GB1582714A - Method and apparatus for deriving compensated measurements in a borehole - Google Patents

Description

(54) METHOD AND APPARATUS FOR DERIVING COMPENSATED MEASUREMENTS IN A BOREHOLE (71) We, SCHLUMBERGER LIMITED, a corporation organised and existing under the laws of the Netherlands Antilles, 277 Park Avenue, New York, N.Y. 10017 U.S.A. do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to well logging methods and apparatus, and more particularly, to methods and apparatus for producing measurements from multiple transducer arrays and combining them to provide compensation for variations in instrumentation and borehole conditions.

It is well known in the art of acoustic logging that some degree of compensation for variations in travel time introduced by changes in borehole diameter may be provided by a borehole tool that includes two receivers and one transmitter. (Such arrays will be abbreviated hereinafter by using a "T" to represent a transmitter and an "R" to represent a receiver with the relative positions of the T's and R's indicated by the sequence, the hyphen "-" separating the transducers indicating a common signal path. Thus a T-RR array specifies a transmitter on one side of two receivers, with the receivers having in common the signal path between the transmitter and its nearer receiver.) Unfortunately, such a T-RR arrangement does not compensate for the tilt of the tool with respect to the axis of the borehole.To overcome the tilt problem an additional transmitter may be provided to form a tool that has a T-RR-T array. As described in U.S. Patent No. 3,257,639, each of the two transmitters may be selectively operated and the travel time to each of the two receivers measured. The individual travel time measurements may then be combined to produce an average travel time for the interval between the two receivers. That average time has the advantage of being compensated for both changes in borehole diameter and tilt of the tool.

As with many different types of measurements under conditions varying nonhomogeneously in a direction radial to the borehole, acoustic measurements appear to vary with distance between transmitter and receiver or, more appropriately for two-receiver arrays, with the distance between the transmitter and a point midway between the receivers.

It is for this reason that the borehole-compensating type tools have two transmitters located equidistant from that mid-point.

As recognized in U.S. Patent No. 3.312,934, one reason why the acoustic velocity may vary with different T-R distances is that different signal paths may result with the longer distance having a path somewhat farther fr6m the borehole and deeper into the formation. This deeper path may be less affected by factors which radially alter acoustic properties when drilled or exposed to the borehole fluid, such as hydrophilic shales which tend to swell. This altered zone may exist deep enough into the formation to cause a short T-R distance to measure, at least in part, properties representative of this altered zone, rather than the desired unaltered formation. Longer T-R distances, such as 8 or 10 feet, are preferred to overcome this particular formation alteration problem.

Longer T-R distances require longer tools, and in the older two-receiver type arrays, i.e., those of the T-RR type, an increase from 3 to 8 feet in T-R distance requires a 5 foot longer tool. However, in the T-RR-T borehole compensating tools, such a substantial increase in T-R distance results in undesirably long tools since the T-R distance occurs twice. Longer tools are undesirable since their length makes them more expensive and difficult to transport and increases the problem associated with getting them down crooked or inclined boreholes.

It is therefore an object of this invention to provide method and apparatus which retain both the advantages of long T-R distances and borehole compensation without requiring unduly long borehole tools.

Borehole compensating type arrays are also employed in sidewall devices such as disclosed in U.S. Patent No. 3,849,721. Here longer T-R distances in the prior art T-RR-T array increase skid length, which unfortunately decreases the chances of keeping the skid in continuous contact with the borehole wall.

It is an additional object of the present invention to provide methods and apparatus which retain the borehole compensation features provided by a T-RR-T array in a sidewall skid, yet allow for increasing the T-R distance without increasing the skid length.

Previous approaches to providing at least a partial compensating system without unduly long tool lengths, such as described in U.S. Patent No. 3,207,256 or U.S. Patent No.

3,330,374, require memorization of at least two different measurements for at least two different distances. This requirement leads to additional memory costs and more vulnerability to depth positioning problems such as introduced by a yo-yo motion of the tool. Further, the compensation for tool tilt is not always complete.

Therefore, it is a further object of this invention to provide methods and apparatus which provide a more complete borehole compensation including tool tilt yet require a minimum of different depth positions and memorization distances.

A further problem with either the T-RR-T or its reciprocal, the R-TT-R array, is that because of the large distances between the T's in the T-RR-T array or the R's in the R-TT-R array, the operating conditions for transducers located at the ends of the array may be quite different, resulting in significant differences in the received signals which are presumed to be equal. For example, if severe tool tile places one of the outer transducers in a substantially eccentered position while the like corresponding transducer at the other end of the tool remains more centered, signals associated with these outer transducers could vary considerably and, in turn, could affect both the travel time and the amplitude measurements.

When a T is between a pair of R's or an R between a pair of T's, there is often a problem with electrical noise, as for example with electrical cross-talk from an electrically noisy transmitter circuit into receiver leads which must pass close to the transmitter or still worse, from a transmitter firing lead having high voltage and current transients, as in the case of acoustic tools, which pass by one of the receivers or received signal amplifiers. For example, the firing pulse leads going to the bottom transmitter in the T-RR-T array must pass by both receivers. A further appreciation of the electrical and mechanical problems introduced by transmitter leads passing receivers may be found in U.S.Patents 3,734,233 and 3,712,410. It would be highly desirable to have a compensating array where all receivers could be isolated from all transmitters and further, where no high voltage pulse leads pass anywhere near a receiver, its associated amplifier or receiver signal lines.

Therefore, it is a further object of the present invention to provide a borehole compensating type array where all receivers and associated receiver signal circuitry may be more readily isolated from the transmitters and their associated firing circuitry.

In prior art compensation type arrays and in some two receiver arrays, it was not possible to obtain measurements over some parts of the borehole. For example, in the T-RR-T array, the tool might not operate properly with the upper transmitter inside the casing and the remaining transmitter and both receivers still out in the open hole. Similarly, measurements of the formation could not be made in the critical bottom part of the hole for a distance corresponding to at least one transmitter-receiver distance. It would be desirable to be able to log as close to the bottom as possible, even if it were necessary to temporarily forego the compensation feature for this interval.

It is therefore an additional object of the present invention to provide methods and apparatus which are capable of making measurements over substantially the entire array length.

In acoustic tools which may be required to operate at some distance from the borehole wall, the acoustic signals arriving at a given receiver effectively leave the borehole wall at a point ahead of the receiver. the displacement of the point varying with the approach direction. This gives rise to what is known as a refraction error. This error and one correction technique for compensating type arrays, as described in U.S. Patent No. 3,304,536 and U.S. Patent No.

3.524.162. involves the use of an additional receiver with each of the two existing receivers.

Each additional receiver is spaced from each existing receiver by a small distance corresponding approximately to twice the displacement introduced by the refraction error -- one displacement for each of the two different reception directions. Thus, four receivers are used, two for each reception direction.

Further, in the T-RR-T type array, omnidirectional receivers are required since each receiver must anticipate signals arriving from either the upper or lower transmitter. Highly desirable directional receivers cannot be used unless four receivers are employed, as in the above refraction correction approach; i.e., two receivers directed towards each of the transmitters. Another approach would be to use the R-TT-R type array, but now desirable directional transmitters cannot be used unless, as with the four-receiver approach, four transmitters are employed. Needless to say, these extra transmitters add considerable control complexity and expense.

Consequently, it is an object of this invention to provide a compensation type array which permits all receivers and transmitters to be directional, yet still requires only four transducers to produce the measurements needed to provide compensation.

Two different T-R distance investigations are desirable and, as described in the above discussed 3,312,934 patent, it is possible the close comparison of the different investigations may even lead to a direct indication of the presence of hydrocarbons when it occurs in the form of gas, or in some cases, to estimate the degree of shaliness as suggested in U.S. Patent 3,096,502. It should be apparent that in order for measurements having different T-R distances twice useful in these applications, the measurements must be as accurate as possible.

The accuracy of the T-RR type of measurements illustrated in the above patents is often such that the observed differences in these different.investigations may actually be due to uncompensated tilt or system measurement errors, rather than radial differences in the acoustic properties of the formations. At least two measurements are required for this application, and it is important that both of these measurements be borehole compensated.

It is therefore an additional object of the present invention to provide methods and apparatus to obtain two differently spaced, that is, long and short T-R, investigations that are generally both compensated for borehole and system measurement errors.

When using prior art compensation type arrays to obtain the different T-R distance investigations, two additional outside transducers at an additional distance beyond those usually provided and a large number of additional measurement subcycles beyond the four normally employed would be necessary. Furthermore, the tool length would be increased by twice the desired difference in distance. Such-requirements for additional transducers and tool length render the second measurement impractical under many circumstances, since the second measurement is usually redundant to the first measurement.However, if it could be provided without such costly complications, this second measurement would increase the value of the primary measurement by providing substantial assurance that at least the longer T-R distance was adequate for altered formations and when favorable conditions did occur, would provide a direct indication of the presence of gas.

It is therefore a still further object of the present invention to provide methods and apparatus to obtain simultaneously two different, generally borehole compensated measurements with different T-R investigation distances and without requiring additional transducers, substantial increases in tool length or a significant number of additional measurement subcycles.

According to one aspect of this invention there is provided a method of producing measurements adapted for determining a compensated measurement of a physical characteristic of subsurface media near a borehole penetrating the earth employing multiple transmitter type and multiple receiver type transducers supported on a support member elongated in a direction generally parallel to said borehole for movement through said borehole, a first pair of transducers of a first type being positioned at a preselected separation along said member, a second pair of transducers of a second type being positioned at said preselected separation along said member and located on one side of said first pair of transducers in the direction of said elongation, and the transducers of said first and second pairs, respectively, having substantially the same operating characteristics; comprising the steps of: (a) producing a first measurement of said physical characteristic of the subsurface media when two of said transducers are at selected respective positions in said borehole; (b) storing said first measurement for combination with a later measurement of said physical characteristic of the subsurface media; and (c) producing said later measurement when two other of said transducers are effectively positioned in said borehole at said selected respective positions for combining with said first measurement to produce a measurement compensated for misalignment of said support member with said borehole and/or variations of said borehole.

According to another aspect of this invention there is provided apparatus for determining a physical characteristic of subsurface media near a borehole penetrating the earth which employs multiple transmitter type and multiple receiver type transducers supported along a support member adapted for movement through said borehole and elongated generally along a direction parallel to its direction of movement through said borehole, comprising:: (a) a first group of transducers of a first type supported for movement through said borehole with adjacent transducers of said first group being separated from each other by a preselected separation along a line generally parallel to the elongated direction of said support member; (b) a second group of transducers of a second type supported for movement through said borehole and located on one side of said first group in a direction therefrom parallel to said elongated direction with adjacent transducers of said second group being separated from each other by said preselected separation along said line; (c) the transducers of said first and second groups, respectively. having common operating characteristics; (d) means for producing measurements ot'said physical characteristic of subsurface media at different depths of the support member in said borehole;; and (e) means for combining said measurements taken at different selected depths of the support member in said borehole to provide compensation for variations in the borehole and/or misalignment of transducers therein.

As used herein. the term ''transducer" means a device that is capable of either transmitting or receiving a particular type of signal. For example. in acoustic measurements the transducer may be either an acoustic transmitter or an acoustic receiver. the transmitter serving to convert electrical energy into mechanical or acoustic energy and the receiver to convert acoustic energy into electrical energy. Similarlv. in electromagnetic wave measurements. the transducer may be an antenna or radiator of electromagnetic waves while the receiver may be an antenna for detecting transmitted electromagnetic waves.

The distance between the two same-type transducer groups may be as long as desired. An array in an apparatus constructed as above using transducers of the first type capable of operating as T's and of the second type as R's may be described as a TT-RR array.

Measurements produced at selected borehole depths between different transmitter-receiver combinations as the transducer array is moved through the borehole may be combined to produce compensated measurements.

