -15 volts to the detector power supplies for the negative plateau test. In the case where the instruction code is 01, a CCLR signal is created as described in connection with Fig. 4, and this signal resets both latch 100E and 100F, causing the output of amplifier 106 to be zero. The output of latch circuits 100E through 100F are also applied to NOR gate 35 108 which supplies a plateau test status signal PCS via inverter 110 to the 790 7 3 91 * ƒ. ·; i -24- 20949 / JF / jl sync / status circuit 80 whether positive or negative platter test is performed.

The instruction code 04 sets latch circuit 100D whereby the calibration tests are now turned on by passing WT 'pulses from the time-5 count generator circuit 60 to the NAND gate 112. The output of gate 112 goes to the amplifier inputs of both detectors. During the calibration test, one count per detection gate loop is added to each of the sixteen gate channels for both detectors. When the detector background is low (the tool is in standby state), all test count rates in all ports 10 should be substantially equal. The output of the latch circuit 100D is the calibration test status signal, i.e., CTS, and is also applied to the sync / status circuit 80. "

Latches 100B and 100C control the scaling factor F in response to instructions 01, 05 and 06. The outputs of these latches 15 go to two D flip-flops 114A and 114B which are clocked by an E 1 pulse from the address generator circuit 76. The outputs of the flip-flops 114A and 114B are the H and L lines which go to the time count generator circuit 60 (see FIG. 3) to control the selection of the time work frequency and therefore the scale factor F. For the case assumed from instruction code 06 it is known from table II that the H, L code must be 10. A high level on line 6 in Fig. 5 accomplishes this by setting the latch 100C and flip-flop 114F. Instruction 06, however, does not reset latch 100B as that instruction is only valid when F equals 1 and F equals 1 only when a 01 instruction resets all latches as a CCLR signal and makes both H and L low . Instruction 05 puts 100B there and resets latch 100C through 0F port 102B. This is necessary because that scale factor change does not depend on the previous value of F. (see Table II). The grounded capacitors at the latch circuit inputs prevent them from being triggered by sparks from the neutron generator.

As indicated in Table II, the process of turning on the neutron generator 22 requires the sequence of instructions 01-07-06-01 to be received by the instruction circuitry 64. The first 01 instruction initializes the state of the latch circuits while flip-flop Β6Β 35 is not reset. The 07 instruction sets latch 100A where 790 7 3 91 "25" 20949 / JF / jl ίό connects a high signal to the D input of flip-flop 116A. When followed by a 06 instruction which sets latch 100C, the high input of flip-flop 116A is clocked into that flip-flop, making the Q output high and the Q output low. The output of latch 100C 5 also resets flip-flop 11B. When the last 01 instruction is decoded, a setting pulse reaches latch 118 via NAND gate 120 and inverter 122 because flip-flop 110A is set. The output of latch circuit 118 sets flip-flop 116a closes NAND gate 120 and terminates the set pulse to latch circuit 118. With latch circuit 118 set, the neutron burst pulses 10NB from the time count generator circuit 60 can pass through NAND gate 124 and inverter 126 to forming ion source pulses ISP ^ and ISP ^ (see FIG. 5) which drive the neutron generator control circuit 30.

While the beam current in the neutron source increases, a relay in the control circuit thereof is closed. This connects a power source to the output line DC of the latch circuit 118 and makes it impossible to turn off the neutron source without turning off the tool power. It can be seen that the flip-flop 116B maintains latch circuit 118 in the reset condition to the correct time flip. Also, the setting input to flip-flop 116b is activated when power is first supplied to the tool because it is connected to the plus voltage through a capacitor. This capacitor and a resistor will pull the setting input low to flip-flop 116B after the power has been applied for around 10 seconds.

Referring now to FIG. 6, two crystal oscillators 128 and 130 have built-in three-by-three timing circuitry 60 in the time counting circuit 60. Thus, both the crystal frequency and the divided-by-three frequency are available from each oscillator circuit, providing a total of four discrete clock frequencies f1, f2, f3 and f4. According to the invention, it is ensured that each of these frequencies corresponds to one of the F values 1 / VlT, 1, /? and 3. This can be done simply by selecting the crystal frequency f1 of the oscillator 128 so that it is V ^ 3 times later than the crystal frequency f2 of the oscillator 130. Then the divided-by-three frequencies f3 of the oscillator 130 will be are related by a factor to the VT to divided by three-frequency f4 of oscillator 128, and it can be seen that the frequencies f1, f2, f3 and f4 are separated by the factor and therefore may correspond to the scaling factor values F = 1/79073 91 '' it \ · «* -26- 20949 / JF / jl 1,] / ~ 3 and 3. The clock frequencies f1 through f4 are applied to square channel data selector or multiplexer 132. The logic levels of the signals H and L applied to the inputs A and B of the multiplexer determine which of the four input frequencies should be connected to the output of the unit.

The operation of the multiplexer 132 can be summarized as follows:

Table III

selected output

A B input frequency instruction F

low low X ^ f ^ 01 -1 10 high low X1 f2 06 1 / V?

low high X2 f ^ 05 VT

high high χ3 f ^ 05 + 06 3

In accordance with the invention and as already described in the foregoing, all time counting waveforms are required for the operation of the neutron generator 22 and the detectors 24 and 26 in accordance with the regime of FIG. 2 collectively derived from the output of the multiplexer 132. Therefore, a change in the F values can be implemented quite easily in all time counting circuits of the implement. The actual generation of the various time count signals is achieved using a scale circuit 134 coupled to the output of a mutliplexer 132. The scale circuit 134 is implemented using the 8 lowest frequency outputs of a 12-stage binary ripple counter.

As seen in Fig. 6, when the Qg and outputs of the scaling circuit 134 are high and the Qg output starts to become high, the RSET pulse is generated in NAND gate 136 and resets the scaling circuit via resistor 138. This indicates the beginning of the neutron burst pulse NB generated in gate 140, which pulse lasts as long as the Qg, Qg, Q1Q and Q1 outputs of the scale circuit 134 are all low. As previously described in connection with FIG. 5, the NB pulse is used in the instruction signal circuit 64 to generate the ion source pulses ISP1 and ISP2. Returning to FIG. 6, negative pulses (N and F) pass from the nearby and distant amplifier discriminators from the near and distant detectors 24 and 26 through capacitors 142 and 144 on their way to ports 146 and 148. There they are blocked during the neutron burst and for an equal time thereafter by 790 7 3 91 - 27- 20949 / JF / jl f * the output of NOR gate 150, of the output is high as long as Q ^, Q ^ q and of scale 134 are all low. When the output of gate 150 is low, it is made possible to pass the near and far pulses N and F to the near detector pulse counter 68 and the far detector pulse counter 70 (see FIG. 3).

The output of scale circuit 134 is designated ECLK and is sent to address generator 76. The Q ^, Qg, Qg, and Q ^ outputs are also used as inputs to a four-channel data selector or multiplexer 152 for generation of the detector gate signal DLCK, which signal 10 is in the form shown at gates 0 ^ to G ^ g in Fig. 2. The A and B inputs to the multiplexer 152 are driven by the third and fourth stages of the scale circuit 154. The waveform at the output of the gate 150, when high, keeps the scale circuit 154 reset and at the same time blocks the output of the gate 156, i.e. it keeps DCLK low 15 during the neutron burst and delay period thereafter, as shown in Fig. 2. When the output of gate 150 goes low, the reset action on scale 154 takes a little gravy. due to the time constant of the R-C circuit 158 such that scale circuit 154, which is clocked by DCLK, does not trigger on the first positive going edge of DCLK. As long as A and B of multiplexer 152 remain low, the DCLK resembles an inverted version of scale 134 which has the period T / 8. The period T is equal to the duration of the widest gate of the regime of Fig. 2, that is, 200 gravy. for each of the gates G ^ through G ^. This duration is counted by the scaling circuit 154 during the first four DCLK cycles, generating the first four gate signals of 25 µs each. The output of scale circuit 154 then goes low, placing a high level on the A input of multiplexer 154 and results in the DCLK being controlled by the Q output of scale circuit 134, with a period T / 4 or 50 us. At the end of four T / 4 cycles as counted by scale circuit 154, which cycles represent the gate signals for gates G ^ -Gg, scale circuit 154 then selects Qg output from scale circuit 134. This output is twice the period of the Q ^ output, i.e. T / 2 and results in the generation of the gate signals for the 100 jus gates G9-G12 * Finally, four cycles later, the output of the scale circuit 134 with a period T is selected. When four cycles of Qj have passed 790 73 91 1 * "-28-20949 / JF / jl, the circuit is effected in gate period through gate 150 as described above.

Memory control pulses WT, WT ', SET and RDY are generated in a Johnson counter 160 which is clocked by the output of multiplexer 132 through gate 162. These memory control pulses are only generated when DCLK is high (DCLK low) since the DCLK- signal inverted by the gate 164 is applied to the setting output 160. This is done because there is no need for memory pulses during the blanking period. As shown in Fig. 6, the RDY signal is the Q2 output of counter 160, the SET signal is the Qj input, the WT signal is the Qg output and the WT 'signal is the output.

When the waveform WT goes high, it blocks further clocks because it is connected to the clock active input C of the counter. WT 'will therefore remain high

There _ until the counter 160 is reset the next time DCLK goes high.

The near and far detector pulse counter circuits 68, 15 and 70 (see FIG. 3), which receive the gated detector signals NG and FG from the time counter 160 (see FIG. 6), are identical. Therefore, the operation of only one of these circuits will be described in conjunction with Fig. 7.

The pulse counter circuits each include a scale circuit 166 which receives detector pulses from the even-numbered gates G2> G ^ etc., and the scale switch 168 which receives one pulses from the odd-numbered gates G ^, G ^ etc.

