Method and apparatus for acoustic well logging

Abstract

Method and apparatus for acoustic wave generation and transmission into a subsurface earth formation. A logging sonde adapted to be suspended in a borehole within the formation houses a generator means for simultaneously generating a plurality of acoustic waves traveling in the direction of and spaced substantially evenly about the logitudinal longitudinal axis of the sonde. An acoustic energy reflector means within the housing reflects the waves radially outwards of the axis and into the formation at angles generally perpendicular to the axis. Detectors within the housing spaced longitudinally from the generator and reflector detect acoustic energy in the formation resulting from the reflected waves. In a preferred embodiment, the generator means comprises four cylindrical magnetostrictively energized elements disposed about the central axis of the sonde, each having an axis parallel to the central axis, so that the four axes of the elements, when viewed in the direction of the central axis, define four corners of a square. The elements are designed so that upon energization, a given element vibrates longitudinally out of phase relative to the two elements adjacent thereto, vibration of the four elements in concert generating two positive and two negative waves which, when reflected into the formation, interfere to produce a quadrupole shear wave.

Description

CROSS-REFERENCES TO RELATED APPLICATIONS

U.S. patent application Ser. Nos. 525,910, filed Aug. 24, 1983 constuctsouce source to be described below does not match the nomenclature of the dipole, octopole, and 16-pole sources. Thus, a dipole (n=1) source comprises two times one or two rods. A quadrupole (n=2) source comprises two times two or four rods. An octopole (n=3), a 16-pole (n=4) and a 32-pole (n=5) source comprises six, eight, and ten rods respectively. Therefore, in general a 2ⁿ -pole source will comprise 2n rods, n being an integer where n=1, 2, 3, and so on indefinitely.

In general, for a 2ⁿ -pole source of the present invention, 2n rods (where n=1, 2, 3, and so on indefinitely) are disposed substantially evenly about the central axis of a logging sonde. Preferably, the rods are disposed substantially evenly about the central axis. Adjacent rods, with respect to angular position about the central axis, produce pressure waves which are substantially 180° out of phase with respect to each other and which initially propagate toward the reflector and are thereafter reflected generally radially outward from the central axis.

Accordingly, referring now to FIG. 7 in comparison to FIG. 4, it may be appreciated that instead of only four rods 66-72, eight rods 158, 160, 162, 164, 166, 168, 170, and 172 are provided (with corresponding coils which are not shown) as well as eight corresponding windows 142, 144, 146, 148, 150, 152, 154, and 156 radially outwards from the rods.

In similar manner to the embodiment of FIGS. 1-6B, the eight rods 158-172 and corresponding coils will be oriented so that their centers are evenly spaced about the circumference of circle 130 and their axes are parallel to central axis 28. Similarly, the rods will alternate between a first and second ferromagnetic material as circle 130 is circumferentially traversed. Finally, the rods may be energized in a manner similar to that functionally depicted in FIG. 5.

It will thus be understood that in the embodiment of FIGS. 7, 8A, and 8B, instead of four pressure waves being produced which travel upwards within the sonde 16 toward reflector 76, eight such waves will be produced. Accordingly, it is necessary to modify reflector 76 as depicted in FIGS. 8A and 8B so as to provide corresponding reflecting surfaces 174, 176, 178, 180, 182, 184, 186, and 188, which will cause each such wave to be reflected out its respective window 142, 146, 148, 150, 152, 154, and 156, into formation 10 in eight separate and distinct radially outward directions from central axis 28.

FIG. 10 is a cross-sectional view of a dipole acoustic shear wave source illustrating another embodiment of acoustic source 26 of the invention. Instead of four rods, only two rods 258 and 260 are provided (with corresponding helical coils which are not shown) as well as two corresponding windows 254 and 256 radially outwards from rods 258 and 260 respectively.

