METHOD AND APPARATUS FOR MULTIPOLE ACOUSTIC LOGGING

Abstract: Apparatus for shear wave borehole logging has a source section 30, a spacer section 32, a detector section 34 and a coupling section 36, adjacent sections being linked by isolation joints 38. The source section 30 has a slotted steel housing 48 containing a source 42 with two transducers 44 a /44 b and 44 c /44 d each arranged to produce acoustic radiation with a dipole radiation pattern. The transducers are oriented so that their radiation patterns are transverse to one another and to the borehole axis. The detector section 34 also has a slotted steel housing 80 and contains six detectors 82 each with two transducers 84 a /84 b and 84 c /84 d arranged similarly to the source transducers. The isolation joint 38 between adjacent sections comprises lengths of steel wire 76 secured at each end to one of the sonde sections and bridging a gap between the sections. In use each of the source transducers is energized in turn as the apparatus is drawn up the borehole, and the waveforms of signals received by the twelve detector transducers are digitized and recorded. Various analyses of the recordings enable the formation shear slowness to be derived, even in 'slow' formations, and permit information to be obtained regarding the magnitude and orientation of anisotropy in the formation.

Description: BACKGROUND OF THE INVENTION This invention relates to methods and apparatus for multipole acoustic logging, for example for logging the shear wave propagation characteristics of earth formations traversed by a borehole. Conventional acoustic logging of earth formations traversed by a borehole is accomplished by means of a logging tool lowered into the borehole on an armored communication cable. Such a tool typically incorporates several acoustic transducers, at least one being operated to produce acoustic signals and one or more others being operated to detect such signals. These transducers are commonly made of piezoelectric ceramic or magnetostrictive materials which expand and contract transversely to their surfaces (i.e. change in thickness) in response to electrical excitation, or conversely generate electrical voltages between those surfaces when subjected to pressure fluctations. Thus the transmitter can be driven with an appropriate oscillating electrical signal to generate pressure fluctuations in liquid in the borehole. These pressure fluctuations propagate as acoustic signals through the liquid and into and through the surrounding formations. The signals traversing the formations couple back into the borehole liquid, for example in the vicinity of the receivers, causing pressure fluctuations which result in electrical voltages at the outputs of the receivers. These voltages are sensed and amplified, and may be processed downhole to extract information for transmission up the cable; alternatively the waveforms of the received signals may be transmitted uphole, for example as digitized time samples, for processing at the surface. The excitation of the transmitter is commonly pulsed, enabling determination for example of the time taken by an acoustic signal to propagate the known distance between the transmitter and the receiver, and thus of the velocity (or its inverse, slowness) of acoustic propagation in the formations. Although the type of transmitter most commonly used (a cylinder) generates pressure or P waves, known tools are not limited to investigating the propagation of such waves. When the P wave reaches the borehole wall, some of the acoustic energy is typically converted into other modes of acoustic propagation. Thus, shear or S waves may be excited in the formation, and Stoneley waves (including the low-frequency type known as tube waves) may be excited at the borehole/formation interface. Because these different modes generally travel at different speeds these additional modes can sometimes be distinguished in the receiver signals. Determination of such parameters as the speed and attenuation of P,S and Stoneley waves is useful in investigating a variety of subsurface formation properties of interest in the exploration for hydrocarbons and other valuable raw materials. However, in certain circumstances transmitters which generate pressure waves are not effective in inducing propagation of S waves in a manner that permits their detection from within the borehole. If the speed of S waves in the formation is less than the speed of acoustic (pressure) waves in the borehole liquid, as in the case of a so-called 'soft' or 'slow' formation, shear wave energy in the formation cannot induce corresponding phenomena in the borehole liquid so the shear wave cannot be detected and its velocity cannot be found. Various techniques have been suggested to facilitate the logging of shear wave properties of subsurface formations irrespective of relative borehole and formation sonic properties. Thus, a variety of sources for exciting shear waves directly have been proposed. These have included various forms of contact devices (US patent specifications 3,354,983, 3,683,326, 4,380,806, 4,394,754, 4,549,630). US patent specification 3,475,722 teaches the use of three geophones arranged along mutually orthogonal axes on a common mount pressed into contact with the borehole wall. However all these devices suffer from the problem of requiring direct contact with the formation, and the consequent difficulties of limitation on logging speed,risk of the tool sticking and bad contact in poorly consolidated formations. More recently, various kinds of multipole or azimuthally asymmetric transducers which can be suspended in the borehole liquid have been proposed for direct or indirect shear wave logging, such as dipole transducers (US patent specifications 3,593,255, 4,207,961, 4,383,591, 4,516,228; GB patent specification 2,124,377; EP patent specification 0 031 989), quadrupole transducers (GB patent specifications 2,122,351, 2,132,763) and octopole transducers (GB patent specification 2,130,725). Another suggestion has been to use geophones suspended in the liquid in a borehole and with a buoyancy adjusted to be nearly neutral for sympathetic movement with the borehole wall (US patent specification 4,369,506); one such proposal (US patent specification 