For example. with the above novel TT-RR array. two measurements with the same T-R spacing that use different T-R combinations are possible. since the separation between each receiver pair equals the separation between each transmitter pair. If one measurement is made with a first T-R pair at a selected borehole depth and a second measurement is made when a second T-R pair has moved to the same depth. the two measurements may be combined to provide a measurement that is compensated for variations between the characteristics of the transducers and other systematic errors.

Furthermore. differential measurements between one transmitter and two receivers when the two receivers are adjacent a selected interval in the borehole at a selected depth can be repeated when the two transmitters are adjacent the interval and all of the measurements combined to produce a borehole compensated measurement for the interval; i.e.. a measurement that is compensated for tool tilt. borehole eccentricity. etc.

Ntoreover. due to the arrangement of transducers in the array. different measurements that are in most cases compensated for borehole errors can be obtained for two different T-R investigation distances: i.e.. a long T-R measurement and a short T-R measurement. In each of the lono- and short- T-R measurements. each transducer in one group respectively spaced the longest and shortest distance from the other group is used. and in such use. may be regarded separately as the long and short spacing transducer in each group. that is. the transducers in each group that are. respectively. the longest and the shortest distance from the other group.

Since the same-t-pe transducers in either group can be closely positioned on the tool. the operating environment. direction of signal propagation and the refraction error are essen tally the same for either of the transducers in a given group during a given measurement. As a result. directional receivers and transmitters may be used. thereby improving the quality of the measurements obtained.

While acoustic and electromagnetic transducers are illustrated in centralized and sidewall skid configurations. this invention applies as well to other types of measurements using at least four transducers. operated either in a pulsed or continuous mode while making one or more measurements such as travel time, phase angle. amplitude ratio or attenuation-like measurements.

Methods and apparatus in accordance with this invention will now be described. by way of example. with reference to the accompanying drawings. in which: FIG. 1 shows. in representative block form. an apparatus for acquiring. storing and combining measurements of physical characteristics of subsurface media near a borehole: FIGS. 2A through 2C show the measurement acquisition sequence using the transducers included in the apparatus of FIG. l: FIGS. 3A and 3B shown surface and downhole circuitry of a form of the apparatus for making acoustic measurements: and FIG. 3C shows the corresponding timing diagram: FIGS. 4A through 4D show the effect of borehole conditions such as misalignment and tilt of a transducer support: FIGS. 5A and 5B show different borehole operating environments for which compensation is provided by the present invention; FIG. 6A shows a prior art skid-mounted borehole compensation array and FIG. 6B shows an array as used in the present invention; FIGS. 7A and 7B show further advantages of a transducer array as used in the present invention; FIGS. 8A and 8B show alternative circuitry for use with circuit 24 of FIG. 3A; FIGS. 9A and 9B show the relationship between different measurements of physical characteristics of subsurface media; and FIGS. 10 and 11 show two forms of apparatus for making electromagnetic wave measurements.

Referring to FIG. 1, there is shown in representative block form apparatus for acquiring, storing, recording and combining measurements of physical characteristics of subsurface media near a borehole that penetrates an earth formation. The apparatus includes a borehole tool 10 with a transducer array having four transducers numbered T1 through T4. The array may be included in a tool that is either a mandrel type adapted for centralized or eccentered operation or in a skid member, with the transducers located on the skid for operation in close contact with the borehole wall.

The explanation which follows assumes that the tool has been run to the bottom of the borehole so that it can then be retrieved slowly towards the surface under the mechanical control of a logging wireline 12 wound on a winch 14 at the surface, which also provides signal and control communication between the tool and surface control 20. In this manner, movement of the tool may be directly related to the movement of the wireline at the surface.

The surface controller 20 acts as a programmed transmitter and receiver selector, which communicates through a slipring collector 16 on the winch 14 to the logging wireline 12 and downhole to subsurface control 11 in the tool 10. Synchronously with the wireline movement, incremental depth pulses are provided to both the controller 20 and to the measurement storage apparatus 22 through any appropriate mechanical or electrical connection 18 and, if present at the well site so that the measurement processing may be done at the same time, to the measurement selection and combination apparatus 24. In this manner, the transducer selection and corresponding measurements are synchronized as described hereinafter.

It is understood that the actual measurement selection and combination need not be done in conjunction with the acquisition of the individual measurements since these measurements may be provided.at any later time from conventional analog or digital storage facilities for processing at a site remote from the borehole. It is important, however, that incremental depths corresponding to the movement of the tool in the borehole be recorded in conjunction with the measurements, since it is necessary, as described hereinafter, to accurately relate the measurements to one another on a depth basis.

As the tool 10 containing the four transducer array is moved upward through depth positions I, J, K and L, various transducers are selected in a systematic manner such that a sequence of measurements is made at regular depth increments. It is customary that a particular point on the tool be selected as a reference point so that measurements taken with various transducers can be related to one another and to the depth of the tool in the borehole as recorded at the surface. Although any point may be selected, FIG. 1 shows and the description hereinafter is based on the selection of the depth reference point as being the location on the tool 10 at the upper-most transducer; i.e., the transducer that is closest to the surface of the earth as the tool advances through the borehole.

In order to describe the sequence of measurements, reference is made to FIGS. 2A through 2C, which again show the four transducer array of the tool 10 as T1 through T4. For descriptive purposes, the letter T with a subscript will be used to indicate transducers, either receivers or transmitters. Further, it will be assumed that the two upper-most transducers, T1 and T2, operate as receivers and that the two lower-most transducers, T3 and T4, operate as transmitters.

Transducers of a particular type, such as those operating as receivers, are grouped or paired together in the tool and groups of transducers move laterally and vertically in the borehole in a coordinated fashion. Furthermore, for reasons which will be apparent hereinafter, the preselected separation between transducers in each group are the same; i.e., the separation between T1 and T2 along the length of the transducer support member of the tool should be the same as the separation between the transmitters T3 and T4. The distances between the groups of different types of transducers, for example, the distance between receiver T2 and transmitter T3, may or may not be the same as the separation between same-type transducers depending on the physical characteristics of the earth formation being measured, the depth of investigation into the earth formation desired, and other factors.

FIGS. 2A. 2B and 2C each show the transducer array T1, T2, T3 & T4 in two separate positions indicated by the depth level indexes at the top of each transducer array. These indexes, I through L, are referenced to the top transducer Tl. in FIGS. 2A and 2B, these positions are land L; i.e., the top transducer T, at depth levels di and dL, respectively. In FIG.

2C, the two positions are labeled I and J because the top transducer T1 is at depth levels di and di, respectively.

As the array is advanced from position I to L in FIGS. 2A and 2B and I to J in FIG. 2C, the array moves up the borehole from depth di through dL, using the T1 as the depth reference point. A signal is generated by transmitter T3, which will propagate uphole towards receivers T2 and Tl. Each of these receivers will convert the received signal into a corresponding electrical signal which may be processed into a measurement m. Since it is normally expected that a signal traveling from T3 toward T2 and T1 will arrive first at T2 and then Tl, the T3 - T2 measurement will be designated as m1 and T3 - T1 as m2.Measurements ml and m2 may then be combined to obtain a measurement of a subsurface physical characteristic in a manner that will depend on the characteristic being measured.

For example, if T3 is transmitting an acoustic pulse, measurements m1 and m2 will represent travel time through the formation and media surrounding the borehole from T3 to T2 and T1, respectively, and then may be combined to determine the interval travel time between T2 and T1, called At.

At some short time separation from the generation of a signal by transmitter T3, a signal is generated by transmitter T4, as shown in FIG. 2B, which is received by receivers T2 and T1 and there converted into measurements m3 and m4, respectively.

A complete sequence of measurements at depth di would include, therefore, all of measurements m1, m2. m3 and m4. Hereinafter, m will designate individual measurements in general, irrespective of type; m1 being made while operating T3 with T2 and m2 with Tl; and m3 being made while operating T4 with T2 and m4 with T1.

Since the four measurements may be acquired in a very short period of time relative to the tool movement, they may be considered as essentially acquired at the same depth. For example, acoustical transmitters may be pulsed on the order of 20 times per second. This rate provides at least five complete sequences per second during which a very small tool displacement would take place at normal logging speeds. The four measurements are transmitted uphole and stored for later use as shown at 22A in FIG. 1 and as will be described in greater detail hereinafter.

At some later time, when the tool has advanced through the borehole to depth dL, as shown in FIGS. 2A and 2B, a second sequence of measurements m1, m2, m3 and m4 may be taken and may be used for compensating for borehole effects on the individual measurements.

For example, when T3 is an acoustic pulse transmitter, the interval travel time, At, between T2 and T1 will be in error if the portions of the signal propagation paths that are located in the borehole are of different lengths at the two receivers. Such a difference would occur in the case of tool tilt.

Prior art borehole compensation techniques in acoustic logging tools use separate transmitters located on opposite sides of the receivers in a T-RR-T array to obtain two At's having reversed near and far receiver relationships.

With the apparatus shown in Figure 1, that type of borehole compensation is possible with an array having a significantly shorter overall length. By combining a first set of measurements, m1 and m2, taken at depth di (aww DIG. 2A at I) with measurements ml and m3 taken at depth dL (see FIG. 2A at L) a novel combination of measurements from transducers having a reversed near and far relationship is obtained that provides the desired borehole compensation.

Furthermore, a second borehole compensated measurement can be made simultaneously with and over the same interval in the borehole as the borehole compensated measurement described above. Such a second measurement cannot be obtained with the prior art T-RR-T array. Referring to FIG. 2B. by combining a second set of measurements, m3 and m4 taken at depth di (see FIG. 2B at I) with measurements m2 and m4 taken at depth dL (see FIG. 2B at L), a second borehole compensated measurement is obtained, but here having a longer T-R distance than the first measurement. This is because this second set of measurements is referenced to transducers more distant than in the first set.

A further advantage of the transducer array shown in Figure l relates to the use of the array to compensate for statistical or systematic errors in the measurements taken and may be described in connection with FIG. 2C.

Note that measurement m2 at depth di is essentially repeated by m3 at dJ, when T2 replaces T1 and T4 replaces Ti as the tool advances through the borehole. Under perfect measurement conditions, therefore. m2 should equal m3. However, under typical borehole measurement conditions there are several known reasons why this may not occur. Even if small statistical variations may be expected. for example when acoustic interval transit time measurements are being made. an improved measurement is obtained by averaging m2 at di and m3 at dJ to provide a measurement that is compensated for such statistical variations While comparable statistical compensation might be accomplished by repeating the measurement at dl, such repeat measurements cut the duty cycle of the tool by half.In contrast, no increase in duty cycle is required to obtain this result by combining the already available m2 and m3. Further, as will be explained hereinafter, there are other reasons why it is preferred to use different transducers and even different tool positions to obtain measurements for such combinations.

While not shown in FIG. 2C, it will be appreciated that other measurements may also be combined advantageously to compensate for random noise or different transducer effects and their relative positions in the borehole. For example, m2 at depth dK may be used with m3 at dL.

In some cases measurements such as m2 and m3 may also be compared to detect borehole distortions, such as tool tilt. A comparison of such measurements can give an indication of the borehole compensation being applied to the basic measurements and, thereby, an indication of the reliability of the borehole compensated measurements.

As described above, all four measurements in each sequence are not essential to provide one compensated measurement, nor is it necessary to make each measurement after individual transmitter firings as described. However, as shown in FIGS. 2A - 2C and summarized below, each individual measurement will be used at least twice in different combinations to provide two different borehole compensated measurements of a selected borehole interval, corresponding to two different transmitter-receiver investigation distances:: TABLE I 1) m2 and m1 at di (interval T2 to T1 operating T3) 2) m3 and m, at dL (interval T3 to T4 operating T2) 3) m4 and m3 at di (interval T2 to T1 operating T4) 4) m4 and m2 at dL (interval T3 to T4 operating T1) 5) m2 at di (T3 to Tl) and m3 at di (T4 to T2) 6) m2 at di: (T3 to T1) and m3 at dL (T4 to T2) As shown in FIG. 1, each measurement m1, m2, m3 and m4, is stored in measurement storage apparatus 22 for each increment of depth dl, di + 1, ... etc.; each increment being on the order of six inches or less.

