DK154584B - PROCEDURE FOR GETTING MEASUREMENT VALUES FOR DETERMINING THE PHYSICAL CHARACTERISTIC VALUES OF UNDERGROUND MEDIA - Google Patents

Description

DK 154584 B

The present invention relates to a method of generating measurement values for determining the physical characteristics of underground media in the vicinity of a borehole passing through the ground, using at least two transmit transducers emitting electromagnetic or acoustic waves, and at least two receiving transducers. there, the waves emitted by the transmit transducers receive a mileage in the underground medium where the transducers are spaced axially apart on a borehole probe and the following steps are performed: a) positioning the probe at a specific depth of the borehole; b) performing two measurements between the transmitting transducers and the receiving transducers, where the measurement distances of the two measurements overlap, and c) combining the two measured measurement values to determine a compensated characteristic value for the overlapped measuring distance.

It is known in the area of acoustic logging that one can provide some compensation for variations in the run-time caused by changes in the borehole diameter by using a borehole probe comprising two receivers and a transmitter. (Such devices are hereinafter abbreviated using a "T" to represent a transmitter and an "R" to represent a receiver having the relative positions at the T's and R's indicated by the order, the hyphen MM, which separate the transducers indicates a common signaling pathway.

Thus, a T-RR group indicates a transmitter on one side of two receivers, the receivers having a common signal path between the transmitter and its closest receiver). Unfortunately, such a T-RR device can cause no compensation with respect to the inclination of the probe in relation to the borehole axis. To remedy this inclination problem, an additional transmitter may be provided to design a probe with a T-RR-T field. As explained in U.S. Patent No. 3,257,639, either of the two transmitters can be selectively controlled and the maturity of each of the two receivers is measured. The individual maturity measurements can then be combined to determine an average running time for the interval between the two receivers. This average running time has the advantage of being compensated both for changes in the borehole diameter as well as for the inclination of the probe.

As with many different types of measurements under conditions that change non-homogeneously in a radial direction with respect to drilling.

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z hole, acoustic measurements appear to change with the distance between transmitter and receiver and, more precisely, for a two-receiver device with the distance between the transmitter and a point in the center between the receivers. For this reason, borehole compensating types of probes have two transmitters located at the same distance from this center.

As explained in U.S. Patent No. 3,312,934, one reason that the acoustic velocity may change at different TR distances is that different signal paths may occur where the longer distance provides a path extending somewhat further from the borehole and deeper within the formation. . This deeper path may be less influenced by factors that radially change the acoustic properties as the borehole is drilled, or this is exposed to the borehole fluid such as hydrophilic shale species that tend to swell. This altered zone may occur deep enough in the formation that a short T-R distance at least partially results in the measurement of properties of this altered zone rather than the unaltered formation as desired. Longer T-R distances, such as 2½ to 3½ meters, are preferred to overcome this particular formation change problem.

Longer T-R distances require longer probes, and in the older two-receiver devices, ie. devices of the T-RR type, increasing the T-R distance from 1 to 2.7 meters requires a probe which is approx. 1.7 meters longer. However, in the T-RR-T borehole compensation probes, such a significant increase in the T-R distance leads to undesirably long probes as this T-R distance occurs twice. Longer probes are undesirable as their length makes them more costly and more difficult to transport and increases the problems associated with lowering them into curved or inclined boreholes.

Borehole compensation type devices are also provided in sidewall apparatus as described in U.S. Patent No. 3,849,721. Here, the longer T-R distances in the known T-RR-T field increase the carriage length, which unfortunately reduces the likelihood that the carriages can be kept in continuous contact with the borehole wall.

Previous attempts to provide at least a partial compensation system without resorting to undesirably long probes as disclosed in U.S. Patent No. 3,207,256 or 3,330,374 require the storage of at least two different measurement values for at least two different

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3 distances. This requirement leads to increased storage costs and greater sensitivity to deep positioning problems, such as is induced by a yo-yo movement of the probe. Finally, the compensation for the Christianity of a probe is not always complete.

A further problem, both with a TRT device and with its counterpart, an R-TT-R device consists in that due to the large distances between the T's in the T-RR-T device and the R's in the R-TT-R, respectively. the device, the operating conditions of the transducers located at the ends of the field can be quite different, leading to noticeable differences in the received signals, which should actually be the same. For example, when a significant probe slant position puts one of the outer transducers in a significantly eccentric position, while the corresponding transducer at the other end of the probe remains better centered, the signals corresponding to these outer transducers can differ significantly and even affect both the duration and amplitude measurements.

When a T is between a pair of Rs or an R is between a pair of Ts, there is often a problem with electrical noise, for example, with electrical cross talk from an electrically strong noise-transmitting circuit in receiver wires extending close to the transmitter, or even worse from a transmitter control line conducting high voltage or current transients, as is the case with acoustic probes, which high voltage lines pass by one of the receivers or amplifiers of received signals. For example, the control line leading to the lower transmitter in the T-RR-T device must pass both receivers. A more detailed explanation of the electrical and mechanical problems associated with transmitter wires passing by receivers is found in U.S. Patent Nos. 3,734,233 and 3,712,410.

It would be very desirable to provide a compensation device where all receivers could be placed in isolation from all transmitters and where no high voltage pulse wires pass anywhere near a receiver, its associated amplifier or receive signal wires.

In the known compensating devices and in some of the two-takeer devices, measurements were not possible over certain parts of the borehole. For example, the probe in the T-RR-T device may not work properly with the top transmitter inside the casing and the second transmitter and both receivers still out in the open hole. Similarly, measurements of the formation could not occur 4

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is taken in the critical bottom portion of the hole for a distance equal to at least one transmitter-receiver distance. It would be desirable to be able to measure as far down to the bottom as possible, although it was necessary to temporarily waive the compensation during this interval.

For acoustic probes that may be required to operate at some distance from the borehole wall, the acoustic signals arriving at a particular receiver leave the borehole wall where the offset of the point changes with the direction of advance. This leads to the effect known as refractive error. This error and a corresponding technique for compensation devices disclosed in U.S. Patent Nos. 3,304,536 and 3,524,162 include the use of an additional receiver for each of the two present receivers. Each additional receiver is located at a small distance from each receiver already present corresponding to approx. twice the displacement introduced by the refractive error, one displacement for each of the two different receiving directions. Thus, four recipients are used, two for each recipient direction.

Furthermore, in the known T-RR-T device directional receivers are required, as each receiver must be able to receive signals arriving either from the top or from the bottom transmitter. Otherwise much desired directional receivers cannot be used when there are no four receivers such as in the above mentioned refraction correction solution, ie. two receivers aimed at each transmitter.

Another solution would be to use the R-TT-R system, but the required directional transmitters cannot be used when they are not constructed as in the four-receiver solution of four transmitters. Needless to say, these additional transmitters significantly increase the complexity of the control and the cost.

Two different TR distance studies are desirable, and as stated in the aforementioned U.S. Patent No. 3,312,934, it is possible that the direct comparison of the various study results may even lead to a direct indication of the presence of hydrocarbons when present in the form of gas, or in some cases it is possible to estimate the degree of shale clay content as mentioned in US Patent 3,096,502. However, it will appear that the measurements must be as accurate as possible in order for the measurements with different T-R distances to be utilized in this way. The accuracy of T-RR measurements according to the above-mentioned patents is often 5

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such that the observed difference in these various studies may in fact be due to uncompensated inclination or system errors rather than radial differences in the acoustic properties of the formations. At least two measurements are required for this use, and it is important that both measurements are borehole compensated.

Using the known compensation devices to obtain the various T-R distance studies, two additional outside transducers at a further distance in addition to the commonly used and a large number of additional measurement subcycles in addition to the four normally used would be necessary. In addition, the probe length would be increased by twice the desired distance difference. Such requirements for additional transducers and probe length make the second measurement impractical in many circumstances, since the second measurement is usually redundant compared to the first. However, if it could be performed without such expensive complications, this second measurement would increase the value of the primary measurement by providing significant assurance that at least the longer TR distance was appropriate for a changed formation, and when favorable conditions occurred, it would provide a direct indication of the presence of gas.

The object of the present invention is to provide a method which, while retaining the advantages of long T distances, does not make prohibitively long borehole probes necessary. Also, the borehole compensation properties of a T-RR-T field at a side wall sliding shoe must be maintained, as the T-R distance can be increased without increasing the sliding shoe length. At a minimum of different depth positions and storage distances, a complete borehole compensation including the probe inclination position must be allowed. The method must also be capable of enabling measurements over the entire transducer device length and to provide two studies at different distances, viz. studies at long and short T-R distances, which are both compensated for borehole and system measurement errors.

This is accomplished by a method of the kind set out in the preamble, characterized in that by placing the transducers in the order, the T ^ receiver receives Tg transmitter T ^ transmitter T ^, respectively, in the reverse order of the probe, the distance between the receiving transducers being equal to the distance between the transmit transducers, the probe between the two measurements is moved a certain distance axially in the borehole, the measurement distances of the two measurements are 6

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equal in length, and the overlapping distance is equal to the distance between the two receiving transducers.

The term "transducer" used hereinafter and hereinafter is understood to be a means capable of either emitting or receiving a particular signal type. For example, the acoustic transducer may be either an acoustic transmitter or an acoustic transmitter. a receiver where the transmitter serves to convert electrical energy into mechanical or acoustic energy, and the receiver serves to convert the acoustic energy back to electrical energy, similarly the transducer by electric wave measurements may be an antenna or a radiator for electromagnetic waves, while the receiver is an antenna for capturing the transmitted electromagnetic waves.

Furthermore, the various combinations of measurements obtained at different borehole positions can be compared or combined to obtain improved measurement results that are compensated for borehole effects and other errors.