4,542,487) provides geophones in orthogonally mounted pairs. These buoyancy adjustments are difficult to make accurately and significantly complicate the use of such devices. Furthermore, previously known devices have relied on the use of non-rigid sondes, for example using cable or rubber hose to interconnect portions of the logging apparatus, in order to reduce problems introduced by propagation of extensionsal and bending acoustic energy directly along the sonde from the source to the detector. Such non-rigid sondes lack ruggedness and are therefore neither easy to use nor able to withstand prolonged operation in the rigorous environment typical of borehole logging. Patent GB-A-2 145 521 describes a multipole acoustic wave logging sonde in which various poled sources can be used including dipole, quadrupole and higher poled sources. The detectors have polarization orientations corresponding to the source to measure compressional waves or shear waves in the formation. Patent EP 0 224 350 was published (3.06.87) after the priority date (22.11.85) of the present application. This patent describes a logging tool for exciting flexural wave energy with dipole motion in a plurality of azimuthal directions around the axis of the borehole. The transmitter comprises a plurality of transducers spaced circumferentially about the axis of the tool to focus dipolar energy into the formations in a plurality of azimuthal directions about the borehole. It is an object of this invention to provide a method and apparatus for shear wave acoustic logging of subsurface earth formations which provides information as a function of azimuthal direction around the tool. A further object of the invention is to provide a method and apparatus for shear wave acoustic logging of subsurface earth formations which enables quantification of formation or rock anisotropy and identification of minimum and maximum stress directions. Such information is valuable in the interpretation of seismic records, the planning of enhanced oil recovery and the planning of hydrofracturing operations for example. SUMMARY OF THE INVENTION According to one aspect of this invention an apparatus for acoustic acoustic logging is set forth in claim 1. According to another aspect of the invention, a method of 5 acoustically investigating a subsurface earth formation is set forth in claims 5 and 14. BRIEF DESCRIPTION OF THE DRAWINGS The invention will become more apparent upon consideration of the following detailed description of the invention, reference being had to the accompanying drawings in which: Figure 1 is a schematic diagram of a borehole logging operation using apparatus in accordance with this invention; Figure 2 is a side view of apparatus in accordance with this invention; Figure 3 is a schematic perspective view of a transducer forming part of the apparatus of Figure 2; Figure 4 is a block diagram of circuitry associated with a source transducer forming part of the apparatus of Figure 2; Figure 5 is a diagram illustrating the radiation pattern of a dipole section of the source transducer of Figure 4; Figure 6 is a sectional view showing the mounting of the transducer of Figure 2; Figure 7 is a block diagram of circuitry associated with a detector transducer forming part of the apparatus of Figure 2; Figure 8 is a diagram illustrating variation of shear velocity with polarization owing to stress and/or formation anisotropy; Figure 9 is a block diagram of a modified form of circuitry associated with a source transducer forming part of the apparatus of Figure 2; Figure 10 is a block diagram of a modified form of circuitry associated with a detector transducer forming part of the apparatus of Figure 2; Figure 11 is a block diagram of another modified form of circuitry associated with a source transducer forming part of the apparatus of Figure 2; Figure 12 is a sectional view of a modified isolation joint forming part of the apparatus of Figure 2; Figure 13 is a sectional view of another modified isolation joint forming part of the apparatus of Figure 2; Figure 14 is a flow chart for a first method of analyzing measurements made with the apparatus of Figure 2; Figure 15 is a flow chart for a second method of analyzing measurements made with the apparatus of Figure 2; Figure 16 is a flow chart for a third method of analyzing measurements made with the apparatus of Figure 2; Figure 17 is a flow chart for a fourth method of analyzing measurements made with the apparatus of Figure 2; and Figure 18 is a flow chart for a fifth method of analyzing measurements made with the apparatus of Figure 2. DETAILED DESCRIPTION Referring to Figure 1, an elongate logging tool or sonde 10 is suspended on an armored communication cable 12 in a borehole 14 penetrating an earth formation 16. The borehole 14 is filled with liquid 18 such as drilling mud used to stabilize the borehole wall and prevent escape of formation fluids up the borehole. The sonde 10 is moved in the borehole 14 by paying the cable 12 out and reeling it back in over a sheave wheel 20 and a depth gauge 22 by means of a winch forming part of a surface equipment 24. Usually the logging measurements are actually made while the sonde 10 is being raised back up the borehole 14, although in certain circumstances they may additionally or alternatively be made on the way down. The depth gauge 22 measures displacement of the cable 12 over the sheave wheel 20 and thus the depth of the tool 10 in the borehole 14. The tool 10 includes an acoustic source section 30 at the bottom, a spacer section 32 separating this source section from an acoustic detector section 34, and a coupling section 36 above the detector section and attached to the cable 12. The sections 30 through 36 are coupled mechanically to each adjacent section by acoustic isolation joints 38. The source section 30 generates acoustic radiation which propagates through the liquid 18 to and then up the formation 16. Some of the acoustic radiation propagates back through the liquid 18 to the detector section 34 which generates digitized signals representative of successive samples of the waveform of the detected radiation. These signals are suitably conditioned by processing and interface circuitry in the sonde 10 for transmission up the cable 12 to the surface equipment 