If measurement storage capacity is limited, it is advantageous to combine some of the measurements to minimize the needed capacity. For example, measurements m, and m2 for the same depth increment (see FIG. 2A at Position I and depth increment dl) may be subtracted in measurement selection and combination apparatus 24 to form a new measurement mS = m2 - ml, which in turn may be stored, replacing both m, and m2 or, if sufficient storage capacity exists, as an additional measurement.

As the array is advanced through the borehole from dl to dJ, other measurements may be combined to form replacement or additional measurements. As the array is advanced to depth dL (see Position L in FIG. 2), a complete set of measurements will be produced. Those previously acquired are now available from storage 22A and those acquired at depth dL now available as current measurements. Thus, it is then possible to combine these produced measurements to provide compensated measurements for the borehole interval shown in FIG, 2 below dl.

For example, by subtracting m1 from m3 produced at dL, the current depth at position L shown in FIG. 2A and combining this result with measurements m2 and m1 at di previously produced at position I, or its previous combination m5 at dl, the borehole compensated measurement for the borehole interval illustrated in FIG. 2 corresponding to the short T-R distance investigation is provided.

The above combinations result, for an acoustic logging embodiment, in adding two At measurements for the same borehole interval, one corresponding to a two-receiver measurement and the other to a two-transmitter measurement, to provide the desired borehole compensation. Depending upon the separation between the like-transducer pairs, the result may need rescaling. If, for example, the separation is one-foot, the correct At, as indicated by output A, will be obtained-by dividing the final combination by two.

In addition to combining these two At measurements as described above, various meas urements at various depth levels could be compared to indicate borehole conditions requiring compensation or combined in a manner to provide the average measurements. For example, m2 at di (see FIG. 2C at I) and m3 at di (see FIG. 2C at J) may be added or averaged to form m6. These average measurements could then be combined to provide At measurements or for other purposes.

In FIGS. 3A and 3B, there are illustrated typical circuits for surface and downhole apparatus for performing transmitter and receiver selection, acquisition and combination of individual measurements. While these circuits are illustrated for providing compensated acoustic travel time measurements, similar circuits may be used for compensated amplitude ratio measurements, for example, by modifying the circuits of FIGS. 3A and 3B to also acquire signal amplitude or gain setting information along with the time measurement information. The amplitude information may be processed in accordance with the teaching of the previously discussed Zill patent.

A general description of the operation of the apparatus of FIGS. 3A and 3B will be given followed by a detailed description.

In general, the measurement sequence commences with a depth pulse corresponding to a depth increment and then clocking the individual transmitter and receiver selection cycles to complete the sequence. Four cycles and corresponding logic modes are shown in Table II below to select either transmitter T3 or the more distant transmitter, T4, and receiver T2 or the more distant receiver, T1.

These logic modes, denoted M and M orN and N, are used to select respectively the appropriate transmitter or receiver as will be described later. The travel path portions given in Table II for each of the measurements are illustrated in FIG. 4A (to be discussed later) and apply as well to either the contact skid embodiment shown or to a mandrel embodiment.

TABLE II CYCLE NO. MODE TRANS. RECR. PATH MEAS.

I M N T3 T2 A + B + D m1 2 M N T3 T, A + B + C + E m2 3 M N T4 T2 F + G + H m3 4 M N T4 T, F + G + I + J m4 The selected transmitter is fired and the propagated signal received at the selected receiver, amplified with a gain setting appropriate for the particular T-R cycle and transmitted uphole.

A reference timing pulse is generated in a fixed time relationship to the time of firing the transmitter and used as a basis for compensating signal losses in the cable and as a time reference point to gate the signal detection circuits used for detecting the time of arrival of the received signal. Time measurements are performed by gating clock pulses into a counter beginning with the transmitter firing reference pulse and stopping with the detection. The counter's contents then become the individual measurements m already discussed in regard to the previous figures. These measurements are stored or recorded according to their cycle position for a later processing.

For a detailed description, refer now to FIG. 3A. The sequence begins with a depth pulse from depth pulse generator 305 starting a rate oscillator control 310 and clock 324. The oscillator 310 and clock 324 cooperate to generate control signals at a rate such that several complete sequences of four cycles each will be performed per second. Clock pulses are fed to cycle keyer330, which generates four cycle control pulses utilized as binary states of M and N; i.e., M, M, N and N by steering logic to produce mode signals representing the modes of Table II. Cycle control pulses are used to synchronize steering logic 331 and 332 to select a new T-R combination, and to synchronize downhole gain system, 334 to establish the gains appropriate for the received signals of each cycle.

Referring to FIG. 3C, a timing diagram is shown to illustrate time relationships between clock pulses (on line 1) from clock 324, M and N binary mode signals (2 and 3) from steering logic 331, cycle control pulses (4.6,8 and 10) and their delayed counterparts (5,7,9 and 11, respectively) from cycle keyer 330 and counter reset and up/down steering control pulses (12, 13 and 14 for counters #l. 2 and 3 respectively) from steering logic 332, used in the surface circuits shown in FIG. 3A to synchronize downhole circuits shown in FIG. 3B.

While the actual circuits, such as square wave generators, for example, which may be used to generate these signals and pulses and their delayed counterparts are not shown for simplicity of the circuit diagrams, how to make these circuits is well known to those in this art.

Similarly for clarity of the diagrams. not all the connections between various circuit components using these signals and pulses to enable gates, select codes, reset counters and the like are shown. The identity of each or alternative signals and pulses is shown where appropriate.

From FIG. 3C. it can be seen that a depth pulse starts a series of clock pulses 1-9 (shown on line 1) to define one complete measurement sequence as previously described in regard to FIG. 2 and Tables I and II. On the initial clock pulse, mode signal M selects T3 (line 2) and N selects T2 (line 3) to begin the Cl cycle to produce m1. Counter #1 (shown at 391 in FIG. 3A) may be reset (RS) (as shown at line 12 in FIG. 3C) on the upward edge of the steering pulse from steering logic 332. Thereafter, counter #1 will start counting clock pulses from high frequency clock 389 when gated to it via clock gate 390 during the initial portion of cycle CI (line 4). Normally. counter #1 is stopped by receiver signal detection before the end of this portion at clock pulse 2, and if not, this pulse or a delayed pulse C1' may be used to stop counter #1, but its contents would be invalid in this case.

Up/ down counters #2 and #3 are shown at 394 and 394A in circuits shown as 24A and 24B respectively in FIG. 3A, and are used in conjunction with memories #1 and #2 also shown there to combine the particular measurements for each counter in the up (+) or down (-) mode as indicated on lines 13 and 14 respectively of FIG. 3C. Similar counters #2A, #2B, #3A and #3B are shown in dashed lines in circuits 24A and 24B of FIG. 3A.

For example, counter #2 of circuit 24A of FIG. 3A is shown (line 13) in a count down mode at clock pulse 1 and, as will be described in detail later, counts down during the initial part of cycle 1 when m 1*, the * indicating the measurement was memorized from a previous ml measurement at a deeper depth, for example, is input from memory #1. Then at clock pulse 2, while still in the down mode the current ml is input to counter #2 direct from counter #1; i.e., without delay or memorization since counter #1 contains the current measurement after the initial part of each cycle. Thus, at clock pulse 3, counter #2 has accumulated -m*1 - ml and is then switched to a count up (+ ) mode.Then during the initial part of cycle 2, mlis input from memory #1 to add + m *2 and, during the latter part of cycle 3 (at clock pulse 6) the current m3 counted during the initial part of cycle 3 is input from counter #1 to add +m3 to the prior accumulation in counter #2.

At the end of cycle 3, counter #2 contains -m ,* - m1 + m + $- m3, which have been input in that order. Subsequently, at a convenient time (shown as during cycle 4 on line 13 in FIG.

3C), counter #2 may be output and reset (RS) to begin at the next depth pulse, as another sequence as described above at clock pulse 1. As will be explained later, and as shown in FIGS. 1 and 3A at A, this combination of measurements corresponds to one of the borehole compensated measurements provided by the techniques of this invention.

Up/-down counter #3 shown as 394A in circuit 24B of FIG. 3A is similarly diagrammed in FIG. 3C on line 14. However, its sequence begins after clock pulse 3 rather than clock pulse 1, as for counter #2 discussed above. At the start bf cycle 2 with clock pulse 3, counter #3 beings, by switching to the down mode. Then during the latter part of cycle 2, and during the initial part of cycle 3, -m2 and -m 3* are input. Then at clock pulse 7, counter #3 is switched to the up mode and- + m 4* + m4 are input from memory #2 and direct from counter #1, respectively during cycle 4. Thus, at the end of cycle 4, counter #3 contains -m2 - m3* + m4* + m4.As shown in FIG. 3C on line 14, contents of counter #3 may be output during the following cycle 1 and counter #3 then reset (RS) and switched to the down mode to begin its sequence again at clock pulse 3. As will be described later, and as shown in FIG. 3A at B, this combination of measurements corresponds to another of the borehole compensated measurements provided by the techniques of this invention.

Cycles 1 through 4 shown in FIG. 3C are summarized in Table III hereinafter and will be further discussed in its description later. With the general use and timing of the control signals, mode pulses, counters and memories now described, the particular circuits of FIGS.

3A and 3B will be described. The M and N mode select signals generated for each cycle by steering logic 331 will be used as the first two bits of a code signal.

At the start of each cycle, a code signal is transmitted from code transmitter 336 in FIG. 3A downhole to code receiver 340 in FIG. 3B. The code signal may contain as few as six bits of information designating which of the two transmitters (one bit), which of the two receivers (one bit), and which of 16 gain settings (four bits) are to be used. Additional bits for additional gain or attenuation settings may be desirable to increase gain resolution when amplitude/attenuation measurements are also being made.

Now concerning the operation of the downhole circuits, refer to FIG. 3B. In general, code transmission results in connecting the selected transmitters and receivers to appropriate downhole circuits and setting the downhole gain. Next, the selected transmitter is fired and the timing count begins. An automatic gain control system is used to standardize the signal amplitudes by varying the gains for each different T-R cycle.

Now discussing the detailed operation of the downhole circuits of FIG. 3B, the code signals from code transmitter 336 are received by downhole code receiver 340 in circuit section 11B shown in FIG. 3B, and a code bit representing the M orlZ mode is routed to the transmitter selector 344 which connects either firing circuit 352 or 354 to T2 for M or T4 for M, respectively. Similarly. the N or N bit is routed to the receiver selector 350 and either receiver T2 for N or T, for N is connected through receiver selector 350 to variable gain amplifier 348.

Gain bits in the signal code (four illustrated) are routed to downhole gain selector 346 which uses these bits to connect selected fixed'attenuators and gain amplifiers to provide the desired gain represented by the code. The resulting 16 possible gain variations are shown for simplicity as represented by variable gain amplifier 348 controlled from gain selector 346.

The gain will be determined automatically by analysis of the received signals as will be explained later but for now it will suffice to-appreciate that longer T-R distances, as for example, T4 to T,. are given relatively higher gains than the shorter T-R distances like T2 - T3.

Reception downhole of a given code by code receiver 340 also causes conditioning of fire pulse receiver gate 360 to interpret the next downhole transmission as a fire pulse command.

Through an appropriate delay provided by delay 341, reception of the code also enables previously disabled downhole output gate 342 to then allow uphole transmission of output from power amplifier 368, which might have previously interfered with the code transmission. Thus, it can be seen that the downhole circuitry of FIG. 3B uses the code to condition associated electronics to connect the appropriate transmitters and receivers and set the desired gain and gates in expectation of a subsequent fire pulse command.

Returning to FIG. 3A, the uphole circuits are enabled synchronously with the operation of the down hole logic, to provide the firing pulse and receive the associated reference pulse and subsequent receiver signal. Cycle keyer 330 shown in FIG. 3A generates, for each cycle, a signal that is sent to steering logic 332 that in turn generates signals to reset to zero a first counter 391 and, depending upon the particular cycle, C1 through C4, to provide various gating signals to gates, counters and signal processing circuitry, most of which has already been described in relation to FIG. 3C.