The invention will then be explained in more detail with reference to the drawing, in which%

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FIG. Fig. 1 is a representative block view showing an embodiment of an apparatus according to the present invention for collecting, storing and combining measurements of physical properties of media below the earth's surface near a borehole; 2A-2C measure the acquisition sequence using the transducers included in the apparatus of FIG. 1, FIG. Figs. 3A and 3B are an embodiment of a circuit at the surface and downhole of the present invention; 3C shows the corresponding timing diagram; FIG. Figures 4A-4D show the effect of borehole states, such as failure to align and incline a transducer support on the method of the present invention; 5A and 5B are different borehole working environments that are compensated for in accordance with the present invention; 6A is a well-known slide-mounted borehole compensation system; FIG. 6B shows the known system modified in accordance with the present invention; 7A and 7B show additional advantages of a transducer system modified in accordance with the present invention; 8A and 8B alternate circuits for use with the circuit 24 of FIG. 3A, FIG. 9A and 9B show the connection between various measurements of physical properties of media below the earth's surface and FIG. 11 and 11 further embodiments of the apparatus of the present invention.

In the drawing, in FIG. 1, in representative block form, a suitable apparatus according to the present invention is shown for collecting, storing, recording and combining measurements of physical properties of media below the surface of the earth in the vicinity of a borehole passing through an earth formation. The apparatus includes a borehole tool 1o with a four transducer system numbered 1-4. The system may be included in a tool which is either of the mandrel type adapted for centralized or eccentric operation or in a sliding chuck with the transducers arranged on the guide shoe for operation in close contact with the borehole wall.

In the following explanation, it is assumed that the tool has been guided to the bottom of the borehole so that it can then be slowly retracted to the surface under the mechanical control of a log wire line 12 wound to reach a soil 14 at the surface, till - DK 154584 B in 2o. In this way, movement of the tool can be directly related to the movement of the wire at the surface.

The surface controller 20 acts as a programmed transmitter and receiver selector which communicates with a logging line 12 and downhole over a drag ring pantograph 16 on the game 14 with an underground controller 11 in the tool 1o. Synchronous with the wire movement providing! gain depth impulses are provided to both the controller 20 and to the measurement storage apparatus 22 over some suitable mechanical or electrical connection 18 and to the measurement selection and combination apparatus 24, if present at the drilling site, so that the measurement processing can be performed at the same time. In this way, the transducer selection and corresponding measurements are synchronized as described below.

It will be appreciated that the actual measurement selection and combination need not be made in connection with the collection of the individual measurements, as these measurements may be provided at any later time from conventional analog or digital storage devices for processing at a location distance from borehole easily. However, it is important that growth depths corresponding to the movement of the tool in the borehole are recorded in accordance with the measurements, as it is necessary, as described below, to accurately reference the measurements on a depth basis.

When the tool lO, which contains the group of four transducers, is moved upward through depth positions I, J, K and L, various transducers are selected in a systematic way, so that a result of measurements is made at regular depth increments. It is customary for a particular point on the tool to be selected as a reference point, so that measurements made with different transducers can be attributed to each other and to the depth of the tool in the borehole recorded at the surface.

Although any point can be selected, FIG. 1 and the following description is based on the selection of the depth reference point-as the position of the tool 1o at the upper transducer, i.e. the transducer which is closest to the surface of the earth as the tool moves through the borehole.

To describe the order of measurements, reference is made to FIG. 2A-2C showing the group of four transducers in the tool lo designated as τι ”Τ4 * for the description, the letter T with an index will be used to indicate transducers either receivers or transmitters.

Furthermore, it is assumed that the top two transducers and T2 act as receivers and that the two bottom transducers and act as transmitters.

It is desirable that transducers of a particular type, such as those working as receivers, be grounded or mocked together in ν »τ · ν + -ίΛ-ϊβ ·· - 9

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easily in a coordinated way. Furthermore, for reasons / as will appear later, the preselected distance between transducers in each group must be the same, ie. the distance between Tj and T2 along the length of the transducer support means in the tool must be the same as the distance between transmitters Tg and T4. The distances between the groups of different types of transducers, e.g. the distance between the receiver T2 and the transmitter Tg may be the same or different from the distance between transducers of the same type depending on the physical properties of the ground formation to be measured, the depth of investigation of the desired ground formation and other factors.

FIG. 2A, 2B and 2C each show the transducer system Ti, T2'T3 and in two separate positions indicated by depth level indices at the top of each transducer system. These indexes I-L are assigned to the upper transducer Tj. In FIG. 2A and 2B are these positions I and L, i.e. the upper transducer Tj at depth levels dj and d ^, respectively. In FIG. 2C, the two positions are designated I and J because the upper transducer Tj is at depth levels dT and dT, respectively.

When the system is moved from position I-L in FIG. 2A and 2B and I-J in FIG. 2C, the system moves up through the borehole from depth dj-dL using Tj as the depth reference point. A signal is produced by the transmitter Tg which will propagate up through the hole to the receivers T2 and Tj. Each of these receivers will convert the received signal into a corresponding electrical signal which can be processed into a measurement m. Since it is normally expected that a signal migrating from Tg towards T2 and Tj will arrive first at T2 and then at Tj, then the Tg-T2 blade will be designated as new and Tg - Tj as m2 · The measurements irij and m2 can then be combined to obtain a measurement of a physical property beneath the soil surface in a manner that will depend on the property being measured.

For example, Tg sends an acoustic impulse, the measurements irij and m2 will represent the migration time through the formation and media surrounding the borehole from Tg to T2 and Tj, respectively, and can then be combined to determine the interval migration time between T2 and Tj, called At.

At some short time distance from the generation of a signal by the transmitter Tg, a signal is generated by the transmitter T4, as shown in FIG. 2B, received by receivers T2 and Tj and converted into measurements mg and m4, respectively.

A complete sequence of measurements at depth dj would therefore include all the measurements mj, m2, mg and m4 · Then m will denote individual measurements in general of the type, with mj being made while Tg works with T2 and m2 with Tj and mg, while T4 works 10

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in

Since the four measurements can be obtained for a very short period of time relative to the tool movement, they can be considered as provided essentially at the same depth. For example, acoustic transmitters can be pulsed on the order of 20 times per second. second. This rate provides at least five complete sequences per second. second, during which a very small tool shift would occur at normal log speeds. The four measurements are sent up through the hole and stored for later use as shown at 22A in FIG. 1, and as explained in more detail below.

At some later point when the tool is moved through the borehole to the depth as shown in FIG. 2A and 2B, another sequence of measurements ιη ^, ι ^, πι ^ and m4 can be made and used in accordance with the present invention to compensate for borehole effects on the individual measurements.

For example, when is an acoustic impulse transmitter, the interval travel time Δt between T2 and Tj will be incorrect if the portions of the signal propagation paths located in the borehole are of different lengths at the two receivers. Such a difference would occur in the case of an inclination of the tool.

Prior art borehole compensation methods by acoustic logging tools use separate transmitters located on opposite sides of the receivers in a T-RR-T system to obtain two At's with opposite near and far receiver ratios.

According to the present invention, this kind of borehole compensation is possible with a system having a significantly shorter overall length. By combining a first set of measurements m ^ and m2 taken at depth dj (see Fig. 2A at I) with measurements m ^ and m ^ taken at depth d ^ (see fig.

2A at L), a new combination of measurements from transducers having an inverse near-distance relationship is obtained, which combination provides the desired borehole compensation.

Further, a second compensated borehole measurement may be made at or above the same borehole interval as the above-described compensated borehole measurement. Such a second measurement is not available with the prior art T-RR-T system. By combining another set of measurements m3 and m ^ taken at depth dj (see Fig. 2B at I) with measurements m2 and m4 taken at depth d ^ (see Fig. 2B at L), another compensated borehole measurement is obtained, but here with a longer TR distance than the first measurement. This is because this second set of measurements is attributed to transducers at a greater distance than in the first set.

A further advantage of the transducer system of the present invention is related to the use of the system to compensate for statistical or systematic errors in the measurements taken and in accordance with the present invention.

It is noted that the measurement m 2 in the depth dj is essentially repeated by m-j at dj when T2 replaces Tj, and replaces as the tool moves through the borehole. Therefore, under perfect measurement conditions, m2 should be equal to m ^ · However, under typical borehole measurement conditions, there are several known reasons why this might not happen. Although small statistical variations can be expected, e.g. when acoustic interval transit time measurements are interpreted, an improved measurement is obtained by forming the mean of m2 at dj and m ^ at dj to provide a measurement that is offset by such statistical variations. Although similar statistical compensation could be performed by repeating the measurement at dj, such repeated measurements cut the key ratio of the tool by half. In contrast, no increase in the key ratio is required to achieve this result by combining the already available m2 and m ^. Furthermore, as explained below, there are other reasons why it is preferable to use different transducers and even different tool positions to obtain measurements for such combinations.

Although not shown in Fig. 2C, it will be appreciated that other measurements may also be combined to advantage to compensate for random noise or different transducer effects and their relative positions in the borehole. For example, m2 at depth d ^ can be used with m ^ at dL- In some cases, measurements such as m2 can also be compared to detect borehole distortions such as tool slope.

A comparison of such measurements may give an indication of the borehole compensation applied to the basic measurements and thereby an indication of the reliability of the compensated borehole measurements.

