CA1204493A - Shear wave logging using acoustic multipole devices - Google Patents

Abstract

ABSTRACT

The multipole shear wave logging device of this invention includes a logging sonde, means for generating a 2n-pole shear wave in an earth formation surrounding a borehole containing liquid where n is an integer greater than 2, and means for detecting in the liquid the refraction of the 2n-pole shear wave. In the preferred embodiment the generating means comprises six similar sectors of a hollow piezoelectric cylinder. The six sectors are polarized radially and are so connected to the sonde that they are in the form of a split cylinder coaxial with the sonde axis. Electrical pulses of similar wave forms are applied across the inner and outer cylindrical surfaces of each sector. The electrical pulses are of such polarities that adjacent sectors vibrate radially in substantially opposite phase. Circumferentially polarized sectors may also be used in the place of radially polarized sectors.
The vibrations of the six sectors will generate compressional waves in the liquid which will interfere to produce an octopole shear wave in the formation. The compressional wave in the liquid caused by refraction of such octopole shear wave is detected by the detecting means. The detect-ing means comprises two detectors in the liquid spaced apart longitudinally from each other and from the generating means. The shear wave velocity of the formation is determined from the time interval between detections of the refraction of the octopole shear wave by the two detectors.

Description

( ~120~93 SYRIA WOW LOGGING SWIG ACOUSTIC MULTIPLE DEVICES

This invention relates to well logging in general and more particularly, to acoustic shear aye well logging.

Bac~round of the Invention -In acoustic well logging, it is customary to measure the come pressional wave velocity of earth formations surrounding Berlioz. A
conventional compression Al wave velocity logging system includes a cylindrical logging Sunday for suspension in a Barlow liquid, a source connected to the Sunday for generating compression Al waves in the Barlow liquid, and one or more detectors connected to the Sunday and spaced apart from the compression Al wave source for detecting compression Al waves in the Barlow liquid. A compression Al wave in the Barlow liquid generated by the source is refracted into the earth formation surrounding the Barlow. It propagates through a portion of the formation and is refracted back into the Barlow liquid at a point adjacent to the detector and is then detected by the detector. The ratio of the distance between the source and detector to the time between generation and detection of the compression Al wave yields the compression Al wave velocity of the formation. The distance between source and detector is usually fixed and known so that measurement of the time between compression Al wave generation and detection is sufficient to determine the compression Al wave velocity of the formation. For better accuracy, such distance is usually much greater than the dimensions of the source or detector.
Information important for production of oil and gas from subterranean earth formations may be derived from the compression Al wave velocities of such formations.
When a compression Al wave generated by a compression Al wave source in the Barlow liquid reaches the Barlow wall, it produces a .`