As the scale circuit 166 counts, scale circuit 168 maintains its count until reset by the WT pulse, and vice versa. When the WT pulse arrives, the held counts are stored in the memory circuits 72, in the case of the nearby detector, and 74, in the case of the remote detector (see Fig. 3). Flip-flop 170 controls operation in accordance with the DCLK signal. After sixteen DCLK pulses, the RSET signal ensures that flip-flop 170 is in a reset condition to repeat operation for the next irradiation interval.

The gated detector pulses NG (> or FG) are positive and suitably around 0.4 gravity in width. They are directed to NAND gates 172 and 174 which are alternatively opened and closed by the Q and Q outputs, respectively from flip-flop 170. Two additional gates 176 and 178 receive WT pulses, following their inversion in gate 180, and are similarly controlled by the Q and Q outputs of flip-flop 170. As will be appreciated, when the scale circuit 166 is clocked by NG (or FG) signal pulse 790 7 3 91 C 'J' '-29- 20949 / JF / jl it is not reset by WT but is thus reset scale 16¾.

After completing a count for a time gate, the total accumulated count accumulated therein appears in parallel form at the Q outputs of the scale 5 devices 166 and 168 and is then applied to one of two quadruple two-channel data selectors 182 and 184. Of which the A and B inputs are controlled by the flip-flop 170 so that they choose the outputs of the scaling circuits 166 or 168 that keep its counts and ignore the outputs of the scaling circuit that counts.

As seen in FIG. 3, the data selector outputs are supplied to the memory circuit 72 for the nearby detector counts and to the memory circuit 74 for the remote detector counts.

Before discussing the memory circuits, however, it is useful to consider the operation of the address generator circuit 76 that controls the memory circuit. This circuit is shown in Fig. 8 and is to control the storage of count rate data after it has been detected and the readout of the data in response to telemetry requests. Although the data storage and data read operations are asynchronous, the memory where the data has been temporarily accumulated should be accessible both for quickly storing 20 new counts at the correct address and for reading accumulated counts at other addresses when required by telemetry. Accordingly, two address scaling circuits are provided to track the individual addresses required for storing and reading, and priority is given to storing new data. The telemetry can then read data between storage operations.

In Fig. 8, the RSET pulse of the time count generator circuit 60 initially sets all outputs of the "storage" address counter 188 high and maintains it high until the start of a new detection interval. The first DCLK pulse at the beginning of a new interval clocks counter 188 in all zeros, and this is the address where the gate G counts of the previous detection interval are stored, while the gate counts in the new interval are accumulated in one of the detector pulse counter circuits 68, 70 (see FIG. 3). As described above in connection with Fig. 7, the counts of the odd numbered gates go to one scale circuit in the counter circuits and those from the even numbered gates to the other. During the DCLK cycle, one 790 7 3 91 · '. *> (V ...

"30" 20949 / JF / jl scaling is actively counting, while the others are counting the previous gate so that they can be added to the memory.

Thus, the G ^ g count for each detector is held in the "even-numbered" scaling for the detector until it is stored in memory 5 for the immediately following G.j period. The address stored in memory during this storage operation is the output of the scaling circuit 188, which is passed through a quadruple two-channel data selector 190 to the memory circuits. As each DCLK pulse arrives, characteristic of a new gate, the scale circuit 188 is incremented to change the storage address.

As noted, an address scale 192 for telemetry reading is provided. Its clock and reset inputs are controlled by a system of gates 194. 196 and 198 which in turn are controlled by a Johnson counter 200. This is done so that the telemetry addressing circuit 192 is clocked only for the data words readout and not for the sync / status portion of the information supplied to the telemetry. . The counter 200 is clocked by all positive going edges of E and is reset by the telemetry reset signal HST, which is not to be confused with the reset RSET of the time count generator 60. The E pulse is excited at -20 in the telemetry coupler 78 (FIG. 3) and goes high for a desired number of times per telemetry window, e.g. 5 and is followed by the telemetry reset signal RST. The NOR gate 194 keeps one input of the NAND gate 196 low until E goes positive for the second time, to prevent generation of memory address signals while the sync / status words are generated and transmitted. Then, the output of gate 194 goes high and remains high for all data words in each telemetry window, e.g. 4 (see FIG. 10), thus allowing counter 192 to be clocked at a corresponding number of times per window to generate the memory readout address. After counter 192 has been clocked 16 times (four windows), output Qj goes. of the counter 192 high, allowing the output of NAND gate 198 to go low at the next "1" output of the counter 200, ie, the E signal goes high. This resets counter 192 through gate 198 and keeps it in sync with the E pulses. The and Qjj outputs of counter 192 control the window identification bits in the sync / status word and are supplied to the sync / status circuit 80 (Fig. 3). 35 Flip-flop 202 controls the data selector 190 so that the appropriate address 790 7 3 91 f. * -31- 20949 / JF / jl counter, i.e. counter 188 or counter 192, controls the memory address lines Aq through A ^. Flip-flop 202 is set by the RDY signal and reset by WT ', both of the time count generator 60. When flip-flop 202 is set, the data selector 190 selects the "save" address and when it is reset, the selector 190 the "read out" address As can be seen in Fig. 6, the RDY signal is the Q2 output of counter 160 and the WT 'signal is the Q1 output, therefore storage occurs for a portion of a cycle and transfer during the other part.

In addition to controlling the memory address lines, the circuit of Figure 8 generates the LDSy LDD-j READ and CLR pulses. The LDS signal loads sync and status bist into a shift register in the sync / status circuit 8o. Shift registers in the memory circuits 72, 72 are loaded with data by memory and selected readout addresses, by the LDD signal. The READ signal causes the memory output to correspond to data at selected addresses and the CLR signal 15 resets the memory content at a selected address. The READ signal starts at the same time as LDD, but takes less time. CLR occurs at the end of LDD. When READ and RDY are mixed, they become CS (chip choice). When CLR and WT are mixed they become. WRD.

Interference between storage and reading is avoided since LDS, 20 LDD and READ are not generated directly by a positive going edge of E but are generated by the first positive going ECLK edge after E goes positive. The ECLK signal is applied to the Cp input of flip-flop 204. The D input is normally high, so that Q is normally high. When E goes low, it is inverted by the gate 206, differentiated by a ^ and Sek ° PPeld 25 at the D input of flip-flop 204 via an RC delay, R ^ and C ^ When the D input of flip-flop 204 flop 204 is low, the first positive going ECLK edge will cause the Q output to go low. This output is differentiated by R ^ and C ^ and is applied to two gates 208 and 210. When the "1" output of counter 200 labeled E ^ is high, an LDS pulse will appear at the output of gate 210 When E ^ is low, there will be an LDD pulse at the output of gate 208. R ^ and Cjj determine the duration of these pulses A simultaneous positive pulse occurs at the output of inverter 212, which is then differentiated by Rg, C ^ to become the READ pulse The READ pulse and RDY pulse are combined in gate 214 to generate the signal CS.

35 A flip-flop 216, which functions as a monostable multivibrator because 790 7 3 91 -32- 20949 / JF / jl Μ ",. j its Q output is connected to its countdown input r, via time delay network Rg, Cg. It is triggered by the trailing edge of LDD after the latter is inverted and somewhat delayed by a time delay network formed by and an input capacitance at the clock input Cp of flip-flop 5 216. The CLR pulse is taken from the Q output of this flip-flop. The duration is controlled by Rg and Cg. The CLR and WT signals are combined in the NOR gate 218 to create the WRT signal.

Flip-flop 220 plays a role in setting the quality bit in each word. This is triggered by the DCLK signal from the telemetry coupler circuit 78 and is output from the output of gate 222. A PRT I signal from the remote memory circuit 74 and the LDD signal send the inputs to gate 222. Operation of these gates and flip-flop 220 will be described in detail in connection with the memory circuit shown in FIG.

9.

The memory circuits 72 and 74 are the same, from which only one of them is shown in Fig. 9. Each memory circuit includes two freely accessible memories (RAM) 224 and 226, each capable of storing four data bits at sixteen different addresses, The address lines AQ up to and including these memories come from the address generator circuit 76, shown in detail in FIG. 8. During readout, the complement of these data appears on the outputs. The outputs of the RAMs 224 and 226 and data inputs of the pulse counter circuits are applied to the callers 228 and 230, each of which is capable of adding two four-bit binary numbers. When these counters are connected in series as shown, they can add two eight-bit binary numbers. The result of this addition is stored in quadruple D flip-flops 232 and 234.

In this arrangement, the complement of the adder 228 and 230 outputs appears to the flip-flop 232 and 234 outputs when the SET pulse of time generator 60 becomes positive. Therefore, it can be seen that the current gamma count in a given gate is added to the previous total count for that gate. Memories 224 and 226 therefore store the total count, for each gate, for a number of neutron bursts, that is, over a number of irradiation intervals.

Incoming data from the pulse counter circuit 68 or 70 comes on the B. | through B ^ inputs of the counters and the memory 224 and 226 outputs 35 are connected to the A ^ through inputs. In order to enter new data 790 7 3 91

J

A beat of RDY-SET-WT pulses is generated. The RDY pulse arrives on the CS line (shared with READ) and the WT pulse is on the WRT line. At the end of RDY, the memories latch to the new address supplied by the address generator 76. The data contained in that address appears on the Q outputs of the RAMs 224 and 226 and the binary number represented thereby is added to the number supplied by the associated pulse counter circuit. After a short time delay, the setting pulse locks the sum in the D-flop flops 232 and 234. The SET pulse is followed by the WT pulse which writes the complement of the sum in the memories at the same address 10. In this regard, it should be noted that the memory output is the complement of the stored number.

A shift register 236 is used for parallel to series conversion of the data in memory when required by telemetry on arrival of an LDD pulse followed by the TLCK signal. A parity tree 238 is also provided to generate the parity bit which is a portion of each gate detector word (see FIG. 10).