As described above, all four measurements in each sequence are not absolutely necessary to provide a compensated measurement, nor is it necessary to perform each measurement according to individual transmitter triggerings as described. As shown in FIG. 2A-2C and summarized below, however, each individual measurement will be used at least twice in different combinations to provide two different compensated borehole measurements of a selected borehole interval corresponding to two different transmitter-receiver survey distances:

TABLE I

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1) m2 and at dj (interval T2 ~ Tj at operation of Tg) 2) mg and m ^ at dj (interval T ^ -T ^ at operation of Tg) 3) m ^ and mg at dj (interval T2 ~ Tj ve ^^ rift of T4) 4) and m2 at dj (range T2 ~ T4 at operation of Tj) 5) m2 at dj (Tg-Tj) and mg at dj (T ^ -Tg) 6) m2 at dR (Tg- Tj) and mg at dj (T ^ -Tg)

As shown in FIG. 1, each measurement mj, m2, m3 and m ^ is stored in the measurement storage apparatus 22 for each increment of depth dj, dj + j, ···, etc., where each increment is of the order of 15 cm or less.

If the measurement storage capacity is limited, it is advantageous to combine some of the measurements to minimize the required capacity. The measurements m ^ and m2 for the same depth gain (see Fig. 2A at position I and depth gain dj) can be e.g. is subtracted in the measurement selection and combination apparatus 24 to form a new measurement m ^ = itig-nij, which in turn can be stored, replacing both nij and mg, or if sufficient storage capacity is provided as an additional measurement.

As the system moves through the borehole from dT to d_, other measurements can be combined to form replacement or additional measurements. As the system moves to the depth dT (see position L in Fig. 2), a complete set of measurements will be generated. The previously obtained measurements are now available from storage 22A and the measurements generated at depth dT are now available as continuous measurements. Thus, it is possible to combine these generated measurements to provide compensated measurements for the borehole interval shown in FIG. 2 under dj.

For example, subtracting ny from m ^ generated by dj, the running depth at position L shown in FIG. 2A, and combining this result with the measurements m2 and ntj at dj previously produced at position I, or its previous combination m ^ at dj, provides the compensated borehole measurement for the borehole interval shown in FIG. 2 corresponding to the short T-R distance survey.

The above-mentioned combinations result in an acoustic logging embodiment in addition of two At measurements for the same borehole interval, one corresponding to a measurement with two receivers and the other to a measurement with two transmitters to provide the desired borehole compensation. . Depending on the distance between the pairs of similar transducers, the result may need rescaling. For example, the in-

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the burden distance is o, 3 m, the correct At as indicated at the output A will be obtained by dividing the final combination by two.

In addition to combining these two At measurements as described above, different measurements at different depth levels could be compared to indicate borehole conditions requiring compensation, or combined in a manner that provides the mean value measurements. For example, m 2 at dj (see Fig. 2C at I) and m ^ at dj (see Fig. 2C at J) can be added, or the mean of them is formed to form m ^. These mean values could then be combined to provide At measurements or for other purposes.

In FIG. 3A and 3B, typical circuits for the apparatus are shown at the surface and downhole to perform transmitter and receiver selection, collection and combination of individual measurements in accordance with the present invention. Although circuits for providing compensated acoustic walking time measurements have been shown, similar circuits can be used for compensated amplitude ratio measurements, e.g. by modifying the circuits of FIG. 3A and 3B to also generate signal amplitude or gain setting information together with the timing information. The amplitude information may be treated in accordance with the description of the aforementioned United States Patent No. 3,524,162.

A general description of the operation of the apparatus of FIG. 3A and 3B should be given below, followed by a detailed description.

In general, the measurement sequence begins with a depth pulse corresponding to a depth gain and then rate control of the individual transmitter and receiver selection cycles to complete the sequence. Four cycles and corresponding logical modes of operation are shown in Table II below for selecting either the transmitter T3 or the distant transmitter and the receiver T2 or the distant receiver Tj.

These logical modes designated M and M or N and N are used to select the correct transmitter or receiver, respectively, as described below. The migration paths given in Table II for each of the measurements are shown in FIG. 4A (which will be explained in greater detail later) and applies equally to both the contact slip shoe embodiment shown and to a mandrel embodiment.

TABLE II

CYCLE NO. WORKING SEND RECEIVER ROAD MEASUREMENT

1 Μ N T3 T2 A + B + D new 2 MN T3 Tj A + B + C + Em2 "Si Si Μ rp m τι. / - 1 tt _

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The selected transmitter is started and the transmitted signal is received at the selected receiver, amplified by a gain setting appropriate to the particular T-R cycle and transmitted up through the hole. A reference time control pulse is generated in a fixed time relation to the start time of the transmitter and used as a basis to compensate for signal loss in the cable and as a time reference point to control the signal detection circuits used to detect the arrival time of the received signal. Time measurements are performed by applying clock pulses to a counter starting with the transmitter's start reference pulse and ending with the detection. The contents of the counter then become the individual measurements m, which have already been discussed above in connection with the previous figures. These measurements are stored or recorded according to their cycle position for later processing.

Referring now to FIG. 3A in connection with a detailed description. The sequence begins with a depth pulse from a depth pulse generator 3o5 which starts a speed oscillator control 31o and a clock sensor 324. The oscillator 31o and the clock sensor 324 work together to produce control signals at such a rate that several complete sequences of four cycles each will be performed. second. Rate pulses are fed to a cycle key 33o which produces four cycle control pulses used as the binary state of M and N, i.e. Μ, Μ, N, and N by controlling the logic to produce working mode signals representing the working modes of Table II. Cycle control pulses are used to synchronize control logic 331 and 332 to select a new T-R combination and synchronize a downhole gain system 334 to provide appropriate gain for the received signals in each cycle.

In FIG. 3C, a timing diagram is shown to illustrate the time ratios of clock pulses (on line 1) from clock transducer 324, M and N binary state signals (2 and 3) from control logic 331, cycle control pulses (4,6,8 and lO) and their delayed counterparts (5.7.9 and 11, respectively) from cycle master 33o and counter reset and up / down control pulses (12,13 and 14) for counters # 1,2 and 3) from control logic 332 used in the surface circuits shown in FIG. 3A to synchronize the downhole circuits shown in FIG. 3B.

Although the real circuits such as square wave generators which can be used to generate these signals and pulses and their delayed counterparts are not shown to simplify the circuit diagrams, it is well known how these circuits are performed. Also, for the sake of clarity of the diagrams, not all connections between different circuit components utilizing these signals and pulses to open gates, select codes, reset counters and the like are shown.

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where required.

In FIG. 3C, it will be seen that a depth pulse starts a series of clock pulses 1-9 (shown on line 1) to determine a complete measurement as previously described in connection with FIG. 2 and Tables I and II. On the initial clock pulse, the state signal M will select T ^ (line 2) and N will select T2 (line 3) to begin the C1 cycle to produce m- ^. The counter # 1 (shown at 391 in FIG. 3A) can be reset (RS) (as shown on line 12 in FIG. 3C) on the upward flank of the control pulse from control logic 332. Then, counter no.

2 begin to count clock pulses from the high frequency clock 388 as they are applied to it over a clock gate 39o during the initial portion of cycle C1 (line 4). Usually counter # 1 of receiver signal detection before the end of this portion is stopped by clock pulse 2, and if not, this pulse or a delayed pulse C1 can 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 the circuits shown as 24A and 24B in FIG. 3A and used in conjunction with memories # 1 and # 2 also shown there to combine the respective measurements for each counter in the up - (+) or down - (-) state as indicated on lines 13 and 14, respectively. in FIG. 3C. Similar counters Nos. 2A, 2B, 3A, and 3B are shown in dotted lines in circuits 24A and 24B of FIG. 3A.

For example, counter # 2 in circuit 24A of FIG. 3A shown (line 13) in a countdown state at clock pulse 1, and as explained in more detail later, it counts down during the initial part of cycle 1 when m ^, with% indicating that the measurement was delayed or stored from a previous measurement at a greater depth and is e.g. input signal from memory # 1, then passed at clock pulse 2 while the counter is still in the down state, the current m ^ as the input signal to counter # 2 directly from counter # 1, ie. without delay or storage, since counter # 1 contains the continuous measurement after the initial part of each cycle. Thus, at clock pulse 3, counter # 2 has accumulated -m ^ - m ^ and is then switched to an enumeration - (+) state. During the initial part of cycle 2, m * is then the input signal from memory # 1 to add tm ^ f and during the latter part of cycle 3 (at clock pulse 6) the current m ^ which is counted during the initial part of cycle 3, input signal from counter # 1 to add + m to the previous accumulation in counter # 2.

At the end of cycle 3, counter # 2 contains -m ^ - m ^ + m ^ + m ^ which has been entered therein in the order specified. For a suitable 4 ”* l / *? than r · »Ir 4- / tri e 4- σ / -vr« «ιμ J ___ a o Ί .ϊ J- ^ 4 £ 1« · 4- * nl 1 av · 16

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they at the next depth pulse as a second sequence as described above by clock pulse 1. As explained below and as shown in FIG. 1 and 3A at A, this combination of measurements corresponds to one of the compensated borehole measurements provided by the method of the present invention.

Up / down counter # 3 shown as 394A in circuit 24B of FIG. 3A is illustrated in a similar manner in diagram form in FIG. 3C on line 14. However, its sequence begins after clock pulse 3 instead of clock pulse 1 as at counter # 2 explained above. At the start of cycle 2 with clock pulse 3, counter # 3 begins by switching to the down-to-off state. During the latter part of cycle 2 and during the initial part of cycle 3, -m2 and -m * are then input signal. At clock pulse # 7, counter # 3 is switched to the up state, and + m ^ + m ^ is supplied from memory # 2 and directly from counter # 1 during cycle 4, respectively.

Thus, at the end of cycle 4, counter # 3 contains -m2 - m ^ + m * + m4 · As shown in FIG. 3C on line 14, the content of counter # 3 can be output as the output signal during the following cycle 1, and counter # 3 is then reset (RS) and switched to the down state to begin its tracking again at clock pulse 3. As explained below and as shown in FIG.