- ( refracted compression Al wave in the surrounding earth formation as described above. In addition, it also produces a refracted shear wave in the surrounding earth formation, and guided waves which travel in the Barlow liquid and the part of the formation adjacent to the Barlow.
S Part of such shear wave is refracted back into the Barlow liquid in the form of a compression Al wave and reaches the detector in the logging Sunday. The guided waves are also detected by such detector. Any wave that is one of the three types of waves detected by the detector may be called an arrival: the compression Al waves in the Barlow liquid caused by refraction of compression Al waves in the formation the compression Al wave arrivals, those caused by refraction of shear waves in the formation the shear wave arrivals, and those caused by guided waves the guided wave arrivals. Thus, the signal detected by the detector is a composite signal which includes the compression Al wave arrival, the shear wave arrival and the guided wave arrivals. Compression Al waves travel faster than shear waves and shear waves usually travel faster than the guided waves. Therefore, in the composite signal detected by the detector, the compression Al wave arrival is the first arrival, the shear wave arrival the second arrival, and the guided wave arrivals the last arrivals. In measuring the compression Al wave velocity of the formation, the time interval between generation of`compressional waves and detection of the first arrival detected by the detector gives the approximate travel time of the refracted compression Al wave in the formation. Hence the later shear wave and guided wave arrivals do not affect measurement of compress signal wave velocity of the formation.
In addition to traveling over a vertical distance in the formation approximately equal to the distance between the source and detector, the compression Al wave also travels over short distances in the liquid. The extra time required to travel such short distances introduces errors in the velocity log. To reduce such errors, conventional logging devices employ at least two detectors spaced vertically apart along the Barlow from each other. The time interval between detection - f 4~9:1 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 yields the compression Al wave velocity. Since the compression Al wave travels over approximately equal short distances in the Barlow liquid before reaching the two detectors, the time interval between detection by the two detectors is a more accurate measure of the actual travel time in the formation. Therefore, using two detectors and measuring the time between detection by the two detectors yield a more accurate comprsssional wave velocity. Other spurious effects such as borehole-size changes and Sunday tilt may be reduced by conventional devices. One such device is described in Interpretation, Volume 1 - Principles, Schlumberger limited, New York, NAY. 10017, 1972 Edition, pages 37-38.
It is well known that shear wave velocity logging may also yield information important for production of oil and gas prom subterranean earth formations. The ratio between the shear wave velocity and compress signal wave velocity may reveal the rock lithology of the subterranean earth formations. The shear wave velocity log may also enable seismic shear wave time sections to be converted into depth sections. The shear wave log is useful in determining other important characteristics of Sarah formations such as shear stress, porosity, fluid saturation and the presence of fractures. The shear wave log may also be helpful for determining the stress state around the Barlow which is very important in designing hydraulic fracture treatments.
The conventional compression Al wave logging source and the compression Al waves it generates in the Barlow liquid are symmetrical about the logging Sunday axis. When such compression Al waves are refracted into the surrounding earth formation, the relative amplitudes of the refracted shear and compression Al waves are such that it is difficult to , ,-~20~49~ `

distinguish the later shear wave arrival from the earlier compression Al wave arrival and from the reverberations in the Barlow caused by refraction of the compression Al wave in the formation. Therefore it is difficult to use a conventional symmetrical compression Al wave source for logging shear wave velocity. Correlation techniques have been employed to extract the shear wave arrival from the full acoustic wave train recorded. Such techniques, however, usually require processing of data by a computer so that shear wave velocities cannot be logged on line. It may also be difficult to extract the shear wave arrival if it is close in time to the compression Al wave arrival.
Asymmetric compression Al wave sources have been developed for logging shear wave velocity. Using such sources, the amplitude of the shear wave arrival may be significantly higher than that of the compress signal wave arrival. By adjusting the triggering level of the detecting and recording systems to discriminate against the compression Al wave arrival, the shear wave arrival is detected as the first arrival. It may thus be possible to determine the travel time of shear waves in the formation and therefore the shear wave velocity. Such asymmetric sources each generates in the Barlow liquid a positive compression Al wave in one direction and a simultaneous negative compression Al wave in the opposite direction. The interference of the two compression Al waves may cause the shear wave arrival to be stronger than the compression Al wave arrival. Asymmetric sources are disclosed by Angina et at, European Patent Application No. 31989 published 15 July 1981, White, US. Patent No. 3,593,255, and Kitsunezaki, US. Patent 4,207,961.
Angina et at disclose a bender-type source which comprises two circular pieæoelectric plates bonded together and attached to a logging Sunday. When voltage is applied across the two piezoelectric plates, the plates will bend. The bending of the transducer plates creates a positive to ';

