Acoustic shear wave logging device

λ_;'.jnvcntion generally cancecne the

·. well logging, and more particularly well logging by acoustic shear waves.

In well acoustic logging, it is common to measure the speed of compression waves in terrain formations surrounding boreholes. A conventional compression wave velocity logging apparatus includes a cylindrical logging probe which can be suspended in a fluid contained in the borehole, a source connected to the probe to produce compression waves in the borehole fluid, and a or more detectors connected to the probe and spaced from the source of compression waves to detect compression waves in the probe fluid. A compression wave produced by the source in the borehole fluid is refracted in the underground formation surrounding this borehole. It propagates through part of the formation and is returned by refraction in the sounding fluid, at a point adjacent to the detector, and it is then detected by the latter. The ratio of the distance between the source and the detector to the time spent between the emission and the detection of the compression wave gives the speed of the compression wave in the formation. The distance between the source and the detector is usually fixed and known, so that the measurement of the time between the emission and the detection of the compression waves is sufficient to determine the speed of the compression waves in the formation. For greater accuracy, this distance is usually much greater than the dimensions of the source or detector. Important information for the production of oil and gas from underground formations can be obtained from the compression wave velocities in such formations.

When a compression wave produced by a source placed in the borehole fluid reaches the wall of the latter, it produces a compression wave refracted in the surrounding subterranean formation, as described above. In addition, it also produces a refracted shear wave in the surrounding subsoil formation and guided waves which pass through the borehole fluid and the part of the formation adjacent to the borehole. A part of this shear wave is returned by refraction into the borehole fluid in the form of a compression wave and it reaches the detector which is in the logging probe and which also detects the guided waves. Any wave belonging to the three types of waves detected by the detector can be called an arrival? the arrivals of compression waves consisting of the compression waves generated in the sounding fluid by refraction of the compression waves traversing the formation, the arrivals of shear waves constituted by the waves generated by refraction of the shear waves in the formation, and guided wave arrivals due to guided waves. Thus, the signal detected by the detector is a complex signal which includes the arrival of a compression wave, the arrival of a shear wave and the arrival of guided waves. In field formations, compression waves travel faster than shear waves and the latter usually travel through the formation more quickly than guided waves. Therefore, in the complex signal detected. by the detector, the arrival of the compression wave is the first arrival, the arrival of the shear wave *

is the second arrival and the guided wave arrivals are the last arrivals. When measuring the speed of compression waves in the formation, the time interval between the emission of the compression waves and the detection of the first arrival detected by the detector gives the approximate time of travel of the compression wave refracted in the formation. Therefore, subsequent arrivals of the shear wave and guided waves do not affect the measurement of the speed of the compression wave in the formation.

In addition to covering a vertical distance, in the formation, roughly equal to the distance between

07486 the source and the detector, the compression wave also travels short distances in the fluid. The extra time required to cover these short distances introduces errors into the speed log. To reduce such errors, conventional logging devices use at least two detectors spaced vertically from each other along the borehole. The time interval between the detections performed by the two detectors is measured, rather than the time interval between transmission and detection. The ratio of the distance between the two detectors to this time interval gives the speed of the compression wave. Since the compression wave travels approximately equal short distances in the sounding fluid before reaching the two detectors, the time interval between the detections made by the two detectors constitutes a more precise measure of the real travel time. in the formation. Consequently, the use of two detectors and the measurement of the time between the detections carried out by these two detectors gives a more precise speed of the compression wave. Other parasitic effects such as variations in the borehole size and a tilting of the probe can be reduced by conventional devices. One of these devices is described in Log Interpretation, volume 1 - Principles, Schlumberger Limited, New York, NY 10017, 1972 edition, pages 37-38.

It is well known that a shear wave velocity log can also provide important information for the production of oil and gas from underground formations. The ratio of the speed of the shear wave to the speed of the compression wave can reveal the rock lithology of the underground formations. The shear wave velocity diagram can also be used to convert time sections of seismic shear waves into depth sections. The diagram of a shear wave is also useful for determining other

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important features of terrain formations, such as porosity, fluid saturation and the presence of fractures.