Telemetry reading requires a sequence of LDD, READ and CLR pulses. Data of the desired address appears on the memory output lines at the end of the READ pulses. The READ pulse is overlapped by the 20 LDD pulse which loads the data into the shift register 236. At the end of LDD, the CLR pulse occurs, resetting the D flip-flops 232 and 234, in order to Set Q outputs to 1 while applying a negative pulse to the WRT line to write the ones into memory. This boils down to wiping that memory address, because it will be read the next time with the result all zeros.

Fig. 3 shows how the shift register and parity trees of the nearby and distant memory circuits 72 and 74 are interconnected, as well as with the SO (slide out) output of flip-flop 220 of the address generator circuit of FIG. 8. Both memory circuits 72 and 74 simultaneously store data from a data detection gate and both are read simultaneously. As shown, the shift registers of the two memory circuits 72 and 74 are connected in series, the SO output of the nearby detection memory circuit 72 being connected to the SI (insert) input of the remote detector memory 74. The SI The input of the nearby memory 790 7 3 91 », -34- 20949 / JF / jl gene circuit 72 comes from the SO output of flip-flop 220 in the address generator 76. This output controls the parity bit . The parity trees 2.38 in the memory circuit are also connected in series to form a single parity bit for the combined near and far word.

5 The resulting output of the distant detector memory parity tree (PRT

I) determines whether the flip-flop 220 will be put in the address generator by the LDD pulse or will remain zero.

Following LDD, the TCLK signal from the telemetry coupler 78 arrives and causes the data to be extended. The first series 10 data bit is the most significant bit of the remote memory 74. After the eight remote bits have been sent, the eight nearby bits are extended, followed by the parity bit.

As can be seen in Figure 3, the data bits of the memory circuits are shifted in series via the sync / status circuit 80 which adds the sync 15 and status bits to the front of data (see Figure 10). Fig.

11 shows the details of the sync / status circuit with the data words applied to the SI input leading to NAND gate 240. The sync / status bits were applied to NAND gate 242 and these two signals are combined in gate 244 to provide the data signal outputs. To generate the sync / status bits, the LDS pulse loads the shift registers 246 and 248 with parallel data, which are then extended by the TCLK signal while the E1 signal is high. The LDS signal also triggers the flip flop 250 which acts as a monostable multivibrator and resets the scaling circuit 252, which controls the "spare" bits, at the end of the sync / status 25 word.

Flip-flop 254 is loaded via port 255 by the LDS signal with the Q output of parity tree 256, which is the parity bit for the sync / status bit in Fig. 10. The inputs to Pg of shift register 248 are connected to +5 V. These control first four data bits which are the sync / sig- 3 ° sew. The other bits are the status signals and are applied to the rest of the shift register 248 inputs and the shift register inputs 246 as follows: see Table IV on page 35.

35 790 7 3 91 -35- 20949 / JF / jl /

* I

3

Table IV

Circuit Input Signal 248 Pjj Plateau test status 248 P High voltage status (neutrons on) 5 248 p L »instruction 05 is active 248 P H = instruction 06 is active 246 'Pg. - higher order window I.D. bit 246 P ^ = lower order window I.D. bit 246 Pg BC = burst control (ISP on) ^ 246 P_ CTS s calibration test status b 246 P.j — P ^ Reserves 254 D Parity bit ·

The status signals applied to these shift circuits come from the instruction signal circuit 64 and the address generator 76. Following the sync status word goes low. This connects the SI input to the data input so that the series of data can be shifted from the memory circuits 72 and 74. to the end of the sync / status word.

The data signal or DATA signal comprising the data words and the sync / status word is sent to the telemetry coupler 78, shown in detail in Fig. 12, for transmission upward. This circuit determines the number of words to be sent in each telemetry window and the number of bits in each word. There are only two input lines to the telemetry interface, that is, the FCLK signal from the telemetry scale 34 and the series of data, or the DATA signal from the s7ni / status 25 circuit. The outputs of the coupling circuit are SIG and RST which go to the telemetry circuit 34, the E signal which goes to the address generator 76 and TCLK which goes to the address generator 76, the memory circuits 72 and 74 and the sync / status circuit 80.

The telemetry circuits start transmitting FCLK signals when it is 3 ° when it is time to transmit data upwards. Before FCLK arrives, E and E * (the output of flip-flop 62 as described below; will be described below) are low and TCLK is high. The first half cycle of FLCK makes E high because one input of a NAND gate 258 goes low after a short time delay due to the RC network at that input. The first time FLCK goes low, flip-flop 260 is clocked, making output E 'high. The Q output of flip-flop 260 goes 79073 91 1 <. j * -36- 20949 / JF / jl low while E is held high by lock or containment port 258. The E signal is made high as soon as this is possible because the LDD signal in address generator 76 may occur after an ECLK cycle after E gets high.

With the E-signal high, the A-signal at the output of NEN-5 gate 262 starts by going low and then the other input. The positive going edges of clocks follow a flip-flop 264 of which the Q- TCLK output. The TCLK signal also clocks the scale circuit 266 on the negative going edges. The purpose of the scaling circuit 266 is to count the number of bits per word, including the sync word. After clocking nineteen times, the outputs B., B_ and Bc of the scale circuit 266 are all high due to the fact that the output Bg of the NAND gate 268 goes low. Half a FCLK cycle later, A1 goes low, causing output A ^ of gate 270 to go high. The A2 pulse. resets flip-flop 260 through port 272 and thereby resets E. Signal A2 also causes an A ^ -15 pulse to be generated in gate 274 and an A ^ pulse to be generated in gate 276.

The latter pulse resets flip-flop 264 as well as counter 266. In addition. the signal adds a count to a word counter 278. The next time FCLK goes low, E 'goes high again and the process is repeated. At the end of five words, the Q ^ and Q ^ outputs of the counter 278 cause the output Dj. of port 280 goes low. The next positive FCLK edge will pass through gate 282 and will trigger the monostable multivibrator including gates 284 and 286 to provide the RST pulse. The RST pulse stops the FCLK signal from the telemetry circuits 34 and resets all flip-flops and scales in the coupling circuit 78.

The output C2 of Ρ00Γ288 is a series of eighteen negative pulses, the leading edges of which mark the boundaries of the seventeen bits in a word. For C2 to go low, the output Bg of gate 290 must be high,

ie whether the B_ or of ,, - output of counter 266 should be low, TCLK

k b should be high and A ^ should be low. The Bg output provides the word sync interval by going low to stop the C2 pulse. The DATA, A.1 and DCLK signals are applied to port 292 to generate the C .j signal.

For DATA to occur, AATA and TCLK must both be low. The C1 signal is midway between the C2 «pulses when this occurs. The signal C ^ is an inverted mixture of C ^ and C2 pulses provided by inverter 294 and goes to the collector of transistor 296. Waveform 790 7 3 91 <* -37- 20949 / JF / jl

It goes to the base of the transistor. The output (SIG) of the transistor at its emitter is connected to the emitter of a similar transistor in the telemetry circuit 34. When the telemetry uses the SIG line, the E 'signal is low, so that the transistor 296 is equivalent is on 5 a: open circuit. Similarly, when the coupling circuit transmits the SIG signal, a similar transistor in telemetry circuit 34 is turned off.

Thus, an FCLK signal from the telemetry circuit 34 can be seen to generate the TCLK signal which shifts the data from the memories and sync / status 10 circuits through the telemetry couple transistor 296 to the telemetry circuit 3% itself. During this time, the number of bits in each word is counted and controlled as well as the number of words in each window.

The preferred method for calculating 'J' of the nearby detector 5 * rates through N16 will now be described.

As noted above, the signal counting circuits 3 & the count rate data accumulate for a time interval Δ t for transferring the data from the buffer store 40 and which are reset by starting a new counting sequence. Correspondingly, the counts per time gate, that is, up to and including for the nearby gates NG ^ through NG ^ and 20 F1 through F ^ for the remote gates FG1 through FG ^ g are as transferred to the storage 40 and the computer 42 are not actual counting rates but simply the counts accumulated at each gate or data accumulation period Δt. Thus, when reference is made to the data symbols N1 through W ^ g and F ^ through F ^ g, the following description regarding the calculation of C is to be understood as representing the total counts over a time interval Δt, in contrast to the feel rates generated during the respective time gate in the downhole scales. To this end, an internal clock is provided in the surface equipment to measure the time period A t of each accumulation period in order to obtain accurate count rates for each gate.

Roughly said, the method followed when calculating * C and then for choosing the scale factor F is as follows:

New values of are periodically calculated for each detector based on ratios • R formed for each detector from the respective 35 net (background corrected) count rates accumulated at the surface 79073 91 -38- 20949 / JF / jl over the period At, selected time gates. These values are denoted hereinafter by Ί N for the near detector values and for the far detector values. As will be described more fully below, there are preferably seven sets of gates, each corresponding to a different ratio R for each of the four scale factors F. The particular combination of gates each determining the ratio R is that combination found to minimize the dispersion of "i at a given interval of chosen as the validity interval of the corresponding ratio. Although the same set of gates is used when calculating both and Ί ^ this is chosen based on only a previously measured value of ^ and is that set valid for such a value of ^ for the F value which is then used. Using the gates thus identified, the ratios R ^ and Rp are calculated, and the news value of and ^ are calculated from linear 15 equations of the form:

Ί = a + b R ”1, CD

where a and b are coefficients which establish a linear relationship between R ~ and ^ over the interval of validity for those particular ratios R. The values of a and b for each ratio are pre-calculated and stored in a computer as a lookup library. The criteria for determining whether a change in the scale factor F is required is then examined based on the new value of ^ from equation (1). When it is determined that the scale value F is to be changed, the appropriate instruction is sent down to the control circuit 33 for selecting the new F value as described above. Preferably, instructions for changing F are sent only at the beginning of new data accumulation periods At in order to avoid mixing of data taken with two different F values.