~2~4~t~

compression Al wave in one direction and a simultaneous negative compress signal wave in the opposite direction. White discloses a compression Al wave source comprising two piezoelectric segments each in the shape of a half hollow cylinder. The two segments are assembled to form a split cylinder. The two segments have opposite polarization and electric voltage is applied to each segment causing one segment to expand radially and simultaneously causing the other segment to contract radially thereby producing a positive compression Al wave in one direction and a simultaneous negative compression Al wave in the opposite direction. In Kitsunezaki, coils mounted on a bobbin assembly are placed in the magnetic field of a permanent magnet and current is passed through the coils to drive the bobbin assembly. The movement of the bobbin assembly ejects a volume of water in one direction and simultaneously 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.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of an acoustic logging system illustrating this invention. , FIG. 2 is a simplified perspective view of an octopole shear wave logging device illustrating the preferred embodiment of this invention.
FIG. 3 is a cross-sectional view of the octopole shear wave logging source of FIG. 2 taken along the line 3-3.
FIG. 4 is a simplified perspective view of the octopole shear wave logging device of FIGS. 2 and I illustrating the orientation of the detectors relative to that of the octopole source, and the electrical connections to the source and detectors.
FIG. 5 is a cross-sectional view of an octopole shear wave logging source illustrating an alternate embodiment of this invention.
FIG. 6 is a cross-sectional view of an octopole shear wave logging source illustrating another alternate embodiment of this invention.
FIG. 7 is a cross-sectional view of an octopole shear wave logging source illustrating still another alternate embodiment of this invention.

12~

SYRIA OF TIE I~E~'TION
The method and apparatus of this invention are for logging the shear wave velocity of an earth formation surrounding a well or Barlow.
The method of this invention comprises transmitting a 2n-pole shear wave through the earth along the well wherein n is an integer greater than 2, and detecting the 2n-pole shear wave aureole at at least one point .-longitudinally spaced along the well from the point of transmission. If the shear wave arrival is detected at two points, the tire lapse between the detections at the two points is measured to determine the shear wave velocity of the earth surrounding the well. If the shear wave arrival is detected at only one point, the time lapse between transmission and , detection of the shear wave signal is measured to determine the shear ; wave velocity of the earth. The apparatus of this invention comprises a housing adapted to be raised and lowered into a well, signal generating means in the housing for transmitting a 2n-pole shear wave into the earth 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 shear wave.
DESCRIPTION OF THE PREFERRED E~lBOnIMENTS
FIG. 1 is a schematic view of an acoustic logging system illustrating this invention. A logging Sunday 10 is adapted to be raised and lowered into a well. The Sunday contains a multiple shear wave source 12 and two detectors, 14, 16. To initiate logging, Sunday 10 is 25 suspended into a liquid 18 contained in a Barlow 20, which is surrounded by an earth formation 22. Detectors 14, 16 are so connected to Sunday lo that they are spaced longitudinally along Barlow 20 from each other and from source 12. Source 12 is connected to a firing and recording control unit 24. Although the firing and recording control unit is 30 shown in FIG. l as a separate unit from the logging Sunday, the part of the unit that powers the multiple shear wave source may, for convenience " of operation, be housed by the logging Sunday. Signals recorded by detectors 14, 16 are fed to a band pass filter 26, an amplifier 28 and a time interval unit 30.

owe In a manner explained below the firing and recording control unit is used to fire source 12 which produces a shear wave in formation 22. The shear wave arrival is detected by detectors 14 and 16. Sunday 10 also contains a preamplifier (not shown in FIG. 1) which amplifies the shear wave arrival detected by detectors 14, 16. The amplified signals are then filtered by filter 26 and amplified again by amplifier 28. The time interval between the detection of the arrival by detector 14 and its detection by detector 16 is then measured by time interval unit 30. Such time interval may be stored or displayed as desired.
10 FIG. 2 is a simplified perspective view of an octopole shear , wave logging device illustrating the preferred embodiment of the invention.
I As shown in FIG. 2, logging Sunday 10 comprises a number of hollow cylinder-eel sections. The top section 32 contains an octopole shear wave it logging source snot shown in FIG. 2), and has six windows 42 which allow the compression Al waves generated by the source to propagate readily there through into the Barlow liquid. Sections 34, 36, each containing a detector snot shown), are located below the source and also have windows 44, 46 as shown in FIG. 2. The combined compression Al waves generated by the source in section 32 propagate through windows 42 and Barlow liquid 18 to reach the wall of Barlow 20. A portion of such combined compression Al waves is refracted into earth formation 22 in the form of a shear wave. After such shear wave travels a distance through the formation, portions of it are refracted back into the Barlow, into Barlow liquid 18, to reach the detectors in sections 34, 36 through windows 44 and 46, respectively. The time interval between the detections by the two detectors is then measured as described.
The nomenclature for the multiple is based upon consecutive powers of two, that is, on, n being an integer, and n = 1, 2, 3 and on indefinitely. Thus the multiples include the dipole on = 1), the quadruple (n = 2) and the octopole (n = 3). The nomenclature for higher order multiples is based upon on with n =-4, 5, 6 and so on indefinitely.
I. .