The conventional source of compression wave logging and the compression waves it produces in the survey fluid are symmetrical with respect to the axis of the logging probe. When such compression waves are refracted in the surrounding subsoil formation, the relative amplitudes of the refracted shear and compression waves are such that it is difficult to distinguish the last arrival of shear wave from the previous arrival of a compression wave and reverberations caused, in the sounding, by refraction of the compression wave in the formation. It is therefore difficult to use a conventional symmetrical source of compression waves to carry out a logging of shear wave speed. Correlation techniques were used to extract the arrival of the shear wave from the complete train of recorded acoustic waves. Such techniques, however, usually require data processing by the use of a computer, so that shear wave speeds cannot be used for direct diagramming. It can also be difficult to extract the arrival of the shear wave if it is close, in time, to the arrival of the compression wave.

Asymmetric sources of compression waves have been developed to carry out a shear wave velocity log. By using such sources, the amplitude of the arrival of the shear wave can be given a value substantially greater than that of the arrival of the compression wave. By adjusting the level of^; triggering of the detection and recording devices in order to discriminate with respect to the arrival of the compression wave, the arrival of the shear wave is detected as the first arrival. It is thus possible to determine the travel time of the

07486 shear waves in the formation and therefore the speed of the shear waves. Asymmetric sources are described in European patent application No. 31989 and in United States patents No. 3,593,255 and No. 4,207,961.

It is described in application No. 31989 a source of the bending type which comprises two circular piezoelectric plates linked to each other and fixed by their perimeters to a logging probe. When a voltage is applied to the two piezoelectric plates, the central part of the circular plates vibrates to generate a positive compression wave in a first direction and a simultaneous negative compression wave in the opposite direction. The two compression waves interfere to produce a shear wave in the terrain surrounding the borehole. This shear wave has frequencies which are inversely related to the diameter of the circular plates. This diameter cannot exceed the diameter of the logging probe, which is itself limited by the diameter of the borehole. Due to this limitation, the shear waves produced by the bending type source described in the aforementioned application No. 31989 have a limited frequency range, and the source may be ineffective in producing low frequency shear waves.

The aforementioned patent No. 3,593,255 describes an asymmetric source comprising two piezoelectric segments each having the shape of a hollow half-cylinder. The two segments are assembled to form a split cylinder. They have opposite polarizations and an electrical voltage is applied to each segment, so that a first segment expands radially and that, simultaneously, the other segment contracts radially, thus producing a positive wave, of compression in a first direction and a simultaneous negative compression wave in opposite direction. The two compression waves interfere to produce a shear wave in, forming the surrounding terrain. The frequencies of the shear wave thus j

07486 produced are in inverse relation to the radius of the split cylinder. Since this radius cannot exceed the radius of the logging probe which itself is limited by the radius of the borehole, this apparatus is not suitable for the production of low frequency signals suitable for wave logging shear.

In the apparatus described in the aforementioned patent No. 4,207,961, windings mounted on a coil assembly are placed in the magnetic field of a permanent magnet 10 and a current is circulated in the windings to control the assembly coil. The movement of the coil ejects a volume of water in a first direction and simultaneously sucks an equivalent volume of water in the opposite direction, thus producing a positive compression wave in a first direction and a simultaneous negative compression wave in the opposite direction. . However, the asymmetric source described in this patent cannot be controlled at high frequencies or with sufficient power, required for harder formations. In addition, it cannot operate at great depths or under high pressures.

In another type of shear wave log source, instead of coupling the source to the borehole wall using the borehole fluid, the source 25 is coupled either directly to the borehole wall or by the through mechanical means such as mounting pads. Such sources of shear wave logging are described in United States Patent Nos. 3,354,983 and 3,949,352.

The invention relates to an apparatus comprising a body designed to be mounted and lowered in a well containing a fluid, an elongated element having two ends, at least one of which is connected to the body. The apparatus according to the invention also comprises means intended to make the unconnected part of the element vibrate, in a direction substantially perpendicular to its length, in order to generate in the fluid a positive compression wave in a first direction and a negative wave^r'486 simultaneous compression in opposite direction. The two compression waves interfere to produce a bipolar shear wave in the terrain surrounding the well. The arrival of the bipolar shear wave is detected by detection means arranged in at least one, and preferably two positions in the fluid, spaced from the element, along the well. From the time interval between the detections in the two positions of the arrival of the shear wave, one can determine the speed of the shear wave in the ground surrounding the well.