This procedure is, of course, performed repeatedly during the course of a scan run, with new values of ^ and ^ calculated at the end of each accumulation interval Δt and new Value of the scale factors F selected as required. Since, as noted, instructions for changing F are sent only at the beginning of an accumulation period, and since the 'Τ' calculations and F-selection procedure may require a significant fraction of an accumulation period, instructions 790 7 3 91 -39- 20949 / JF / jl for changing F down are sent at the beginning of each other accumulation period. For example, at a typical scan rate of 1800 feet per hour, the surface accumulation time A t may be about 1 sec. for providing accumulation times Δt and thereby γ measurements are 5 corresponding to 6 inch deep intervals. Therefore, changes in F can be made as frequent as one per foot depth, which is very adequate for tracking the most rapid H changes commonly encountered, i.e., about 100 µ per foot.

Referring now to Figures 13A, 13B and 14, the manner in which a library of gate sets for determining the ratio R ^ and R ^ and the corresponding values of the coefficients a and b for use in calculating ^ according to equation (1) are seen. Since it is a primary object of the tr calculation procedure to optimize the static accuracy of the value obtained therefrom, it is desirable to use as many ratios as possible in order to reduce the statistical uncertainty of the result, but any ratio. use only over its valid 'j' range. In order to determine which ratios provide optimal statistical accuracy in 'J' over the range of interest, some preliminary determinations should be made. First of all, it is desirable to take into account early non-exponential decay of the thermal neutron concentrations. This is done by choosing the first gate in any set such as that which starts closest to a time equal to twice the preceding of the end of the neutron burst. Second, the gates used in the numerator and the gates used in the denominator are preferably contiguous in time. This is equivalent to using counts of only one gate for the numerator and denominator, such as gate ΔΤ1 in Figure 13A and gate ΔΤ ^ in Figure 13B. Following this, the number and identity of the gates contained in each term of the relationship is determined empirically. In the initial step, the average counting ratio in each gate 30 is determined by the expression:

"T

Nij = A ° 1_e B / t) (® "Vt - e-Tj / r) + ΒΛ (2) • T (i - Vx) -. - tTj-Ti>

P TP

35 where Me represents the average number of counts per second in a gate- 7907391 -40- 20949 / JF / yl

Cl represents which starts at the time T. and ends at the time T. with

- J

drawing to the end of the neutron burst, Tn and T are the duration o Ρ and the neutron burst repetition period and (where the times are expressed in units of seconds) AQ is the total number of decay signals-5 counts that would be detected following a single long burst of neutrons (ΤΒ >> 0 ^) and Bq are the total background and per second when an equilibrium neutron flux is assumed at the same intensity as that occurring during the neutron burst T_.

D

Ag depends on the peak neutron yield and detector size, efficiency and distance as well as the environment of the tool. BQ depends on all the parameters described above and also on time, since the main contribution to Bq with a Nal detector is the 25 minute neutron activation of I in the detector crystal. Characteristic, observed values of AQ are between 50 to 100 counts for the nearby detector and between 9 to 4 counts for the distant detector. B rises to about 5 x 10 4 h counts per second and 0.6 x 10 counts per second for the nearby and distant detectors, respectively.

Using equation (2), the gate count rates are calculated for a given F value. For example, in the case of F = 1, C = * 4 20 137.5 / is, Aq * 50 counts and Bq = -5 x 10 counts per second and using the gate times and durations of FIG. 2, the following count rate table is obtained:

Table V

Port Net counts background counts Gross counts 25 _____ per second per second_ gen per second, 4 453.9 55 · 4 509.3 5 693.9 110 · 8 804.7 6 482.4 110 · 8 593.2 30 7 335.2 110.8 446.0 8 233.0. 110.8 343.8 9 274.6 221.7 496.3 35 10 132.8 221.7 354.5 790 7 3 91 η - -41- 20949 / JF / jl (Table V continued)

Port Net counts Background counts Gross counts _ per second_ per second _ gen per second 11 S4-: l 221.7 285.8 5 12 _ 31.1 221.7 252.8 13 * 22.2 ..., -. 443.3. 465.5 14 5.2, 443.3 448.5 25 1.3 ,,,, 443.3 444.6 10 lg 0.2 ... _ 443.3 443.5

In Table V, the background count rate is taken as the sum of the count rates of the gates G ^ and as operatively supplied to each gate. Since the gates G ^ and ° 16 are each 200 coats long, the background count rate for gates G ^ and G ^ is also 200 με long, half of G ^ and G ^ g total, while the background count rate for gates Gg through G ^ each 100 με long is a quarter of the idie of gate G ^ and G ^ g total etc.

A reasonable set of gates is then chosen and the fractional standard deviation or / R in the ratio R of the count rates of such K

20 gates is calculated from: * R _ _ — _—: t% ·: d 2 (3)

D

L NJ L ^ nJ

25 where: NR is the net count speed in the numerator and the net count speed in the denominator. The standard deviations in the counting speed in the numerator and the counting speed in the denominator 0 ^ are given by: <7W ~ NG * | ~ NR / 3 2 and (4)

30 L.

° D 'Dg + £ ° b (5) where Νβ is the background counting speed in the numerator, DR is the background speed 35 in the denominator, B is the sum of the gates counting speeds and G ^ g, N ^, 790 7 3 91 20949 / JF / jl is the sum of and Ν0 and DQ is the sum of and Dg.

Using equation (3), the standard standard deviation 0 "_ / R of the ratio is used for different background smoothing times Π, for example 1, 2, 4 and 8 seconds. For example, for the data 5 of Table V, the term ψβ / Β in equations (4) and (5) 0.0336 for an average time of 1 second, but is only half of that, or 0.0186 for an average time of 4 seconds. fractional standard deviation / 'f in ^ calculated from: 10 ^ - k! | (6) where: K * _τ_ · ΔΤj + {T £ “Ti)“ * ΔΤ2 (y) 15 e0T, / T_i (eST2A-i) where : ΔΤ ^, ΔΤ ^, and T ^. 'The times and times of the numerator and denominator gates are as taken as some long gates in the manner shown in Figures 13A and 13B »20 Repeated solutions of the equation (3) eh (6) are made for different gate sets and the respective K values of equation (7) until it is determined that the minimum is found. Some compromise may be required, since a ratio (gate set) that gives the best results with a given smoothing time may not be optimal with another.

As an example, six different gate sets each implying a different ratio R have been chosen for the case of Table V ('i' = 137.5 ps and F = 1), as indicated below in Table VI.

The results obtained from the solutions of equations (3) and (6) are shown against each ratio for both the 1 second and 4 second background-30 smoothing times. PE K value obtained by the solution of equation (7) for each ratio is also shown.

See table on page ^ 3.

790 7 3 91. f -43 ~ 20949 / JF / yl

Table VI

Background mediation Background mediation

Ratio ^ of 1 second__ of 4 seconds 5 or / R στ / τ oR / R στ / τ (1) G ^ + Gs. 553 * 7.33% 4,051 6.58% 3.64% g8 + g9 + g10ÏGh (2) Gfc + Gs. 722 5.72% 4.13% 5.35%, 3.86% 10.

Gy + Gg + Gg + Gjg (3) * 'Gk + G5 .527 7.94% 4.19% 6.87% 3.62%

Gg + Gg + Gxg + Gz ^ + G22 15 (4) Gi * + G5 .670 6.02% 4.03% 5.47% 3.67% G7 + G8 + G9 + Gl0 + Gll (5) Gtf + Gs + Gg .599 7.15% 4.28% 6.37% 3.81%

Ge + Gg + Gio + Gll 20 - (6) Gi »+ G5 -598 6.92% 4.14%. 6.43% 3.84%

Ge + Gg + Gio

As can be seen, the ratios (1 JenW are close to the same minimum value when the background is averaged for 1 second, while with the four second averages the ratios C1j (3) are best, with the ratio (1)) being far behind. In this case, the ratio (1) is therefore the preferred ratio for use where Ί is in the vicinity of 137.5 ps · and F = 1 (see also Table VII below). The data of Table VI also show that there is no great variation between the best and the bad value among all ratios. This occurs because the corresponding variation in K tends to compensate for the variation in * T_ / R at the different ratios. Tables such as Tables V and IV are also prepared for others within each range and for all other ranges across the full range of interest, for example from 50 gravy to 600 gravy. In 7907391.- * \ v '* 20949 / JF / jl in general, the calculations are preferably made for three 3 * * s within each range, suitably the mean value and two near or at each extreme of the range. For example, for the Y range from 131.3 to 143, ö jus, calculations can be made for Ί's of 131 jus, 137.5 5) and 144 μβ. The ratio which is best & y / / 4? over the entire γ range is then chosen as the one used for that particular range.

The same process is repeated for the remaining F values 1 / fT, V? and 3 to build a complete library of ratios 10 for all scale factors. It is recalled that Tables V and VI represent only a single value F, that. ie F = 1 and only a single, ie * 1 «137.5 usec.

It will therefore be appreciated that the foregoing calculations will result in a number of ratios, or gate sets, for each F value, each ratio corresponding to a particular range as discussed above. The number of ratios used for each F will depend on the number of F values used with respect to the desired degree of accuracy. in Ί. At four F's, it has been found to be preferable to use seven ratios for each F value. This number of ratios allows an accurate calculation of the entire ^ range associated with each F value. Of course, the given 't limits for each ratio for each F value may vary from those described here, which are illustrative only.

In accordance with the foregoing, a representative library of ratios for the full range from about 50 µs to about 600 µs can be as follows (for simplicity, the symbol G is omitted from the port numbers):

Table VII

3 * range F ratio. equation for Ύ 30 ----; ------------ T <61.31 «lUT r- 21.9-6 60.1 <r 61.3 - 6S.S" T- + «.1 63.3 - 73.8, * 7l8 + ^ o.ll. '* - »· **« · »· ** 35 73.3 - 83.0' -S ^ iiii t - 33.6. 71.6 δ'1 790 7 3 91, \ -45- 20949 / JF / yl

Continued Table VII.