owe FIG 3 is a cross-sectional view of the octopole shear wave source of JIG. 2 taken along the line I Six substantially similar sectors 62, Al 66, I 70, 72 of a radially polarized piezoelectric hollow cylinder are so spatially arranged that they are substantially coaxial and that they surround a common axis. Substantially the same electrical pulse is applied across the cylindrical surfaces of each sector substantially simultaneously such that the pulses supplied to any two adjacent sectors are opposite in polarity. This arrangement is illustrated in FIG. 3. With such an arrangement, if one sector is lo caused by the electrical pulse to expend radially then the two sectors adjacent 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 hollow arrows in FIG. 3. During contraction of a sector, its entire inner cylindrical surface will move inward. During its expansion, its entire outer cylindrical surface will move outward. The combined compressior.al wave so generated by the expansion and contraction of the six sectors will refract into the surrounding earth formation to generate an octopole shear wave. To detect the octopole shear wave arrival, the detectors may be of construction similar to the octopole shear wave Lou cues illustrated in FIG. 3, or in FIG. 5, which will be described later.
The central space between the six sectors is filled by an annular body of backing material 74 to damp out the reverberations of the vibrations of the six sectors so that the octopole shear wave generated will be short in duration. This annular body 74 may be attached to section 32 by a conventional means such as inserting a mandrel 76 through the center of body 74, and screwing the two ends of the mandrel onto two disks that fit snugly into section 32. The six sectors are placed on the outer cylindrical surface of body 74 and may be wept in place by two annular rings trot shown in FIG. 3) of elastic backing material fitting snugly over the six sectors. The six sectors are so positioned in section 32 that each sector faces one of the six windows 42, as shown in FIG. 3. The vectorial spaces between the windows and the sectors are -~Z~49~93 filled by oil 78. The vibrations of the six sectors will generate compression Al waves in oil 78 which are transmitted through window 42 to generate an octopole shear wave in the earth. The vectorial spaces between the oil filled spaces are filled by backing material 80 for damping out the reverberations of the vibrations of the six sectors FIG. 4 is a simplified perspective view of the octopole shear . wave logging device of FIGS. 2 and 3 illustrating the orientation of detectors relative to that of the octopole source, and the electrical connections to the source and the detectors. To detect the compression Al wave in a Barlow liquid caused by refraction of the octopole shear wave generated by source 12, detector 14 is preferably also an octopole detector of construction similar to source 12. The six sectors are ' placed so that they have substantially the same axis as the six sectors j of source 12, and that they have substantially the same lateral positions around the common axis as the sectors of source 12 to maximize the strength of the octopole signal detected. The outer and inner cylindrical surfaces of the six sectors of the detector are connected to band pass filter 26 in a manner similar to the connections from the respective surfaces of source 12 to the firing and recording control unit 24.
20 Detector 16 is similar to detector 14 but is not shown in FIG. 5 for simplicity. To allow the six sectors of each of the two detectors to detect the octopole shear wave arrival, sections 34, 36 of FIG. 2 each will have preferably six windows 44, 46 respectively. While the detector of FIG. 4 is shown as similar in construction to the source of FIG. 3, it will be understood that detectors of construction similar to the sources of FIGS. 5, 6, 7 (described below) may also be used. The six sectors or plates of each type of detector are preferably aligned laterally around the common axis, that is, azimuthal) with the six sectors of the source to maximize the detected signal.
FIG. 5 is a cross-sectional view of an octopole shear wave . source illustrating an alternate embodiment of this invention. Six elongated piezoelectric composite plates 82, 84, 86, 88, 90, 92 are so spatially arranged that they form substantially the parallelograms of a ~2~44~