An increase in the length of the element causes a decrease in the frequencies of the shear bipolar wave produced. By using a low frequency bipolar source to log shear wave velocities in soft ground formations, the signal ratio of the bipolar shear wave to the noise of the compression wave is improved.

Two substantially identical plates, placed so as to be adjacent to each other in a well containing a fluid, so that their flat surfaces are substantially parallel to each other, are vibrated substantially by the same way, but in such a way that the vibrating parts of the two plates approach each other and move away from each other in an almost simultaneous manner. Such vibrations generate in the fluid a Stoneley wave which is detected in two positions spaced from each other and from the two plates, along the well. From the time interval between the detections carried out in the two positions, the speed of the Stoneley wave is determined. The speed of the shear wave can be obtained from the speed of the Stoneley wave. .

The invention will be described in more detail with reference to the accompanying drawings by way of non-limiting examples and in which:

- Figure 1 is a schematic elevation of an acoustic logging apparatus according to the invention;

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- Figure 2 is a simplified perspective view of a bipolar shear wave logging apparatus corresponding to an embodiment of the invention;

- Figure 3 is a partial section of the bipolar shear wave apparatus, taken along line 3-3 of Figure 2; and

- Figure 4 is a partial section, similar to that of Figure 3, of a source of logging bipolar shear waves, illustrating the preferred embodiment of the invention.

Figure 1 is a schematic view of an acoustic logging apparatus according to the invention. A logging well 10 is designed to be raised and lowered in a well. The probe contains a source 12 of bipolar shear waves and two detectors 14, 16. To carry out a logging, the probe 10 is suspended in a fluid 18 contained in a borehole 20 which is surrounded by a formation 22 of terrain. The detectors 14 and 16 are connected to the probe 10 so as to be spaced from each other and from the source 12, along the borehole 20. The source 12 is connected to a device 24 for controlling the firing and d 'recording. Although the firing and recording control device is shown in FIG. 1 as a device separate from the logging probe, the part of the device which supplies energy to the source of bipolar shear waves can, for reasons of convenience, be housed in the logging probe. The signals recorded by the detectors 14 and 16 are applied to a bandpass filter 26, to an amplifier 28 and to a device 30 for calculating time intervals.

As described below, the firing and recording control device is used to ignite the source 12 which then produces a bipolar shear wave in the formation 22. The arrival of the bipolar wave shear is detected by detectors 14 and 16. The probe 10 is also a pre9

07486 amplifier (not shown in FIG. 1) which amplifies the arrival of the bipolar shear wave detected by detectors 14 and 16. The amplified signals are then filtered by filter 26 and again amplified by amplifier 28. L time interval entered the detection of arrival by the detector 14.and its detection by the detector 16 is then measured by the time interval device 30. This time interval can be stored or displayed as desired.

FIG. 2 is a simplified perspective view of a device for logging bipolar shear waves, illustrating an embodiment of the invention. As shown in Figure 2, the logging probe 10 includes a number of hollow cylindrical sections. The upper section 32 contains the source 12 for logging bipolar shear waves of FIG. 1 and it has two opposite windows 42 which allow the compression waves produced by the source 12 to easily propagate through them to pass into the fluid. of the survey. Sections 34> and 36 containing the detectors 14 and 16 of FIG. 1 are arranged below the source 12 and also have windows 44 and 46, as shown in FIG. 2. The combined compression waves produced by the source 12 propagate through the windows 42 and the borehole fluid 18 to reach the wall of the borehole 20. A portion of these combined compression waves is refracted in the formation 22 of land in this form of a bipolar shear wave. After this shear wave has traveled a certain distance through the formation, parts of said wave are returned by refraction in the borehole fluid 18 in order to reach the detectors 14 and 16 via the windows 44 and 46 respectively. The time interval between the detections carried out by the detectors 14 and is then measured as known, described.