Delayed control pulses from cycle keyer 330 divide each cycle into subcycles for detection and automatic gain determination, and gating completed current measurements or previously stored measurements into the signal processing apparatus 24 of FIG. 3B, as appropriate for the particular cycle. For example, when signal processing circuitry 24A and 24B is utilized, these steering signals may be used to clear counters 394 and 394A and condition them to process the next input by counting down or, if the input is a digital word transfer, to combine the word with a negative sign. Subsequent steering signals cause these counters to accept further input by counting up or adding to the previous contents.

Besides providing clock pulses which begin each cycle to cycle keyer 330, clock 324 also provides, after a suitable delay, a control pulse to fire pulse circuit 320. This delay, provided by circuits internal to clock 324, is such as to allow time for both the downhole and surface circuitry to be conditioned, as already described, to receive the fire pulse. With the downhole and uphole circuits ready. this delayed clock pulse causes fire pulse circuitry 320 to initiate a fire pulse command (FP) which is transmitted downhole and properly interpreted,by the previously conditioned fire pulse receiver gate 360 shown in FIG. 3B.

Returning to FIG. 3B. the fire pulse command is gated through to fire pulse detector 362 and upon detection, causes To generator 364 to initiate a downhole firing pulse. This causes transmitter selector 344. previously connected to selected transmitter firing circuits, 352 for T3 on mode M or 354 for T4 on mode M (see Table II), to fire the selected transmitter, T3 or T4. The To generator 364 also initiates a To pulse for use as a reference signal both by downhole and surface circuitry. For surface use, the To pulse is transmitted uphole via power amplifier 368 and through the now enabled output gate 342 to signal receiver 370 and system automatic gain control 372 located at the surface (see FIG. 3A).

At the surface. the To pulse is used as an amplitude reference. Since it is established downhole by To generator 364 with a standard reference amplitude, the system automatic gain control 372. To gate 374. peak reader circuit 376 and To gain set control 378 (shown in FIG. 3A) are utilized to re-establish at the surface this standard amplitude reference. Thus, this internal system control provides compensation for cable losses, phase distortion, drift, etc.

The To pulse is gated via To gate 374, which has been previously conditioned to allow To through to peak reader 376. This conditioning is synchronized with the operation of fire pulse circuit 320. The peak amplitude of surface received To pulse is read by peak reader 376 and compared with a reference amplitude by To gain setting circuitry 378 which adjusts, if necessary. the automatic gain control circuit 372 to re-establish the amplitude for subsequent To signals to the reference amplitude level. Additional signal conditioning circuitry may be included for cable losses using this known To signal standard.

The peak To amplitude read from peak reader 376 is also provided to an amplitude detector 380 as an amplitude reference for use in detecting the receiver signal which will follow To. as will be explained.

The To pulse provides an accurate time reference related to the transmitter firing. This time reference is determined by a zero crossing detector circuit 375 connected through To gate 374 to consistently detect the To zero crossing point. This To detection point is used as a beginning for the time measurement by providing a time-related To detection signal as a start signal to clock gate control 388 of FIG. 3B which enables a clock gate 390 to pass high frequency clock pulses from clock 389 to counter #1 which. having been previously set to zero. begins counting the clock pulses. The clock pulses should have a high enough frequency, for example. 2.5 MHz. to provide the desired time resolution. The To detection signal is used in turn to gate off the To gate 374 and gate on a received signal detector gate 379 such that subsequent signals will be interpreted as the next expected receiver signal. With the uphole apparatus of FIG. 3A already beginning the time measurement at the start of the clock pulse counting, refer now to downhole circuits shown in FIG. 3B.

The To signal generated by To generator 364 is delayed by delays D1 and D2 provided by delay circuit 365, as shown in FIG. 3B, and used to respectively open and close a receiver gate 366 for a time interval corresponding to the expected arrival time of the received signal.

These delays will understandably vary with the design distance between the transmitter and receiver and can be determined in a well known manner.

After propagation in the formation, the acoustic pulse transmitted from the selected transmitter is propagated through the borehole and formation and received by the selected receiver, which has been previously connected through receiver selector 350 to an already set variable gain amplifier 348. The selection and gain setting was discussed earlier in regard to the operation of code receiver 340. The received signal is allowed through receiver gate 366 now enabled as described above to the previously described power amplifier 368 and still open output gate 342 where it is transmitted to surface circuitry shown in FIG. 3A.

Returning again to FIG. 3A, the amplified receiver signal, here denoted as Rx, is received at the surface and reconditioned at signal receiver 370 and amplified by system AGC 372, already described, to provide a cable compensated amplitude. It is then gated through a receiver detector gate 379, previously conditioned by a To detection signal generated by zero crossing detector 375 to allow the signal to pass to detection circuits 380 through 384.

As illustrated in FIG. 3 A, the arrival or -of received signal Rx is detected by simultaneously comparing Rx amplitude in amplitude detector 380 with a To reference amplitude supplied by peak reader 376 and examining Rx with a zero, crossing detector 382 and a peak detector 384.

A typical Rx signal is shown in FIG. 9A and has positive and negative half-cycles which increase in amplitude during the first few half-cycles. As illustrated in the circuits of FIG. 3A, three conditions are required for detection: 1) a zero crossing must be detected by zero crossing detector 382, this detection being delayed internally by a delay corresponding approximately to one half-cycle; 2) the subsequent amplitude of Rx as compared by amplitude detector 380 must exceed a small fraction of the To reference amplitude; and 3) an Rx amplitude peak must be detected by peak detector 384 within the half-cycle delay following the zero crossing detection.All three detection indications are provided to AND gate 385, such that the first occurrence of an amplitude exceeding a threshold amplitude referenced to the To amplitude which is preceded by a zero crossing in the appropriate polarity, and which is followed by an amplitude peak of the same polarity within the half-cycle delay, completes the detection.

An Rx detection indication is output from AND gate 385 and causes hold circuit 386 to hold the peak amplitude detected by peak detector 384 for use in setting the downhole gain for subsequent reception with the same transmitter-receiver combination in cooperation with downhole gain setting circuit 334. Independent gain settings are made and stored in the downhole gain setting circuit 334 for subsequent use with corresponding cycles. Further, these gains are determined on the same part of the signal used for the measurement. A further description of this atomatic gain setting technique is provided in U.S. Patent Specifications Nos. 4 040 001 and 4 172 250.

The Rx detection signal output from AND gate 385 is used to reset the receiver signal gate 379, previously described, and, more importantly, to cause clock gate control 388 to gate off clock pulses coming from clock 389. These pulses had been gated on earlier to counter 391 by means of the clock gate 390 by the To detection as previously described. Thus, the To and Rx detection are used to cause the determination of a clock pulse count corresponding to the time measurement for this given cycle. In this manner, the counter #1 now contains the number of 2.5 MHz clock pulses corresponding to the travel time relative to the To and received signal detections.The counts contained in counter #1 may be in turn regarded as the individual time measurement corresponding to the particular measurement cycle such as m1 for cycle I. m2 for cycle 2, etc. The contents of counter #1 at the completion of the counting may then be transferred through various gates to utilizing devices at the times shown in FIG.

3C, as provided by appropriately delayed control pulses C' from cycle keyer 330.

For example. where the individual measurements are to be recorded for later processing, the delayed control pulses Cí through C4 each cause the counter contents, corresponding to m1 through m4 respectively. to be gated through gate 392B to a suitable recording device connected at point C such as a digital tape'recorder, not shown.

Alternatively. particular cycle control pulses are used to selectively gate the counter contents into the memory and counter circuits 24A to provide one compensated signal, and to similar circuitry in 24B if two different spacing compensated signals are desired. These circuits accomplish the relationships for combining individual measurements illustrated in Table III below: TABLE III CONTR. GATE CONNECTION CONTENTS OF COUNTERS PULSE MEAS.FROM TO C#l C# 2 C#3 C1 ml* M#l C#2 -m1* B C1' m1 C#l M#l ml OUTPUT C#2 -m1-m1* RESET C2 m2 * M#l C#2 +m2* -m1 -m1 * C2' m2 C#1 M#l m2 C#3 - m2 C3 m3* M#2 C#3 -m3 * - m2 C3' m3 C#1 M#2 m3 +m3+m2* C# 2 -mlml * C4 m4* M#2 C# 3- A +m4* -m3 - m2 C4' m4 C#l M#2 m4 OUTPUT C#3 RESET +m4*+m4 -m3 * - m2 * measurement from previous position Table III above illustrates the general cycle for each measurement m. During the primary portion of the cycle, here denoted as subcycle C, clock pulses are being accumulated in counter #1 (C#1) for the new measurement at the current depth dJ, for example, as illustrated in FIGS. 2A and 2B.The corresponding measurement m * made at the previous position di in the illustrative example, is transferred from memory M to a second counter which has been previously conditioned for this cycle to count down or subtract; or count up or add; for example. This makes room in memory for current measurement such that memory need only have a capacity for the number of measurements acquired between di and ds because the current measurement m (at di) may replace the measurement stored at dl).

The next subcycle C' begins after a long enough delay has been provided to allow the completion of the current measurement; i.e., after the expected signal has been received from downhole and detected and C#1 has stopped counting. Then m is gated from C#1 to memory M, replacing the corresponding previous measurement m*. During subcycle C', m is also gated to the particular second counter C#2 or C#3 for this cycle. As illustrated for the two compensated measurements A and B, each m goes in turn to one memory M and one additional counter; e.g.. m1 goes to M#1 and C#2; m2 to M#1 and C#3; m3 to M#2 and C#2 and m4 to M#2 and C#3.Thus, each M stores two different m's and the counters C#2 and C#3 combine two current m's and two previously stored m*'s.

From examination of FIGS. 3A and 3B and Table III above, it is readily seen that both the measurements and apparatus' components serve multiple uses. The same control, amplifiers, cable compensation. automatic gain system, time reference, detection circuitry, high frequency clock and clock pulse counter are employed for each individual measurement. This not only provides lower cost apparatus but provides compensation for systematic measurement errors. as will be explained in more detail later. For now, it need only be appreciated that if a component inaccuracy causes m, to be in error. m2, m3 and m4 will also be in error by the same amount in the same direction. However, in accordance with the advantages of this invention, when these systematically erroneous measurements are combined as illustrated above. these errors will be compensated out just as a systematic error induced by sonde tilt, for example. is compensated.

As previously discussed and as illustrated in Table III, various measurements are typically used twice. first in reference to the receivers and then with the transmitters. Steering logic 332 provides the control pulse mode signals allowing the contents of counter #1 to be transferred to the gated memories or counters used to perform the measurement combinations. For example, at the completion of the first measurement m, cycle which corresponds, as indicated by Table II, to the T3 - T2 measurement, as illustrated in FIG. 3A, the clock control pulse C1 or preferably, a delayed version of it, Cl', as in Table III above, may be utilized to gate the counter contents to memory in circuit 24. Preferably the earlier Cl pulse is used to gate out a previously stored measurement from memory.The timing relationships for various M, N, C and C', (delayed) states and combinations for the associated measurements m, through m4 have already been described in regard to FIG. 3C. These relationships will be detailed now in regard to the particular circuit components.

Memory is utilized to delay measurements m* made at an earlier position such as at di illustrated in FIG. 2, so that they may be combined with current measurements which are in counter #1. In the preferred arrangement, counter #1 contents are gated through gate 392 to memory #1, both at the completion of cycle #1 and cycle #2, to store measurements ml and m2. After the number of complete cycles corresponding to the movement of the transducer array from the position illustrated as di to the position illustrated as dL in FIG. 2A have been stored, these measurements are available from the output of memory #1 such that control pulses provided to gate 396 gate out these previously stored measurements for utilization in counter #2.In this manner, C1' would cause m1 at di and C2' similarly cause m2 at di to be gated through gate 392 into memory #1 in serial arrangement. This memorization process of m1 and m2 in memory #1 continues until, for example, at dL, the previously memorized measurements become available as output of the memory. At this time control pulse C1' would continue to provide new m1 measurements to memory as well as to counter #2 through gate 393.

