NO833903L - PROCEDURE AND DEVICE FOR LOGGING A DRILL - Google Patents

Description

This invention relates to borehole logging in general and more particularly to acoustic shear wave borehole logging.

In borehole acoustic logging, it is common to measure the compression wave velocity of soil formations surrounding boreholes. A conventional compression wave velocity logging system comprises a cylindrical logging probe that is immersed in a borehole fluid, a source connected to the probe to generate the compression wave in the borehole fluid, and one or more detectors connected to the probe and separated from the compression wave source to detect compression waves in the borehole fluid. A compression fluid in the borehole fluid generated by the source is deflected into the soil formation surrounding the borehole. It propagates through part of the formation and is deflected back into the borehole fluid

at a point that is close to the detector and is then detected by the detector. The ratio between the distance between the source and the detector and the time between generation and detection of the compression wave gives the compression wave velocity of the formation. The distance between source and detector is usually fixed and known, so that measuring the time between compression wave generation and detection is sufficient to determine the compression wave velocity of the formation. To achieve greater accuracy, such a distance is usually much larger than the dimensions of the source or detector. Information that is important for producing oil and gas from underground soil formations

tions can be derived from the compressional wave velocities of such formations.

When a compression wave generated by a compression wave source in the borehole fluid reaches the borehole wall, it produces a deflected compression wave in the surrounding soil formation as described above. In addition, it also produces a deflected shear wave in the surrounding soil formation, and guided waves that propagate in the borehole fluid and the part of the formation that lies close to the borehole.

Part of such a shear wave is deflected back into the borehole fluid in the form of a compression wave and reaches the detector in the logging probe. The guided waves are also detected

tert of a detector. Any wave that is one of the three types of waves detected by the detector can be called an arrival: the compression waves in the borehole fluid caused by deflection of compression waves in the formation can be called the compression wave arrivals, those caused by deflection of shear waves in the formation can be called the shear wave arrivals and these caused by guided waves can be called the guided wave arrivals. Thus, the signal detected by the detector is a composite signal comprising the compression wave arrival, the shear wave arrival and the shock wave impacts. Compression waves propagate faster than shear waves

and shear waves usually propagate faster than the guided waves. Therefore, in the composite signal detected by the detector, the compression wave arrival is the first arrival, the shear wave arrival is the second arrival and the guided wave arrivals are the last arrivals. By measuring the compressional wave velocity of the formation, the time interval between the generation of the compressional waves and the detection of the first arrival detected by the detector gives the approximate propagation time of the deflected compressional wave in the formation. Consequently, the later shear wave and shock wave arrivals will not affect the measurement of compressional wave velocity in the formation.

In addition to propagating over a vertical distance in the formation approximately equal to the distance between the source and the detector, the compression wave also propagates over short distances in the fluid. The extra time required to propagate over such short distances introduces error into the velocity log. To reduce such errors, conventional logging devices use at least two detectors that are separated vertically from each other along the borehole. The time interval between detection by the two detectors is measured instead of the time interval between transmission and detection. The ratio between the distance between the two detectors and such time interval gives the compression wave speed. Since the compression wave propagates over approximately equally short distances in the borehole before it reaches the two detectors, the time interval between detection by the two detectors is a more accurate measure of the real propagation time in the formation. Using two detectors and measuring the time between detection of the two detectors therefore gives a more accurate compression wave speed. Other spurious effects such as changes in borehole size and probe tilt can be reduced using conventional devices. Such an arrangement is described in Log Interpretation, Volume 1 - Principles, Schlumberger Limited,

New York, N.Y. 10017, 1872 Edition, pages 37-38.

It is well known that shear wave velocity logging can also

provide information that is important for producing oil and gas from underground soil formations. The relationship between the shear wave velocity and the compression wave velocity can reveal the rock lithology of the underground soil formations. The shear wave velocity log can also make it possible to transform seismic shear wave time sections into depth sections. The shear wave log is useful in determining other important properties of soil formations such as shear stress, porosity, liquid saturation and the presence of cracks. The shear wave log can also be useful for determining the state of stress around the borehole, which is very important when designing hydraulic fracture treatment devices.