FIG. 3 is a partial section of a device for logging bipolar shear waves, according to

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1 ° line 3-3 of FIG. 2. As shown in FIG. 3, the source 12 for logging bipolar shear waves comprises two elongated piezoelectric plates having two ends connected together by their flat suffaces, to form a composite plate

12. Each of the two plates is polarized approximately perpendicular to its flat surface and the polarizations of the two plates are of substantially opposite directions. One end of the composite plate 12 is clamped tightly between two clamping plates 54 which hold the composite plate in position while it vibrates. The outwardly facing flat surfaces of the composite plate 12 are connected by wires 56 to the firing and recording control device 24, by means of a polarity switch 58. The fire and recording control device applies an electrical pulse through the composite plate 12. It is well known that, if an electrical pulse is applied through the two flat surfaces of an element consisting of two piezoelectric plates of opposite polarities, it causes this element to bend. If the polarity of the pulse is applied as shown in FIG. 3, the loose part of the elongated composite plate 12 flexes in the direction of the arrow 60 shown in FIG. 3.

When the non-fixed part of the composite plate 12 bends in the direction of arrow 60, it generates a positive compression wave, in the same direction, and a simultaneous negative compression wave, in the opposite direction. A part of the combined compression waves, resulting from the interference of the two compression waves, is refracted in the formation of the ground 22 in the form of a bipolar shear wave. The two compression waves interfere so that the compression wave refracted in the underground formation, then detected by the detectors 14 and 16, has an amplitude substantially lower than that of the refracted bipolar shear wave. Therefore, by adjusting the

07486 trigger level of the time interval measuring device 30, the bipolar shear wave is detected as the first arrival.

For greater efficiency, the flat surfaces of the source 12 are preferably parallel to the axis of the well, although other orientations are acceptable. Although the polarizations of the composite plate 12 are as shown in FIG. 3 and are preferably perpendicular to its flat surfaces, it is obvious that, for the signals to vibrate the plate 12, it is sufficient that the polarizations are such that 'they have components oriented in the direction of the electric field of the pulse applied through the compound plate. After the application of the signal formed by the electrical pulse, the non-fixed part of the plate 12 continues to vibrate. The extreme positions taken by the plate 12 during its vibration are shown in dashed lines in FIG. 3.

The composite plate, comprising the two piezoelectric plates of opposite polarities, is readily available commercially. Compound piezoelectric plates supplied by the Vernitron Company, Bedford, Ohio, known as the Bender Bimorphs have worked. The commercially available composite piezoelectric plates which can be used for the purposes of the invention are usually sold in the form of two piezoelectric plates connected to each other by a conductive layer which is caught between the two plates . The outer flat surfaces of the composite plates are also usually coated with conductive layers.

These conductive layers are shown at 62 in FIG. 3. Such conductive layers allow the application of the electrical pulse through the compound plate in order to produce more uniform electric fields in such a plate, and they improve the efficiency of the log source. If the two piezoelectric plates constituting the compound plate 12

07486 are polarized in substantially the same direction, to cause bending of the composite plate, the electrical impulse must be applied between the intermediate conductive layer and the two outer and visible conductive layers. The space surrounding the source 12 and enclosed by the cylindrical section 32 is filled with oil 64. The upper part of section 32 is filled with a support material 66 which advantageously has good damping qualities in order to eliminate by damping the reverberations of the source 12 so that the compression waves produced by the latter are of a short time. A diaphragm 68 hermetically separates the oil 64 from the lower part of the section 32 which can be filled with air.

As shown in FIG. 3, the detector 14 is of an embodiment similar to that of the source 12. The section 34 and its mode of connection to the detector 14 are similar to the section 32 and to its connection to the source 12. The the only difference is that, instead of being connected to a firing circuit, the visible external conductive layers of the detector 14 are connected to a bandpass filter 26. The bandpass filter is then connected to an amplifier and a time interval measuring device as shown in Figure 1.

The flat surfaces of the detector 14 are preferably substantially parallel to the flat surfaces of the source 12 and the directions of polarization of the detector 14 are substantially parallel to the directions of polarization of the source 12. Thanks to this orientation, the arrivals of the detected shear waves by the detector 14 have an optimal amplitude. It should be noted that other orientations can be used provided that the flat surfaces of the detector 14 are not. not perpendicular to the flat surfaces of the source 12. With such orientations, the detector 14 detects the arrival of a shear wave. To achieve greater efficiency, the flat surfaces of the

07486 source 12 and detector 14 are preferably parallel to the axis of the well, although other orientations may also occur. To improve the sensitivity of the detection, the non-fixed end of the detector 14 must be directed towards the source 12 and the non-fixed end of the source 12 must be directed towards the detector 14. The detector 16 and the section 36 (not shown in FIG. 3) are of the same embodiment as the detector 14 and the section 34 and they are arranged below the detector 14.