In a similar manner to the embodiment of FIGS. 1 through 6B, rods 258 and 260 and corresponding coils are oriented substantially 180° away from each other on the circumference of circle 230 and their axes are parallel to central axis 28. Similarly, one of rods 258 and 260 is made of a first magnetostrictive material having a positive strain constant (such as a 2 V Permendur) and the other is made of a second magnetostrictive material having a negative strain constant (such as nickel). Rods 258 and 260 are energized in a manner similar to that functionally depicted in FIG. 5.

It will thus be understood that in the embodiment of FIG. 10, two pressure waves (one 180° out of phase with respect to the other) will be produced as to propagate initially within sonde 16 toward reflector 278. Reflector 278 is positioned above the upper ends of rods 258 and 260 by threading threaded recess 208 onto matingly threaded upper end portion of mandrel 74.

Reflector 278, depicted in FIGS. 11 and 12 is provided with reflecting surfaces 274 and 276, for respectively reflecting the pressure waves from rods 258 and 260 out windows 254 and 256, so as to propagate substantially radially outward from central axis 28. Reflector 278 is generally shaped as an inverted solid cone, truncated by the surface adjacent recess 208, and having reflecting surfaces 274 and 276 formed on oppositely facing regions of its generally conical outer surface. The axes of rods 258 and 260 intersect faces surfaces 274 and 276, respectively, when reflector 278 is properly positioned relative to the rods.

FIG. 13 is a cross-sectional view of an octopole acoustic shear wave source illustrating yet another embodiment of acoustic source 26 of the invention. Referring to FIG. 13 in comparison with FIG. 4, it may be appreciated that instead of only four rods 66-72, six rods 358, 360, 362, 364, 366, and 368 are provided (with corresponding coils which are not shown) as well as six corresponding windows 344, 346, 348, 350, 352, and 354 radially outwards from the rods.

In similar manner to the embodiment of FIGS. 1-6B, the six rods 358-368 and corresponding coils will be oriented so that their centers are evenly spaced about the circumference of circle 370 and their axes are parallel to central axis 28. Similarly, the rods will alternate between a first and second ferromagnetic material as circle 370 is circumferentially traversed. Finally, the rods may be energized in a manner similar to that functionally depicted in FIG. 5.

It will thus be understood that in the embodiment of FIGS. 13, 14, and 15, instead of four pressure waves being produced which travel upwards within the sonde 16 toward reflector 76, six such waves will be produced. Accordingly, it is necessary to employ a modified reflector 376 as depicted in FIGS. 14 and 15 (rather than reflector 76 in the embodiment depicted in FIGS. 6A and 6B) so as to provide six corresponding reflecting surfaces 380, 382, 384, 386, 388, and 390, which will cause each such wave to be reflected out its respective window 344, 346, 348, 350, 352, and 354, into formation 10 in six separate and distinct radially outward directions from central axis 28.

It will be recalled from the foregoing that in the embodiment of FIG. 3 it was mentioned that in an alternate embodiment thereof it is desirable to provide two biasing magnets (such as those two depicted therein as 120, 122). This alternate embodiment will now be discussed in greater detail.

In the alternate embodiment presently being discussed, all the rods thereof such as rods 66-72 of FIG. 3 may be made of the same ferromagnetic material, a material chosen with a relatively high strain constant in order to produce relatively higher vibrational amplitudes of the rods and accordingly a stronger acoustic source.

One problem with using rods of the same material is in achieving the desired hereinbefore noted out-of-phase relationship between the generated acoustic waves (generated by each rod and achieved previously due to use of rods with two differing strain constants). By providing a biased magnetic field on two diametrically opposed rods such as 66 and 70 of the four depicted in FIG. 3, this out-of-phase operation may nevertheless still be achieved.

More particularly, rods 66 and 70 may, for example, be prestrained by corresponding permanent magnets 120 and 122 carried above them in reflector 76 (alternatively electromagnetic coils may be substituted for magnets 120 and 122 in some applications in which permanent magnets might be prohibitively bulky).