As already explained, counter #2 has been previously conditioned prior to C1 to interpret subsequent input in a countdown or negative sense. Thus, when CI is supplied to memory #1 output gate 396, m,* corresponding to previously stored ml* measurement at di is gated to counter #2. Then, at a delayed version of C1 denoted C1', the current m1 is also gated to counter #2 and to memory. In this manner, m, * at di and m, at dL are gated to and combined at counter #2 in the same sense; i.e., either by continuing to count down for their combined count or added with negative signs.Thus, in counter #2 at the end of the C1 cycle is -m, at dL m,* at di. The next clock cycle C2 would add run2* at di through gate 396 to counter #2, but now conditioned to consider input in a positive or count up sense. Then the current m2 at dL would be stored.Thus, at the end of a C2 cycle, counter #2 would contain m2 * at di - m, at dL m, * at dl. During the next clock cycle C3, measurement m3 at current depth dL would be gated through gate 393 to counter #2 still in its add mode such that the result becomes m3 at dL + m2 * at di - m, at dL - m, * at di. Then, at a subsequent convenient clock pulse, illustrated here as C4', the contents of counter #2 is gated out through gate 397 to point A as the compensated signal. Where the transmitter-receiver selection has been in accordance with Table II compensated signal A corresponds to a short T-R distance investigation.Counter #2 is subsequently reset and the processing for another compensated signal sample corresponding to the next sequential depth increment begun in the above described manner.

For a long T-R distance investigation B, corresponding circuitry 24B, shown in FIG. 3A with separate memory #2 and counter #3 and corresponding gates, may be utilized. In 24B, these components have been designated with the same numbers used in 24A but now include an additional designation "A". Of course, these "A" gates are controlled by different control pulses as indicated therein since they involve different measurements obtained at different times. Like the timing diagram of FIG. 3C for the timing of the circuits shown in FIG. 3A, Table III summarizes the operations of both circuits 24A and 24B in terms of the control pulses, senses and contents of the various counters and memories illustrated in FIG. 3A and used to derive the two compensated signals, A and B.

It will be realized where both A & B are desired, memories 1 and 2 may be readily combined since their input and output functions occur at separate control pulses and measurements m l through m4 may be stored in that order and retrieved in that same order. One suitable memory is described in U.S. Patent Specification No. 4 040 002. Each time a new measurement. as for example, m, is ready, the oldest corresponding measurement is retrieved from storage such that the newest measurement may replace the oldest measurement and the memory managed on a replacement basis, thereby conserving the memory capacity.

It will be readily recognized how the additional measurements m2 through m4 may be acquired and utilized from the description of m, above, the control logic and definitions to acquire these measurements being provided by Table II and the processing logic being provided by Table III. It should be realized that the invention may be practiced by providing a single compensated measurement, here illustrated as either A or B, therefore employing only a single memory or additional counter other than counter #1. In this case. two measurements may be combined as acquired and only the result stored. The two current measurements would not need to be stored.

It also should be realized that the processing provided by circuits 24A and 24B may be provided by a digital microprocessor with its normally associated memory replacing memories 395 and 395A and its arithmetic registers replacing counters 394 and 394A, its control program utilizing the control pulses to perform the indicated transfers to and from memory and the registers. As previously stated, these processes may also be provided by utilizing output C recorded on a digital tape recorder which is subsequently produced as input to a general purpose digital computer and processed with an equivalent control program.

Referring now to FIG. 4A, there is illustrated a transducer skid support which is tilted from the desired wall contact position parallel to the borehole wall. Such tilt may be due to a variety of mechanical problems associated with the skid to mandrel linkage, inadequate sidewall pressure, etc. Unfortunately, when this tilt problem occurs, it may not be reflected in linkage caliper or pressure measurements. However, in accordance with one feature of this invention, not only can the tilt be detected but its effect compensated.

The transducer array shown in FIG. 4A is arranged as was assumed for illustration in FIGS.

2A and 2B; i.e., the receiver pair T, and T2 is at the top and the transmitter pair T3 and T4 is at the bottom of the skid.

As denoted in Table I already described, four measurements are made between different combinations of these transducers. Two binary modes M and N are used to code the transmitter and receiver selection which control the signal paths. In accordance with one advantage of the novel transducer array, compensation for borehole path length differences due to either tilt or borehole washout between the near and far transducers in the pair is provided by reversing the sense of the near and far transducer measurements, i.e., the far transducer becomes the near transducer and vice versa. This ability is provided by utilizing a pair of transmitters in the same sense that a pair of receivers are used to provide one of two sets of the transducer measurements. FIGS. 4A and 4B illustrate how this compensation is obtained for the skid type and FIGS. 4C and 4D for the non-skid type arrays.

Consider the paths shown in FIG. 4A and Table II. Signals leaving T3 travel path A through the borehole to the formation and then towards the receivers along path B, reaching T2 via borehole path D and T1 via additional formation path C and borehole path E. If the borehole paths E & D are equal, the differences between the signals ofT2 and T1 will be essentially a measurement of the travel through formation path C, corresponding to the interval between T2 and T1.,If, however, path D is substantially different from path E, this distorts the short T-RR measurement thought to correspond to formation path C as in the illustrated case where D is larger than E. The short-spacing travel time measurement ms equals m2 - m1 = C C + (E - D) because the common paths A and B subtract out.Ideally, E = D and there would be no error. However, in the above illustrated case, the error equals their difference, E which is negative, indicating travel time will be too short.

An error would also be present for the long T-RR measurement me made relative to T4, since the borehole path lengths H and J are also unequal. Here, me = m4 - m3 = I +(J-H), since common paths F and G subtract out. As illustrated, H is larger than J, making the error due to their difference also negative, and indicating this travel time is also too short.

Despite the separation in paths illustrated in FIG. 4A, formation paths C and I for the formation interval between T2 and T, and borehole paths D and H at T2 are respectively almost the same, as are E and J at T1. Even formations regularly varying in acoustic properties radially from the borehole wall can be assumed to still have nearly identical receiver borehole paths for signals received over either the long or short T-RR distances. Consequently, both the short-distance ms using T3 and the long-distance me using T4 can be expected to have the same error.

Referring to FIG. 4C, consider the nature of the error when the transducer array is moved from position (a), when the receiver pair is adjacent interval I, to position (b), when the transmitter pair is adjacent interval I. The borehole paths for position (a) are denoted as in FIG. 4A and for position (b) by the same letter-but primed; e.g., A and A' at T3, respectively.

With the interval of interest I between T3 and T4, the short-distance measurement for position (b) is m'5 = m3 - rn' using T2 and the long-distance measurement me e = m4 - m; using T,. Referring to Table I, it can be seen that the error for both m'5 and m'e is F' - A'. If F' is greater than A' . the error is positive and thus in the opposite sense from the errors in position (a). As FIGS. 4B and 4D will showm the error is also of the same magnitude.

Consider FIG. 4B for the skid case shown in FIG. 4A. and recalling that position (a) errors were (E-D) or (J-H). it can be readily seen that since the tilt angle y is the same, the paths E or J at T, for position (a) taken in ratio to path A' at T3 for position (b) is proportioned to paths D or H at T2 for position (a) taken in ratio to F' at T4 for position (b), due to the geometrical similarity. Thus, (E - D) = -(F' - A') and in fact, the tilt angle y, may be computed. If ms (or me) is less than m's (or m'c), the illustrated case of tilt is present where the upper pair of same-type transducers is closer to the wall than the lower pair.If ms is greater than m's, the reverse case would be indicated. This will be more fully appreciated from FIG. 4D.

In FIG. 4D, paths are illustrated with the transducers superimposed to show the differences in parallel paths A' and F' and paths E (or J) and D (or H). It can be seen that each path is related to the tilt angle y, the distance from the wall contact point of the transducer array, and the refraction angle p. Since y and ss are constant and the distance separating like transducers is the same (here shown as I) it can be shown that the difference between path lengths for like transducers is also the same, such that D - E (or H - J) = F' - A'.

From the above, it can be seen that measurements between first the receiver pair and then the transmitter pair reverses the sense of the tilt error introduced in these measurements.

The effects of borehole shape rather than tilt is illustrated in FIGS. 5A and 5B. In FIG. 5A there is shown in horizontal section the ideal position of a transducer T; i.e., centered in a round hole. Path 1 from a transmitter and the path 2 to a receiver are of equal length as are all the paths around the circumference of the transducer. This causes the transmitted energies radiated in different directions to be received essentially at the same time and thus reinforce one another to provide the best signal amplitude and phase stability.

FIG. 5B shows the same transducer T parallel to the borehole wall as in FIG. 5A (not tilted) but now the borehole is non-circular, with the shape resembling two intersecting cylinders with different diameters and non-coincident centers. This shape is typically found in directional holes. It can be readily seen that the borehole paths 5 from a transmitter and 6 to a receiver not only vary in length but frequently do not even intersect the transducer. This results in a marked reduction of the transmitted energy coupled to the formation and a destructive out-of-phase relationship for the signals arriving at the receiver, since a signal traveling via path 7 will arrive much sooner than via path 8, for example. Consequently, large reductions in amplitudes are experienced under such conditions.

To a lesser degree, the above signal problem also occurs in tilt cases, since, in those situations, it is impossible to have all the transducers in the ideal position. For example, as illustrated in FIG. 4C, various degrees of eccentering, even in a round hole, would be present for each of the four transducers. Thus, measurements m2 and m3 would be equal under the ideal FIG. 5A conditions, but unequal under the out-of-round hole of FIG. 5B conditions or the eccentering associated with sonde tilt. In this manner, this comparison of different measurements at different depths may actually detect different transducer operating environment conditions such as caused by sonde tilt.

In the prior art T-RR-T arrays, the transmitters are located at extremes of the array. Thus, if tilt causes one end to eccenter, the two very much spaced apart transmitters operate under substantially different positions even in a round hole.

By comparison. same transducers in the TT-RR array described herein are closely spaced and operate advantageously in much the same positions with respect to the borehole wall.

As previously mentioned, it is desirable, particularly in acoustic investigations, to have long T-R distances to overcome the effects. for example, of shale alteration. The same desire exists in skid devices and in other types of measurements such as high frequency, electromagnetic investigations. etc.

FIG, 6A illustrates one prior art borehole compensating array. The T-R distance is shown as occurring twice. first from T, and second from T2 to the array midpoint between R, and R2.

For comparison FIG. 6B illustrates the compensating array of the present invention, as applied to the sidewall skid. The same span or receiver investigation interval and skid length are used in both FIGS. 6A and 6B. However for the same transducer array length, the novel array shown in FIG. 6B provides a substantial increase in the T-R distance even for the shortest T-R investigation. For the longest T-R investigation. this distance is the entire array length. less only one-half the span. In contrast. the maximum T-R distance of the prior art array is only one-half the entire array length. By the novel overlapping of both the short and long T-R distances, the array illustrated in FIG. 6B, constructed in accordance with this invention. provides not only longer T-R distances for the same array length. but provides two different T-R distances contained swithin this length.Typical span between like transducers for acoustic time measurements is one or two feet while the shortest T-R distances are at least four feet but preferably six or eight feet. Thus. the reduction in length obtained with the TT-RR array of this invention is on the order of six or more feet as shown graphically between Figs. 7A and 7B.

In FIG. 7A. both circuit connections and the use of directional transducers in the prior art type of compensation array are illustrated. In order to use directional receivers. two separate sets of receiver pairs must be employed. Rn and Rf for receiving signals from the direction of Tu. the upper transmitter. and Rn and R' for ,signals from Tt. the lower transmitter. Added to the complexity necessitated by the two extra receivers is the usual electronic noise problem associated with connections between the uphole circuits above the transducers to the bottom transducers. For the lower transmitter. for example these connections must be strung through or around the upper transducers.A high voltage generator is usually located near one ot the transmitters, here not shown but above Tu. In any case one high voltage lead, here Ft, must be run past the receivers to the remote transmitter. High voltage pulses typically employed to fire such transucers must be shielded in order to prevent electrical crosstalk into the receivers or receiver leads Rn, R'n, Rf, and R'f, and even then crosstalk can become severe.