Although detectors 14 and 16 are shown as detectors of the bipolar type, it is obvious that other detectors can be used. For example, it is possible to use a geophone whose axis 15 forms an angle with the flat surfaces of the source 12, preferably an angle of 90 °.

FIG. 4 is a partial section of a source of logging bipolar shear waves, illustrating the preferred embodiment of the invention.

As shown in Figure 4, a cylindrical section 80 contains a source 82 of shear bipolar wave log, which includes two pairs of elongated piezoelectric plates 84 and 86 each having two ends. The two pairs 84 and 86 are each connected to the hollow cylindrical section 80 by being clamped tightly, at their two ends, between clamping plates 88. The two pairs of piezoelectric plates 84 and 86 are similar in construction to that of the pair of piezoelectric plates 12 in FIG. 3. Thus, the visible flat surfaces of the two pairs are each coated with a conductive layer, the flat surfaces outside of the two pairs being indicated respectively at 84a, 84b and 86a, 86b in FIG. 4. The recording and recording control device 24 applies a signal consisting of an electrical pulse through each of the two pairs 84 and 86, by means of wires 90 which are connected to the layers

07486 conductive covering the two pairs. As in the embodiment described above, this signal acts on the two pairs so that they each produce a positive compression wave in a first direction and a negative compression wave in the opposite direction, these two waves interfering to produce a bipolar shear wave in the field formation. If the electrical signal applied through each of the two pairs 84 and 86 is such that the surface 84a is at an electrical potential greater than that of the surface 84b and that the surface 86a is at an electrical potential greater than that of the surface 86b, the bipolar shear waves produced by the two pairs are then added to generate in formation 22 a stronger bipolar shear wave. The distance between the two pairs of plates and the space above and · below the two pairs of plates and enclosed by the cylindrical section .80 are filled with a support material 92 which has good qualities of damping so that the compression waves produced by the vibration of the two pairs of plates are of short duration. The compression waves produced by the two pairs of plates are transmitted through oil 94 and through windows 96 to the borehole fluid 18. The arrival of the shear wave is detected by detectors, in the same way as in the embodiment described above. Since the two ends of each of the two pairs of elongated piezoelectric plates 84 and 86 are clamped on the part 80, only the middle part of the two pairs of plates can vibrate freely. The extreme positions of the two pairs of plates during vibration are illustrated in phantom in Figure 4.

As described above, the source of bipolar shear waves according to the invention can be used for the online production of shear wave velocity logging if the amplitude of the arrival of the shear wave is significantly greater.

07486 s " ¹⁰ to that of the arrival of the compression wave. However, this is only so if the frequencies of the bipolar shear wave produced in the surrounding sounding ground are included in certain ranges. For any terrain formation, there is a preferred range of frequencies for logging its shear wave speed, so that the arrival of the shear wave is significantly stronger than the arrival of the wave. compression. This preferred frequency range varies with the speed of the shear wave in the logging formation. Thus, if the approximate range of shear wave velocities in the formation is known, a preferred range of frequencies can be chosen. In the case of a 25 cm diameter well, the preferred frequency ranges are given in the table below.

Approximate range of shear wave speeds

Preferred frequency range

<td> 1500 -</td><td> 1800</td><td>m / s</td><td> 1,5</td><td> - 7,5</td><td>kHz</td>

<td> 1800 -</td><td> 2100</td><td>m / s</td><td> 3,0</td><td> - 12</td><td>kHz</td>

<td> 2100 -</td><td> 2400</td><td>m / s</td><td> 4,0</td><td> - 16</td><td>kHz</td>

<td> 2400 -</td><td> 2700</td><td>m / s</td><td> 4,5</td><td> - 20</td><td>kHz</td>

If a source of bipolar shear waves produces bipolar waves having strong components at frequencies between 4.5 and 7.5 kHz, such a source then operates in the preferred frequency ranges corresponding to the entire range of shear wave speeds, from 1500 to 2700 m / s.