These rods 66 and 70 will either be strained further or relieved from prestrained conditions as a function of the direction of the magnetic field applied by corresponding coils 86 and 90 to rods 66 and 70. Whereas magnetostrictive material having a positive strain constant will elongate (and magnetostrictive material having a negative strain constant will contract) with magnetization independent of the sign (positive or negative) of the magnetic field applied, the amount of such movement is related to the absolute magnitude of the applied magnetic field.

Thus, by alternating the direction of the energizing current to coils 86 and 90, the magnitude of the net magnetic field exerted on rods 66 and 70 may be made to vary on either side of the pre-biased or prestrained value, thereby causing the rods 66 and 70 to move in either desired direction of central axis 28 from a prestrained position either to a less or greater strained position. This, in turn, permits creation of the desired out-of-phase motion between diametrically opposed rod pairs 66-70 and 68-72.

Referring now to FIG. 9, yet another embodiment of the present invention may be seen depicted therein. More particularly, FIG. 9 depicts an alternative method of constructing the vibrating rods utilized in the embodiments of the acoustic wave source illustrated in FIGS. 3, 7, or 10.

Each magnetostrictive rod with associated coil, such as rod 66 and coil 86 of FIG. 3, may have substituted therefor a piezoelectric rod such as the four shown in FIG. 9 in exploded view.

Each rod will be seen to be comprised of a plurality of polarized discs such as disc 198, 200, 204, and 206 of FIG. 9 fashioned from a suitable piezoelectric crystal material, such as that commercially supplied by the Vernitron Company of Bedford, Ohio. These discs will be stacked and coaxially aligned along respective axes 190, 192, 194, and 196. These axes will be seen to correspond to longitudinal axes of previously described rods 66-72 extending parallel to center axis 28.

Piezoelectric crystals have the property that they will either expand or contract in response to an applied electrical potential and whether the crystal expands or contracts is controllable by the direction of the applied potential and the direction of the crystal polarization.

Accordingly, with the crystal discs 198-206 polarized according to the arrows as shown, stacked, and wired, it will be understood that because wiring of stacks aligned along axis 190 and 194 is opposite to those aligned along axis 192 and 196, upon energization of all four stacks from energy source 132 by closing switch 42, two diametrically opposed stacks will expand longitudinally in the direction of central axis 28, whereas the remaining two will contract, thus achieving the desired generation of two sets of out-of-phase longitudinal acoustic waves previously described with respect to the embodiment of FIG. 2.

As previously noted, due to the longitudinal displacement mode of the rods of the present invention and further due to the relatively greater longitudinal dimensions of sonde 16 (as opposed to transverse dimensions) available for housing a vibrating member, it is possible to build acoustic sources in accordance with the teachings of the present invention which may generate extremely powerful out-of-phase acoustic pressure waves in the sonde 16 sufficient to easily establish strong dipole, quadrupole, or higher order shear waves in the formation of interest.

The desired frequency of the acoustic waves to be generated will govern the choice of the particular lengths of rods 66-72 in a manner well known in the art, inasmuch as the natural frequency of the rods, a function of their length, will be related to this desired frequency. However, for acoustic shear wave logging the typical desired frequency ranges of oscillation for rods 66-72 in the quadrupole embodiment shown in FIG. 2 will be in the range of just below 3 KHz to about 14 KHz or even higher, with frequencies about 3 KHz being often typical for direct shear wave logging relatively "soft" formations and about 6 KHz or higher for direct shear wave logging in "hard" formations.

Due to the strength of acoustic waves which may be generated with the sonde of the present invention, it has been found that the first harmonic of the nominal oscillating frequency of the rods (which first harmonic is also present in the oscillations) may be of sufficient magnitude such that the source 26 may be operated for both soft and hard formations at the same frequency.