24. The equipment 24 typically receives, decodes, amplifies and records the signals on chart an/or magnetic tape recorders as a function of the depth signals generated by the depth gauge 22. In addition the equipment 24 may process the data represented by these signals to yield indications of the required formation parameters which are also recorded. Further processing of these and other signals from the sonde 10 enables the surface equipment 24 to monitor the operation of the sonde 10 and generate signals which are transmitted down the cable 12 to control the sonde 10, for example to synchronize the operation of its component circuits or modify circuit parameters such as amplifier gain. Figure 2 shows the sonde 10 in greater detail. Referring to Figure 2, the source section 30 has a sleeve 48 closed at one end and made of steel pipe perforated by numerous slots as at 40. This section contains an acoustic source 42 for propagating acoustic radiation through the borehole liquid 18 to the formation 16. As shown schematically in Figure 3, one possible configuration of this source incorporates four matched active radiating elements 44. These elements may for example be rectangular, piezoelectric bimorph bender elements of known kind which deflect transversely to their surfaces when a voltage is applied between those surfaces. The elements 44 are secured along opposite edges to support plates 46, and opposed pairs of the elements, such as 44 a and 44 b , are oriented to deflect in the same direction when the same polarity voltage is applied to like-facing surfaces of the elements. In the illustrated embodiment of the invention like-facing surfaces of opposed pairs of the elements are connected together to receive the same driving voltage. This orientation and interconnection is indicated in Figure 4 by + and - symbols adjacent each element 44. Also shown in Figure 4 is a form of driving circuitry for the source 42. A trigger signal is supplied on a line 50, for example by a timer circuit in the sonde 10 or the surface equipment 24. This trigger signal fires a pulser 52 which supplies a pulse comprising one cycle of a sinusoid waveform to a push-pull amplifier 54. The frequency of the waveform is preferably chosen in accordance with the criteria discussed in US patent 4 703 460. To optimize the shear measurement it is generally desirable to use a frequency within a range whose upper bound f c is given by: f c = v s /2πa where v s is the formation shear velocity and a is the borehole radius. For most formations the operating frequency is typically in the range 1 to 5 kHz. However, the frequency may exceed the limit f c in certain circumstances, for example to obtain a stronger signal by exciting borehole flexural waves and then derive shear velocity in the manner described in US patent 4 698 792. The output of the amplifier 54 is coupled to four gates 56 a to 56 d each of which is coupled to a respective pair of the elements 44 in the acoustic source 42. The gates 56 a and 56 b are coupled to respective surfaces of the opposed pair of elements 44 a and 44 b and receive opposite polarity signals from the amplifier 54. Both of these gates are controlled by a Q output of a bistable circuit 58 having a clock input C connected to the trigger signal line 50. The gates 56 c and 56 d are likewise connected to respective surfaces of the opposed pair of elements 44 c and 44 d and receive opposite polarity signals from the amplifier 54, but are controlled by an inverse-Q output of the bistable circuit 58. Thus each successive trigger signal on the line 50 energizes the Q and inverse-Q outputs of the bistable circuit 58 alternately, and the amplified pulse output of the amplifier 54 is coupled to the pairs of elements 44 a /44 b and 44 c /44 d alternately. The interconnection and energization of the elements 44 as shown in Figure 4 causes both elements of each opposed pair to deflect in the same direction, producing a dipole compressional (P) wave radiation pattern with maxima along the direction of deflection. This is illustrated in Figure 5 for the pair of elements 44 a and 44 b , where the arrow 60 indicates the direction of deflection and the dotted circles 62 indicate the azimuthal variation in the magnitude of the compressional wave. Although the pattern is symmetric in magnitude it is asymmetric in polarity: a positive pressure fluctuation to one side of the transducer 42 is always accompanied by a negative pressure fluctuation to the other side. Thus when the compressional wave reaches the borehole wall it induces a shear (S) wave in the formation 16 propagating in a direction generally parallel to the borehole 14. The direction (or polarization) of formation particle vibration in this S wave is transverse both to the borehole axis and to the direction of propagation of the wave, as indicated by the double-headed arrow 66, and the magnitude of the shear wave vibration exhibits a dipole radiation pattern with maxima transverse to the direction of deflection of the elements 44 a and 44 b , that is up and down the borehole 14, as shown by the solid circles 64 in Figure 5. With the transverse arrangement of the pairs of elements 44 a /44 b and 44 c /44 d shown in Figure 3, the polarizations of the respective S waves produced by these element pairs are also transverse to one another. The slots 40 in the source section 30 provide a path for the propagation of acoustic energy from the transducer 42 to the borehole liquid 18 and the formation 16 surrounding the sonde 10. In addition they slow down the propagation of acoustic waves through the steel sleeve 48, thereby facilitating the use of this sleeve to protect the transducer 42 and provide mechanical strength. The magnitude of signals propagating through the sleeve 48 is further limited by minimizing the mechanical coupling between the transducer 42 and the sleeve 48. Thus, as illustrated in Figure 6, the transducer 42 is supported by flexible rods 70 secured in holes 72 in the center of the support plates 46 (Fig. 3). These rods, which may be made for example of steel tubes loosely coupled by transverse pins, are in turn secured at each end to a cross-plate 74 secured inside the sleeve 48. Propagation of acoustic energy along the tool is also inhibited by the acoustic