The conventional compression wave logging source and the compression waves it generates in the borehole fluid are symmetrical about the logging probe axis. When such compressional waves are deflected into the surrounding soil formation, the relative amplitudes of the deflected shear and compressional waves are such that it is difficult to distinguish the latest shear wave arrival from the early compressional wave arrival,

and from the blowbacks in the borehole caused by deflection of the compressional wave in the formation. It is therefore difficult to use a conventional symmetrical compression wave source for logging shear wave velocity. Correction techniques have been used to extract the shear wave arrival from the complete acoustic wave train recorded. However, such techniques usually require processing of data using a computer, so that the shear wave velocities cannot be logged continuously. It can also be difficult to extract the shear wave arrival if it is close in time to the compression wave arrival.

Asymmetric compression wave sources have been developed to log shear wave velocity. When using such sources, the amplitude of the shear wave arrival can be significantly higher than the amplitude of the compression wave arrival. By adjusting the trigger level of the detection and recording systems to discriminate against the compression wave arrival, the shear wave arrival is detected as the first arrival. It may thus be possible to determine the propagation time of shear waves in the formation and therefore the shear wave velocity. Such asymmetric sources each generate in the borehole fluid a positive compression wave in one direction and a simultaneous negative compression wave in the opposite direction. The interference between the two compression waves can ensure that the shear wave arrival is stronger than the compression wave arrival. Asymmetric sources are shown by Agona and Co., EPC Patent Application No. 31989, White, U.S. patent No. 3,593,255

and Kitsunezaki, U.S. Patent No. 4,207,961.

The Angona patent application shows an inverted-r type source comprising two circular piezoelectric plates glued together and attached to a logging probe. When voltage is applied across the two piezoelectric plates, the plates will bend. The bending of the transducer plates creates a positive compression wave in one direction and a simultaneous negative compression wave in the opposite direction. The White patent shows a compression wave source comprising two piezoelectric segments, each in the form of half a hollow cylinder. The two segments are assembled to form a split cylinder. The two segments have opposite polarization and electrical voltage is applied to each segment causing one segment to expand radially and at the same time causing the other segment to contract radially, thereby producing a positive compression wave in one direction and a simultaneous negative com- pressure wave in the opposite direction. In the Kitsunezaki patent, coils are mounted on a coil device and placed in the magnetic field of a permanent magnet and current is fed through the coils to drive the coil device. The movement of the spool device ejects a quantity of water i

one direction and at the same time sucks an equivalent volume of water in the opposite direction, thereby generating a positive pressure change in one direction and a simultaneous negative pressure change in the opposite direction.

The method and device according to this invention are used to log the shear wave velocity of an earth formation surrounding a well or borehole. The method according to the invention comprises sending a 2n<->pole shear wave through the earth along the well where n is an integer greater than 2, and detecting the 2n<->pole shear wave arrival at at least one point longitudinally separated along the well from the transmission point. If the shear wave arrival is detected at two points, the time delay is

time between the detections at the two points measured to determine the shear wave velocity in the soil surrounding the well. If the shear wave arrival is detected only at one point, the time interval between transmission and detection of the shear wave signal is measured to determine the shear wave velocity in the earth. The device according to the invention comprises a housing adapted to be raised and lowered

down a well, signal generating means in the housing for sending a 2n<->pole shear wave into the soil formation surrounding the well where n is an integer greater than 2, and signal detecting means in the housing longitudinally spaced along the well from the signal generating means for detecting the arrival of such a shear wave.

The method and device are specified in more detail in the subsequent patent claims.

The invention will now be described with reference to the drawings, where

figure 1 schematically shows an acoustic logging system according to the invention;