hexagonal prism. Each of the Sue composite plates comprises two oppositely polarized piezoelectric plates bonded together. The six composite plates arc attached to section I of the logging Sunday by two clamping plates (not shown in FIG. ;). Each of the two clamping plates his six slots into which the ones of the six composite plates are fitter snugly.
The two clamping plates are then inserted into and attached to section I in such position that the elongated composite plates are substantially -parallel to the logging Sunday assay. The portion of each composite plate between the two ends will hereinbelow be called the "unclamped portion"
or -the "unattached portion." It will be understood, however, thaw the six composite plates need not be attached to the Sunday at their ends.
Attachment of one end, or at a location between the two ends, will suffice. Then the portion of each plate other than the attached part may be called the "unclamped portion" or the "unattached portion."
Substantially the same electrical pulse is applied across the flat surfaces of each of the six composite plates substantially simulate-nuzzle. The pulses applied to any two adjacent composite plates are opposite in polarity such that if the unattached portion of one 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 illustrated by hollow arrows in FIX. 5. The bending motion of each composite plate will generate a compression Al wave in the Barlow liquid . The combined compression Al wave generated by the octopole source will refract into the formation surrounding the Barlow to produce an octopole shear wave. To detect the octopole shear wave arrival in the Barlow liquid detector 14 is preferably an octopole type which may be of construction similar to the octopole sources thus-treated in FIG. 3 or in FIG. 5. The outer surfaces ox the composite plates of detector 14 are connected to band pass filter 26 instead of to the firing and recording control unit 24. The six sectors or plates of ! the detector are preferably aligned azimuthal with the six plates of the shear wave source of FIG. 5.

~Z~4~

The composite plates comprising a pair of oppositely polarized piezoelectric plates are readily available commercially. Piezoelectric composite plates supplied by- the Vernitron Company of Bedford, Ohio, known as Bender Bimorphs have been satisfactory. The six piezoelectric sectors of the Tao illustrated in FIG. 3 and of the types illustrated in FIGS. 6, 7 to be described layer are also supplied by ~ernitron Company.
IT. 6 is a cross-sectional view of an octopole shear wave source illustrating another alternate embodiment of the invention. In FIGS. 3 and 4, the six sectors of the octopole source are polarized radially. Alternatively, the six sectors may be polarized circumferential as shown by the polarizations of sectors 102, 104, 106, 108, 110 and 112 in FIG. 6. The six sectors polarized circumferential are in what is known as the hoop mode. The six sectors may be obtained from a hollow cylindrical piezoelectric cylinder by cutting out six narrow long-tudinal sectors. An electrical pulse is applied across the side surfaces of each of the six sectors so that the resulting electric field in each sector is substantially parallel to its polarization. The electrical pulse will cause each sector to expand or contract radially depending upon the polarity of the pulse If the sectors 102, 106, 110 are polarized in a circumferential clockwise direction but the electric fields therein are in circumferential counterclockwise direction as shown in FIG. 6, the three sectors will contract radially. If the polarizations of and the electric fields in sectors 104, 108, 112 are all in the circumferential counterclockwise direction, then the three sectors will expand radially.
FIG. 7 is a cross-sectional view of still another alternate embodiment illustrating an octopole shear wave source in the hoop mode.
The six sectors, 122, 124, 126, 128, 130, 132 are six of the twelve longitudinal sectors of a piezoelectric hollow cylinder, each of the twelve sectors having been polarized circumferential. Adjacent members have opposite circumferential polarizations. the six sectors 122, 124, 126, 128, 130, 132 are the only sectors of the cylinder which will ~;2V44~