I ·· 3-0 - Μ · 8 "snólnm” · 0 + 94-s * '' '"· 8-103 · 3" Siokt + i2 + 13 »·" »♦ 23 · 2 τ> ιω · 3." iönSlSÜ γ- 36 · 5 + 9 = · ° »" 7 <"« -3 1 ΪΒ ^ Β5 τ - 38.0 * 69.5 R-1 106.3 - 113.8 ·· 3 + 7 + fïf + ÏÓ 7- 48 · 4 + Μ.1 ** '118.8 -131.3> iSsISm Τ “53.2 + 76.1 R-' 131 · 3 -143 · 8 8Ϊ5ϊΐδΰΐ 1 -“ · 3 ♦ 124 · 1 * "163.8-162.5 * aslimj τ ·« · «+ 164 2 * "1Μ '5 - 1S7 · 5'" jSIÓ + uL + u * ”· 6 ♦ '2 ° 2 *"', τ> 187-5 * Ϊ0 + ΠΪΠ + Ϊ3 Γ- 63.2 + 164.6 R - * t <184.0m / Γ ^ 4 + 9 65.8 + 121.12- 184.0 - 205.7 'gy ^ 1 () Τ - 83.7 + 126.7 R- * 205.7 - 227.3. · ^ + '9 ^ + α Τ' 10 ° '· 9 + 131 · 8 227.3 - 249.0 "MitóÏLL Τ" 10δ · 2 + 215 * ° R_l 249.0 - 281.5 "τ» 78.9 + 266.0 R-1

281.5 - 324.8 I '* 9 + 10 + 11 + 12 τ * 72'7 + 265.5 R.

I 7 + fl t. Τ> 324 · 8 I ”10 + 11 + 12 + 13 7- 109.4 + 283.0 R- '7 <318.8 3' - ^ 3 + 9 7 - 114.0 + 209.7 Sf '3! 3.8 - 336.3“ ^ ”+ 10 7 - 145.0 + 219.4 R "'356.3 - 395.8 - g> ^ + 10 7. 111.0, + 206.3 r'1 393.3 - 431.3" t - 136.2 + 305.1 r "431.3 - 487.5 * SÖ $ 5TÜ' · U6 · 7 + 461- ° 437.5 - 562.5 * 9 ^ 0 ^ 1 + 12 7 - 126.1 + 460.0 8-1 T> 362.5 "10+ L L + 12 + 13 T - 139.5 + 4.4 ° 790 7 3 91 '"

O

y '-46- 20949 / JF / jl

There is no simple relationship between R and * ƒ from which ^ can be directly obtained once the ratio has been calculated. However, since any ratio R is used over only a limited range of, a linear relationship can be established between R and welk, which approximates the true relationship between them. The dashed curve 298 in Fig. 14 shows an example of the true relationship between *! and R-1 given by the equation: T2 - T1 (-MX \ R = —-1_V - e 1 / (8) w. -δτ2 * \ V - ~) in which the terms A T2, and T2 are defined according to the figures 13A and 13B.

Equation (8) is solved 'with the assumed values of * / over the range of 50 coat to 200 gravy, using the gate set (Gjj + G ^) / (Gg + Gg + G ^ + G ^), where = 25 μ3, G ^ ren Gg = 50 ps each and Gg, G ^ u and = 100 jus each and curve 298 of Fig. 1.4 is the result. In this case, F = 1. In Table VII, the area of the dashed curve 298 for which the greatest accuracy in vereist is required is between 131.3 gravy and 1.43.8 µm. ., which is the interval valid for the determined gate set and the determined F value represented by the curve 298. Accordingly, the solid line curve 300 in FIG. 14 is fabricated to fit as precisely as possible at these points by introducing the and R values into equation for each of these points and solving the resulting simultaneous equations for the values of the coefficients a and b. For the example of Fig. 14, this gives an a value of 61.3 and a b value of 124.1.

A solution of the equation for curve 300 in Fig. 14, i.e., 61.3 + 124.1 R ~ for the R ~ value used for writing Fig. 14, gives calculated t values such as shown in Table VIII, from which it can be seen that the accuracy of the calculated is + 1% or better over approximately 2-to-1 range, namely from 100 ps to 200 ps.

See table VIII on page 47 35 790 7 3 91 k # «.

J · s -47- 20949 / JF / jl

Table VIII

Actual 'ί „-1 calculated ΐΤ (jis) R (psj_ 50 0.0388 6S.1 5 75 0.1520 80.2 100 0.3194 100.9 115 0.4340 115.2 10 125 0.5133 125.0 137.5 0.6139 137.5 150 - 0.7147 150.0 175 0.9126 174.6 15 200 1.1014 198.0

The values of the coefficients a and b for the remaining combinations of F and gate sets of Table VII are similarly determined to develop the complete library of the expressions of equation (1) for use in solving and - fp based on respective measured values of RT and R_. These terms are also indicated in Table VII in addition to the corresponding ratios and are also stored in the computer 42 in accordance with the associated F values and ratio gate sets. It will be understood that the values of the coefficients a and b 25 will differ from those indicated in Table VII when F values different from 1 / VT »1, V ^ IT and 3 are used» or when different gates are used at the determination of the ratios R.

As noted, the counting rates of the various gates used in calculating the ratios R 1 and R p are the net counting rates 30 over the accumulation period Δ t. To obtain the net counting rates, it is necessary to determine the background counting rate as well as subtract the appropriate amount from the gross gate counting rates. When the background count rate is unknown and varies, it should be estimated. Since the background count rates generally vary slowly, it is permissible to average this over a relatively long time, that is, 4 to 8 seconds in comparison with the 790 73 91 * 1 * -48-20949 / JF / jl equation with the normal accumulation period Δ t of 1 sec. for the gross gate count speed. It has been found that for the first four sets of gates and equations for each F value in Table VII, it is sufficient to assume that gates 15 and 16 contain only background signals and the gross count rates and the 15 3 and 16 of the 2nd gates can be easily accumulated and averaged over the background accumulation period and then subtracted from the gates' gross count rates to be used in determining the ratios. However, in the case of the last three gate steeps and equations for each F value, it is necessary to set the coefficients a and b to have the presence of a small but significant amount of adulterated needle in the gross count rates of the correct ports 15 and 16. The manner in which this background adjustment is carried out can be seen from the consideration of mainly the case of F = 1 and R = (.Gj + Gg) / (G1q + G. ^ + G12 + G13). First of all, the real average counting speeds are 15

Mg, N1 o etc., and each of the ports G ^, Gg, G ^ q etc. to be determined. This can be done as shown above using equation, (2). For the present example and assuming "J" = 185 ps, AQ = 100 and F = 1 and using the neutron burst and the detector gate times of FIG. 2, equation (2) yields the following "works" Decay signal: ΟΛ count rates (in counts per second) in each of the gates of interest; N ^ = 881.0; Ng = 672.3; N1Q is 526.9; = 306.8; N12 = 178.8; N13 = 164.8, N15 = 18.9 and N ^ g = 6.4 The magnitude of the decay signal that will be subtracted from a 200 ps time gate together with the background will therefore be (N ^ + Ν ^) / 2 = 12.65 counts per second This will provide the following '' net '' count rates (in counts per second) for the gates of interest of N ^ (net) = 881.0 - 12.65 / 4 = 877.8, Ng = 672.3 - 12.65 / 4 = 669.1, Mlfl = 526.9 - 12.65 / 2 - 520.6, N11 = 306.8 - 1265/2 = 300.5, N12 = 178, 8 - 12.65 / 2 = 172.5 and N13 = 164.8 - 12.65 = 152.2 Similar equation = 210 ps (while all other parameters remain the same) yields equation (2) where rof signal count rates (counts 30 per second of) N7 = 1019.2, Ng = 803.0, N1q = 702.8, N ^ = 436.8, N12 = 271.6, N ^ 3 = 273, = 40.6 and N g = 15.7. Adding the count rates of the gates G ^ and G ^ g and dividing by 2 subtracting the decay signal along with the background from 200 ps * yields the net count rate which equals 28.15 counts per second, The respective net counting rates (in counts 35 per second) are therefore = 1012.2, Ng = 796.0, N ^ = 688.7, N ^ = 422.7, 790 7 3 91 -49- 20949 / JF / jl k > N12 = 257.5, N13 = 244.9.

Forming the ratio R = (Νγ + Ng) / (N1Q + Nn + N | 2 + N13 ^ yields for Ί * = 185 gravy> ewi - 210 gravy, the respective values of R

are 1,350 and 1,1205, with the corresponding values of R 1 0.7407 and 0.8925.

The two sets of 's and Rs' can then be used to solve the set values of the coefficients a and b in equation (1). This gives a = 63.0 and b = 164.7, so that when the gates and G ^ g are used for background, the resulting equation for the gate set (Νγ + Ng) / (ït10 + + N12 + N13) 'i = 63.0 + 164.7R ~ ^. This is not entirely identical to the equation for this gate set in Table VII, since the equation

-1 _ -J

was made to fit the "actual" 5 against R curve at 'j' = 187 gravy and 212.5 gravy, while = 185 and 210 were used for the purpose of this example. However, the differences between these two comparisons are very small. It will therefore be appreciated that the presence of decay signal in the "background" 15 gates and G g can be properly accounted for in the manner previously described for each of the last three gate sets and equations for all F values.