Moreover, also due to the strength of the instant source, well-to-well logging may even be achieved wherein the formation may be acoustically excited at one borehole situs and the acoustic signature detected at an adjacent borehole situs.

Because oscillating magnetostrictive rods may be provided which are energized by magnetic fields, relatively small power supply requirements of low voltage are required to energize their respective coils. This is a distinct advantage over conventional piezoelectric vibrating elements which characteristically require higher voltage supplies with attendant noise problems and the like. However, when "stacked array" rods of a piezoelectric disc material are substituted for magnetostrictive rods, as in the case of the alternate embodiment of FIG. 9, these problems may be reduced by careful design.

It will be appreciated that the operating principles of the sonde 26 of the present invention disclosed herein may be adapted with relatively minor changes to construct acoustic wave detectors, and such detectors are accordingly specifically within the scope and spirit of the subject invention.

For example, with reference to FIG. 2, it is readily apparent that if the source depicted therein is used as a detector, acoustic waves from the formation to be detected will travel opposite to those generated when it is acting as a source. More particularly, acoustic waves will enter through windows 79, 81, etc., and be reflected downward by reflector 76 onto rods 66-72.

This energy impinging upon rods 66-72 will cause vibrations therein which may be used to induce measurable potential signal levels in coils 86-92 functionally related to the acoustic waves.

It is therefore apparent that the present invention is one well adapted to obtain all of the advantages and features hereinabove set forth, together with other advantages which will become obvious and apparent from a description of the apparatus itself. It will be understood that certain combinations and subcombinations are of utility and may be employed without reference to other features and subcombinations. Moreover, the foregoing disclosure and description of the invention is only illustrative and explanatory thereof, and the invention admits of various changes in the size, shape and material composition of its components, as well as in the details of the illustrated construction, without departing from the scope and spirit thereof.

Claims

1. A method of establishing multiple acoustic waves in a subsurface earth formation traversed by a borehole with a sonde, having a central longitudinal axis, disposed therein, comprising the steps of:

simultaneously generating 2N acoustic pressure waves, where N is an integer not less than one, originating from a corresponding number of discrete locations spaced radially outward from said central longitudinal axis so that said waves will propagate initially along respective wave axes in directions substantially parallel to said central longitudinal axis, said wave axes being oriented so that the projections, in a plane perpendicular to the central longitudinal axis, of lines intersecting the central longitudinal axis and the wave axes define a plurality of approximately equal angles α, wherein α is approximately equal to 360°/2N, and so that any first one of the waves traveling along a corresponding first one of the wave axes is substantially out of phase with respect to any second one of the waves traveling along a corresponding second one of the wave axes, where the first wave axis and the second wave axis are separated by angle α with respect to the central longitudinal axis; and reflecting said waves substantially radially outward from said central longitudinal axis into said formation.

2. The method of claim 1, wherein said pressure waves are initially propagated within said sonde and are thereafter reflected radially outward from said sonde.

3. The method of claim 2, wherein said wave axes are substantially equidistant from said central axis.

4. The method of claim 1, wherein said reflected waves include lobes reflected at an angle θ from a plane perpendicular to said central longitudinal axis, where θ is greater than zero.

5. The method of claim 4, wherein N=2.

6. The method of claim 4, wherein the directions of said radially outward reflected waves, when projected onto a plane perpendicular to said central axis, define said plurality of angles α, and wherein said generated pressure waves have an amplitude and frequency such that said reflected waves will interfere to generate an acoustic multipole shear wave in said formation.