expand and contract and are all polarized in the circumferential clockwise direction. The connecting edges of adjacent sectors may be coated by conducting layers snot shown). Electrical pulses are so applied that the electric field in each of the six sectors is substantially parallel to its polarization. hit the polarizations of sectors and polarities of pulses as shown in FIG. 7, sectors 122, 126, 130 will expand radially while sectors 124, 128, 132 will contract radially. The remaining six sectors will not expand or contract since no potential difference is applied across such sectors.
In both the preferred embodiment and the three alternate embodiments described above, piezoelectric materials are used to construct the octopole shear wave source, and the source is vibrated by electrical pulses. It will be understood, however, that other constructions of the source and other vibrating means may be used. Thus purely mechanical means may be used to vibrate the six sectors of the preferred embodiment, and the six plates or sectors of each of the three alternate embodiments.
An octopole shear wave will be venerated so long as the sectors or the plates are caused to vibrate in the same manner as in the preferred and alternate embodiments.
The octopole shear wave source of this invention may be used to log shear wave velocities on line (that is, the shear wave velocities may be determined without data processing if the shear wave arrival is significantly greater in amplitude than the compression Al wave arrival.
The shear wave arrival is significantly greater in amplitude than the compression Al wave arrival only when the frequencies of the octopole shear wave produced in the earth surrounding the Barlow are within certain frequency ranges. For any earth formation there is a preferred frequency range for logging the shear wave velocity so that the shear wave arrival is significantly stronger than the compression Al wave arrival. The preferred frequency range varies with the shear wave velocity of the formation to be logged. Thus if the approximate range of the shear wave velocities of the formation is known, a preferred range of frequencies can be chosen For a well with ten inches diameter the preferred frequency ranges are shown in the table below.

owe App~-o~imate Range of Preferred Frequency Shear Wave Velocities (ft/sec~ Range (kHz) 5000 - 6000 3.7 - 12.6 7000 owe 3.9 - 26.5 ~000 - 9000 4.1 - 33 The approximate range of shear wave velocities of a formation ma be estimated by conventional methods such as measuring the ccmpres-signal wave velocities of the formation. The shear wave velocity is approximately one-half the compression Al wave velocity. From tune measured compression Al wave velocities the approximate range of shear wale velocities may be estimated. The preferred frequencies vary inversely with the diameter of the well. Therefore for a well with diameter d inches instead of ten inches the preferred frequency ranges are given by those listed in the table above multiplied by a factor 10/d.
FIGS. 3, 4, 6, 7 illustrate octopole shear wave sources employing six sectors which are vibrated radially to generate octopole shear waves in earth formations. The frequencies of the octopole shear waves so generated vary inversely with the radii of the sectors. For the frequencies to be within the preferred frequency ranges listed above, it is preferable that the radii of the sectors be large. Therefore, their radii are preferably only slightly smaller than the radius of the logging Sunday. It will be understood that FIGS. 3, 4, 6, 7 are not drawn to scale.
The higher order multiple sources may be constructed in a manner similar to the four embodiments of the octopole shear wave source illustrated in FIGS. 3, 5, 6 and 7. Thus the 16-pole source may be constructed by spatially arranging 8 elongated pie~oelectric composite plates to form the 8 parallelograms of an octagonal prism. Substantially the same electrical pulse is applied to each of the eight composite i plates with such polarity that adjacent plates vibrate in substantially opposite phases. An alternative embodiment of the 16-pole source is constructed if the eight composite plates are replaced by eight substantially identical sectors of a radially or circumferential polarized piezoelectric hollow cylinder. Substantially the same electrical pulse is applied to each sector such that adjacent sectors vibrate in substantially opposite phases. Other jays of constructing and vibrating the plates and sectors may be used so long as the plates and sectors are vibrated in the same manner. Other higher order multiples may be constructed in a manner similar to the octopole and 16-pole. Preferably the detectors used to detect the higher order shear wave arrivals will be of an order what matches the order of the source.
The number of composite plates or sectors in the embodiments of the octopole and the 16-pole sources described above does not match the nomenclature of the octopole and 16-pole sources. Thus the octopole source comprises 6 plates or sectors end the 16-pole source S plates or sectors. The 32-pole source comprises 10 plates or sectors. Thus while the nomenclature of the multiple sources is based on on, n being an integer, with n = 1, 2, 3...., the corresponding number of plates or sectors is on. Thus, a dipole (n = 1) source comprises 2 x 1 or 2 plates or sectors. A quadruple (n = 2) source comprises 2 x 2 or 4 plates or sectors. An octopole (n = 3), a 16-pole on = 4) and a 32-pole on = 5) source comprises 6, 8 and LO plates or sectors respectively.
Therefore, in general, a 2 -pole source will comprise on plates or sectors, n being an integer, whereon = 1, 2, 3 and so on indefinitely.
The above description of method and construction used is merely illustrative thereof and various changes in shapes, sizes, materials, or other details of the method and construction may be within the scope of the appended claims.
,..