The approximate range of shear wave velocities in a formation should be estimated by a conventional method, for example by measuring the speeds of compression waves in the formation. The speed of the shear wave is approximately equal to half the speed of the pressure wave. From the measured speeds of the compression waves, the approximate range of the speeds of the shear waves can be evaluated.

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The preferred frequencies vary inversely with the diameter of the well. Therefore, in the case of a well of d cm diameter instead of 25 cm, the preferred frequency ranges, given in the table above, should be multiplied by a factor of 25 / d.

The frequencies of the bipolar shear waves produced in the formations of grounds by the sources 12 and 82 of waves depend on the lengths of the elongated piezoelectric plates and the thicknesses of the pairs of plates in the direction of the vibration. In the frequency spectrum of such a bipolar shear wave, the frequency f corresponding to the peak amplitude is given by;

f = K (t / 1²) where K is constant rine t is the thickness of the pair of piezoelectric plates in the direction of the vibration, and is the length of the pair of plates.

For a source of bipolar shear waves comprising two elongated piezoelectric plates of 12.7 cm in length, 4.76 mm in width and .0.51 mm in thickness, the maximum frequency of the peak amplitude may be around 2 kHz. This frequency of the peak amplitude can be increased or decreased by varying the thickness and length of the pair of piezoelectric plates, in accordance with the above equation. The frequency can be increased by increasing the thickness or by decreasing the length of the pair of piezoelectric plates, and it can be lowered by decreasing the thickness or increasing the length of this pair of plates. Thus, if the length of the pair of plates in the example above is reduced from 12.7 to 10.2 cm, the frequency is increased from 2 kHz to approximately 3.75 kHz. Since the frequency corresponding to the peak amplitude is inversely proportional to the square of the length, this

07486 frequency is very sensitive to variations in the length of the pair of piezoelectric plates. A source of bipolar shear waves, having a length enabling it to produce bi5 polar shear waves at the desired frequency for the peak amplitude, can be chosen if the approximate range of shear wave velocities is known.

If the preferred range of frequencies for the terrain formation is known, piezoelectric plates of suitable dimensions, producing bipolar shear waves having frequencies within this range, can be chosen. Thus, if the length and thickness, in the direction of the vibration of the plates are such that the frequency corresponding to the peak amplitude is within such a preferred range of frequencies, the arrivals of shear waves produced by such plates then have amplitudes significantly greater than those of the arrival of the compression waves also produced by the plates. Since the frequency corresponding to the peak amplitude is particularly sensitive to the length of the piezoelectric plates, this frequency can be easily chosen by selecting piezoelectric plates of appropriate length. This length is not limited by the diameters of the logging probe and the borehole. Low frequency shear bipolar waves can be produced using plates of sufficient length. Such low frequency bipolar shear waves can be important for the study by logging of formations having low shear wave velocities. Thus, the elongated shape of the device according to the invention allows it to efficiently produce bipolar shear waves at low frequency.

As previously described with reference to the figure

3, the positive and negative compression waves produced by the source 12 interfere to produce in the formation 22 a bipolar shear wave. The₁₈ 07486 two compression waves produced are usually not, in practice, exactly in phase opposition. Consequently, their interference also generates a compression wave in the formation 22 and surface waves such as Stoneley waves which are maximum at the interface of the fluid and the borehole. The arrivals of compression waves and Stoneley waves are also detected by detectors 14 and 16 in the form of noise. This noise can be reduced as follows. A first logging recording is carried out using the arrival of the shear wave as described above. This recording contains, in the form of noise, the arrivals of the compression waves and of Stoneley. The polarity of the electrical signals applied by the source 12 is then reversed by means of the polarity switch 58 and a second log recording of the arrival of the shear wave is made. Given the reversal of polarity of the electrical signal, the non-fixed end of the pair of plates flexes in the opposite direction to the direction 60 in order to generate a positive compression wave in the direction of movement and a negative compression wave in the direction opposite. The two compression waves thus produced interfere to give formation 22 a bipolar shear wave which is of opposite polarity to that produced during the first recording. The compression wave and the Stoneley wave produced during the second recording, however, do not have their polarity reversed. Therefore, subtracting the two log records reduces the noise due to compression waves and Stoneley waves. If the two log records are added instead of being subtracted, the arrival of the bipolar shear wave virtually disappears , but the arrivals of compression waves, Stoneley and others remain. In the preferred frequency ranges for logging bipolar shear waves, the arrivals of Stoneley waves dominate the sum of the records. Therefore, the wave of