Figure 2 is a simplified perspective drawing of an octopole shear wave logging device and shows the preferred embodiment of the invention; Figure 3 shows a cross section of the octopole shear wave log source in Figure 2 taken along the line 3-3; Figure 4 is a simplified perspective drawing of the octopole shear wave logging device of Figures 2 and 3, showing the orientation of the detectors in relation to the octopole source, and the electrical connections to the source and detectors; Figure 5 is a cross-section of an octopole shear wave log source, illustrating an alternative embodiment of the invention; Figure 6 is a cross-section of an octopole shear wave log source and illustrates yet another alternative embodiment of the invention; and Figure 7 is a cross-section of an octopole shear wave log source, showing yet another alternative embodiment of the invention. Figure 1 schematically shows an acoustic logging system according to the invention. A logging probe 10 is adapted to be raised and lowered into a well. The probe contains a multi-pole shear wave source 12 and two detectors 14, 16. To start logging, the sod 10 is immersed in a fluid 18 in a borehole 20, which is surrounded by an earth formation 22. Detectors 14, 16 are thus connected to the probe 10 that they are separated longitudinally from each other along the borehole 20 and from the source 12. The source 12 is connected to a firing and recording control unit 24. Although the firing and recording control unit is shown in Figure 1 as a unit that is separate from the logging probe, the part of the unit that provides energy to the multipole shear wave source, for appropriate operational reasons, be arranged in the housing of the logging probe. Signals recorded by the detectors 14 and 16 are fed to a bandpass filter 26, an amplifier 28 and a time interval unit 30. In a manner described below, the firing and recording control unit is used to fire the source 12 which produces a shear wave in the formation 22. The shear wave arrival is detected by the detectors 14 and 16. The probe 10 also contains a pre-amplifier (not shown in figure 1) which amplifies the shear wave arrival detected by the detectors 14 and 16. The amplified signals are then filtered by the filter 26 and amplified again by the amplifier 28. The time interval between the detection of the arrival of the detector 14 and the detection of the detector 16 is then measured by the time interval unit 30. Such a time interval can be stored or displayed as desired.

Figure 2 is a simplified perspective drawing of an octopole ice wave logging device and illustrates the preferred embodiment of the invention. As shown in figure 2, the loff probe 10 comprises a number of hollow cylindrical sections. The top section 32 contains an octopole shear wave log source (not shown in Figure 2), and has 6 windows 42 which allow the compression waves generated by the source to propagate therethrough into the borehole fluid. Sections 34 and 36

each containing a detector (not shown), are disposed below the source and also have windows 44 and 46 as shown in Figure 2. The combined compressional waves generated by the source in section 30 propagate through windows 42 and borehole fluid 18 to reach the wall of the borehole 20. A part of such combined compression waves is de-

bent into the soil formation 22 in the form of a shear wave. After such a shear wave has propagated some distance through the formation, parts of it are deflected back into the borehole, into the borehole fluid 18, and reach the detectors in sections 34 and 36 through windows 44 and 46 respectively. The time interval between the detections of the two detectors is then measured as described.

The terminology for the multipole is based on successive powers of two, that is, 2n, where n is an integer, and n = 1, 2, 3, and so on. Thus, the multipoles include the dipole (n = 1), the quadrupole (n = 2) and the octopole (n = 3). The terminology for higher order multipoles is based on

2n with n = 4, 5, 6 and so on.

Figure 3 is a cross-section of the octopole shear wave source in Figure 2 taken along line 3-3. Six substantially equal sectors 62, 64, 66, 68, 70 and 72 of a radially polarized piezoelectric hollow cylinder are spaced so as to be substantially coaxial and to surround a common axis. Substantially the same electrical pulse is impressed across the cylindrical surfaces of each sector substantially simultaneously, so that the pulses supplied to any two adjacent sectors are opposite in polarity. This arrangement is shown in Figure 3. With such an arrangement, if one sector of the electric pulse is caused to expand radially, the two adjacent sectors to it will contract radially and vice versa.

If the six sectors are polarized radially outward,

then the directions of expansion and contraction will be as illustrated by the white arrows in figure 3. During contraction of a sector, its entire inner cylindrical surface will move inwards. During its expansion,

its entire outer cylindrical surface will move outwards. The combined compressional wave generated on

this way by the expansion and contraction of the six sectors, will deflect into the surrounding soil formation and generate an octopole shear wave. To detect the octo-pole shear wave arrival, the detectors can be of a construction similar to the octo-pole shear wave source illustrated in figure 3, or in figure 5, which will be described later.

The central space between the six sectors is filled with an annular body of support material 74 to dampen the back radiation of the vibrations from the six sectors so that the generated octopole shear wave is short in duration. This annular body 74 can be attached to the section 32 by means of a conventional device such as an inserted spindle 76 through the center of the body 74, and where on the two ends of the spindle are screwed two washers which fit snugly into the section 32. The the six sectors are arranged on the outer cylindrical surface of the body 74 and may be held in place by means of two annular members (not shown in Figure 3) of elastic support material which fit snugly over the six sectors.