Equation (7) for K assumes that there is no significant decay signal in gates G ^ g and G ^ g, but only background signal. An alternative method of determining K which can be used whether there is a non-negligible signal mixed with background in gates G ^ and G ^ or not is to calculate Δ R- ^ with respect to LT and after K solve from:

Ar _ · A «-i τ - K (8) ΈΓ 25 For example, to find K for the case of F = 1 and 200.

using the gate set (G ^ + Og) / (G ^ Q + G ^ + G ^ 2 + G ^) and further using the gates G ^ and G ^ g for background, the values of the R- ^ calculated as a solution of the equation (2) of the respective gate counting rates and taking the inverted value of the ratio 30 for two values of Ύ equidistant from the ^ in question, i.e. = 200 gravy. As a result, for example, 213 gravies and 187 gravies become a 26 gravy. value of R 1 of 0.9105 and 0.7527, respectively, obtained for an AR-1 of 0.1578. For the average of 200 gravy. the mean R ”is 0.8316. K, is then simply obtained with equation 35 (8) by introducing 200 gravies for .85316 for R-1, 26; is for Λ ^ 790 73 91 -50- 20949 / JF / jl * * \ ' j '/ and 0.1578 for 4r, 1 and K the value 0.685 is given. This value is then used in equation (6) to determine the fractional standard deviation in 7.

As noted above, the same time gates are used for the remote detector 26 as for the nearby detector 24, i.e. NG1 = FG.j, NG2 = FG2, _____, NGl6 = FG16 and the same F value and chosen based on

of the new 7 „is also used for both detectors. In accordance with N

T'p is calculated based on the same type of equation as 7 ^ t i.e. the equation of Table VII. but, of course, using the 10 velocities of the distant detector ports. Background correction of the distant detector count rates and of the coefficients a and b is performed, if necessary, in the same manner as described in connection with the nearby detector. Since tends to be greater than, the first gate is chosen to calculate 7 ^ usually less than 2 × 7 ^ from the end of the neutron burst. Although this does not seriously affect the value of T, starting from the far detector gates closer than 2 × 7 ^ gives the substantial statistical advantage of significantly increasing the tar velocities in the far detector gates. For example, when p equals 1.15x, the distant detector count rates are F ^, F2. .F ^ in the order of magnitude 1.3 x greater than these would be if part F were equal to 7 '. The fact that 7 ^ is greater than 7 ^ results in a relatively more decay signal in gates FG ^ and FG ^. However, this effect does not appear to be significant and most 7 ^ can be reduced as a result by around 1% when fs 1.15 µ 7 ^. An important advantage of this is that it is essentially free of neutron diffusion effect. - ten. It is therefore very useful for applications of diffusion effects to be observed.

Once 7 and Yp are determined, \ N and Σ p can be easily calculated using the expression: 30 Z 4550 <9) τ where l is in units and 7 ^ or 7 ^ in gravy

Likewise, it may be desirable to obtain a ratio generally indicated by the N / F ratio of the tidal rates of 790 7 3 91} ~ 5U 20949 / JF / µl being near and far the detector ports. Such a ratio plotted against en and / or p p is useful for obtaining clear values of the porosity and saltiness of the water in accordance with U.S. Pat. No. 3,991,935. The ratio N / F can take various forms, but is preferably formed from the gates used for calculating plus all intermediate gates, for example, for the case of F s 1 and R s (Gg + G ^) / (Gg + G10 + G ^ + G12 + Cf ^ ) will be a suitable form for the ratio: N _ Νς + N7 + Ne + N9 + Nio + NX1 + N12 + Nl3 10 F $ + F7 + Fe + F9 + Flo + i'll + F12 + F13 where Ng to and with and Fg to F ^ are net count rates averaged over the accumulation period Δt.

As noted above, the telemetry circuits 34 and 36 may include any suitable two-way telemetry systems, the details of which are not part of the invention. * ·

Although the invention has been described and shown by various embodiments thereof, many modifications and variations thereof can be made by those skilled in the art without departing from the inventive concepts described herein. Accordingly, it is postulated that all such modifications and variations are within the spirit and scope of the appended claims.

25 30 - CONCLUSIONS - 35 79073 91

Claims (81)