By comparing FIG. 7B illustrating the array according to the invention with the prior art array of FIG. 7A described above, it is readily apparent how the advantages of the inventive array can be used to overcome this electrical connection and crosstalk problem. Since both the transmitters are together and located-on the same side of the receiver pair, no high voltage leads need pass near the receivers or the receiver electronics. The high voltage generator may be located below the receivers and their associated electronics. Thus, only a relatively low voltage DC supply needs to be connected from above. This arrangement provides good electrical signal isolation and freedom from cross talk into the much lower level receiver signals.

Further, the novel transducer arrangement will allow the use of both directional transmitters and receivers without the necessity of adding an extra pair of transducers to provide the needed directivity. Since both receivers are on the same side of both transmitters, each receiver and transmitter has a unique directivity requirement, requiring no additional transducers as in the prior art array. Still further, since the same pairs of transducers are always used, differences in additional pairs of otherwise needed transducers to obtain directivity in both directions will not occur to affect the measurement.

An additional advantage of the TT-RR type array is its ability to compensate for refraction effects. As apparent from the prior art compensation array shown in FIG. 7A, the signals approach the receivers from different directions and incliniations. This inclination is due to the well-known refraction effect which gives the appearance that the borehole signal path intersects the formation at an angle somewhat less than 90 , the actual angle depending upon the formation/borehole fluid velocity contrast.

Two pairs of receivers Rn and Rf and R'n and R'f respectively, are illustrated in FIG. 7A to accommodate the refraction effect. Each receiver is aimed with its most sensitive direction along a particular inclined borehole path. Each pair is displaced to more effectively match the position of the formation interval simultaneously under investigation between the two pairs.

This displacement may be termed a refraction displacement and determines the small spacing between the two receivers illustrated therein. which are used to take the place of the usual single receiver for directed reception from above and below; i.e., between either of Rn and R'f or R'n and Ri. Unfortunately. the refraction displacement varies not only with hole size but with formation velocity, such that a fixed spacing between these two receivers can be designed but for one displacement corresponding at best to a nominal borehole size, formation velocity, etc.

However, in accordance with this invention, variations in the refraction displacement can be compensated by varying the delay distance or number of depth increments between measurements made between the different same-type transducer pairs. As can be seen in regard to FIG. 7B,the lower pair of same-type transducers sees the formation interval slightly above the actual borehole depths for these transducers while the upper pair sees the intervals slightly below their actual depths. Thus. the refraction displacement compensation may be readily provided by simply adjusting the delay between measurements made between these pairs before their combination, as for example. decreasing the delay for larger displacements between the actual position and the effective position of a transducer caused by a larger borehole size. higher velocity formations. etc.

An additional feature of the invention may be seen by comparing the formation intervals investigated as shown in FIGS. 7A and 7B. In the prior art compensation arrays shown in FIG. 7A only interval I centered about its midpoint is investigated. Thus, this excludes any possibility of investigating the critical interval between this point and the bottom of the hole.

However. as can be seen in FIG. 7B, the lower interval It on the TT-RR array lies very near the bottom of the hole and can be investigated by measurements made between the bottom pair of transducers. While not compensated in this particular case, both short and long T-R investigations can be made.

Circuits to provide the At measurement from the upper and lower intervals. At for Iu and Ate for It are shown by dashed lines in FIG. 3A. For example. the m, and m2 measurements gated to memory #I via gate 392 may also be gated to up/down counter #2A shown at 398 in circuit 24A. This counter. like up/down counter #2 shown at 394. is directed by steering pulses from steering logic 332 to count down or load with a negative sign form1 during C1' and up or add with a positive sign form2 during C2'.Thus. at the end of C2'. the contents of counter #2A are m2-m1 for the interval currently between T2 and T1 or Atu. Since m2 and m, are both referenced to the short-distance transmitter T3. this is a short T-R distance Atu as can be seen in FIG. 2A at position I. The timing and steering may be seen in Fig. 3C.

A long T-R distance Atu is similarly provided using the remaining measurements in another up/down counter #3A shown at 398A of FIG. 3A. This counter, steered like up/down counter #3, with m3 and m4 input from gate 392A in circuit 24B provides m4-m3 for the interval currently between T2 and T1 or Atu as can be seen in FIG. 2B at position I.

For position L, and the lower interval It, the short T-R distance Ate is provided by up/down counter #2B at 399, steered like counter #2 with m1 and m3 input from gate 393 to provide m3-m ,; and for the long T-R distance Ate, by up/ down counter #3B at 399A steered like counter #3 with m2 and m4 input from gate 393A in circuit 24B. Thus, long and short T-R distance At investigations are provided for both the upper and lower intervals shown in FIG.

7B.

Even when neither Atu nor Ate can be borehole-compensated, they are obviously useful to log the borehole intervals respectively present, for example, just below the casing and at the bottom of the borehole. When used together, they are useful as borehole compensation indicators, since their difference indicates the degree of tool tilt; e.g., Atu < Ate corresponds to the FIG. 4C illustration.

Referring to FIGS. 8A and 8B, there is shown alternative circuitry for one section of signal compensation circuit 24, previously described in connection with FIG. 3A. As previously mentioned, it is sometimes advantageous t9 compare, as well as to combine, the measurements. By comparing different measurements that should be substantially the same, for example, measurements between different transmitter-receiver pairs over the same interval in the borehole, certain borehole operating conditions that cause the measurements to vary may be detected. If the measurements compare to a reasonable degree, their differences may be attributedito statistical variations such that they may be combined to produce an improved or compensated measurement.However, if the comparison disclosed an unreasonable difference, the operating condition causing the error may be indicated.

Accordingly, a circuitry illustrated in FIG. 8A allows, upon the occurrence of a depth pulse, gating at 181 of delayed measurement m* corresponding to a previous position and transducer combination available at the output of memory, as shown in FIG. 3A, to be passed to comparator 182. Similarly, the current measurement m directly comparable to the memorized measurement m* is also input to comparator 182.

If, for example, the delayed input corresponds to m2 at dl and the direct input corresponds to measurement m3 at dJ, as illustrated in FIG. 2C, it may be expected that under normal conditions the measurements would be substantially equal. However, if a detection error has occurred in one of the measurements, a substantial difference will be noted.

As shown in FIG. 8A, an unreasonable difference provides a no-comparison signal, which may be used to indicate a detection problem, such as cycle skipping. However, if the comparison is reasonable, that indication is used to gate measurements m2 and m3 to adder 183 for combination to produce a compensated average measurement from the measurements.

Alternative circuitry illustrated in FIG. 8D is more appropriate for indicating the borehole compensation required to compensate either time or amplitude measurements. The memory delayed and the direct (current) measurements are gated to and compared at 182A. If the comparison is reasonable, the two measurements may be then combined as described above.

However, if the comparison is unreasonable, this indication may be used to gate, via gates 181C and 181D, the measurements to difference amplifier 183A, whose output is summed at 184 and used to indicate the relative error in the two measurements.

The circuitry shown in FIGS. 8A and ,8B may also be used for other compensating purposes. As previously described in regard to FIGS. 4C and SB, the condition of tool tilt produces different degrees of eccentricity for various transducers and corresponding differences in the arrival times and amplitude measurements, which will be indicated by relative measurement indicator 184A. If the tilt-results in an upper transducer pair that is more eccentered than the lower pair, it would be expected that the upper or memory delayed measurement would be shorter in time and less in amplitude relative to the direct measurement. Thus, the difference between the delayed and direct measurements will provide a negative indication. Conversely, if the lower transducer pair is more eccentered, the indication would be positive. This will be seen from the following example.

Consider measurements m2 and m3, defined as shown in Table II. taken when their known positions along the borehole correspond to the same formation interval. This takes place when the transducer array is moved, as for example, from position dl to dJ in FIG. 2C. In effect, transducer T2 replaces T, and T4 replaces T3. Formation paths B and C for m2 at di (here m2*) are substantially equal to path G for m3 at dJ (here m3), and any errors between m2 * and and m3 will be due to differences in the comparable paths A and F in combination with E and H, as can be seen from FIGS. 4A or 4C. Thus, the difference m2 * - m3 equals A + (B + C) + E - F - G - H = (A - F) + (E - H), assuming B + C = G.

As illustrated in FIGS. 4A or 4C, A is less than F and E is less than H, such that the differences (A - F) and (E - H) do not cancel, but are of like sign (both negative here) and combine to indicate both the nature of the error between these two measurements and its magnitude.

While the preceding discussion has generally been directed to acoustic measurements, additional methods and apparatus directed to other types of measurements, such as high frequency, electromagnetic measurements, etc. are possible and will be described. First, some inherent differences in the measurement techniques used in these additional applications will be reviewed.

FIG. 9A illustrates the type of detections used typically in acoustic travel time measurements or other measurements, where the signal period or wavelength is long compared to the resolution required. The signal is normally propagated as a pulse having positive and negative oscillations beginning with its arrival and relatively little signal prior to that time. Thus, as illustrated at I and II, corresponding to the reception signals that might be expected respectively at the near and far receivers, relatively little signal is present prior to its arrival. By design, the first and relatively weaker half-cycle will be provided a polarity opposite to that used for detection. A detection threshold amplitude different from zero to avoid noise and in the opposite polarity from the first half-cycle is employed.The detection corresponds to the point Tx when the amplitude first swings beyond this threshold.

Thus, for I in FIG. 9A, the detection at the first receiver occurs as illustrated at Txl and the corresponding detection for II at Tx2. These detection points are related in time either to each other as for example, where Txl would begin a time interval, and Tx2 stop the time interval for the case of differential measurements, or in the case of individual sequential measurements, Tx may be made relative to some earlier time such as To. In this manner, the measurement m, at III, corresponding to signal received at T2, would begin at To and stop at Txi while, for T3 and m2 at IV, the measurement would begin at the reference To time and stop at Tx2. In this manner, the difference m2 - m1 provides the interval measurement At as illustrated at V.

In electromagnetic measurements, the signals travel at significantly higher velocities and their periods are quite short compared to the required time resolution. Consequently, phase detection is usually employed rather than the zero crossing or threshold method illustrated in FIG. 9A. The phase relationship may be measured between signals received from the near and far receivers to obtain a differential measurement or if individual measurements are preferred, to a known reference signal of the same frequency. As illustrated at land II of FIG.

9B, the two signals are displaced slightly as will be seen by comparing the zero crossing detection points at III for the signal on the line I with IV for the signal on the line II. Thus, as illustrated on line V, the phase shift A between the illustrated zero crossing points corresponds in much the same manner as the At measurement illustrated in FIG. 9A. Particular circuitry to perform the above illustrated differential phase measurements will be found in the previously discussed US Patent No. 3,849,721, and in U.S. Patent No. 3,944,910.

For an illustration of an application of the novel transducer array to an electromagnetic measurement. refer now to FIG. 10. The novel TT-RR compensation array takes the form of transmitter T and receiver R antennae supported on sidewall skid 37. As with the acoustic embodiment already described, two separations are identified between same-type transducer groups, here In between receivers T, and T2 and Ie between transmitters T3 and T4. For electromagnetic measurements, Iu and Ie will be in the order of a few centimetres. Two T-R distances respectively 2 and 4 times I may be provided on reasonable length skids. The actual distances vary, as indicated by the division between T3 and T2, depending upon the frequency used in the measurement.This frequency-distance relationship is described further in the above patents. Where phase detection is employed, care must be taken that the distances provide the proper basis for phase comparison. For example, combinations of frequencies and distances which result in crossing through zero differences should be avoided.