The six sectors are so positioned on the section 32 that each sector faces one of the six windows 42, as shown in figure 3. The sector distance between the windows and the sectors is filled with oil 78. The vibrations of the six sectors will generate compression waves in the oil 78, which waves are sent through the window 42 to generate an octopole shear wave in the earth. The sector spacing between the oil-filled space is filled with support material 80 to dampen the back radiation of the vibrations of the six sectors. Figure 4 is a simplified perspective drawing of the octopole ice wave logging device in figures 2 and 3, and shows the orientation of the detectors in relation to the octopole source, and the electrical connections to the source and the detectors. In order to detect the compression wave in a borehole fluid caused by deflection of the octopole shear wave generated by the source, the detector 14 is preferably also an octopole detector of a construction similar to the source 12. The six sectors are so positioned that they have substantially the same axis as the six the sectors of the source 12, and so that they have essentially the same lateral positions around the common axis as the sectors of the source 12, in order to maximize the strength of the detected octopole signal.

The outer and inner cylindrical surfaces of the six sectors of the detector are connected to the bandpass filter 26 in a manner similar to the connections from the respective surfaces of the source 12 to the firing and recording control unit 24. The detector 16 is similar to the detector 14, but for clarity is not shown in Figure 5. In order to allow the six sectors of each of the two detectors to detect the octo-Polish ice wave arrival, each section 34, 36 in Figure 2 preferably has 6 windows 44 and 46 respectively. While the detector in figure 4 is shown structurally similar to the source in figure 3, it must be understood that detectors with a construction similar to the sources in figures 5, 6 and 7 (described below) can also be used. The six sectors or plates of each type of detector are preferably aligned laterally about the common axis, (ie azibutally) with the six sectors of the source to maximize the detected signal.

Figure 5 is a cross section of an octopole shear wave source,

and shows an alternative embodiment of the invention. Six elongated piezoelectric composite plates 82, 84,

86, 88, 90 and 92 are so spatially arranged that they essentially form the parallelograms of a hexagonal prism. Each of the six co-assembled plates includes two opposite ones

polarized piezoelectric plates that are glued together.

The six composite plates are attached to the section 32'

of the logging probe using two clamping plates (not shown in figure 5). Each of the two clamping plates has six slots into which the ends of the six composite plates are tightly inserted. The two clamping plates are then inserted into and attached to the section 32' in such a position that the elongated composite plates are substantially parallel to the log probe axis. The portion of each composite plate between the two ends will hereinafter be called "the unclamped portion" or "the unfastened portion". However, it must be understood that the six composite plates need not be attached to the probe at their ends. Attached at one end or somewhere between the two ends will suffice. The part of each plate other than the clamped part can thus be called "the unclamped part"

or "the unattached party".

Essentially the same electrical pulse is impressed across

the flat surfaces of each of the six composite plates essentially simultaneously. The pulses applied to any two adjacent composite plates are opposite in polarity, so that if the unattached portion of a composite plate bends and moves radially outward, then

the unattached portions of the two adjacent composite plates will bend and move radially inward. The directions of the bending movements of the six composite plates are shown by hollow arrows in Figure 5. The bending movement of each composite plate will generate a compression wave in the borehole fluid. The combined compressional wave generated by the octopole source will deflect into the formation surrounding the borehole and produce an octopole shear wave. To detect the octopole shear wave arrival in the borehole fluid, the detector 14 is preferably an octopole type which may be of a construction similar to the octopole source shown in

figure 3 or on figure 5. The outer surface of the combined

set plates of the detector 14 are connected to the bandpass filter 26 instead of the firing and recording control unit 24. The six sectors or plates of the detector are preferably aligned azimuthally with the six plates of the shear wave source in Figure 5.

The composite plates comprise a pair of oppositely polarized piezoelectric plates, and are commercially available. Piezoelectric composite plates supplied by the Vernitron Company of Bedford, Ohio, known as Bender Bimorphs have proven satisfactory. The six pietro-electric sectors of the type shown in Figure 3 and of the type shown in Figures 6 and 7, to be described hereinafter, are also supplied by the Vernitron Company.

Figure 6 is a cross section of an octopole shear wave source,

and shows another alternative embodiment of the invention.