3A at B, this combination of measurements corresponds to another of the compensated borehole measurements provided by the method of the present invention.

The cycles 1-4 shown in FIG. 3C is summarized in Table III and will be further explained in the following description. With the general use and timing of the control signals, state pulses, counters and memories described above, the particular circuits of FIG. 3A and 3B are now described. The M and N state select signals generated for each cycle of control logic 331 are used as the first two bits of a code signal.

At the start of each cycle, a code signal is sent from code transmitter 336 in FIG. 3A into the borehole of encoder receiver 34o of FIG. 3B. The code signal can contain down to six bits of information indicating which of the two transmitters (enbit), which of the receivers (enbit), and which of the sixteen gain settings (Eire bits) to use. Additional bits for additional gain or attenuation settings may be desirable to increase gain resolution when amplitude / attenuation measurements are also performed.

With reference to the operation of the downhole circuits, reference is now made to FIG. 3B. In general, code broadcasting results in the selected transmitters and receivers being connected to appropriate downhole circuits and the downhole gain is set. DER-

17 DK 154584 B

tical gain control system is used to standardize the signal amplitudes by varying the gain for each different T-R cycle.

The detailed operation of the downhole circuits of FIG.

3B will now be explained. The code signals from code transmitter 336 are received by code receiver 34o downhole in circuit section 11B of FIG. 3B, and a code bit representing the M or M state is passed to the transmitter selector 344 which connects either the starting circuit 352 or 354 to T ^ for M or for M. Similarly, the N or N bit is passed to the receiver selector 35o and either receives T2 for N or T ^ for N is connected across the receiver selector 35o to the variable gain amplifier 348.

Amplification bits in the signal code (four of which are shown) are passed to the amplifier selector 346 downhole, which selector uses these bits to connect selected fixed attenuation joints and amplifiers to provide the desired amplification represented by the code. The resulting sixteen possible gain variations are shown, for simplicity, as represented by the variable gain amplifier 348 controlled from the gain selector 346. The gain will be determined automatically by analyzing the received signals as explained later, but at present it will suffice to mention that longer TR distances, such as T ^ -T ^, are given relatively larger reinforcements than the shorter T-R distances, such as T2 ~ T3 *

Downhole reception of a given code by means of the code receiver 34o also causes adaptation of the start pulse receiver port 36o to interpret the next downhole transmission as a start pulse order. Over a suitable delay provided by a delay link 341, receiving the code also causes opening of the previously blocked output port 342 downhole to then allow transmission up through the borehole of the output of the power amplifier 368 which may have previously interfered with the code transmission. Thus, it can be seen that the circuit of FIG. 3B downhole uses the code to tune associated electronics to connect the correct transmitters and receivers and set the desired gain and ports in anticipation of a subsequent start pulse order.

As FIG. 3A, again, the circuits above the borehole are activated synchronously with the downhole logic operation to provide the starting pulse and receive the corresponding reference pulse and subsequent receiver signal. The cycle probe 33o shown in FIG.

3A generates, for each cycle, a signal sent to control logic 332 which in turn generates signals to reset a first count- 1 Air * 3 01 file nnl r \ rr q-PVi ie \ i> irr? / * + · -P «3/3 λυι / 3 a line P n ~ _l i_ · n____ _

18 DK 154584 B

bringing various port signals to ports, counters and signal processing circuits, most of which have already been described in connection with FIG. 3C.

Delayed control pulses from cycle sensor 33o divide each cycle into subcycles for detection and automatic gain determination and insertion of complete continuous measurements or previously stored measurements into the signal processing apparatus 24 of FIG. 3B correctly for that cycle. For example, when the signal processing circuit 24A and 24B is used, these control pulses can be used to empty counter 394 and 394A and prepare them for processing the next input signal by countdown, or if the input signal is a digital word transfer to combine the word with a negative sign. Subsequent control signals cause these counters to receive additional input signals by counting or adding to the previous content.

In addition to providing clock pulses which begin each cycle to the cycle sensor 33o, the clock sensor 324 also provides, after a suitable delay, a control pulse for starting the pulse circuit 32o. This delay, produced by circuits in clock transducer 324, is of such magnitude as to give the circuit both downhole and at the surface sufficient time to prepare to receive the starting pulse, as already described. When the downhole and above-bore circuits are ready, this delayed clock pulse causes the start pulse circuit 32o to trigger a start pulse order (FP) which is sent down into the borehole and correctly interpreted by the previously prepared start pulse receiver port 36o shown in FIG. 3B.

Turning back to FIG. 3B, the start pulse order is passed through a gate to a start pulse detector 362 and, upon detection, Tq generates the generator 364 to produce a start pulse downhole. This brings the transmitter selector 344 previously connected to selected transmitter start circuits 352 for Tg in state M or 354 for T ^ in state M (see Table II) to start the selected transmitter Tg or T ^. The TQ generator 364 also triggers a Tq pulse for use as a reference signal both of the downhole circuit and of the circuit at the surface. For surface use, the TQ pulse is transmitted through the hole over the power amplifier 368 and through the now open output port 342 to the signal receiver 37o and the automatic gain control 372 located at the surface (see Fig. 3A).

At the surface, the T ^ pulse is used as an amplitude reference.

Since it is generated downhole by the TQ generator 364 with a standard reference amplitude, the automatic gain control system T79 port 374, peak read circuit 376 οσ τΛ gain

19 DK 154584 B

this is the standard pamphlet reference at the surface. This internal control thus provides compensation for cable loss, phase distortion, drift etc.

The Tq pulse is transmitted over the TQ gate 374, which has previously been prepared to pass the TQ to the peak value reader 376 over line 374A. This preparation is synchronized with the operation of the start pulse circuit 32o. The peak amplitude of the TQ pulse received at the surface is read by the peak value reader 376 and compared with a reference amplitude by the TQ gain tuning circuit 378 which, if necessary, sets the automatic gain control circuit 372 to restore the amplitude of subsequent TQ signals to the reference. Additional signal conditioning circuits can be added due to cable loss using this known TQ signal standard.

The peak value of the TQ amplitude read by the peak value reader 376 is also passed to an amplitude detector 38o as an amplitude reference for use in detecting the receiver signal following the TQ as explained below.

The Tq pulse provides an accurate time reference attributed to the transmitter start. This time reference is determined by a zero crossing detector circuit 375 which is connected across the Tq port 374 to continuously detect the T1 zero crossing point. This TQ detection point is used as a starting point for the timing measurement by providing a timed Tq detection signal as a start signal for a timer gate control 388 in FIG. 3A, which opens a clock sensor gate 39o to pass high frequency clock pulses from clock sensor 389 to counter # 1, which, having previously been reset, starts counting the clock pulses. The clock pulses must have a sufficiently high frequency, e.g. 2.5 MHz to provide the desired time resolution. The Tq detection signal is again used to block the Tq port 374 and open a detector port 379 for received signal so that subsequent signals will be interpreted as the next expected receiver signal. As the apparatus above the borehole of FIG. 3A already starts the time measurement at the start of the clock pulse count, reference must now be made to the downhole circuits shown in FIG. 3B.

The Tq signal generated by the TQ generator 364 is delayed by delay links D1 and D2 in the delay circuit 365 as shown in FIG. 3B and used to open and close a receiver port 366, respectively, for a time interval corresponding to the expected arrival time of the received signal. These delays will obviously vary by 20

DK 154584 B

transmitted from the selected transmitter, propagating through the borehole and formation, and received by the selected receiver previously connected over the receiver selector 35o to an already set variable amplifier 348. The selection and gain setting were discussed earlier in connection with the operation of the code receiver 34o. The received signal is passed over the receiver port 366, now opened as described above, to the previously described power amplifier 368 and the still open output port 342 where it is sent to the surface circuit shown in FIG. 3A.

The amplified receiver signal shown in FIG. 3A is designated as R, is received at the surface and reconditioned at the signal receiver 37o and amplified by the automatic gain control system 372 already described above to provide a cable compensated amplitude. It is then passed through a receiver detector port 379 previously conditioned by a detection signal produced by the zero crossing detector 375 to allow the signal to pass to detection circuits 38o to 384.

As shown in FIG. 3A, the arrival of the received signal R is detected by simultaneously comparing the amplitude of R in the amplitude detector 38o with a reference amplitude applied from the peak value reader 376 and upon examination of R with a zero crossing detector 382 and a peak value detector 384. A typical R signal is shown in FIG. 9A and have

X

positive and negative half periods, which grow in amplitude during the first few half periods. As shown in the circuits of FIG. 3A, three conditions are required for detection: 1) a zero crossing must be detected by a zero crossing detector 382, this detection being delayed internally with a delay equal to approximately half a period; 2) the subsequent amplitude of R compared with the amplitude detector 38o must exceed a small fraction of the TQ reference amplitude, and 3) an R amplitude peak value must be detected by the peak value detector 384

A

within the half-period delay, which follows the zero-crossing detection. All three detection indications are fed to an AND gate 385 such that the first occurrence of an amplitude exceeding a threshold amplitude attributed to the TQ amplitude, which lies after a zero-crossing with correct polarity, followed by an amplitude peak value of the same polarity within the half-period delay, the detection completes.

An R detection indication is output from AND gate 385 and provides a holding circuit 386 for holding the peak amplitude detected by the peak value detector 384 for use in adjusting the downhole gain for subsequent reception with the same transmitter.

DK 1 54584 B

bet 334 down the borehole. Independent gain settings are made and stored in the gain tuning circuit 334 for subsequent use in corresponding cycles. Furthermore, these amplifications are determined on the same portion of the signal used for the measurement. A further description of this automatic gain adjustment method can be found in U.S. Patent Application No. 528,693 of December 2, k974 and U.S. Patent Application No. 528,694 continued as No. 678,643 of 2o. April 1976.