7. Apparatus for establishing multipole acoustic waves in a subsurface each formation traversed by a borehole with a sonde, having a central longitudinal axis, disposed therein, comprising:

acoustic wave generator means for simultaneously generating 2N acoustic pressure waves, where N is an integer not less than one, originating from a corresponding number of discrete locations spaced radially outward from said central longitudinal axis so that said waves will propagate along respective wave axes in directions substantially parallel to said central longitudinal axis, said wave axes being oriented so that the projections, in a plane perpendicular to the central longitudinal axis, of lines intersecting the central longitudinal axis and the wave axes define a plurality of approximately equal angles α, wherein α is approximately equal to 360°/2N, and so that any first one of the waves traveling along a corresponding first one of the wave axes is substantially out of phase with respect to any second one of the waves traveling along and corresponding second one of the wave axes, where the first wave axis and the second waves axis are separated by angle α with respect to the central longitudinal axis; and

acoustic wave reflector means for reflecting said waves in substantially radially outward directions from said central longitudinal axis into said formation.

8. The apparatus of claim 7, wherein N=2.

9. The apparatus of claim 7, wherein said vibrations acoustic waves have an amplitude and frequency such that said reflected waves are capable of interfering to generate an acoustic multipole shear wave in said formation.

10. The apparatus of claim 7, wherein said wave axes are substantially equidistant from said central axis.

11. The apparatus of claim 7, wherein said generator means includes a plurality of rod means each longitudinally coaxial to a different corresponding one of said wave axes for establishing vibrations in the direction of said wave axes.

12. The apparatus of claim 11, wherein each of said plurality of rod means is comprised of piezoelectric material which vibrates in the direction of the wave axis corresponding thereto when energized by a changing electrical potential.

13. The apparatus of claim 11, wherein said acoustic wave reflector means comprises a plurality of acoustic reflector face means each for a different one of said waves and facing a different one of said substantially radially outward directions from said central axis, for reflecting each of said different ones of said plurality of waves in a respective said different one of said substantially radially outward directions.

14. The apparatus of claim 11, wherein each of said rod means is comprised of a magnetostrictive material having a strain constant, which vibrates in the direction of the wave axis corresponding thereto when in the presence of a changing magnetic field in said direction.

15. The apparatus of claim 14, wherein a next one of said plurality of rod means, and each one of said plurality of rod means having a wave axis separated from the wave axis corresponding to said next one of said plurality of rod means by an angle equal to 2nα with respect to central axis are comprised of a first magnetostrictive material having a first strain constant, and wherein every other one of said plurality of rod means is comprised of a second magnetostrictive material having a second strain constant different from said first strain constant, where n is a positive integer.

16. The apparatus of claim 15, wherein said first magnetostrictive material is nickel and second magnetostrictive material is 2 V Permendur.

17. The apparatus of claim 15, wherein the absolute value of the first strain constant is larger than that of the second strain constant, and also comprising:

an electrically conducting metal element wrapped about each of said rod means comprised of a material having said first strain constant so that the absolute value of the effective strain constant of each of said wrapped rod means is reduced to a value less than the absolute value of said first strain constant.

18. The apparatus of claim 14, wherein each of said plurality of rod means has substantially the same strain constant.

19. The apparatus of claim 18, also comprising:

pre-biasing means for establishing a constant selected magnetic field adjacent predetermined ones of said plurality of rod means.

20. The apparatus of claim 19, wherein said pre-biasing means comprises a permanent magnet.

21. The apparatus of claim 19, wherein said pre-biasing means comprises an electromagnetic coil.

22. The apparatus of claim 12, wherein each of said rod means is comprised of a plurality of discs of said piezoelectric material stacked longitudinally in the direction of the wave axis corresponding thereto.

23. The apparatus of claim 22 13, wherein each of said plurality of face means is oriented for reflecting a different one of said waves at an angle θ from plane perpendicular to said central axis, where θ is greater than zero.

24. The apparatus of claim 13, wherein the number of said plurality of rod means is equal to 2N and the number of said plurality of face means is equal to 2N.

25. The apparatus of claim 13, wherein said acoustic wave reflector means defines an inverted pyramid in coaxial alignment with said central axis, said pyramid having faces, each of which comprises a different one of said plurality of face means, each of said faces being oriented above a respective one of said rod means and being further intersected by one of said wave axes corresponding to said respective one of said rod means.