In Figures 3 and 4, the six sectors of the octopole source are polarized radially. Alternatively, the six sectors may be circumferentially polarized as shown by the polarizations of sectors 102, 104, 106, 108, 110 and 112 in Figure 6. The six circumferentially polarized sectors are in a state known as the ring mode. The six sectors can be produced from a hollow cylindrical piezoelectric cylinder by cutting out six narrow longitudinal sectors. An electric pulse was applied across the side surface to each of the six sectors, so that the resulting electric field in each sector is essentially parallel to the polarization. The electrical pulse will cause each sector to expand or contract radially depending on the polarity of the pulse. If the sectors 102, 106 and 110 are polarized along the circumference clockwise, while the electric fields along the circumference have a counterclockwise direction as shown in Figure 6, the three sectors will contract radially, if the polarizations of and the electric fields in the sectors 104 , 108 and 112 are all along the circumference in a counter-clockwise direction, then the three sectors will expand radially.

Figure 7 is a cross-section of yet another alternative embodiment, and illustrates an octopole shear wave source in the ring mode. The six sectors 122, 124, 126, 128, 130 and 132 are six of the twelve longitudinal sectors of a piezoelectric hollow cylinder, and each of the twelve sectors has been polarized along the circumference. The elements adjacent to each other have opposite circumferential polarization. The six sectors 122, 124, 126, 128, 130 and 132 are the only sectors of the cylinder that will expand and contract, and are all circumferentially polarized in a clockwise direction. The connecting edges of adjacent sectors can be covered by a conductive layer (not shown). Electric pulses are applied so that the electric field in each of the six sectors is essentially parallel to the polarization. With the sector polarizations and pulse polarities as shown in Figure 7, sectors 122, 126 and 130 will expand radially, while sectors 124, 128 and 132 will contract radially. The remaining six sectors will not expand or contract since no potential difference is imposed across these sectors.

In both the preferred embodiment and the three alternative embodiments described above, piezoelectric materials are used to form the octopole shear wave source,

and the source is vibrated using electrical pulses.

However, it must be understood that other constructions of the source and other vibration devices can be used. Thus, purely mechanical devices can be used to vibrate the six sectors of the preferred embodiment,

and the six disks or sectors of each of the three alternative designs. An octopole shear wave will be generated as long as the sectors or plates become vi-

bred in the same manner as in the preferred and alternative embodiments.

The Octopol shear wave source according to the invention can be used for logging shear wave velocities continuously or on line (that is, the shear wave velocities can be determined without data processing) if the shear wave arrival is significantly larger in amplitude than the compression wave request. The shear wave arrival is significantly greater in amplitude than the compression wave arrival, only when the frequencies of the octopole shear wave produced in the earth environment of the borehole lie within certain frequency ranges.

For any soil formation, there is a preferred frequency range for logging the shear wave velocity such that the shear wave arrival is substantially stronger than the compression wave arrival. The preferred frequency range varies with the shear wave velocity of the formation to be logged. If the approximate range of the shear wave velocities in the formation is known, a preferred frequency range can be selected. For a well with a diameter of

ten inches are the preferred frequency ranges shown in the table below.

The approximate range of shear wave velocities in a formation can be calculated using conventional methods such as measuring the compressional wave velocities of the formation. The shear wave speed is approximately half of the compression wave speed. From the measured compression wave velocities, the approximate range for shear wave velocities can be calculated. The preferred frequencies vary inversely with the diameter of the well. For a well with a diameter of d inches instead of 10 inches, the preferred frequency ranges will therefore be given by these listed in the table on the previous page multiplied by a factor of 10/d.

Figures 3, 4, 6 and 7 show octopole shear wave sources using six sectors which are vibrated radially to generate octopole shear waves in soil formations. The frequencies of the octopole shear waves generated in this way vary inversely with the radii of the sectors. In order for the frequencies to be within the preferred frequency ranges listed above, it is preferable that the radii of the sectors are large. Therefore, the radii are preferably only slightly smaller than the radius of the logging probe. It will be understood that Figures 3, 4, 6 and 7 are not shown to the correct scale.