The detection signal emitted from OG gate 385 is used to reset the receiver signal port 379 described above, and more importantly, to cause clock gate control 388 to block clock pulses coming from clock sensor 389. These pulses have previously been applied to counter 391 by means of clock gate 39o at Tq detection as previously described. Thus, the TQ 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 way, counter # 1 now contains the number of 2.5 MHz clock pulses corresponding to the travel time relative to TQ and received signal detections. The counts contained in counter # 1 can again be considered as the individual time measurement corresponding to the respective measurement cycle, such as m ^ for cycle 1, m2 for cycle 2, etc. The contents of counter # 1 upon completion of the count can then be transferred over various ports to utilization means at the times shown in FIG. 3C, which is obtained by appropriately delayed control pulses C from cycle probe 33o.

Where the individual measurements are to be recorded for later processing, the delayed control pulses C 1 to 1 each cause the counter contents corresponding respectively to port 392B to a suitable recording means connected at point C, such as a digital tape recorder, not shown.

Alternatively, special cycle control pulses are used to selectively feed the counter contents into memory and counter circuits 24A to provide a compensated signal and to similar circuits in 24B if two compensated signals are required for different distances. These circuits provide the interrelationships for combining individual measurements shown in Table III below:

22 DK 15 4 5 8 4 B

TABLE III

Control- Port Connection__Contents of Counters_ Pulse Measurement__ From To__C # 1__C # 2__C # 3_

Cl m-j * M # 1 C # 2 -m-j * B

Cl 'm ^ C # 1 M # 1 ir ^ Output C # 2 -m, - m ·, * Return- _____ position C2 m2 * M # 1 C # 2' -m1 - new * C2 * m2 C # 1 M # 1 m2 C # 3 -m2 C3 m3 * M # 2 C # 3 -m3 * - m2 C3 'm3 C # 1 M # 2 m3 + m3 + m2 * C # 2 -ir ^ - m1 * C4 m * M # 1 C # 3 A + m * 4 4 in ”m3” m2 jiit ------- in C41 m ^ C # 1 M # 2 m ^ Output C # 3 Back- + m * + position m * m3 ”m2 * measurement from previous position 23 DK 1 54584 3

Table III shows the general cycle for each measurement m. During the primary part of the cycle, here referred to as sub-cycle C, clock pulses accumulate in counter # 1 (C # 1) for the new measurement at the Iden continuous depth, e.g. , as shown in FIG. 2A and 2B. The corresponding measurement m * made at the previous position dj in the example shown is transferred from memory M to another counter which has been previously conditioned for this cycle to e.g. to count down or subtract or count or add. This leaves space in the memory for the continuous measurement, so that the memory need only have a capacity for the number of measurements made between dj and dj, because the continuous measurement m (by dj) can replace the measurement m * (stored by dj) .

The next subcycle C begins after a sufficiently long delay has been provided to allow completion of the continuous measurement, ie. after the expected signal has been received from the borehole and detected and C # 1 has stopped the count. Then, m from C # 1 is passed to memory M and replaces the corresponding previous measurement m *. During subcycle C *, m is also passed to the respective second counter C # 2 or C # 3 for this cycle. As shown for the two compensated measurements A and B, each m again goes to a memory M and an additional counter, e.g. goes m ^ to M # 1 and C no.

2, m2 to M # 1 and C # 3, m ^ to M # 2 and C # 2 and m ^ to M # 2 and C # 3. Each M thus stores two different m's, and counters C # 2 and C # 3 combine two running m's and two previously stored m * 's.

In view of FIG. 3A and 3B and Table III above, it will be readily apparent that both the measurements and the device components serve multiple uses. The same control amplifies cable compensation, automatic gain system, time reference, detection circuit, high frequency encoder and clock pulse counter for each individual measurement. This not only provides a device at a lower cost, but provides compensation for systematic measurement errors as explained in more detail below. It is only necessary to note here that if a component inaccuracy causes m ^ to be flawed, π ^, πι ^ and m ^ will also be flawed with the same magnitude in the same direction.

In accordance with the advantages of the present invention, when the systematic error measurements are combined as shown above, these errors will be offset, as will a systematic error introduced by e.g. probe inclination, compensated.

As previously explained and shown in Table III, different 24

DK 154534B

signals which allow the contents of counter # 1 to be transmitted to the gate-controlled memories or counters used to perform the measurement combinations. For example, at the completion of the first measurement cycle m ^ which, as indicated in Table II, corresponds to the - T2 measurement, as shown in FIG. 3A, the clock control pulse C1 or preferably a delayed version of the C1 as in Table III above can be used to drive the counter contents to memory in circuit 24. Preferably, the previous C1 pulse is used to carry a previously stored measurement out of memory. The time relations for different Μ, N, C and C (delayed) states and combinations for the corresponding measurements new to m ^ have already been described in connection with Figs. 3C. These relative conditions must be explained in detail below in connection with the circuit components concerned.

Memory is used to delay measurements m * made at a previous position, such as by dj shown in FIG. 2 so that they can be combined with continuous measurements which are in counter # 1. In the preferred arrangement, the contents of counter # 1 are passed through port 392 to memory # 1 upon completion of cycle # 1 and cycle # 2 to store the measurements irij and m2 · After the number of complete cycles corresponding to the movement of the transducer system from the position shown as dj to the position shown as dL in FIG. 2A has been stored, these measurements are available from the output of memory # 1, so that control pulses fed to port 396 carry these previously stored measurements for use in counter # 2. In this way, C1 and C21 would cause m ^ at dj and m2 at dj, respectively, to be passed through the gate 392 to memory No. 1 in series. This storage process of new and m2 in memory # 1 continues until e.g. by d ^ the previously stored readings become available as output signals from memory. At this point, the control pulse C1 'would continue to provide new readings to memory as well as to counter # 2 above port 393.

As already explained, counter # 2 has previously been conditioned prior to C1 to interpret subsequent input in a countdown or negative direction. Thus, when C1 is passed to the output port 396 for memory # 1, new corresponding to the previously stored measurement * at dj is passed to counter # 2. In a delayed version of C1 designated as C1, the running new thereon is also led to counter # 2 and to memory. In this way itij * at dj and nij at dL is led to and combined at counter # 2 in the same direction, ie. either by continuing to count down for their combined count or added with negative sign. In counter # 2 at the end of Cl

DK 154584 B

25 the term is -m ^ thus at dL - at dj. The next clock cycle C2 would add m2 * at dj over port 396 to counter # 2, but now conditioned to consider the input signal in a positive or counting direction. Then the running m2 at d ^ would be stored. Thus, at the end of a C2 cycle, counter # 2 would contain m2 * at dj - irij at dL - at dj. During the next clock cycle C3, the measurement m ^ at the running depth dL would be passed through port 393 to counter # 2, which is still in its addition state, so that the result becomes m ^ at dj | + m2 * at dj - new at d ^ - m2 * at dj. At a subsequent appropriate clock pulse illustrated as C4 *, the contents of counter # 2 are then output through port 397 to point A as the compensated signal. When the sehder receiver selection has been in accordance with Table II, the compensated signal A corresponds to a study at short T-R distance. Counter # 2 is then reset and processing for another compensated signal test corresponding to the next depth gain in the sequence begins in the manner described above.

For a study B at long T-R distance, the corresponding circuit 24B shown in FIG. 3A with separate memory # 2 and counter # 3 and corresponding ports. In 24B, these components have been designated by the same numbers as used in 24A, but now provided with a further designation "A". These "A" ports are, of course, controlled by different control pulses as indicated therein, since they involve different measurements provided at different times. Like the time diagram of FIG. 3C for the timing of the circuits shown in FIG. 3A, Table III summarizes the functions of both circuits 24A and 24B expressed by the control pulses, directions and contents of the different counters and memories shown in FIG. 3A and used to derive the two compensated signals A and B.

It will be appreciated that when both A and B are desired, memories 1 and 2 can be easily combined, since their input and output functions occur at separate control pulses, and measurements m ^ to m ^ can be stored in this order and recovered in the same order. A suitable memory is disclosed in U.S. Patent Application No. 571,497 of April 25, 1975 (Continued Application No. 384,228 of July 31, 1973). Each time a new measurement, such as m ^, is ready, the oldest corresponding measurement is taken out of storage so that the latest measurement can replace the oldest measurement and the memory is processed on a replacement basis, thereby preserving the memory capacity.

It will be immediately recognized how the further measurements m 2 to m 2 can be made and utilized from the description of new above,

DK 154584 B

26, it is understood that the invention may be practiced by providing a single compensated measurement shown here as either A or B, thereby using only a single memory or additional counter besides counter # 1. In this case, two measurements may be combined, and only the result is stored. The two continuous measurements do not need to be stored.

It will also be appreciated that the processing provided by the circuits 24A and 24B can be performed by a digital microcomputer with its normally associated memory replacing the memories 395 and 395A and its arithmetic registers replacing the counters 394 and 394A as its control program uses the control pulses to perform the specified transfers to and from memory and registers. As previously stated, these treatments can also be provided by using the output C recorded on a digital tape recorder, which is later provided as the input signal to a digital computer for general purposes and processed by an equivalent control program.

Referring now to FIG. 4A, there is shown a transducer slide shear support having a slope from the desired wall contact position parallel to the borehole wall. Such a slope may be due to many different mechanical problems associated with the sliding shoe and mandrel joint system, insufficient sidewall pressure, etc. Unfortunately, this slope problem may not be reflected in joint system caliber or pressure measurements. However, in accordance with a feature of the present invention, not only can the slope be detected, but its effect can also be compensated.