A method for detecting the decay over time of thermal neutrons in an earth formation for use in measuring the thermal neutron decay characteristics of the formation, including; irradiating an earth formation with a discrete burst of fast neutrons during each of a succession of irradiation intervals, detecting indications of the concentration of the thermal neutrons in the formation following each neutron burst and generating signals in response thereto and transmitting the signals during an contiguous sequence of discrete time gates during an irradiation interval, which sequence begins following a discrete time delay after termination of the neutron burst in the best irradiation interval and extends over a portion of the remainder of the irradiation interval, characterized, further comprising controlling the signal transmitting step in response to an instruction signal related to the measurement of a thermal neutron decay characteristic of the formation based on indications; of the thermal neutron conferencing detected during one or more previous irradiation intervals in the sequence of irradiation intervals, in order to adjust the duration of each discrete time gate occurring in a subsequent irradiation interval by a common discrete scale selected from a plurality of discrete scale. A method according to claim 1, characterized in that the control step further comprises the step of adjusting the duration of the discrete time delay occurring in the subsequent irradiation interval with the selected scale factor value. A method according to claim 1 or 2, characterized in that the control step further comprises adjusting the duration of the discrete neutron burst occurring in the subsequent irradiation interval with the selected scale factor value. Method according to claim 1, 2 or 3, characterized in that the sequence of time gates comprises a number of contiguous groups of gates, each group per se comprising a number of discrete contiguous gates, the duration of the time gates being substantially equal is within each individual gate group and progressively increases from group to group in the sequence. The method of claim 4, characterized in that the number of discrete time gates within each group is equal to the number of gate groups in 790 7 3 91 -53-20949 / JF / µl sequence. Method according to claim 4 or 5, characterized in that the duration of the individual discrete time gates in the gate group after the first occurring gate group in the finite factor sequence is greater than the duration of the 5 discrete time gates within the direct preceding gate group in the sequence. Method according to claim 6, characterized in that the finite factor is a multiple of the duration of the individual time gates within the immediately preceding gate group. Method according to claim 6 or 7, characterized in that there are four groups of gates in each series of time gates and four discrete time gates within each group and that the finite factor by which the duration of the individual discrete time gates in successive gate groups is greater than that of the individual discrete time gates in the immediately preceding group is two, where the gate duration in the second occurring group is twice the port duration in the first occurring group, the gate duration in the third occurring group is twice the gate duration the second occurring gate group and the gate duration in the fourth occurring gate group is twice the gate duration in the third occurring gate group. Method according to any one of the preceding claims, characterized in that the selected scaling factor is selected by changing the output frequency of a multiple frequency oscillator means in response to the instruction signal, said oscillator means having a separate discrete output frequency has for each of the finite scale factor values. A method for detecting thermal neutron decay over time in an earth formation, comprising: irradiating an earth formation with a discrete burst of fast neutrons, detecting indications of the concentration of thermal neutrons in the formation subsequent to the neutron burst and generating signals in response thereto, characterized in that it further comprises transmitting signals from the detector member during a time gate sequence beginning following a discrete time delay after termination of the neutron burst and which number of contiguous groups of time gates, each group per se comprising a number of contiguous discrete time gates, the duration of the time gates being substantially equal within each individual gate group and progressively increasing from group to 790 7 3 91 (., · · I * * Λ /, ') J - * 54-20949 / JF / jl group in the sequence. Method according to claim 10, characterized in that the number of discrete time gates within each gate group is equal to the number of gate groups in the sequence. Method according to claim 10 or 11, characterized in that the duration of the individual time gates in each gate group after the first occurring gate group in the finite factor sequence is greater than the duration of the individual discrete time gates within the immediately preceding gate group in the .sequence. 13. A method according to claim 12, characterized in that the finite factor is a multiple of the duration of the individual time gates within the immediately preceding gate group] Method according to claim 13, characterized in that there are four groups of gates in the sequence of time gates and four discrete time gates within each group and that the finite factor by which the duration of the individual discrete time gates in successive gate groups is greater than that of the individual time gates in the immediately preceding gate group is two, where the gate duration j in the second occurring gate group is twice the gate duration in the first occurring gate group, the gate duration in the third occurring gate group twice the gate duration in the second occurring gate group and the gate duration in the fourth occurring gate group is twice the gate duration in the third occurring gate group. 15. A method of detecting the decay over time of thermal neutrons in an earth formation, for use in measuring the thermal neutron decay characteristics of the formation comprising the steps of: irradiating the earth formation with a discrete burst of fast electrons with a duration T during each of a sequence of irradiation intervals and detecting findings of the concentration of thermal neutrons in the formation following each neutron burst and generating signals in response, characterized in that it further comprises transmitting the signals over a contiguous sequence of discrete time gates during each irradiation interval, which sequence begins following a discrete time after the neutron burst ends in each irradiation interval and extends over substantially the rest of the irradiation interval, with at least one number 35 of the discrete time gates in the check have respective time durations, 790 7 3 91 * -55- 20949 / JF / jl (, which progressively increase with time ending in that of the neutron burst from a shortest duration of less than T to a longest duration of at least as great as T. 16. A method of measuring a thermal neutron decay characteristic of an earth formation, comprising the first step of irradiating a formation with a discrete burst of fast neutrons, characterized in that it further comprises the following steps: 2) selecting, as a function of a known value of the thermal neutron decay characteristic of the formation to be measured, a given set of two different measurement intervals following each neutron burst from a number of such sets of two measurement intervals, each of such imagine. of measurement intervals corresponding to a certain value range of the decay characteristic, the determined set of measurement events which is chosen is that which corresponds to the decay characteristic value range surrounding the known value of the decay characteristic, (3) jet 15 measuring indicia of the thermal neutron concentration in the formation during each of the measurement intervals in the selected set following each neutron burst, forming a function R of the measurements made in the step described immediately above, (5) determining a new value of the decay characteristic as a function of the value of the function R formed in the immediately preceding step and (6) repeating all the steps described above using the new values of the decay characteristic determined by the immediately preceding step as the known value of the decay characteristic in the new second step. / 17. A method according to claim 16, characterized in that the formation 25 is irradiated with a discrete burst of fast neutrons during each of a succession of irradiation intervals and that the function R is formed from the measurements made in the third step accumulated over a number of irradiation intervals. 18. A method according to claim 16 or 17, characterized in that the decay characteristic is the thermal neutron decay time constant f. 19. A method according to claim 16, 17 or 18, characterized in that the fifth or penultimate step comprises: pre-establishing for each set of measuring intervals an empirical relationship between the decay characteristic and the ratio R of neutron concentration measurements for that set which is valid over substantially the full range of decay characteristic values corresponding to the set of measurement intervals and the determination of the new value of the decay characteristic from 790 73 91 ^ I-— »I__ ί / -56- 20949 / JF / jl the empirical relationship thus established prior to the set of measurements selected in the second step. 20. A method according to claim 19, characterized in that the empirisbh relationship is a linear relationship having substantially the form 'i - a + bR ~, where R is the inverse of the ratio of the measurements made in the selected set of measurement intervals. and a and b are constant which relate "S to R-1 over the range of" Τ "values, corresponding to the selected set of measurement intervals. Method according to any one of claims 16 to 20, characterized in that the times of occurrence of the measurement intervals of each set of measurement intervals is chosen such that a minimum deviation in the decay characteristic value is provided when determined in the manner specified in the third, fourth and fifth steps, for substantially all decay characteristic values over the entire range of decay characteristic values corresponding to each set. Method according to any one of claims 16 to 21, characterized in that the first occurrence of the two measuring intervals selected in the second step starts at a time which is substantially twice as long after termination of the neutron burst as the known value of the decay characteristic used in the second step. Method according to any one of claims 16 to 22, characterized in that it further comprises the steps of Π) selecting as one of the new value of the decay characteristic determined in the fifth step one of a finite number of discrete scale factor values F and (8) adjusting the duration of at least the two measurement intervals selected in the new second step by the selected F values. The method according to claim 23, characterized in that the duration of each neutron burst in the new first step is adjusted by the selected 30 F values. Method according to claim 23 or 24, characterized in that there are a number of different decay characteristic ranges for each scale factor value F and each of such ranges has a given set of two measuring intervals corresponding to it and in that the determined set of two measuring intervals -35 falls selected in the new second step is that which corresponds to both 790 7 3 91 • kt * -57- 20949 / JF / jl the new value of the decay characteristic determined in the previous fifth step as well as the selected F value . 26. A method according to claim 25, characterized in that there are four scaling factor values F and seven sets of measurement intervals corresponding to seven different ranges of decay characteristic values for each F value. A method according to claim 26, characterized in that the scale factor value progressively increases in size from F value to F value with a constant incremental factor. A method according to claim 27, characterized in that there are four scale factor values and the magnitude of the incremental factor is / 31. 29. A method according to claims 23-24, characterized in that the seventh step comprises: pre-establishing for each F value a certain range of decay characteristic values, within which each F value can be used As well as selecting the F value which is to be used as a value whose corresponding decay characteristic range surrounds the new decay characteristic values determined in the fifth step. 30. A method according to any one of claims 17-29, characterized in that the measuring intervals comprise a contiguous sequence of time gates and that the signals representative of the concentration of the thermal neutrons in the formations following each neutron burst are stored during the time gates in each of the irradiation intervals, which sequence begins following a discrete time delay after the termination of the neutron burst in the irradiation interval and extends over substantially the remainder of the irradiation interval and the measurements are made from the signals. A method according to claim 30, characterized in that the adjustment of the duration of the measurement intervals comprises simultaneously adjusting the durations of the discrete time gates in a consecutive number of irradiation intervals by the selected F value. The method of claim 31, characterized in that the eighth step further comprises adjusting in successive number of irradiation intervals the duration of the discrete time delay between the end of the neutron burst and the start of the gate sequence with the selected F value wherein both the time of occurrence and the duration of such discrete time gates are commonly adjusted not the selected F value. 790 7 3 91 * 5 "* *>" 58 "20949 / JF / jl The method of claim 32, characterized in that the eighth step further comprises adjusting the duration and repetition period of the neutron burst in the successive number of irradiation intervals with the selected F values. 34. A method according to any one of the preceding claims 23-33, characterized in that the seventh step further comprises the preliminary creation within each range of decay characteristic values for each F-value of a number of sub-ranges of decay characteristic values and another gate set corresponding to each such subrange and choosing as a given gate set to be used in the second step during the consecutive number of irradiation intervals of the gate set which both correspond to the F value selected in the seventh step and the new decay characteristic value determined in the fifth step. 35. A method according to claim 34, characterized in that the F values progressively increase in value by a constant incremental factor from a lowest value to a highest value and the decay characteristic ranges established for contiguous F values in the sequence of F values overlap, where there is an area within any given decay characteristic range within which one of the two adjacent F values can be used. 36. A method according to any one of claims 16-35, characterized in that the decay characteristic is the thermal neutron decay constant 'J' and that the empirical relationship is a linear relationship with substantially the form 'Jf = a + bR_1, wherein R 1 the inverse of the ratio R of measurements is made in the determined selected gate set and a and b are constant® which relate Cf to R ~ over the range of values corresponding to the determined selected gate set. 37. Apparatus for detecting thermal neutron decay over time, in an earth formation for use in measuring the formation's thermal neutron decay characteristics, comprising a means for irradiating an earth formation with a discrete burst of fast neutralization thrones during each of a succession of irradiation intervals, a detector means for detecting indications of the concentration of thermal neutrons in the formation following a neutron burst and generating signals in response thereto, and a signal gate means responsive to the control signals and to the signals generated by the detector for transmitting signals from the detector member for an contiguous sequence of 790 7 3 91 Λ * -59-20949 / JF / µl discrete time gates during an irradiation interval, which sequence follows a discrete time delay after termination of the neutron burst in each irradiation int and extends over a portion of the remainder of the irradiation interval, characterized in that it further comprises a controller 5 responsive to an instruction signal related to a measurement of a thermal neutron capture characteristic of the formation based on indications of the thermal neutron concentration detected during one or more previous irradiation intervals in the sequence of irradiation intervals to generate control signals for controlling the operation of the sig-10 signal gate to adjust the duration of each discrete time gate occurring in a subsequent irradiation interval by a common from a finite number of discrete scale factor values chosen dischrete scale factor values. 38. Device according to claim 37, characterized in that the control means comprises means for adjusting the duration of the discrete time delay occurring in the subsequent radiation interval with the selected scale factor value. 39. Device according to claim 37 or 38, characterized in that the control means comprises a means for adjusting the duration of the discrete neutral burst of energy in the subsequent irradiation interval by the selected scale factor value. 40. Device according to claim 38 or 39, characterized in that the sequence of time gates comprises a number of contiguous groups of gates, each group of which itself comprises a number of discrete contiguous time gates, the duration of the time gates being substantially equal within each individual gate group and in the sequence progressively increases from group to group. 4i. The device according to claim 40, characterized in that the number of discrete time gates within each group is equal to the number of gate group in the sequence. Device according to claim 40 or 41, characterized in that the duration of the individual discrete time gates in each group after the first gate group in the finite factor sequence is greater than the duration of the individual discrete time gates within the immediately adjacent one preceding gate group in the sequence. 