Much of the circuitry illustrated in FIG. 10 is described in U.S. Patents Nos. 3849721 and 3944910 mentioned above. and will not be detailed here. Provisions have been added to allow making individual T-R measurements rather than the usual R-R differential measurements. This is accomplished by providing a transmitter related signal for use as a reference signal in place of a missing receiver signal. The mode control signals M and N already described in conjunction with the acoustic embodiment are utilized as well in FIG. 10, here to steer the transmitter and receiver signals and the processing circuits. These control signals may be provided by conventionally designed square wave generators 60A and 60B.

As shown in FIG. 10, the transmitter signals are switched from the high frequency oscillator 45 by switch 47 as controlled by mode M to either leads 47A or 47B and transmitted respectively at T3 or T4. Simultaneously, signals are also delayed and attenuated to simulate formation conditions for short and Long T-R distances by delays Ds at 40A and De at 40B and switched through switch 41 to serve as reference input 41A to mixer 50.

Transmitted signals propagate through the formation and are received at both T2 and T, but only one of these signals is switched to mixer 51 depending upon switch 43 as controlled by N. The phase difference measurement is made using mixer circuits 48 and 49, zero crossing detectors 71 and 72 and sign reversing flip-flop 77 with integrator 78 to produce at 78A the phase or travel time measurement for the particular T-R combination. Further changes in modes M and N result in a sequence of such measurements, each made in the above manner by utilizing delays Ds and De to provide the preferred range of phase differences for the corresponding T-R distance. The four T-R combinations have already been described in conjunction with M and N in relation to Table II.

Rather than using the transmitter reference signal approach as a phase comparison basis, as illustrated by circuirts 40, 41 and 48, alternate circuits 44 through 44E may be used. As shown by dashed lines in FIG. 10, a 100 kHz oscillator 44 may be used in conjunction with the high frequency oscillator 52 to provide synchronous 100 kHz clock pulses 44A which are then delayed by either delay Ds to provide signal 44B or delay De to provide signal 44C. These selectively delayed signals are then routed by switch 44D as determined by control pulse M such that output 44E may be used to replace the similar pulses normally output at 71A from zero crossing detector 71.

As disclosed in U.S. Patents Nos. 3849721 and 3944910 mentioned above, it is beneficial to also measure, along with the phase difference or travel time measurements, the amplitude or attenuation of the electromagnetic signals. Thus, a second set of measurements corresponding to peak amplitudes are desired. These are obtained simultaneously with the individual phase measurements by circuits 80 through 90 shown in FIG. 10 such that a continuous corresponding sequence of amplitude measurements for each T-R combination are provided at 90A.

Since the T-R combination measurements are acquired at different depths, a memory and gate circuit similar to that shown in FIG. 3A may be employed. Since the compensation provided by the use of the TT-RR array applies both to time or phase type measurements and to amplitude or attenuation type measurements, it is desired that these different type measurements, with each type having two different T-R distances, be provided the compensation.

Since the signals provided at 78A and 90A may appear as sequences of analog voltage levels, they may be converted from analog to digital measurement sequences by A/D converter 94 synchronized to multiplex the input sequence using multiplexor 93. The depth synchronization is provided for memory delay purposes by depth pulses 92, and the measurement sequence synchronization is controlled by control pulses M and N. The resulting digital output is then gated from the A/D converter to individual gate, memory and counter circuits 24C through 24F, each constructed as shown in Figure 3A for circuits 24A and 24B.

These compensation circuits respectivelv output first and second investigations representing different transmitter-to-receiver distances corresponding to output A and output B already described in regard to circuits 24A and 24B. However, in this case the investigations represent separate phase and attenuation measurements as shown at 96through 99 of Figure 10.

Referring now to FIG. 11, there is shown a further apparatus for the type of measurement where a given transducer may be operated either as a transmitter or a receiver, such as an antenna capable of transmitting or receiving electromagnetic waves. Thus, in FIG. 11, the transducer pairs are denoted as antenna As and Ae for one long- and short-distance antennae in one pair, respectively. and A's and A'e f6r the other pair.

The ability to switch a given transducer of one type to another provides the advantage of differential measurements and a better duty cycle. Thus, a given transmission may be simultaneously received by both receivers and measured either as differential measurement, or individually relative to the same reference signal. Since in effect two measurements are made at the same time, each measurement may be averaged over a longer period.

Modifications illustrated in FIG. 11 to the circuits in already described FIG. 10 provide for switching the transmission signal generated by oscillator 45 to either 47A or 47B. Switch 41A. which is separately but synchronously controlled by steering pulse N, applies the short-distance transmitter signal to either As or A's and the long-distance transmitter signal to either Ai or A' e. Similarly. switch 42A selects two adjacent antennae for use as receiver pairs and routes the detected signals to the separate mixer circuits 48 and 49 previously described.

In this manner, differential receiver investigations may be obtained alternatively from the upper interval Iu using As and Ai as near and far receivers while at the same time alternating between A and A' f as short- and long-distance transmitters. Then, without movement of the tool, differential receiver investigations may be obtained from the lower interval Ie by using A's and A'i as the pair of receivers while alternating between As and Ae as the transmitters.

Thereafter. the array is moved such that A'eel and A's are adjacent the interval Iu previously investigated by As and Ai. Processing circuitry 95 depth synchronizes the measurements and combines them to produce the compensated first and second investigation phase and attenuation measurements already described and illustrated in FIG. 10.

There has been illustrated method and apparatus for maximizing the use of a fourtransducer array and measurements taken between different combinations of the transducers. By utilizing in a novel arrangement the same four transducers normally employed to provide borehole compensation measurements, these transducers can be used to provide measurements for determining not one, but two borehole compensated measurements, each investigating the same formation interval with a different transmitter-receiver distance. Since both investigations are compensated in the same manner, this compensation adds meaningfulness to any differences occurring between these different investigations and the interpretation significance attributed thereto, such as, for example, indicating the presence of gas in a subsurface formation.

In general, the novel transducer array allows double use of the measurements derived therefrom. The two-receiver measurements are used twice at each depth increment, once each in relation to the near and far transmitters. Then, in turn, the two-transmitter measurements are used twice, once each relative to the near and far receivers. Even the transmitterreceiver distance is in effect used twice by overlapping this distance, which allows desirable increases in the T-R distances without the undesirable increases in the array length associated with prior art arrays.

Further, since all transmitter-type transducers are located on the same side of the receivertype transducers, signal propagation takes place to or from one side only of each transducer for all measurements, which readily facilitates the use of directional transducers. Further, since same-type transducers are grouped together, they operate in substantially similar borehole environments, which allows both the combination and the comparison of individual measurements made with different transducer combinations.

While the illustrative embodiments comprised acoustic and electromagnetic measurements, the invention can also be used in other types of measurements. Furthermore, although the receiver pair was generally illustrated as being the upper pair of transducers and the transmitter pair as the lower pair of transducers, it will be appreciated that it is also possible to use the reversed arrangement. Similarly, the acquisition of measurements may be made as the transducer array is moved either upwardly, as illustrated, or downwardly in the borehole.

Although the described embodiments provide for combining measurements as they are acquired at the well site. it will be appreciated that the individual measurements may be recorded and combined at a different time and place.

WHAT WE CLAIM IS: 1. A method of producing measurements adapted for determining a compensated measurement of a physical characteristic of subsurface media near a borehole penetrating the earth employing multiple transmitter type and multiple receive type transducers supported on a support member elongated in a direction generally parallel to said borehole for movement through said borehole, a first pair of transducers of a first type being positioned at a preselected separation along said member, a second pair of transducers of a second type being positioned at said preselected separation along said member and located on one side of said first pair of transducers in the direction of said elongation, and the transducers of said first and second pairs. respectively, having substantially the same operating characteristics; comprising the steps of: (a) producing a first measurement of said physical characteristic of the subsurface media when two of said transducers are at selected respective positions in said borehole; (b) storing said first measurement for combination with a later measurement of said physical characteristic of the subsurface media; and (c) producing said later measurement when two other of said transducers are effectively positioned in said borehole at said selected respective positions for combining with said first measurement to produce a measurement compensated for misalignment of said support member with said borehole and/or variations of said borehole.

2. The method of Claim l characterized in that said step of producing said later measurement comprises storing said later measurement for subsequent combination with said first measurement to produce a compensated measurement.

3. The method of Claim 2 wherein said first pair of transducers are characterized as a pair of transmitters and said second pair as a pair of receivers; and said producing measurement steps comprise using combinations of different transmitters and receivers for producing one measurement when one combination of said transmitters and receivers has a selected position along said borehole and another measurement when another combination has effectively moved to said'selected position; and said step of producing said later measurement comprises cpmbining said one and another measurement to provide a borehole compensated measurement of the physical characteristics of the subsurface media near the borehole for said selected position.

4. The method of Claim l wherein said first type transducer is characterized as transmitters and said second type characterized as receivers; and

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (38)

**WARNING** start of CLMS field may overlap end of DESC **. There has been illustrated method and apparatus for maximizing the use of a fourtransducer array and measurements taken between different combinations of the transducers. By utilizing in a novel arrangement the same four transducers normally employed to provide borehole compensation measurements, these transducers can be used to provide measurements for determining not one, but two borehole compensated measurements, each investigating the same formation interval with a different transmitter-receiver distance. Since both investigations are compensated in the same manner, this compensation adds meaningfulness to any differences occurring between these different investigations and the interpretation significance attributed thereto, such as, for example, indicating the presence of gas in a subsurface formation. In general, the novel transducer array allows double use of the measurements derived therefrom. The two-receiver measurements are used twice at each depth increment, once each in relation to the near and far transmitters. Then, in turn, the two-transmitter measurements are used twice, once each relative to the near and far receivers. Even the transmitterreceiver distance is in effect used twice by overlapping this distance, which allows desirable increases in the T-R distances without the undesirable increases in the array length associated with prior art arrays. Further, since all transmitter-type transducers are located on the same side of the receivertype transducers, signal propagation takes place to or from one side only of each transducer for all measurements, which readily facilitates the use of directional transducers. Further, since same-type transducers are grouped together, they operate in substantially similar borehole environments, which allows both the combination and the comparison of individual measurements made with different transducer combinations. While the illustrative embodiments comprised acoustic and electromagnetic measurements, the invention can also be used in other types of measurements. Furthermore, although the receiver pair was generally illustrated as being the upper pair of transducers and the transmitter pair as the lower pair of transducers, it will be appreciated that it is also possible to use the reversed arrangement. Similarly, the acquisition of measurements may be made as the transducer array is moved either upwardly, as illustrated, or downwardly in the borehole. Although the described embodiments provide for combining measurements as they are acquired at the well site. it will be appreciated that the individual measurements may be recorded and combined at a different time and place. WHAT WE CLAIM IS:

1. A method of producing measurements adapted for determining a compensated measurement of a physical characteristic of subsurface media near a borehole penetrating the earth employing multiple transmitter type and multiple receive type transducers supported on a support member elongated in a direction generally parallel to said borehole for movement through said borehole, a first pair of transducers of a first type being positioned at a preselected separation along said member, a second pair of transducers of a second type being positioned at said preselected separation along said member and located on one side of said first pair of transducers in the direction of said elongation, and the transducers of said first and second pairs. respectively, having substantially the same operating characteristics; comprising the steps of: (a) producing a first measurement of said physical characteristic of the subsurface media when two of said transducers are at selected respective positions in said borehole; (b) storing said first measurement for combination with a later measurement of said physical characteristic of the subsurface media; and (c) producing said later measurement when two other of said transducers are effectively positioned in said borehole at said selected respective positions for combining with said first measurement to produce a measurement compensated for misalignment of said support member with said borehole and/or variations of said borehole.

2. The method of Claim l characterized in that said step of producing said later measurement comprises storing said later measurement for subsequent combination with said first measurement to produce a compensated measurement.

3. The method of Claim 2 wherein said first pair of transducers are characterized as a pair of transmitters and said second pair as a pair of receivers; and said producing measurement steps comprise using combinations of different transmitters and receivers for producing one measurement when one combination of said transmitters and receivers has a selected position along said borehole and another measurement when another combination has effectively moved to said'selected position; and said step of producing said later measurement comprises cpmbining said one and another measurement to provide a borehole compensated measurement of the physical characteristics of the subsurface media near the borehole for said selected position.