The transducer system shown in FIG. 4A is arranged as assumed by the illustration in FIG. 2A and 2B, i.e. the receiver pair and T2 are at the top, and the transmitter pair T ^ and are at the bottom of the guide shoe.

As indicated in Table I described above, four measurements are made between different combinations of these transducers. Two binary states M and N are used to encode the transmitter and receiver selection which control the signal paths. In accordance with an advantageous drawback of the new transducer system, compensation for borehole paths is provided due to either slope or borehole leaching between the near and far transducer in the pair by reversing the direction of the near and far transducer measurement, ie. the distant transducer becomes the near transducer and vice versa. This possibility is achieved by using a transmitter pair in the same direction as a receiver pair is used to provide one of two sets of transducer measurements. FIG. 4A and 4B show how this compensation is obtained for the slide type and FIG. 4C and 4D for a non-slip type system.

Hereafter, the roads shown in FIG. 4A and Table II. signals

27 DK 154584 B

and then toward the receivers along the path B, reaching T across the borehole paths D and T ^ over a further formation path C and the borehole path E. If the borehole paths E and D are the same, the differences between the signals from T2 and T · a measurement of the migration through the formation path C corresponding to the interval between T2 and T1. However, if the path D is significantly different from the path E, this distorts the short T-RR measurement, which was assumed to correspond to the formation path C, which, in the case shown, where D is greater than E.

ISLAND

over the short distance is equal to m2 - m ^ = C + (E - D) because the common paths A and B are deleted. In the ideal case, E = D and there would be no error. In the above case, however, the error is equal to their difference E - D, which is negative, indicating that the travel time will be too short.

An error would also exist for the long T-RR measurement m ^ made relative to T ^, since borehole path lengths H and J are also different. Here m ^ = m ^ - m ^ = I + (J-H) as the common paths F and G are deleted. As illustrated, H is larger than J, which causes the error due to their difference is also negative and indicates that this travel time is also too short.

Despite the difference in the roads shown in fig. 4A, the formation paths C and I for the formation interval between T2 and T ^ and the borehole paths D and H at T2 are almost the same, as is the case for E and J at T ^. Even formations that vary radically in terms of acoustic properties radially from the borehole wall may still assume nearly identical receiver boreholes for signals received over either the long or short T-RR distance. Accordingly, both the short distance ms using T 1 and the long distance m 1 using T 4 can be expected to have the same error.

Referring to FIG. 4C, the nature of the error is considered when the transducer system is moved from position (a) when the receiver pair is in the vicinity of interval I to position (b) when the transmitter pair is in the vicinity of interval I. The borehole paths for position (a) are designated as in FIG. . 4A and for position (b) with the same letter but with marking, e.g. A and A 'at T ^. With the interval I of interest between T ^ and T ^, the short distance measurement for position (b) is m'g = ... m's - m '- ^ using T2 and the long distance measurement m ^' = m4 "" M2 * ve <^ use of T ^. Referring to Table I, it can be seen that the error for both m 'and now 1 is F' - A '. If F 'is greater than A', the error is positive and thus in the opposite direction of the errors in position (a). As FIG. 4B and FIG. 4D will show, the error is also of the same size.

DK 154584 B

28 and it is recalled that the errors at position (a) were (ED) or (JH), so it can be readily seen that since the slope angle γ is the same, the paths E or J at for position (a) are taken relative to path A 'at for position (b) aligned to paths D or H at T2 for position (a) taken relative to F' at for position (b) due to the geometric similarity. (E-D) = ~ (F '- A') and thus the slope angle lenykan is thus calculated. If m_ (or m,) is less than m '(or

5 X S

m ^ 1), the case of inclination is shown, with the upper pair of transducers of the same type being closer to the wall than the lower pair. If m is greater than m ', the opposite case would be indicated. This will be better seen in FIG. 4D.

In FIG. 4D, paths with the transducers superimposed are shown 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 angle of inclination V, the distance from the wall contact point of the transducer system and the angle of refraction β. Since Train β is constant and the distance separating one's transducers is the same (shown here as I), it can be shown that the difference between the path lengths of one's transducers is also the same such that D - E (or H - J ) = F '- A1.

From the foregoing, it will be seen that measurements first between the receiver pair and then the transmitter pair reverse the direction of the slope error introduced in these measurements.

The effects of the borehole shape rather than the slope are illustrated in FIG. 5A and 5B. In FIG. 5A, the horizontal position of the transducer T, i.e. centered in a round hole. The path 1 from a transmitter and the path 2 to a receiver are of equal length as are all the paths around the perimeter of the transducer. This causes the emitted energies radiated in different directions to be received substantially at the same time and thus amplify each other 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 (no slope) ,. but the borehole is now circular, the shape resembling two intersecting cylinders of different diameters and non-coinciding centers. This form typically occurs 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 cut the transducer. This results in a clear reduction of the transmitted energy transmitted to the formation and a detrimental phase-shifting ratio of the signals arriving at the receiver, as a signal moving through the path 7 will arrive much earlier than e.g. across the road 8. FrilaeHa ontr;

DK 154584 B

29. To a lesser extent, the above signal problem also occurs in the case of inclination, since in these situations it is impossible to have all the transducers in the ideal position. Such as. shown in FIG. 4C, different degrees of eccentricity, even in a round hole, will occur for each of the four transducers. Thus, the measurements m2 and m3 would be similar under the ideal conditions of FIG. 5A, but different under the conditions of FIG. 5B does not know the round hole or the eccentricity associated with probe inclination. In this way, this comparison of different measurements at different depths can in fact detect different ambient conditions for transducer operation, such as caused by probe inclination.

In the prior art T-RR-T systems, the transmitters are located at the ends of the system. Thus, if inclination causes one end to be located eccentrically, the two transmitters, which are very far apart, operate in significantly different positions, even in a round hole.

In comparison, the same transducers in the TT-RR system described herein are located at a short distance apart and advantageously operate in nearly the same positions with respect to the borehole wall.

As previously mentioned, it is desirable, especially in acoustic studies, to have long T-R distances to overcome the effects e.g. of slate clay change. The same desire exists in sliding shoe devices and in other types of measurements such as high frequency, electromagnetic studies, etc.

FIG. 6A shows a prior art borehole compensation system.

The T-R distance is shown to occur twice, first from T 1 and then from T 2 to the system midpoint between R 1 and R 2.

6B the compensation system of the present invention applied to the sidewall guide shoe. The same distance or receiver survey interval and guide shoe length are used in both of FIG. 6A and 6B. For the same transducer system length, the new system shown in FIG. 6B, however, significantly increases the T-R distance, even for the shortest T-R study. For the longest T-R study, this distance is the entire system length minus half the receiver distance. In contrast, the maximum T-R distance in the prior art system is only half the entire system length. In the new overlap of both the short and long T-R distances, the system shown in FIG. 6B, in accordance with the invention not only extend T-R distances for the same system length, but provide two different T-R distances located within this length. Typical distances between similar transducers for acoustic time measurements are o, 3 or o, 6 m, while the shortest T-R distances are at least 1.2 m, but preferably 1.8 or 2.4 m.

DK 154584B

30 of the invention is thus of the order of 1.8 m or more as shown graphically between FIG. 7A and 7B.

Referring now to FIG. 7A and 7B, showing further features of the invention. In FIG. Fig. 7A shows both circuit connections and the use of directional transducers in the prior art compensation system type. To use directional receivers, two separate sets of receiver pairs Rn must be used and to receive signals in the direction of T, the upper end, and Rn 'and R ^' for signals from T ^, the lower transmitter. In addition to the complicated nature required by the two additional receivers, the usual electronic noise problem occurs in connection with connections between the circuits across the transducers to the lower transducers. For example, for the lower transmitter, these connections must be passed through or around the upper transducers. A high voltage generator is usually located near one of the transmitters, not shown here, but above T. In any case, a high voltage line, here F ^, must be passed past the receivers to the remote transmitter. High voltage s pulses typically used to activate such transducers must be shielded to prevent electrical cross talk to the receivers or receiver wires Rn, R ', R ^ and R ^', and even then cross talk can become powerful.

By comparing FIG. 7B showing the system of the present invention with the prior art system of FIG. 7A described above, it will be immediately seen how the advantages of the system according to the invention can be used to alleviate this electrical connection and cross talk problem. Since both transmitters are arranged together and can advantageously be located on the same side of the receiver pair, it is not necessary to conduct high voltage wires near the receivers or receiver electronics. The high voltage generator can be placed under the receivers and their associated electronics. Thus, it is only necessary to connect a relatively low voltage DC supply from above. This arrangement provides good electrical signal isolation and freedom of cross talk in the receiver signals which are of much lower level.

The new transducer arrangement will further enable the use of both directional transmitters and receivers without the need to add an additional pair of transducers to provide the required directional effect. Since both receivers are on the same side of the two transmitters, each receiver and transmitter have a special directional effect requirement which does not require additional transducers as in the prior art system. Since the same pairs of transducers are always used, differences in additional pairs of the otherwise required transducers to achieve directional effect in both directions will not occur and affect the measurement.

DK 154584 B

31 to compensate for refractive effects. As can be seen from the prior art compensation system shown in FIG. 7A, the signals approach the receivers from different directions and inclines. This slope 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 °, with the actual angle depending on the formation / borehole fluid velocity contrast.

Two pairs of receivers Rn and R ^ and Rn 'and R ^', respectively, are shown in FIG. 7A to record the refraction effect. Each receiver is set with its most sensitive direction along a particular inclined borehole path. Each pair is displaced to more effectively adjust the position of the formation interval simultaneously during study between the two pairs. This offset may be referred to as a refractive offset and determines the small distance between the two receivers shown therein which is used to replace the usual single receiver for directional reception from above and from below, i.e. between one or the other of R n and R 1 or R 1 and R 2. Unfortunately, the refractive displacement varies not only with hole size but with formation velocity, so that a fixed distance between these two receivers can be determined, but for a displacement corresponding at best to a nominal borehole size, formation velocity, etc.