Claims (18)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of logging the earth surrounding a well which comprises operating a source of substantially only 2n-pole acoustic waves in the well to transmit from the well a 2n-pole shear wave through the earth along the well wherein n is an integer greater than two, and detecting the 2n-pole shear wave arrival at at least one point longitudinally spaced along the well from the point of transmission.

2. The method of claim 1, further comprising measuring the time lapse between the transmission and detection of the 2n-pole shear wave to determine the shear wave velocity of the earth surrounding the well.

3. The method of claim 1, wherein the 2n-pole shear wave arrival is detected at two points having known spacing therebetween, said two points being spaced longitudinally along the well from each other, said method further comprising measuring the time lapse between the detections at the two points to determine the shear wave velocity of the earth surrounding the well.

4. The method of claim 1, wherein the well contains a liquid and wherein the 2n-pole shear wave is transmitted into the earth by generating in the liquid a number of compressional waves which will interfere to produce the 2n-pole shear wave in the earth surrounding the liquid.

5. The method of claim 1, wherein the multiple shear wave is an octopole shear wave.

6. The method of claim 5 further comprising the step of determining the approximate range of shear wave velocities of the earth surrounding the liquid and wherein the frequencies of the octopole shear wave are in the preferred frequency range corresponding 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. An apparatus for acoustically logging an earth formation surrounding a borehole which contains a liquid, said apparatus comprising:
a logging sonde adapted to be suspended into the liquid in the borehole;
a shear wave source comprising 2n members connected to the logging sonde where n is an integer greater than two, each member comprising a sector of a hollow cylinder wherein the 2n sectors are so connected to the logging sonde that they are substantially coaxial and they surround a common axis;
means connected to the logging sonde for vibrating the 2n sectors radially, substantially simultaneously and in substantially the same manner such that adjacent sectors vibrate in substan-tially opposite phase to generate a 2n-pole shear wave in the earth formation;
means connected to the logging sonde for detecting at at least one selected location in the liquid spaced longitudinally along the borehole from the 2n members the refracted compressional wave in the liquid caused by refraction of the 2n-pole shear wave.

8. The apparatus of claim 7 wherein n is equal to 3 and the vibrations of the six sectors generate an octopole shear wave in the formation.

9. The apparatus of claim 7, wherein the detecting means comprises 2n sectors of a hollow cylinder wherein said 2n sectors of the detecting means are substantially coaxial and they surround the common axis of the sectors of the shear wave source, and wherein said 2n sectors of the detecting means are aligned laterally around the common axis with the six sectors of the shear wave source.