isolation joints 38. As shown in Figure 2 these comprise lengths of steel wire 76 approximately 2 mm in diameter, bridging a gap between each section of the sonde 10. The gap is approximately 25 mm wide and the ends of the wires overlap the ends of the sonde sections by approximately 10 mm. The wires 76 are spaced approximately 30° apart around the outer periphery of the sonde sections and secured thereto in any convenient manner, such as by clamping them under the heads of bolts threaded into the sonde sections. It has been found that such a joint is simple and robust, resists bending and twisting of the sonde sections relative to one another and attenuates acoustic energy propagating along the sonde 10 by as much as 30 to 40 dB. To prevent ingress of material that might lodge in the gap between the sonde sections, the joint 38 is closed by a flexible sleeve 78, for example of rubber. The lowest joint 38 connects the source section 30 to the spacer section 32 which comprises a blank steel pipe separating the acoustic source from acoustic receivers in the transmitter. This separation helps to isolate the detector section 34 from the propagation of acoustic energy along the body of the sonde 10. Another isolation joint 38 couples the spacer section 32 to the detector section 34, which has a sleeve 80 of steel pipe perforated with slots 40 in the same manner as the source section 30. The detector section 34 contains an array of six acoustic detectors 82 for detecting acoustic radiation propagating through the borehole liquid 18 from the formation 16. Each of these detectors 82 is similar in structure and mounting to the source 42, and as shown in Figure 7 has like-facing surfaces of opposed pairs of matched elements such as 84 a and 84 b connected in parallel to respective inputs of an amplifier 86. Thus each detector 82 has two associated amplifiers 86, the output of each of which is coupled to a respective analog-to-digital converter 88. This converter includes a sample-and-hold circuit and an oscillator which generates timing pulses to coordinate sampling and digitizing of the magnitude of the signal from the amplifier 86 at regular intervals, for example with a repetition rate of 100kHz. The A-D converter 88 has a reset input R connected to the trigger signal line 50 to re-synchronize the oscillator in the converter 88 each time the source 42 is triggered. The pairs of opposed elements such as 84 a and 84 b have a dipole sensitivity pattern as shown in Figure 5, and the detectors 82 are oriented within the section 34 so that the polarization of shear waves sensed by each pair of elements is aligned with the polarization of the shear wave radiation produced by a corresponding pair of elements 44 in the source 42. The third isolation joint 38 (Fig.2) couples the detector section 34 to the coupling section 36 which comprises a blank steel pipe closed at one end and attached to the cable 12. This section may conveniently house circuitry for conditioning and communication of signals between the sonde 10 and the surface equipment 24, and for coordinating and controlling the operation of the sonde 10. In particular the digitized output signals of each A-D converter 88 are sent uphole via the circuitry in the section 36 and the cable 12 to the surface equipment 24 for recording and analysis as required. Precise details of the circuit components such as amplifiers and analog-to-digital converters discussed herein are not a part of this invention, and being well known to those skilled in the art need not be described. Likewise the details of construction of the sonde 10, and its possible combination with other known logging devices such as a caliper and a gamma ray sonde, will be obvious to those skilled in this art. In particular, a device of known type for measuring the azimuthal orientation of the sonde 10 in the borehole 14 is desirably included. In operation the pulser 52 is supplied with a train of trigger signals via the line 50. As explained above these trigger signals cause the opposed pairs of elements in the source 42 to be excited alternately, resulting in the propagation of shear wave energy through the formation and the detection of corresponding signals by the detectors 82. The interval between successive trigger signals is arranged to allow recording of waveforms from the detectors 84 of sufficient duration to include all components of interest before arrival of energy resulting from the next following energization of the source 42. This interval will therefore depend on the velocity of acoustic energy propagating through the formation 16, and may be fixed in advance in anticipation of expected shear velocity values for the formation 16 to be investigated or adjusted during logging in accordance with actual measured values; The repetition rate of the trigger signals will also depend on the rate at which the sonde 10 is drawn up the borehole 14 and the desired distance between formation points at which measurements are to be made. Thus for a measurement spacing of 15cm (six inches) at a logging speed of 15 cm/s (1800 feet per hour) one trigger pulse per second suffices. A typical maximum trigger pulse rate is ten pulses per second. It is also preferable that the measurement spacing equal the spacing between detectors 82, by arranging that successive pairs of trigger pulses occur after the sonde 10 has moved a distance equal to the detector spacing, since this allows the use of analytical techniques which take advantage of the multiple redundant measurements obtained with an array of detectors. The A-D converter 88 is free-running, so it typically digitizes many samples between measurement trigger signals that are not required. These surplus measurements need not be recorded. However, the measurements occurring after a trigger signal but before acoustic energy reaches the detectors 82 preferably are recorded. This permits the initial part of the digitized signal, before arrival of acoustic energy at the detectors 82, to be used in establishing a zero level for analysis of subsequent parts of the digitized signal. As each opposed pair of elements 44 is energized, corresponding signals are recorded both for the aligned elements 84 in the detectors 82 and for the orthogonal elements 84, so a total