However, according to the present invention, variations in the refraction offset can be compensated by varying the delay distance or the number of depth increments between measurements performed between the different transducer pairs of the same type. As seen in FIG. 7B, the lower pair of transducers of the same type of formation interval looks slightly above the actual borehole depths of these transducers, while the upper pair sees the intervals slightly below their operating depths. Thus, the refraction offset compensation can be readily obtained by simply setting the delay between measurements performed between these pairs prior to their combination, e.g. reducing the delay for larger displacements between the real position and the effective position of a transducer caused by a larger bore hole size, higher velocity formations, etc.

A further feature of the invention can be seen by comparing the examined formation intervals shown in FIG. 7A and 7B. In the prior art compensation systems shown in FIG. 7A, only the interval I, centered around its midpoint, is examined. This thus excludes any possibility of examining the critical interval between this point and the bottom of the hole. As can be seen in FIG. 7B, it lies

bottom xvalue I-t .-. S ΦΦ— / DT? cvcf amaf -ι -m -i -ΐ rn Ωί-τΩ-ί- to ί ^ πη ^ αη a * F

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32 transducers. Although no compensation occurs, both short and long T-R examinations can be performed.

Circuits for providing the dt measurement from the upper and lower ranges At ^ for Iu and At- ^ for 1 ^ are shown in dotted lines in FIG. 3A. For example, the m 2 and m 2 measurements taken to memory # 1 above port 392 can also be fed to the up / down counter # 2A shown at 398 in circuit 24A. This counter is like up / down counter # 2 shown at 394, controlled by control pulses from control logic 332 to count down or enter a negative sign for m ^ below C1 'and up or add with a positive sign for m2 below C2'. Thus, at the end of C21, the content of counter # 2A is m2 - m ^ for the interval at that time between T ^ and or Atu. Since m2 and m- ^ are both assigned to the transmitter for short distance, this is an At ^ for short T-R distance, as shown in FIG. 2A at position I. The timing can be seen in FIG. 3C.

An Atu for long T-R distance is similarly provided using the other measurements in another up / down counter # 3A shown at 398A in FIG. 3A. This counter, controlled like the up / down counter # 3 with m ^ and m ^ supplied from port 392A of circuit 24B, provides m ^ - m ^ for the interval at that time between T2 and T · ^ or Atu, as shown in fig. 2B at position I.

For position L and the lower interval 1 ^, At- ^ is provided for the short TR distance of up / down counter # 2B at 399 controlled like counter # 2 with m ^ and m ^ supplied from port 393 to provide m ^ - m ^, and At ^ for the long TR distance is provided by up / down counter # 3B at 399A controlled like counter # 3 with m2 and m ^ supplied from port 393A of circuit 24B. Thus, long and short T-R distance studies are provided for both the upper and lower ranges shown in FIG. 7B.

Although neither Atu nor At ^ are borehole compensated, they are obviously useful for logging borehole intervals located immediately below the casing and at the bottom of the borehole. Used together, they are useful as borehole compensation indicators as their difference indicates the degree of tool slope, e.g. Atu <At- ^ corresponds to the illustration in fig. 4C.

Referring now to FIG. 8A and 8B, alternate circuits for a section of the signal compensation circuit 24 previously described in connection with FIG. 3A. As mentioned earlier, it is sometimes advantageous to compare as well as combine the measurements. When comparing different measurements that should be in

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33 recipient pairs over the same interval in the borehole, certain borehole operating conditions which cause the measurements to vary can be detected. If the measurements are similar to a reasonable degree, their differences may be due to statistical variations so that they can be combined to produce an improved or compensated measurement. However, if the comparison shows a not reasonable difference, it may show working conditions causing the error.

The circuit shown in FIG. 8A thus allows for the occurrence of a depth pulse supply at 181 of a delayed measurement m * corresponding to a previous position and transducer combination present at the output of the memory, as shown in FIG. 3A, to the comparator 182. Similarly, the current measurement m, which is directly comparable to the stored measurement m *, is also applied to the comparator 182.

For example, the delayed input signal corresponds to m2 at dj, and the direct input signal corresponds to measurement m ^ at dj, as shown in FIG. 2C, it can be expected under normal conditions that the measurements will be essentially the same. However, if a detection error occurs in one of the measurements, a significant difference will be noticed.

As shown in FIG. 8A provides a non-reasonable difference, a no comparison signal which can be used to indicate a detection problem such as cycle skipping. However, if the comparison is reasonable, this indication is used to pass measurements m2 and m2 to an adder 183 for combination to produce a compensated mean value measurement from the measurements.

The alternative circuit shown in FIG. 8B is more suitable for indicating the borehole compensation required to compensate either time or amplitude measurements. The memory delayed and the direct (continuous) measurement are added and compared at 182A. If the comparison is reasonable, the two measurements can then be combined as described above. However, if the comparison is not reasonable, this indication can be used to pass over ports 181C and 181D to a differential amplifier 183A whose output signal is summed at 184 and used to indicate the relative error of the two measurements.

The circuit shown in FIG. 8A and 8B may also be used for other compensation purposes. As previously described in connection with FIG. 4C and 5B, the state of tool inclination produces different degrees of eccentricity for different transducers and corresponding differences in arrival times and amplitude measurements which will be indicated by a relative measurement indicator 184A. If the slope results in an upper transducer pair being more eccentrically located than the lower pair, then it is expected that the teariest or memory-impaired target will

34 DK 154584 B

The delay between the delayed and direct measurement will thus provide a negative indication. Conversely, if the lower transducer pair is more eccentrically located, the indication will be positive. This will be apparent from the following example.

Measurements m 2 and m 2 are determined as shown in Table II and taken when their known positions along the borehole correspond to the same formation interval. This occurs when the transducer system is moved e.g. from position dj to dj in FIG. 2C. In fact, transducer T2 replaces Tj, and T ^ replaces T ^. The formation paths B and C for m2 at dj (here m2 * r) are essentially equal to path G for m ^ at dj (here m ^), and any errors between m2 * and m ^ will be due to differences in the comparable paths A and F in combination with E and H, as shown in FIG. 4A or 4C. The difference m2 * - m ^ is thus equal to A + (B + C) + E - F - G - H = (A - F) + (E - H), assuming that B + C = G.

As shown in FIG. 4A or 4C, A is smaller than F, and E is smaller than H, so that the differences (A - F) and (E - H) do not cancel each other, but have the same sign (both negative here) and unite to indicate both the nature of the error between these two measurements and its magnitude.

While the foregoing description 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 below. First, some natural differences in the measurement methods used in these additional applications are considered.

FIG. 9A shows the type of detections typically used in acoustic walking time measurements or other measurements where the signal period or wavelength is long compared to the required resolution. The signal usually propagates as a pulse with positive and negative oscillations beginning with its arrival and relatively small signal prior to this time. Thus, as shown by I and II, corresponding to the reception signals which can be expected at the near and far receiver, respectively, a relatively small signal appears prior to its arrival. In the design, the first and relatively weaker half period gives an opposite polarity to that used for detection. A detection threshold amplitude different from zero is used to avoid noise and with the opposite polarity of the first half period. The detection corresponds to the point T when the amplitude first swings past this threshold.

For I in FIG. 9A, the detection occurs at the first receiver, as shown by Txj Qg a corresponding detection for II at TX2 ·

DK 154584 B

These detection points are assigned to each other in time, such as e.g. where Τχ ^ would begin a time interval and Τχ2 stop the time interval in the case of differential measurements or in the case of individual successive measurements, T can be related to some previous time such as Tq, in this way the measurement m ^ at III corresponding to signal received at T2 begin at TQ and stop at Txl 'while the measurement for T ^ and m2 at IV would begin at the reference time.TQ and stop at Τχ2. In this way, the difference m2 - m- ^ provides the interval measurement At as shown by V.

In electromagnetic measurements, the signals migrate at significantly higher speeds and their periods are rather short compared to the required time resolution. Accordingly, phase detection is usually used instead of the zero crossing or threshold method shown in FIG. 9A. The phase ratio can be measured between signals received from the near and far receiver to obtain a differential measurement, or if individual measurements are preferred to a known reference signal of the same frequency.

As shown by I and II in FIG. 9B, the two signals are slightly offset, as will be seen by comparing the zero crossing detection points at III for the signal on line I with IV for the signal on line II. As shown on line V, the phase shift $ between the displayed zero crossing points in many respects corresponds to the measurement shown in FIG. 9A. Specific circuits for performing the differential phase measurements shown above are disclosed in United States Patent Nos. 3,849,721 and 3,944.91ο.

In order to illustrate an application of the new transducer system to an electromagnetic measurement, reference should now be made to FIG. lo. The new TT-RR compensation system is in the form of transmitter T and receiver R antennas supported on the sidewall guide shoe 37. As in the acoustic embodiment already described, two mutual distances are indicated between transducer groups of the same type, here Iu between receivers T- and T2 and l- j_ between transmitters T3 and T4 · For electromagnetic measurements, I and 1- ^ will be of the order of a few centimeters. Two T-R distances 2 and 4 times I, respectively, can be provided on reasonable length sliding shoes. The actual distances vary as indicated by the separation between T 1 and T 2 depending on the frequency used in the measurement. Frequency to distance ratios are described in more detail in the above-mentioned patents. When phase detection is used, care must be taken to ensure that the distances provide the correct basis for phase comparison. For example, combinations of frequencies and distances that result in crossing through zero-phase differences should be avoided.