10. An apparatus for acoustically logging an earth formation surrounding a borehole which contains a liquid, said apparatus comprising:
a logging sonde adapted to be suspended into the liquid in the borehole;
a shear wave source comprising 2n members connected to the logging sonde where n is an integer greater than two, each member comprising an elongated plate attached to the logging sonde at one location and in such manner that the 2n members substantially form the parallelograms of a 2n-sided polygonal prism;
means connected to the logging sonde for vibrating the unattached portion of each of the 2n plates in a direction substantially normal to the flat surface of the plate substantially simulta-neously and in substantially the same manner such that the unattached portions of adjacent plates vibrate in substantially opposite phase to generate a 2n-pole shear wave in the earth formation;
means connected to the logging sonde for detecting at at least one location in the liquid spaced longitudinally along the borehole from the 2n members the refracted compressional wave in the liquid caused by refraction of the 2n-pole shear wave.

11. The apparatus of claim 10 wherein n is equal to 3 and the vibrations of the six members generate an octopole shear wave in the earth formation.

12. The apparatus of claim 10 wherein the detecting means comprises 2n elongated plates attached at one location of the plate to the logging sonde in such manner that they substantially form the parallelograms of a 2n-sided polygonal prism and that they are aligned azimuthally with the 2n plates of the shear wave source.

13. An apparatus for acoustically logging an earth formation surrounding a borehole which contains a liquid said apparatus comprising:
a logging sonde adapted to be suspended into the liquid in the borehole;
2n sectors of a polarized, hollow, piezoelectric cylinder connected to the logging sonde so that the 2n sectors are substantially coaxial and that they surround a common axis, wherein n is an integer greater than two;
means connected to the logging sonde for applying substantially the same electrical pulse substantially simultaneously across each of the 2n sectors, causing the 2n sectors to vibrate radially, said electrical pulses being of such polarities that adjacent sectors will be caused to vibrate in substantially opposite phases, thereby producing a 2n-pole shear wave in the earth formation; and means connected to the logging sonde for detecting at at least one location in the liquid spaced longitudinally along the borehole from the 2n sectors the refracted compressional wave in the liquid caused by refraction of the 2n-pole shear wave.

14. The apparatus of claim 13, wherein the 2n sectors are polarized radially, and wherein the electrical pulses are applied across the outer and inner cylindrical surfaces of the sectors.

15. The apparatus of claim 13, wherein the 2n sectors are polarized circumferentially and wherein the electrical pulses are applied to the 2n sectors such that the electric field in each sector is substantially parallel to its polarization.

16. The apparatus of claim 15, wherein adjacent sectors are polarized in opposite circumferential directions and wherein the polarities of the electrical pulses applied are such that the electric fields in the 2n sectors are in the same circumferential direction.

17. The apparatus of claim 15, wherein the 2n sectors are polarized in the same circumferential direction, wherein any two adjacent sectors are separated by another sector of the hollow piezoelectric cylinder, and wherein the polarities of the electrical pulses applied are such that the electric fields in adjacent sectors are in opposite circumferential directions.

18. An apparatus for acoustically logging an earth formation surrounding a borehole which contains a liquid said apparatus comprising:
a logging sonde adapted to be suspended into the liquid in the borehole;
2n pairs of elongated piezoelectric plates, each pair bonded to each other by their flat surfaces wherein n is an integer greater than two, each pair being polarized in directions substantially perpendicular to the flat surfaces of the pair, each pair attached at one location to the logging sonde, and each pair so attached to the sonde that the 2n pairs form substantially the parallelograms of a 2n-sided polygonal prism;
means 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 its flat surfaces, said electrical pulses being so applied that the unattached portions of adjacent pairs will vibrate in substantially opposite phases to generate a 2n-pole shear wave in the earth formation; and means connected to the logging sonde for detecting at at least one location in the liquid spaced longitudinally along the borehole from the 2n pairs the refracted compressional wave in the liquid caused by refraction of the 2n-pole shear wave.