4. The method of Claim l wherein said first type transducer is characterized as transmitters and said second type characterized as receivers; and

said producing measurement steps comprise using combinations of different transmitters and receivers for producing one set of measurements when said receivers have a selected position along said borehole and another set of measurements when said transmitters have said selected position; and said step of producing said later measurement includes the step of combining said one and another set of measurements to provide a borehole compensated measurement of the physical characteristics of the subsurface media near the borehole for said selected position.

5. The method of Claim 4, characterized in that said one set of measurements includes measurements separately referenced to each transmitter, each respectively less and more distant from said receivers, and said another set of measurements includes measurements separately referenced to each receiver each respectively less and more distant from said transmitters, and said step of combining measurements comprises: combining measurements from said one set referenced to said transmitter less distant from said receivers and from said another set referenced to said receiver less distant from said transmitters to provide one borehole compensated measurement; and combining measurements from said one set referenced to said transmitter more distant from said receivers and from said another set referenced to said receiver more distant from said transmitters to provide another borehole compensated measurement, said one and another borehole compensated measurements corresponding to two separate investigations at said selected position with two different transmitter-receiver distances.

6. The method of Claim 1, wherein said different types of transducers are receivers and transmitters, respectively, and characterized in that the measurement producing step comprises: firing a first transmitter at one borehole position and measuring at a first receiver the resulting signal after passage in a selected direction through an interval of borehole and subsurface media said first transmitter located along an elongated support member on said borehole tool from said first receiver at a preselected distance;; moving a second transmitter and receiver to said borehole position such that said second transmitter and said second receiver have positions in said borehole generally corresponding respectively to the positions of said first transmitter and said first receiver at said one borehole position said second transmitter located along said elongated support member on said borehole tool from said second receiver by said preselected distance; and firing said second transmitter and measuring at said second receiver the resulting signal after passage in said selected direction through said interval of borehole and subsurface media; and said combining step comprises combining said measured signals to provide a signal indicating misalignment of the borehole tool with the borehole and variations in the cross-sectional shape of the borehole.

7. The method of Claim 1, wherein said first and second groups of transducers each comprises a pair of transducers and characterized in that the measurement producing step comprises: producing a set of measurements between one transducer in said first pair of transducers and the transducers in said second pair with a selected borehole interval between said second pair; and producing a set of measurements between one transducer in said second pair and the transducers in said first pair with said selected borehole interval between said first pair; and the combining step comprises combining said sets of produced measurements to provide an improved measurement compensated for borehole measurement conditions at said selected borehole interval.

8. The method of Claim 7 characterized in that said measurements producing step further comprises: producing additional sets of measurements between the other transducer in said first pair of transducers and the transducers in said second pair with said selected borehole interval between said second pair and between the other transducer in said second pair and the transducers in said first pair with said selected borehole interval between said first pair; and said combining step further comprises combining said additional sets of produced measurements to provide an additional improved measurement compensated for borehole measurement conditions at said selected borehole interval.

9. The method of Claim l characterized in that the measurement producing step comprises: producing first measurements made at different borehole depths by moving the support member through said borehole while operating one transmitter and a pair of receivers; producing second measurements made at said borehole depths while operating one receiver in said pair of receivers and a pair of transmitters including said one transmitter; and said combining step comprises: combining the ones of said first and second: measurements representative of substantially the same borehole interval of provide a measurement representative of a physical characteristic of the subsurface media compensated for variations in the borehole measurement environment which were present near said transmitters and receivers when said borehole apparatus moved through said borehole interval.

10. The method of any of the previous claims and further characterized by the step of recording said measurements and said positions along said borehole.

11. The method of Claims 4 and 10 wherein the said one and said other sets of measurements are produced, respectively, at first and second depths of the support member in the borehole and characterized in that the combining step comprises delaying the recall of recorded measurements produced at a first depth for a depth interval generally corresponding to the difference between said first and second depths.

12. Apparatus for determining a physical characteristic of subsurface media near a borehole penetrating the earth which employs multiple transmitter type and multiple receiver type transducers supported along a support member adapted for movement through said borehole and elongated generally along a direction parallel to its direction of movement through said borehole, comprising:: (a) a first group of transducers of a first type supported for movement through said borehole with adjacent transducers of said first group being separated from each other by a preselected separation along a line generally parallel to the elongated direction of said support member; (b) a second group of transducers of a second type supported for movement through said borehole and located on one side of said first group in a direction therefrom parallel to said elongated direction with adjacent transducers of said second group being separated from each other by said preselected separation along said line; (c) the transducers of said first and second groups respectively, having common operating characteristics; (d) means for producing measurements of said physical characteristic of subsurface media at different depths of the support member in said borehole; and (e) means for combining said measurements taken at different selected depths of the support member in said borehole to provide compensation for variations in the borehole and/or misalignment of transducers therein.

13. The apparatus of Claim 12, characterized in that said producing means produces measurements corresponding to when different transducers have approximately the same positions along said borehole.

14. The apparatus of Claim 12, wherein said first type of transducers are receivers, and said second type of transducers are transmitters; and characterized in that: said producing means produces measurements using different combinations of said transmitters and receivers comprising one set of measurements with the support member at a selected borehole depth when said receivers have a selected position along said borehole and another set of measurements with the support member at another selected borehole depth when said transmitters have approximately said selected position; and said combining means comprising means for combining said one and another sets of measurements to provide a borehole compensated measurement of the physical characteristics of the subsurface media near the borehole for said selected position.

15. The apparatus of Claim 14, characterized in that said one set of measurements includes measurements separately referenced to two transmitters, one transmitter being less distant and another transmitter being more distant along said line from one of said re'ceivers, and said another set of measurements includes measurements separately referenced to two receivers, one receiver being less distant and another receiver being more distant along said line from one of said transmitters, and said combining means comprises: 1) means for combining measurements from said one set referenced to said less distant transmitter and from said another set referenced to said less distant receiver to provide one borehole compensated measurement; and 2) means for combining measurements from said one set referenced to said more distant transmitter and from said another set referenced to said more distant receiver to provide another borehole compensated measurement, whereby said one and another borehole compensated measurements correspond to two different transmitter-received distances.

16. The apparatus of Claim 12, characterized in that said first-type transducers are a pair of receivers. one receiver in said pair being less distant and another more distant along said line from one of said second-type transducers; and said second-type transducers are a pair of transmitters. one transmitter in said pair being less distant and another more distant along said line from one of said first-type transducers; said producing means produces a first measurement using a first combination comprising said more distant receiver and said less distant transmitter when said first combination has a selected position along said borehole, and a second measurement using a second combination comprising said less distant receiver and said more distant transmitter when said second combination has approximately said selected position; and said combining means combines said first and second measurements to provide an indication of a borehole compensation required to compensate measurements of said physical characteristics of the subsurface media near said borehole for said position.

17. The apparatus of Claim 16, characterized in that said producing means produces measurements comprising one set of measurements when said receivers have a selected position along said borehole and another set of measurements when said transmitters have approximately said selected position; said combining means combines said one and another sets of measurements to provide a borehole compensated measurement of the physical characteristics of the subsurface media near the borehole for said selected position; and wherein said indication of a borehole compensation indicates the borehole compensation provided in said borehole compensated measurement.

18. The apparatus of Claim 12, characterized in that said first type transducers include one first type supported at one selected distance and another first type supported at another selected distance along said line from one of said second type and wherein said second type transducers include one second type supported at said one selected distance and another second type supported at said another selected distance along said line from one of said first type, and said measurements include measurements produced using different combinations of said first and second types supported at said one and another selected distances.

19. The apparatus of Claim 18, characterized in that said measurements include one set of measurements produced when said first type transducers have a selected position along said borehole and another set of measurements wherein said second type have approximately said selected position.

20. The apparatus of Claim 19, characterized in that said one set is produced when the support member is at a first depth and said another set when it is at a second depth.

21. The apparatus of Claim 20, and further characterized by a means for storing at least some of said measurements with reference to said depths.

22. The apparatus of Claim 21, further characterized by means for reproducing measurements produced at said first depth from the storing means by delaying said first depth measurements for a depth interval generally corresponding to the difference between said first and second depths.

23. The apparatus of Claim 22, characterized in that the combining means comprises means for combining said reproduced measurements from one set produced at said first depth and measurements from said another set produced at said second depth to provide a borehole compensated measurement.

24. The apparatus of Claim 23, characterized in that said one set includes measurements produced using said one second type supported at said one selected distance and said another set includes measurement produced using said one first type supported at said one selected distance.

25. The apparatus of Claim 24, characterized in that said one set includes measurements produced using said another second type supported at said another selected distance and said another set includes measurements produced using said another first type supported at said another selected distance and said combining includes separately combining measurements from said first set produced using said first type transducers supported at said one selected distance and from said second set using said second type transducers supported at said one selected distance to produce one borehole compensated measurement corresponding to said one selected distance. and measurements from said first set produced using said first type transducers supported at said another selected distance and from said second set using said second type transducer supported at another selected distance to produce another borehole compensated measurement corresponding to said another selected distance.

26. The apparatus of Claim 25 characterized in that said transducers in one type of said first and second types are transmitters for transmitting signals from said transmitters through said borehole and subsurface media; and said transducers in another of said first and second types and receivers for receiving said signals after transmission through said borehole and subsurface media.

27. The apparatus of Claim 26, characterized in that said measurements include a first measurement using a first combination comprising one receiver supported at said one selected distance along said line from one of said transmitters and one transmitter supported at said another selected distance along said line from one of said receivers; and a second measurement using a second combination comprising one receiver supported at said another selected distance along said line from said one of said transmitters and one transmitter supported at said another selected distance along said line from said one of said receivers; and said first measurement being produced when said first combination has a selected position along said borehole and said second combination has substantially said selected position; whereby said first and second measurements are substantially the same measurements under ideal measurement conditions; and said stored and reproduced measurements include said first measurement and said combining further includes additional combining means for combining said reproduced first measurement and said second measurement to provide an additional measurement.

28. The apparatus of Claim 27, characterized in that said additional measurement provides an indication of the borehole compensation provided in said borehole compensated measurements.

29. The apparatus of any of the previous claims characterized in that said signals are acoustic signals and said measurements are travel times for said signals to travel through said borehole and subsurface media.

30. A method of producing measurements adapted for determining a compensated measurement of a physical characteristid of subsurface media near a borehole penetrating the earth, substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

31. A method of producing measurements adapted for determinging a compensated measurement of a physical characteristic of subsurface media near a borehole penetrating the earth, substantially as hereinbefore described with reference to Figures 3A, 3B and 3C of the accompanying drawings.

32. A method of producing measurements adapted for determining a compensated measurement of a physical characteristic of subsurface media near a borehole penetrating the earth, substantially as hereinbefore described with reference to Figure 10 of the accompanying drawings.

33. A method of producing measurements adapted for determining a compensated measurement of a physical characteristic of subsurface media near a borehole penetrating the earth, substantially as hereinbefore described with reference to Figure 11 of the accompanying drawings.

34. Apparatus for determining a physical characteristic of subsurface media near a borehole penetrating the earth, substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

35. Apparatus for determining a physical characteristic of subsurface media near a borehole penetrating the earth, substantially as hereinbefore described with reference to Figures 3A, 3B and 3C of the accompanying drawings.

36. Apparatus for determining a physical characteristic of subsurface media near a borehole penetrating the earth, substantially as hereinbefore described with reference to Figures 1 2 and 7B of the accompanying drawings.

37. Apparatus for determining a physical characteristic of subsurface media near a borehole penetrating the earth, substantially as hereinbefore described with reference to Figure 10 of the accompanying drawings.

38. Apparatus for determining a physical characteristic of subsurface media near a borehole penetrating the earth, substantially as hereinbefore described with reference to Figure 11 of the accompanying drawings.