26. A method of detecting multipole acoustic waves in a subsurface earth formation traversed by a borehole with a sonde having a central longitudinal axis disposed therein, said multipole acoustic waves creating 2N acoustic pressure waves in the borehole that propagate radially inwardly toward the sonde, comprising the steps of:

reflecting the 2N acoustic pressure waves traveling radially inwardly toward the sonde so that the reflected 2N acoustic pressure waves travel along respective wave axes substantially parallel to the central longitudinal axis of the sonde; and

simultaneously detecting 2N acoustic pressure waves, where N is an integer not less than one, at a corresponding number of discrete locations spaced radially outward from said central longitudinal axis, said reflected wave axes being oriented so that the projections, in a plane perpendicular to the central longitudinal axis, of lines intersecting the central longitudinal axis and the wave axes define a plurality of approximately equal angles α, wherein α is approximately equal to 360°/2N, and so that any first one of the waves traveling along a corresponding first one of the wave axes is substantially out of phase with respect to any second one of the waves traveling along a corresponding second one of the wave axes, where the first wave axis and the second wave axis are separated by angle α with respect to the central longitudinal axis. 27. The method of claim 26, wherein said reflected wave axes are substantially equidistant from said

central axis. 28. The method of claim 26 wherein said acoustic pressure waves propagating toward the sonde include lobes reflected at an angle r from a plane perpendicular to said central longitudinal axis, where r is greater than zero. 29. The method of claim 28, wherein N=2. 30. The method of claim 28, wherein the directions of the wave axes of the acoustic pressure waves reflected substantially parallel to the central longitudinal axis, when projected outwardly toward the formation and then projected onto a plane perpendicular to said central axis, define said plurality of angles

α. 31. Apparatus for detecting multipole acoustic waves in a subsurface earth formation traversed by a borehole with a sonde having a central longitudinal axis disposed therein, wherein said multipole acoustic waves in the formation create 2N acoustic pressure waves propagating radially toward the sonde, said apparatus comprising:

acoustic wave detector means for simultaneously detecting 2N acoustic pressure waves, where N is an integer not less than one, at a corresponding number of discrete locations spaced radially outward from said central longitudinal axis, said waves propagating along respective wave axes in directions substantially parallel to said central longitudinal axis, said wave axes being oriented so that the projections, in a plane perpendicular to the central longitudinal axis, of lines intersecting the central longitudinal axis and the wave axes define a plurality of approximately equal angles α, wherein α is approximately equal to 360°/2N, and any first one of the waves traveling along a corresponding first one of the wave axes is substantially out of phase with respect to any second one of the waves traveling along a corresponding second one of the wave axes, and the first wave axis and the second waves axis are separated by angle α with respect to the central longitudinal axis; and

acoustic wave reflector means for reflecting said acoustic pressure waves propagating radially toward toward the sonde so that such waves propagate along respective wave axes substantially parallel to the central

longitudinal axis of the sonde. 32. The apparatus of claim 31, wherein N=2. 33. The apparatus of claim 32, wherein said wave axes are substantially equidistant from said central axis. 34. The apparatus of claim 31, wherein said detector means includes a plurality of rod means each longitudinally coaxial to a different corresponding one of said wave axes for detecting vibrations in the direction of said wave axes. 35. The apparatus of claim 34, wherein said plurality of rod means is comprised of piezoelectric material which produces measurable signals in response to vibrations in the direction of the wave axis corresponding thereto. 36. The apparatus of claim 34, wherein said acoustic wave reflector means comprises a plurality of acoustic reflector face means each for a different one of said waves and facing a different substantially radially outward direction from said central axis. 37. The apparatus of claim 34, wherein each of said rod means is comprised of a magnetostrictive material having a strain constant, which when vibrated in the direction of the wave axis corresponding thereto is capable of creating a changing magnetic field in said direction.