of four recordings are obtained from each of the six detectors 82 for each complete energization of the source 42. Since the sonde 10 is being gradually drawn up the borehole as the source 42 is repetitively triggered, successive sets of twenty-four recordings are provided by the array of detectors 82 for successive positions of the sonde 10. These recordings can be analyzed in a variety of ways to derive useful information about the formation 16. Since the source 42 has a dipole radiation pattern as explained above, it can excite shear wave radiation in the formation 16 in a manner which permits detection by the detectors 82 irrespective of the relative acoustic velocities in the formation 16 and the liquid 18. Thus the determination of shear velocity for the formation 16 is possible even in soft or other types of slow formation, without resorting to indirect methods such as derivation from the velocity of other acoustic wave modes. In addition the detectors 82 are relatively insensitive to such modes as tube waves, which could otherwise constitute a source of perturbation of the wanted shear wave signals. Other types of formation characteristic which can be investigated by measurements made with the sonde 10 include the presence and direction of formation or rock anisotropy (that is anisotropy in the physical character of the formation material), and the distribution of stress in the formation 16, in particular the direction and magnitude of any anisotropy in that stress. It is known that stress in a solid affects the velocity of shear wave acoustic energy propagating through that solid. A subsurface formation may sustain stresses that vary with direction (that is they are anisotropic), for example owing to tectonic forces. In such a case shear wave energy propagating with the polarization will have a velocity different from such energy propagating with a different polarization. Referring to Figure 8, the borehole 14 is illustrated in cross-section and the axes S x and S y represent the directions of polarization of radiation generated by the element pairs 44 a /44 b and 44 c /44 d respectively in the source 42. Typically there will be two possible shear wave velocities, a minimum value v₁ for waves polarized in a direction at an angle  to the axis S x , and a maximum value v₂ for waves polarized in the orthogonal direction. Excitation of shear waves in any other direction will generate two components polarized along these two directions. The variation with azimuthal direction in the relative amplitudes of these components is indicated in Figure 8 by the circle 90. The distance from the v₁, v₂ axes of any point on the circle represents the amplitude of the respective shear wave components for excitation along a line passing through that point and the origin. The quantities v₁, v₂ and  are of interest in, for example, the planning of fracturing operations for producing hydrocarbons from the borehole 14. It can be shown that if the minimum and maximum shear velocities v₁ and v₂ are known the principal radial stresses σ₁ and σ₂ along the axes of v₁ and v₂ can be derived from the following relationships: ρv₁²=µ+(σ z /3K)(4λ+4µ+m+nλ/4µ)+ (σ₁/3K)(λ+2µ+m+nλ/4µ)+(σ₂/3K)(m-n(λ+µ)/2µ-2λ) (1) ρv₂²=µ+(σ z /3K)(4λ+4µ+m+nλ/4µ)+ (σ₁/3K)(m-n(λ+µ)/2µ-2λ)+(σ₂/3K)(λ+2µ+m+nλ/4µ) (2) where ρ is the formation density, µ and λ are the second-order elastic (Lamé) constants of the formation, m and n are third-order elastic constants of the formation, K is the formation bulk modulus (=λ+2µ/3) and σ z is the stress due to overburden pressure at that point in the formation. If it is assumed that the formation is isotropic, it can also be shown that the stress difference in the two directions of particle motion is given by: σ ₁-σ₂=4µρ(v₁²-v₂²)/(4µ+n) (3) All of the quantities ρ, µ, λ, m, n and σ z can be measured, or their values derived or estimated from known formation parameters or core samples for example. Further discussion of the expressions (1) to (3) can be found in 'On the feasibility of using sonic techniques to estimate in situ stresses' by Mao et al, Lawrence Livermore National Laboratory, June 1983, and in 'Second-order elastic deformation of solids' by Hughes et al, Phys. Rev., vol. 92, No. 5. Dec. 1953, pp. 1145-1149. Even if only the shear velocities along the S x and S y axes are derived, rather than the maximum and minimum shear wave velocities v₁ and v₂, valuable information about the presence of anisotropy can be derived. In one type of analysis of measurements made with sonde 10, known techniques are used to derive the differing times of occurrence (or 'moveout') of shear wave components in the recorded waveforms obtained from correspondingly aligned element pairs 84 a /84 b of detectors 82 at different spacings from the source 42. Specifically, and as illustrated by step 200 in the flow chart shown in Figure 14, the first motion of the shear wave component in each waveform is located, and then the time of occurrence of this first motion is determined (step 210) for each detector. Knowing the spacing of the detectors 82, the moveout enables the shear velocity to be found for acoustic energy polarized along the axis S x , as at step 220. The shear velocity is likewise found for acoustic energy polarized in the transverse plane S y , by reference to the recordings obtained from the other. transversely aligned elements 84 c and 84 d - steps 230 through 250. Comparison of these velocities, and in particular any difference between them, can then be used as at step 260 to derive valuable indications of the presence or absence of stress anisotropy, rock strength and fractures, for example. As indicated in Figure 8 and mentioned above the velocity values obtained will not necessarily be the maximum and minimum values v₁ and v₂ for the formation 16 at the measurement depth, but will be related to those values by the (in this case unknown) azimuthal orientation  of the sonde 10 relative to the axes of anistropy. Alternatively, and as shown in Figure 15, the waveforms received by both orthogonal pairs of detector elements 84 upon energization of one of the pairs of source elements 44 can be analysed to derive their amplitudes, as at steps 300 and 310, for example by measuring their peak amplitudes or integrating the