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Stoneley appears as the first arrival in the sum of the two records. It is well known that the speed of the shear wave of a formation can be derived from the speed of the Stoneley wave of this formation. The way in which this result can be achieved can be known from the general description of Biot, MA 1952, Propagation of Elastic Waves in a Cylindrical Bore Containing a Fluid, Journal of Applied Physics, volume 23 / pages 997-1005. Another method of measuring the speed of shear waves in formation 22 is a log 10 of the Stoneley wave speed.

Noise due to compression and Stoneley waves is also reduced by rotating, between two logging records, the shear wave source 12 through 180 ° so that the flat surfaces of the source are rotated in substantially opposite directions to what they had before the rotation. The latter reverses the polarity of the bipolar shear waves, but not that of the compression and Stoneley waves produced.

Therefore, subtracting the two log records reduces noise due to compression and Stoneley waves. As before, in the sum of the two logging records, the Stoneley wave 25 appears as the first arrival.

When two pairs of piezoelectric plates are used as in the preferred embodiment shown in Figure 4, there is no need to add and subtract the log records and Stoneley wave speed can be used directly online for a log. Thus, by flipping the polarity switch 110, polarities opposite to the electrical pulses applied by the firing and recording control device 24 are given to the two pairs of piezoelectric plates 84 and 86. Consequently, if such a pulse gives at surface 86a an electrical potential greater than that of surface 86b after the switch!

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110 has been fired, the pulse applied to the pair of plates 84 gives the surface 84b an electrical potential greater than that of the surface 84a. After the assembly indicated above, electrical pulses applied by the shot control and recording device 24 act on the surfaces 84a and 86b so that, roughly simultaneously, they move outwards, in the direction of the windows 96, and contract away from the windows. The bipolar component · of the acoustic waves produced by the two pairs 84 and 86 vanishes substantially and the only significant arrivals detected are the arrivals of Stoneley waves. Therefore, the Stoneley wave speed can be used directly as a log.

To make a log of Stoneley wave speeds, the preferred embodiment of FIG. 4 has a better signal-to-noise ratio than that of conventional devices using hollow cylindrical sources of compression waves. Such conventional sources normally operate at a frequency of about 15 to 20 kHz. At such frequencies, compression waves, shear waves and guided waves are not negligible compared to waves^;de Stoneley produced by the same classical source. The operating frequencies of such conventional sources are in inverse relation to the diameter of the source. Such frequencies cannot be significantly lowered by increasing the diameter of the source, since the latter cannot exceed the diameter of the probe. The elongated configuration of the preferred embodiment of the invention allows it to operate at frequencies much less than 15 kHz. At lower frequencies such as 4 to 7 kHz, the compression waves or other waves produced are negligible compared to the Stoneley waves.

The shear wave logging apparatus illustrated in Figure 4 can also be used to determine a terrain formation surrounding a

07486 well is anisotropic. Some formations are composed of thin horizontal layers of different materials so that the shear wave traveling in the vertical direction may have a different speed from that of the shear wave traveling in a horizontal direction. Since a shear wave is a transverse wave, it can vibrate in a plane of polarization. A horizontally propagating shear wave can have a vertical or horizontal plane of polarization or any other intermediate angle. The speed of a horizontally propagating shear wave, with a horizontal plane of polarization, can be obtained from the speed of the Stoneley wave measured in a substantially vertical well. The procedure to be followed to achieve this result is described by White, JE and Sengbush, RL in Velocity Measurements in Near-Surface;

Geophysics vol. 18, pages 64-69, 1953. The speed of the shear wave in a formation, described above, is the speed propagating roughly vertisi such speed differs from the speed of the wave measured directly as of the shear wave firmly. Therefore, from that obtained from

Stoneley in training, the latter is anisotropic.

It goes without saying that numerous modifications can be made to the apparatus and to the method described above, in particular as regards the shapes, dimensions, materials or other details, without departing from the scope of the invention.