Higher order multipole sources can be constructed in a manner similar to the four embodiments of the octopole shear wave source shown in Figures 3, 5, 6 and 7. Thus, the 16-pole source can be constructed by spatially arranging 8 elongated piezoelectric composite plates to form the 8 parallelograms of an octagonal prism. Essentially the same electrical pulse is applied to each of the 8

with such a polarity that adjacent plates vibrate in essentially opposite phases. An alternative embodiment of the 16-pole source can be constructed by replacing the eight composite plates with eight substantially identical sectors of a radially or circumferentially polarized piezoelectric hollow cylinder. Essentially the same electrical pulse is applied to each sector, so that adjacent sectors vibrate with essentially opposite phases. Other methods for constructing and vibrating the plates and sectors can be used as long as the plates and sectors are vibrated on it. the same way. Other higher order multipoles can be constructed in one way

equal to the octopole and the 16-pole. The detectors used to detect the higher order of the shear wave arrival will preferably be of an order that is adapted to the order of the source.

The number of composite plates or sectors in the designs

of the octopole and 16-pole source as described above does not fit the terminology of the octopole and 16-pole sources. Thus the octopole source comprises 6 plates or sectors and the 16-pole source 8 plates or sectors. The 32-pole source comprises 10 plates or sectors. Thus, while the terminology of the multipole sources is based on 2n, where n is an integer, with n = 1, 2, 3.... , the corresponding number of plates or sectors is 2n. Thus a dipole (n = 1) source comprises 2 x 1 or 2 plates or sectors. A quadrupole (n = 2) source comprises 2x2 or 4 plates or sectors. An octopole (n = 3), a 16-pole (n = 4) and a 32-pole (n = 5)

source comprises 6, 8 and 10 disks or sectors respectively. In general, therefore, a 2n<->pole source will comprise 2n plates or sectors, where n is an integer, where n = 1, 2, 3 and so on.

The above description of the method and construction

which is used, is intended as purely illustrative, and various changes in shape, size, materials or other details regarding the method and construction will lie within the scope of the subsequent patent claims.

Claims (18)