waveforms with respect to time. Thus the energy, or a quantity functionally related thereto, of shear waves propagating with polarizations parallel and perpendicular to the source elements can be found. Comparison of these amplitudes or energies at step 320, for example by taking their ratio, is indicative of the presence of formation anistropy. A third possibility is to combine the signals from the orthogonal pairs of elements 83 in a detector 82 using sinusoidally weighted vector addition to simulate the effect of physically rotating the source 42 and the detectors 82. The shear wave X that would be received at time t by a single pair of detector elements aligned with a single pair of source elements and at an angle ϑ to the actual orientation of the elements 84 a /84 b is X(ϑ,t)=X x (t)cos²ϑ+X y (t)sinϑcosϑ+Y x (t)cosϑsinϑ+Y y (t)sin²ϑ (4) where X x and X y represent the orthogonally polarized shear waves which are received at time t by the detector element pairs 84 a /84 b and 84 c /84 d respectively upon energization of the source element pair 44 a /44 b ; and Y x and Y y represent the orthogonally polarized shear waves which are received at time t by those respective pairs upon energization of the source element pair 44 c /44 d . Thus, referring to Figure 16, the digitized sample values of the like and cross polarized waveforms supplied by the A-D converters are recorded at step 400. A value for the angle ϑ is set at step 410 and the waveforms for one of the detectors 82 are selected for analysis at step 420. The analysis proceeds to step 430 where the digitized sample values at the same time t in the waveforms form the selected detector 82, relative to the time of triggering of the source 42, are combined by vector addition, using the weightings given by equation (4), to produce a value representing a sample at time t of a new, synthesized, digitized waveform X(ϑ,t) for the angle ϑ. Step 430 repeats this process for all values of t to produce a complete new waveform for the selected detector 82 at the current angle ϑ. At step 440 a check is made as to whether the waveforms for all detectors 82 have been processed for the current value of ϑ. If not the analysis returns to step 420 to select the waveforms from another detector 82 for vector addition at step 430. If the waveforms for all the detectors have been processed the analysis proceeds to step 450, where the set of synthetic waveforms thus generated is analyzed as described above with reference to steps 200 through 250 of Figure 14 to derive values for shear velocity along the S x and S y axes, and also the signal amplitudes, for the current angle ϑ. At step 460 a check is made to determine whether the processing for all desired values of ϑ has been accomplished. If not the procedure returns to step 410 to set another value of ϑ and repeat steps 420 through 450. This process is thus performed for various values of ϑ, after which the maximum and minimum values of the velocities derived at step 450 (interpolated relative to angle ϑ if desired) are determined at step 470, together with the corresponding value of ϑ. These minimum and maximum values are the desired values v₁ and v₂, and the corresponding angle ϑ is the value of . These results taken in conjunction with orientation readings from an orientation device incorporated with the sonde 10 provide information on the direction and magnitude of any anisotropy in the formation 16, by means of the expressions (1) and (2) for example. Another method of analyzing the angular dependency of the received signals involves expressing those signals for each detector 82 as a function of the signals S x f(t) and S y f(t) generated by the source element pairs 44 a /44 b and 44 c /44 d , as follows: X x (z,t)=(S x /z[cos²f(t-z/v₁)+sin²g(t-z/v₂)] (5) X y (z,t)=(S x /z)sincos[-f(t-z/v₁)+g(t-z/v₂)] (6) Y x (z,t)=(S y /z)sincos[-f(t-z/v₁)+g(t-z/v₂)] (7) Y y (z,t)=(S y /z)[sin²f(t-z/v₁)+cos²g(t-z/v₂)] (8) where z is the distance along the sonde 10 between the source 42 and the relevant detector 82 and f(t) and g(t) represent functions of time. Referring to Figure 17, each of the sets of signals such as X x (z,t) is recorded at step 500 with the array of detectors 82. The waveforms for a given depth are then used to derive an estimate of the velocities v₁ and v₂ at that depth, at step 510. This may be accomplished using any of several known techniques for analyzing signals from arrays of detectors, such as slowness-time coherence analysis (USP 4 594 691), shot to shot processing (USP 4 543 648), the maximum likelihood method or Prony's method for example. At step 520 the estimated velocity values are compared. If they are substantially equal, that is only a single shear wave velocity is derived, it can be concluded that the formation is isotropic, as at step 530. However, if there are two distinct velocities, a choice is made at step 540 among the estimates derived from each of the signals such as X x (z,t). This choice may be made, for example in the case of slowness-time coherence analysis, by selecting the estimates which have the largest semblance values together with appropriate values for velocity and time relative to the known properties of the formation 16. In the case of an analysis which yields amplitude values, such as Prony's method, the estimates having the largest associated amplitudes may be chosen. The chosen estimate of the velocity v₁ is then used at step 550 to control velocity filtering of the waveforms form the array of detectors 82. This is done for example by stacking the waveforms with a time offset between waveforms from adjacent detectors that is a function of the estimated velocity v₁ and detector spacing. This emphasizes the component of the waveforms corresponding to the velocity v₁. The amplitude of the v₁ component in the filtered X x waveforms is proportional to the term cos²f(t-z/v₁) of equation (5), and the amplitude of this component in the filtered Y y waveforms is proportional to the term sin ²f(t-z/v₁) in equation (8). Thus the ratio A of the filtered v₁ amplitude of the X x waveforms to the filtered v₁ amplitude of the Y y waveforms is equal to 1/tan², from which  can be directly obtained at step 560. The chosen estimate of the velocity v₂ is likewise used to control velocity