43. An apparatus according to claim 42, characterized in that the finite 79073 91 "* 60-20949 / JF / µl factor is a multiple of the duration of the individual time gates within each immediately preceding gate group. 44. An apparatus according to claim 43, characterized in that there are four groups of gates in the sequence of time gates and four discrete time-5 gates within each gate group and that the finite factor by which the duration of the ....... individual discrete time gates in successive gate groups is greater than that of the individual time gates in the immediately preceding gate group is two, where the gate duration in the second occurring gate group is twice the gate duration in the first occurring gate group, the gate duration in the third occurring gate group twice the gate duration in the second occurring gate group and the gate duration in the fourth occurring poor group is twice the gate duration in the third occurring gate group. 45. An apparatus according to any one of claims 37-44, characterized in that the control means comprises: an oscillator means for generating the driver signals, said oscillator means having a separate discrete clock frequency for each of the finite scale factor values and a means responsive to an instruction signal for selecting one of the discrete clock frequencies for use in generating control signals for supply to the signal gate. An apparatus for detecting thermal neutron decay over time in an earth formation, comprising: means for irradiating the earth formation with a discrete burst of fast neutrons, a detector means for detecting indications of the concentration of thermal neutrons in the formation following the neutron burst and for generating signals in response thereto, characterized in that it further comprises a signal gate member for transmitting signals through the detector member during a time gate sequence beginning after a discrete time delay upon termination of the neutron burst and which includes a plurality of contiguous groups of time gates, each group comprising in itself a plurality of contiguous discrete time gates, the duration of the time gates being substantially equal within each individual gate group and progressively increasing in sequence from group to group. 47. An apparatus according to claim 46, characterized in that the number of discrete time gates within * each group is equal to the number of gate groups in the sequence. 790 7 3 91 -61- 20949 / JF / jl The device according to claim 46 or 47, characterized in that the duration of the individual discrete time gates in each group after the first occurring gate group in the finite factor sequence is greater than the duration of the individual discrete gates within the immediately adjacent one previous 5 gate group in the sequence. An apparatus according to claim 48, characterized in that the finite factor is a multiple of the duration of the individual time gates within the immediately preceding gate group. 50. An apparatus according to claim 49, characterized in that there are four groups of gates in the sequence of time gates with four separate time gates within each gate group and the finite factor by which the duration of the individual discrete time gates in successive gate groups is greater. than that of the individual discrete time gates in the immediately preceding gate group is two, where the gate duration in the second occurring gate group is twice the gate duration in the first occurring gate group, the gate duration in the third occurring gate group is twice the gate duration in the second occurring gate group and the gate duration in the fourth occurring gate group is twice the gate duration in the third occurring gate group. 51. Apparatus for detecting the temporal decay of thermal neutrons in an earth formation for use in measuring thermal neutron decay characteristics of the formation, comprising: means for irradiating an earth formation with a discrete burst of rapid neutrons of the duration T during each of a succession of irradiation intervals, a detector means for detecting indications of the concentration of the thermal neutrons in the formation following, -> on each neutron burst, and generating signals in response, with the characterized in that it further comprises a signal gate member responsive to the control signals and to the signals generated by the detector for transmitting signals from the detector members during contiguous sequence of the second time gates during each irradiation interval, which sequence follows a discrete time delay after termination of the neutron burst in e Each irradiation interval extends over substantially the remainder of the irradiation interval at least one of the discrete time gates having a duration in the sequence which progressively increases with the time following the termination of the shortest burst of shortest duration. from less than T to a longest duration of at 790 73 91 -62 “20949 / JF / jl at least as large as T. 52. Apparatus for measuring a thermal neutron decay characteristic of an earth formation, comprising a first means for irradiating the formation with a discrete burst of fast neutrons, characterized in that it further comprises a second means for choosing as a function of a known value of the thermal neutron decay characteristic of the formation to be measured from a given set of two different burst intervals following each neutron burst from a number of such sets of two measurement intervals, each of such sets of the measurement intervals 10 falls corresponds to a certain range of the value of the decay characteristic, which particular set of measurement intervals is selected, which is the set corresponding to the decay characteristic value range which. surrounds the known decay characteristic value, a third means for measuring indications of the thermal neutron concentration in the formation during each of the measuring intervals, in the selected set and for generating signals representative thereof, a fourth means responsive to the signals following the neutron burst to generate a signal related to a function R of the measurements and a fifth means responsive to the function signal for determining a new value of the decay characteristic. and generating a signal representative thereof. An apparatus according to claim 52, characterized in that the means responsive to the signal generating signal related to the function R responds to the signals following a series of neutron bursts. 54. An apparatus according to claim 52 or 53, wherein the first member comprises a member for irradiating the formation with a second series of discrete bursts of fast electrons, characterized in that the second member comprises a member responsive to the signal for the new value of the decay characteristic for choosing the new value as the known value of the decay characteristic following the second series of neutron bursts. 55. An apparatus according to claim 52, 53 or 54, characterized in that the decay characteristic is the thermal neutron decay constant. 56. Device according to claim 52, 53, 54 or 55, characterized in that the first member comprises: a member which, for each set of measuring intervals, has a previously established empirical relationship between the decay characteristic and the function R of the concentration measurements for the set embodied, which is valid 79073 91 Λ “-63- 20949 / JF / jl t \ over substantially the entire range of decay characteristic values, which corresponds to the set of measurement intervals and a means for determining the new value of the decay characteristic from The pre-established empirical relationship for the set of measurement traps selected by the second organ. 57. Device according to claim 56, comprising; characterized in that the empirical relationship is a linear relationship with essentially the form ^ = a + bR \ where R- ^ is the inverse of the ratio of the measurements made in the selected set of measurement traps and a and b are constants <3 relate to R over the range of the value i corresponding to the selected set of measuring traps. Apparatus according to any one of claims 52-57, characterized in that the times of occurrence of the measurement intervals with respect to the end of the neutron burst for each set of measurement intervals are preset in the second member at those times that the minimum provide deviation from the decay characteristic values, when. determined by the third, fourth and fifth means, for substantially all decay characteristic values over the full range of decay characteristic values corresponding to each set. 59. Device according to any one of claims 52-58, characterized in that the first occurrence of the two measuring interval selected by the second member starts at a substantially twice longer time after the termination of the neutron burst than the known value. of the decay characteristic used by the second member. 60. An apparatus according to any one of claims 52-59, characterized in that it further comprises: a sixth means for responding to the signal characteristic of the new value of the decay characteristic determined by the fifth means. selecting a finite number of discrete scale factor values E and for generating a control signal representative thereof and a seventh means responsive to the control signal for adjusting the duration of at least the two measurement intervals selected by the second means following the second series of neutron bursts by the selected F value. The device according to claim 60, characterized in that the seventh member comprises a means for adjusting the duration of the second neutron burst with the selected F values. An apparatus according to claim 60 or 61, characterized in that there are a number of different decay characteristic ranges for each scale factor value 35 F and each such range has a given set of two measurement intervals 790 7 3 91 -64- 20949 / JF / jl ' v. *> 'corresponding to it and that the determined set of two measurement intervals selected by the second member following the second neutron burst that set v corresponds to both the new value of the decay characteristic determined by the fifth member following the first series ηβη ^ οηβηΒΗΐνο · s as with the selected P-values. An apparatus according to claim 62, characterized in that the four scale factor values are F and seven sets of measurement intervals corresponding to seven different ranges of decay characteristic values of each F value. An apparatus according to claim 60, 61, 62 or 63, characterized in that the constant incremental factor scaling factor values progressively increase in size from F value to F value. The device according to claim 64, characterized in that the magnitude of the including factor is VI1. An apparatus according to claim 60 or 61, characterized in that the sixth means for selecting the F values comprises an element for each. F value embodied a certain range of decay characteristic values within each F value that can be used, and a means for selecting the F values to be used following the second series of neutron bursts as a value whose corresponding decay characteristic range it surrounds the new decay characteristic value determined by the fifth member following the first series of neutron bursts. 67. An apparatus according to any one of claims 52-66, characterized in that it further comprises means for storing for a contiguous sequence of discrete time gates comprising the measured Interval signals representative of the concentrations of the thermal neutrons: in the formation following each neutron burst, which sequence begins following a discrete time delay after termination of the neutron burst in the irradiation interval and extends over substantially the remainder of the irradiation interval and the meter responds to these signals. An apparatus according to claim 67, characterized in that it further comprises means for simultaneously adjusting the duration of the discrete time gates in a consecutive number of irradiation intervals with the selected F value 69. An apparatus according to claim 68, characterized in that the adjustment means further comprises means for adjusting in successive number of irradiation intervals the duration of the discrete 790 7 3 91 4 Γ t Μ -65- 20949 / JF / jl. time delay between the end of the neutron burst and the start of the gate sequence with the selected F value, adjusting both the time of occurrence and the duration of such 5 discrete time gates in common with the selected F value An apparatus according to claim 69, characterized in that the adjusting means further comprises means for adjusting the duration and the repetition period of the neutron burst in the successive number of irradiation intervals with the selected F value * · An apparatus according to any one of claims 60-70, characterized in that the sixth member further comprises a member which, within each range of decay characteristic values for each F value, a number of sub-ranges of decay characteristic values and another gate set corresponding to embodied each such subrange as well as a means for selecting as the determined gate set 15 to be used during the consecutive number of irradiation intervals of the shirt set corresponding to both the F value selected by the sixth means and the new decay characteristic value determined by the fifth organ. An apparatus according to claim 71, characterized in that the F values 20 progressively increase in value by a constant incremental factor from a lowest value to a highest value and in that the decay characteristic ranges are established for adjacent F values in the sequence of F values overlap, where there is an area within any given decay characteristic range within which one of the two adjacent F values can be used. 73. An apparatus according to any one of claims 52-72, characterized in that the decay characteristic is the thermal neutron decay constant constant and that the empirical relationship is a linear relationship with substantially the form s1-1 j1 + bi, where fl is the inverse of the ratio R of the measurements is made in the particular gate sets chosen and a and b are constants which relate to. R- ^ over the range of Ύ values corresponding to the particular gate set chosen. A method according to any one of claims 1-9, characterized in that the signals are passed during an contiguous sequence of discrete time gates during each irradiation interval. 75. A method according to any one of claims 1-9 or 74, characterized by 79073 91 •. T <. . The sequence of the discrete time gates extends over substantially the rest of the irradiation interval. 76. A method according to any one of claims 1-9, 74 or 75, characterized in that the finite number of discrete scale factor values is less than the number of discrete time gates within each irradiation interval. A method according to any one of claims 16-36, characterized in that the function R is a ratio of the measurements made in the third step. 78. An apparatus according to any one of claims 37-45, characterized in that the gate member transmits means from the detector member during an adjacent sequence of discrete time gates during each radiation interval. An apparatus according to any one of claims 73-45 or 78, characterized in that the sequence of discrete time gates extends over substantially the rest of the irradiation interval. An apparatus according to any one of claims 37-45, or 78 or 79, characterized in that the finite number of discrete scale factor values is less than the number of discrete time gates in each irradiation interval. An apparatus according to any one of claims 52-73, characterized in that the function R is the ratio of the measurements. A method for providing a background-compensated measurement of the induced radiation level in an earth formation, comprising: (a) irradiating an earth formation with a discrete burst of neutrons during each of a succession of irradiation intervals, (b) detecting indications of the radiation level in the formation during at least a first detection interval occurring at a first time within each irradiation interval, (c) detecting indications of the background radiation level during a second detection interval occurring at a second time within each irradiation interval, with the characterized in that it further comprises the following steps: (d) measuring the average level of the first detected indications over a first number of radiation intervals, (e) measuring the average level of the second detected indications over a second larger number irradiation intervals and (f) combining d The first and second measurements to provide a first background compensated measurement of the average level of induced radiation in the formation. 83. A method according to any one of claims 17 to 36 or 77, characterized by 790 7 3 91 ♦ r'-67- 20949 / JF / jl. note that the third step further comprises measuring indications of background radiation during a third measuring interval following each neutron burst and that the fourth step further firstly comprises forming a background compensation measurement from the background measurements made in the third step, which are battery -5 mutates over two or more numbers of neutron bursts and secondly combines the background compensation measurement with the neutron concentration measurements of the third step accumulated over the number of neutron bursts to form a background compensated function R. 84. An apparatus according to any one of claims 53-73 or 81, characterized in that the third measuring means comprises means for measuring indications of background radiation during the third measuring interval following each neutron burst as well as for generating signals representative therefor and in that the fourth responsive member includes firstly a means for forming a background compensated measurement from the background measurements provided by the third member accumulated over two or more series of neutron bursts and secondly a means for combining the background compensation measurement with the neutron concentration measurements provided by the third organ accumulated over the series of neutron bursts to form a background compensated function R. 20 Eindhoven, October 1979. 25 30 35 790 73 91