38. The apparatus of claim 37, wherein a next one of said plurality of rod means, and each one of said plurality of rod means having a wave axis separated from the wave axis corresponding to said next one of said plurality of rod means by an angle equal to 2nα with respect to said central axis are comprised of a first magnetostrictive material having a first strain constant, and wherein every one of said plurality of rod means is comprised of a second magnetostrictive material having a second strain constant different from said first strain constant, where n is a positive integer. 39. The apparatus of claim 38, wherein said first magnetostrictive material is nickel and said second magnetostrictive material is 2V Permendur. 40. The apparatus of claim 39, wherein the absolute value of the first strain constant is larger than that of the second strain constant, and also comprising:

an electrically conducting metal element wrapped about each of said rod means comprised of a material having said first strain constant so that the absolute value of the effective strain constant of each of said wrapped rod means is reduced to a value less than the absolute value of said first strain constant. 41. The apparatus of claim 37, wherein each of said plurality of rod means has substantially the same

strain constant. 42. The apparatus of claim 41, also comprising:

pre-biasing for establishing a constant selected magnetic field adjacent predetermined ones of said plurality of rod means. 43. The apparatus of claim 42, wherein said pre-biasing means comprises a permanent magnet. 44. The apparatus of claim 42, wherein said pre-biasing means comprises an electromagnetic coil. 45. The apparatus of claim 35, wherein each of said rod means is comprised of a plurality of discs of said piezoelectric material stacked longitudinally in the direction of the wave axis corresponding thereto. 46. The apparatus of claim 36, wherein the number of said plurality of rod means is equal to 2N and the number of said plurality of face means is equal to 2N. 47. The apparatus of claim 36, wherein said acoustic wave reflector means defines an inverted pyramid in coaxial alignment with said central axis, said pyramid having faces, each of which comprises a different one of said plurality of face means, each of said faces being oriented above a respective one of said rod means and being further intersected by one of said wave axes corresponding to said

respective one of said rod means. 48. The apparatus of claim 36, wherein each of said plurality of face means is oriented for reflecting a different one of said waves at an angle r from a plan perpendicular to said central axis, where r is greater than zero.

49. Apparatus for detecting multipole acoustic waves in a subsurface earth formation traversed by a borehole with a sonde having a central longitudinal axis disposed therein, wherein said multipole acoustic waves in the formation create 2N acoustic pressure waves propagating radially toward the sonde, said apparatus comprising:

acoustic wave reflector means for reflecting said acoustic pressure waves propagating radially toward the sonde at a plurality of approximately equal angles α in a plane substantially perpendicular to the axis of the sonde, wherein α is approximately equal to 360°/2N, so that such reflected waves propagate along respective wave axes substantially parallel to the central longitudinal axis of the sonde; and

acoustic wave detector means for simultaneously detecting the 2N acoustic pressure waves, where N is an integer not less than one, at a corresponding number of discrete locations spaced radially outward from said central longitudinal axis, said waves propagating along respective wave axes in directions substantially parallel to said central longitudinal axis and any first one of the waves traveling along a corresponding first one of the wave axes is substantially out of phase with respect to any second one of the waves traveling along a

corresponding second one of the wave axes. 50. The apparatus of claim 49, wherein said detector means includes a plurality of rod means each longitudinally coaxial to a different corresponding one of said wave axes for detecting vibrations in the direction of said wave axes. 51. The apparatus of claim 50, wherein said plurality of rod means is comprised of piezoelectric material which produces measurable signals in response to vibrations in the direction of the wave axis corresponding thereto. 52. The apparatus of claim 50, wherein said acoustic wave reflector means comprises a plurality of acoustic reflector face means each for a different one of said waves and facing a different substantially radially outward direction from said central axis. 53. The apparatus of claim 50, wherein each of said rod means is comprised of a magnetostrictive material having a strain constant, which when vibrated in the direction of the wave axis corresponding thereto is capable of creating a changing magnetic field in said direction.