filtering of the same waveforms at step 570, and the ratio B of the filtered v₂ amplitudes of these waveforms is derived. This ratio B is equal to tan², enabling a second derivation of  at step 580 as a cross-check for the first value obtained. If the spacing z between the source 42 and the detectors 82 is made large enough for the different shear wave velocities v₁ and v₂ to produce distinct components of the waveforms generated by the detectors 82, a modification of the technique just described can be used. In this case during the time period z/v₂<t<z/v₁ the equations (5) through (8) above become: X x (z,t)=(S x /z)sin²g(t-z/v₂) (5′) X y (z,t)=(S x /z)sincosg(t-z/v₂) (6′) Y x (z,t)=(S y /z)sincosg(t-z/v₂) (7′) Y y (z,t)=(S y /z)cos²g(t-z/v₂) (8′) Accordingly, as shown in Figure 18, the like and cross polarized waveforms are recorded at step 600 and the portions of these waveforms for the time period z/v₂<t<z/v₁ are selected at step 610. Detection of the shear wave first arrivals in these signals, by first motion (threshold) detention, or by slowness-time coherence analysis with a short time window extending no later than z/v₁, enables the presence and velocity v₂ of the faster shear wave component to be derived, as at step 620. The original waveforms such as X x (z,t) of equations (5) through (8) are then velocity filtered using this value of v₂ at step 630 to remove this faster shear wave component, and the filtered waveforms used at step 640 to estimate the velocity v₁ of the slower component. The angle  is derived at the step 650 from a combination of portions of waveforms such as X x (z,t) and X y (z,t) selected at step 610. This derivation may use any of several relationships between the waveform amplitudes and , such as tan=X x /X y =Y x /Y y , or tan²=X x /Y y . Equation (4) above for synthesizing the effect of rotating the source 42 and the detectors 82 relates to the specific situation in which the source and detectors are rotated in synchronism to remain in alignment. The signal that would be obtained in the more general case where the source 42 and the detectors 82 are at arbitrary angles ϑ s and ϑ d with respect to the elements 84 a /84 b can be synthesized from the detector signals according to the relation: X(ϑ s ,ϑ d ,t)=[X x (t)cosϑ d +X y (t)sinϑ d ]cosϑ s + [Y x (t)cosϑ d +Y y (t)sinϑ d ]sinϑ s (9) It will be apparent to those skilled in the art that the above described analyses can be performed, for example, by means of an appropriate programmed digital computer or alternatively by using special-purpose circuitry. Furthermore, instead of incorporating angular functions during analysis of the recorded signals subsequent to logging, these functions can be applied during the logging operation itself. Thus, in one modification of the sonde 10 shown in Figure 9, the signal from the pulser 52 is supplied to a pair of variable-gain push-pull amplifiers 100 and 102. These supply drive signals to the elements pairs 44 a /44 b and 44 c /44 d respectively with amplitudes A₁ and A₂ controlled by signals on control lines 104 and 106. The effective azimuthal direction ϑ of the polarization of the shear waves propagated by the source 42 can be steered by appropriate variation of the amplitudes A₁ and A₂, according to the relation ϑ=tan⁻¹A₂/A₁ (10) A particularly advantageous version of this implementation is to vary the angle ϑ at a frequency equivalent to the frequency of the waveform supplied by the pulser 50 to the amplifiers 100 and 102. This has the effect of propagating acoustic energy in the formation 16 with circular polarization. The direction of signals sensed by the detectors 82 can likewise be steered as shown in Figure 10. Each pair of detector elements 84 a /84 b and 84 c /84 d is coupled to a respective amplifier 86 a and 86 c which in turn feeds a respective multiplier 108 and 110. These multipliers multiply the amplified signals by respective functions cosϑ and sinϑ received from an oscillator 112. The outputs of the multipliers 108 and 110 are summed and digitized by the A-D converter 88, giving the vector sum X(ϑ,t)=X x (t)cosϑ+X y (t)sinϑ (11) which would be produced by a single pair of opposed elements such as 84 a /84 b oriented at an angle ϑ to the pair of elements 44 a /44 b of the source 42. Another way of steering the dipole radiation pattern of either the source 42 or the detector 82 is to rotate the transducer physically by means of, for example, a stepping motor having a shaft on which the transducer is mounted. However, the electronic steering described above has the advantages of avoiding the need for movable transducers, permitting various steering options to be investigated during subsequent processing and permitting movement of the dipole radiation patterns at speeds that would be difficult or impossible to accomplish with motor-driven transducers. Instead of direction coupling opposed source elements 44 as shown in Figures 4 and 9, it is also possible to provide each such element with individual drive signals as shown in Figure 11. Referring to Figure 11, the signal from the push-pull amplifier 54 is supplied to four controllable inverters 120 a to 120 d which in turn drive respective elements 44 a to 44 d of the source 42. The polarities of the drive signals from the controllable inverters 120 are controlled by signals on lines such as 122. Appropriate choice of these polarities relative to one another enables the radiation pattern of the source 42 to be selected from a wide variety, such as the dipole pattern of Figure 5, a monopole pattern, or a quadrupole pattern for example. Likewise the individual detector elements 84 can be connected to respective amplifiers and A-D converters, and their signals combined with appropriate phases and weightings to simulate monopole, dipole or quadrupole detectors. Various other modifications to the sonde design described above are possible. Thus the two opposed pairs of elements such as 44 a /44 b and 44 c /44 d may be replaced by a two elements mounted transversely to one another and offset a small distance axially along the sonde 10. Other shapes for the elements 44, such as circular, are possible. Other types of element besides bimorph benders are usable, such as piezoelectric transducers shaped as quadrants of a cylinder, or