1. Method for logging the earth surrounding a well, characterized in that it comprises and transmits from the well a 2n<->pole shear wave through the earth along the well where n is an integer greater than 2, and to detect 2n<->pole the shear wave arrival at at least one point longitudinally separated along the well from the transmission point. 2. Method according to claim 1, characterized in that it comprises measuring the time course between the transmission and the detection of the 2n<->pole shear wave in order to determine the shear wave speed of the earth surrounding the well. 3. Method according to claim 1, characterized in that the 2n<->pol shear wave arrival is detected at two points that have a known distance from each other, where said two points are separated longitudinally along the well from each other, and the time course between the detections at those points is measured to determine the shear wave velocity of the soil surrounding the well. 4. Method according to claim 1, characterized in that the well contains a liquid and the 2n<->pole shear wave is transmitted into the earth by generating in the liquid a number of compression waves that interfere to produce the 2n<->pole shear wave in the earth surrounding the liquid . 5. Method according to claim 1, characterized in that the multipole shear wave is an octopole shear wave. 6. Method according to claim 5, characterized in that the approximate range of shear wave velocities in the earth surrounding the liquid is calculated, and where the frequencies of the octopole shear wave lie in the preferred frequency range that corresponds to the approximate range of shear wave velocities of the earth surrounding the liquid in accordance with the table below: where d is the borehole diameter in inches. 7. Device for acoustic logging of an earth formation surrounding a borehole containing a liquid, characterized in that the device comprises: a logging probe adapted to be immersed in the liquid in the borehole; a shear wave source comprising 2n elements connected to the logging probe, where n is an integer greater than 2, and each element comprises a sector of a hollow cylinder where the 2n sectors are connected to the logging probe in such a way that they are essentially coaxial and that they surround a common axis ; a device connected to the logging probe to vibrate the 2n sectors radially, substantially simultaneously and substantially in the same manner, so that adjacent sectors vibrate substantially opposite in phase to generate a 2n<->pole shear wave in the soil formation; a means connected to the logging probe for detecting at at least one selected location in the fluid spaced longitudinally along the borehole from the 2n elements, the deflected compressional wave in the fluid caused by deflection of the 2n<->pol shear wave. 8. Device according to claim 7, characterized by vedatner equal to 3 and the vibrations of the six sectors generate an octopole shear wave in the formation. 9. Device according to claim 7, characterized in that the detector device comprises 2n sectors of a hollow cylinder where said 2n sectors of the detector device are mainly coaxial and surround the common axis of the sectors of the shear wave source, and where said 2n sectors of the detector device are arranged laterally around it common axis with the six sectors of the shear wave source. 10. Device for acoustic logging of an earth formation surrounding a borehole containing a liquid, characterized in that the device comprises; a logging probe adapted to be immersed in the borehole fluid; a shear wave source comprising 2n elements connected to the logging probe where n is an integer greater than 2, each element comprising an elongated plate attached to the logging probe at a location and in such a way that the two n elements essentially form the parallelograms of a polygonal prism with 2n sides; a device connected to the logging probe for vibrating the unattached portion of each of the 2n plates in a direction substantially normal to the flat surface of the plate substantially simultaneously and in substantially the same manner so that the unattached portions of up to overlapping plates vibrate essentially opposite in phase to generate a 2n<->pole shear wave in the soil formation; a device connected to the logging probe for detecting, at at least one location in the fluid separated longitudinally along the borehole from the 2n elements, the deflected compressional wave in the fluid caused by deflection of the 2n<->pol shear wave. 11. Device according to claim 10, characterized by data equal to 3 and the vibrations of the six elements generate an octopole shear wave in the soil formation. 12. Device according to claim 10, characterized in that the detector device comprises 2n elongated plates attached at a place on the plate to the logging probe in such a way that they essentially form the parallelograms of a polygonal prism with 2n sides, and that they are aligned azimuthally with the 2n the plates of the shear wave source. 13. Device for acoustic logging of a soil formation surrounding a borehole containing a liquid, characterized in that it comprises; a logging probe adapted to be immersed in the borehole fluid; 2n sectors of a polarized, hollow, piezoelectric cylinder connected to the logging probe such that the 2n sectors are substantially coaxial and such that they surround a common axis, where n is an integer greater than 2; a device connected to the logging probe for applying substantially the same electrical pulse substantially simultaneously over each of the 2n sectors, and causing the 2n sectors to vibrate radially, and said electrical pulses are of such polarities that adjacent sectors are caused to vibrate in substantially opposite directions phases, thereby producing a 2n-pole shear wave in the soil formation; and a device connected to the logging probe for detecting, at at least one location in the fluid separated longitudinally along the borehole from the 2n sectors, the deflected compression wave in the fluid caused by deflection of the 2n<->pol shear wave. 14. Device according to claim 13, where the 2n<->sectors are polarized radially, and where the electrical pulses are impressed over the outer and inner cylindrical surfaces of the sectors. 15. Device according to claim 13, characterized in that the 2n<->sectors are polarized along the circumference, and where the electric pulses are impressed on the 2n<->sectors, so that the electric field in each sector is mainly parallel to the polarization. 16. Device according to claim 15, characterized in that adjacent sectors are polarized in opposite circumferential directions and where the polarities of the electrical pulses that are applied are such that the electric fields in the 2n<-> sectors are in the same circumferential direction. 17. Device according to claim 15, characterized in that the 2n<-> sectors are polarized in the same circumferential direction, and where two random adjacent sectors are separated by a sector of the hollow piezoelectric cylinder, and where the polarities of the electric the pulses applied are such that the electric fields in adjacent sectors are in opposite circumferential directions. 18. Device for acoustic logging of an earth formation surrounding a borehole containing a liquid, characterized in that it comprises: a logging probe adapted to be immersed in the liquid in the borehole; 2n pairs of elongated piezoelectric plates, where each pair is glued to each other at their flat surfaces, where n is an integer greater than 2, and each pair is polarized in directions substantially perpendicular to the flat surfaces of the pair, and each pair is fixed somewhere to the logging probe, and each pair is so attached to the probe that the 2n pairs essentially form the parallelograms of a polygonal prism with 2n sides; a device for applying substantially the same electrical pulse to each pair substantially simultaneously to vibrate the unattached portions of each of the 2n pairs in a direction substantially normal to the flat surface, said electrical pulses being applied in such a manner that they do not attached portions of adjacent pairs vibrate in substantially opposite phases to generate a 2n<->pole shear wave in the soil formation; and a device connected to the logging probe for detecting, at at least one location in the fluid separated longitudinally along the borehole from the 2n pairs, the deflected compressional wave in the fluid caused by deflection of the 2n<->pol shear wave.