A large part of the embodiment shown in FIG. The circuits shown are described in the above-mentioned United States Patent Specification No. 3,849,721 οσ 3,944.91ο and should not

36 DK 15 4 5 8 4 B

le T-R measurements instead of the usual R-R differential measurements. This is accomplished by providing a transmitter-related signal for use as a reference signal instead of a missing receiver signal. The state control signals M and N already described in connection with the acoustic embodiment are used equally well in FIG. lo, being used herein to control the transmitter and receiver signals and the processing circuits. These control signals can be provided by custom-designed square wave generators 6oA and 6oB.

As shown in FIG. 1o, the transmitter signals from a high frequency oscillator 45 are switched by means of a switch 47 controlled by the state M to either line 47A or 47B and transmitted to Tg or Tj respectively. At the same time, signals are also delayed and attenuated to simulate formation states for short and long T-R distances by delay lines D at 4oA and D. at 4oB and switched by a switch 41 to serve as reference input signal 41A to a mixer 5o.

Transmitted signals propagate through the formation and are received at both T2 and T1 but only one of these signals is switched to mixer 51 depending on switch 43 controlled by N. The phase difference measurement is performed using mixer circuits 48 and 49, zero crossing detectors 71 and .72, and a sign reverser. flip-flop 77 with an integrator 78 for generating at 78A of the phase or travel time measurement for that TR combination. Further changes in states M and N result from such measurements, each made in the above manner using the delay links Dg and D1 to provide the preferred range of phase differences for the corresponding T-R distance. The four T-R combinations have already been described in connection with M and ten in connection with Table II.

Instead of using the transmitter reference signal method as a phase comparison basis as shown by circuits 4o, 41 and 48, alternative circuits 44 to 44E can be used. As shown by dashed lines in FIG. 1o, a loo kHz oscillator 44 can be used in conjunction with high frequency oscillator 52 to provide synchronous loo kHz clock pulses 44A which are then delayed by either delay line Dg to provide signal 44B or delay line D1 to provide signal 44C. These selectively delayed signals are then routed by a switch 44D controlled by the control pulse M such that the output signal 44E can be used to replace the corresponding pulses normally output at 71A from the zero crossing detector 71.

As shown in the aforementioned United States Patent Nos. 3,849,721 and 3,944,91ο, it is advantageous, together with the phase difference or travel time measurements, to also measure the amplitude or attenuation of the electrical current.

37 DK 154584 B

thus, the cushions are desirable. These are obtained simultaneously with the individual phase measurements by circuits 8o to 9o shown in FIG. 1o, to provide a continuous corresponding sequence of amplitude measurements for each T-R combination at 9oA.

Since the T-R combination measurements are performed at different depths, there can. a memory and gate circuit similar to that shown in FIG. 3A. Since the compensation provided by the use of the TT-RR system applies to both time and phase measurements and to amplitude or attenuation measurements, it is desirable that these measurements of different types, with each type having two different TR distances, provide the compensation. .

Since the signals provided at 78A and 9oA can occur as a result of analog voltage levels, they can be converted from analog to digital measuring sequences by an analog-to-digital converter 94 synchronized to multiplex the input sequence using a multiplexer 93. Depth synchronization is provided for memory delay purposes by depth pulses 92 and the synchronization of the measurement is controlled by control signals M and N. The resulting digital output signal is then fed from the analog-digital converter to individual port memory and counter circuits 24C to 24F, each shown as shown. in FIG. 3A for the circuits 24A and 24B. These compensation circuits produce a first and a second examination, respectively, representing different transmitter-receiver distances corresponding to output A and output B already described in connection with circuits 24A and 24B. In this case, however, the studies represent separate phase and attenuation measurements as shown at 96 to 99 in Figs. lo.

Referring now to FIG. 11, there is shown a further embodiment of the invention corresponding to the type of measurement in which a given transducer can be operated either as a transmitter or a receiver, such as an antenna capable of emitting or receiving electromagnetic waves. In FIG. 11, the transducer pairs are thus designated as antenna Ag and A ^ for a long and short-range antenna in one pair and Ag 'and A ^' for the other pair, respectively.

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 broadcast can be simultaneously received by both receivers and measured either as differential measurement or individually in relation to the same reference signal. Since two measurements are in fact performed at the same time, the mean of each measurement can be performed over a longer period.

38 DK 154584 B

in FIG. Io allows switching of the transmit signal produced by oscillator 45 to either 47A or 47B. A switch 41a, which is separately but synchronously controlled by the control pulse N, carries the short distance transmitter signal to either A_ or A 'and the long distance transmitter signal to either A ^ or A- ^'. Similarly, switch 42A selects two adjacent antennas for use as receiver pairs and carries the detected signals to the separate mixer circuits 48 and 49 described above.

In this way, differential receiver surveys can be provided alternatively from the upper range I using A and A, U S j.

as near and far receiver, while simultaneously switching between A 'and b A ^' as short and long distance transmitters. Without moving the tool, differential receiver surveys can be provided from the lower range 1 ^ using Ag 'and A ^' as the receiver pair, while switching between A and Αη as the transmitters. In accordance with the present invention, the system is then moved such that A 1 - 'and As' are in the vicinity of the interval Iu previously investigated by Ag and A1 · The processing circuit 95 deeply synchronizes the measurements and combines them to produce the combined first and other study phase and attenuation measurements already shown and described in connection with FIGS. lo.

A method and apparatus for maximizing the use of a system of four transducers and measurements performed between different combinations of the transducers are described above. When used in a new arrangement of the same four transducers normally used to provide borehole compensation measurements, these transducers may be used to provide measurements for determining not one, but two borehole compensated measurements, each investigating the same formation range with different transmitter-receiver spacings . Since both studies are compensated in the same way, this combination adds meaning to any differences that occur between these different studies, and the significance of interpretation attributed to them, such as e.g. indication of the presence of gas in a formation below the soil surface.

In general, the new transducer system enables dual use of the derived measurements. The two-receiver measurements are used twice at each depth increment, once each in relation to the near and far transmitter. Then the measurements with two transmitters are used twice, once each in relation to the near and far transmitter. In fact, even the transmitter-receiver distance is used twice when overlapping this distance, which allows for desired increases in the T-R distances without

39 DK 154584 B

known systems.

Furthermore, since all transmitter transducers are located on the same side of the receiver transducers, signal propagation takes place in the same direction for all measurements, which immediately facilitates the use of directional transducers. Furthermore, since transducers of the same type are grouped together, they essentially work in similar borehole environments, allowing both combination and comparison of individual measurements performed with different transducer combinations.

Although the embodiments shown include acoustic and electromagnetic measurements, the new features of the invention apply equally to other types of measurements. Furthermore, although the receiver pair has generally been shown to be the top pair of transducers and the transmitter pair as the bottom pair of transducers, it will be understood that the particular features of the invention will also be provided by the reverse arrangement. Likewise, the provision of the measurements can be performed when the transducer system is moved either upwards as shown or downwards in the borehole. Although the described embodiments provide combined measurements when made at the drilling site, it will be appreciated that the individual measurements can be recorded and combined at another time and place.

Claims (4)

A method of generating measured values for determining the physical characteristic values of underground media near a borehole passing through the ground, using at least two transmit transducers emitting electromagnetic or acoustic waves, and at least two receiving transducers receiving those of the transmit transducers emit waves passing through a mileage in the underground medium where the transducers are spaced axially apart on a borehole probe and the following steps are performed: a) placing the probe at a specific depth of the borehole; b) performing two measurements between the transmitting transducers and the receiving transducers, where the measuring distances of the two measurements overlap, and c) combining the two measured measuring values to determine a compensated characteristic value for the overlapped mileage, characterized in that by placing the transducers in the order of receipt (T, j) receiver (Tg) transmitter (Tg) transmitter (T ^) respectively, in the reverse order of the probe, where the distance between the receiving transducers is equal to the distance between the transmitting transducers, the probe is moved between the two measurements a certain distance in the axial direction in the borehole, the measurement distances for the two measurements are equal and the overlapping measurement distance is each time equal to the distance between the two receiving transducers. Method according to claim 1, characterized in that the first measurement is carried out between the first transmitter (Tg) and the first receiver (T1), and the second measurement is carried out between the second transmitter (T2) and the second receiver (Tg). ), the probe moving after the first measurement a distance equal to the distance between the receivers (T ^ Tg) (Fig. 2C). Method according to claim 1, characterized in that the measurement values for the two measurements are formed by each two sub-measurements, wherein the first sub-measurement (m 2) of the first measurement is carried out between the first transmitter (Tg) and the second receiver (Tg), and the second sub-measurement (mg) of the first measurement is performed between the first transmitter (Tg) and the first receiver (T ^), and after displacing the probe a distance equal to the distance between the two receivers. 41 DK 154584 For (T ^ / Tg), the first sub-measurement (m ^) is performed by the second measurement between the first transmitter (Tg) and the second receiver (Tg) and the second sub-measurement (mg) by the second measurement between the second transmitter (T4) and second receiver (Tg) (Fig.2A). Method according to claim 1, characterized in that the measurement values for the two measurements are formed by each two sub-measurements, wherein the first sub-milling (mg) of the first measurement is carried out between the second transmitter (T ^) and the second receiver (Tg) and the second sub-measurement (m ^) of the first measurement between the second transmitter (T4) and the first receiver (T ^), and then after displacement of the probe a length equal to the distance between the first receiver (T ^) and the first transmitter (Tg), the first sub-measurement (mg) is carried out by the second measurement between the first transmitter (Tg) and the first receiver (T ^) and the second sub-measurement (rr »4) between the second transmitter (T4) and the first receiver (T4). (Fig. 2B).