electrodynamic transducers. Instead of providing an individual A-D converter for each detector element or connected group of elements, fewer converters may be used together with circuitry for multiplexing each converter among several detector elements. The spacer section 32 an/or one of the lower two isolating joints 38 may be omitted. The design of the isolating joints may differ from that shown in Figures 1 and 2. Thus, as shown in Figure 12, the joint may comprise a short steel cylinder 130 of smaller diameter than a sleeve such as the source section sleeve 48. This cylinder is disposed within and between the sleeve 48 and the spacer section 32, and is retained by two pins 132 extending through holes 134 in the cylinder 130 and corresponding holes 136 in the sleeve 48 and the spacer section 32. Sound absorbers such as rubber O-rings 138 are disposed between the cylinder 130 and the sleeve 48 or the spacer section 32. Another design for the joint 38 is shown in Figure 13, in which the facing ends of adjacent sections such as 30 and 32 are made solid, with spherical recesses 140 containing the spherical ends 142 of a dumb-bell shaped ball-joint link 144 extending between the sections 30 and 32. There has been described and illustrated herein methods and apparatus in accordance with the present invention for multipole acoustic logging. While particular embodiments of the invention have been described, it is not intended that the invention be limited thereby. Therefore it will be apparent to those skilled in the art that various changes and modifications may be made to the invention as described without departing from the scope of the appended claims.

Claims: An apparatus for acoustic logging to determine a property of subsurface formations traversed by a borehole, wherein an elongate sonde (10) is adapted for movement through a liquid-filled borehole traversing an earth formation; and source means (42) is mounted on said sonde for causing propagation of acoustic radiation in the formation with azimuthally separated dipole radiation patterns via the borehole liquid; and wherein acoustic detectors (82) are mounted on said sonde and spaced therealong from said source means, with each detector being sensitive along azimuthally separate directions with dipole characteristics for detecting via the borehole liquid, and generating signals representative of acoustic radiation propagating in the formation; characterized in that the sonde has a plurality of sections (30,34) containing respectively the source means and the detectors, said sections being coupled by a joint (38) which provides stiffness and acoustic isolation with a plurality of wires (76) extending between and secured to adjacent sonde sections. The apparatus of claim 1 characterized in that said joint (38) comprises a tube (130) extending into and between adjacent sonde sections and secured thereto by pins (132). The apparatus of claim 1 characterized in that said joint comprises a dumb-bell member (144) extending between adjacent sonde sections and having enlarged ends (142) disposed in cavities (140) in said sections. The apparatus of any preceding claim, characterized in that said source means (42) and said detectors (82) are mounted within said sonde on flexible rods (70). A method of acoustically investigating a subsurface earth formation traversed by a borehole through which an elongate sonde (10) is adapted to move, where the sonde has an acoustic dipole source (44) for generating dipole shaped acoustic pulses and further having dipole receivers (82) located along the sonde to detect acoustic energy with predetermined dipole patterns relative to that of the dipole source, comprising the steps of: energizing a first acoustic dipole source (44a,44b) with a first dipole radiation orientation to cause the propagation of acoustic radiation in a subsurface formation with a first azimuthal polarization orientation; energizing a second acoustic dipole source (44c,44d) with a second dipole radiation orientation so as to cause the propagation of acoustic radiation in a subsurface formation with a second azimuthal polarization orientation that differs from the first azimuthal polarization orientation by a predetermined angle; detecting, with a first acoustic receiver (84a,84b) having a dipole radiation pattern oriented along a first direction, acoustic radiation attributable to the first acoustic dipole source and producing a first waveform signal indicative thereof, and detecting acoustic radiation attributable to the second acoustic dipole source and producing a second waveform signal indicative thereof; and effectively azimuthally steering the acoustic investigation by combining the first and second waveform signals with relatively adjusted amplitudes and producing first composite dipole waveform signals representative of an azimuthal acoustic investigation of the subsurface earth formation. The method of claim 5 further comprising the steps of: detecting, with a second acoustic receiver (84c,84d) having a dipole radiation pattern oriented along a second direction which bears a second predetermined angle relative to the first direction, acoustic radiation attributable to the first acoustic dipole source (44a,44b) and producing a third waveform signal indicative thereof, and detecting acoustic radiation attributable to the second acoustic dipole source (44c,44d) and producing a fourth waveform signal indicative thereof; and combining said third and fourth waveform signals with relatively adjusted amplitudes so as to effectively azimuthally steer the acoustic investigation of the subsurface earth formation and producing second composite dipole waveform signals representative thereof. The method of claim 6 wherein said detecting steps are carried out with a plurality of acoustic receivers (82) to produce correspondingly a plurality of said first and second composite dipole waveforms signals further comprising the steps of: deriving portions in the composite dipole waveform signals representative of a common acoustic wave; and determining a characteristic of said acoustic wave from said derived portions as a function of azimuthal directions. The method of claim 7 wherein said common acoustic wave is a shear wave and said characteristic is the velocity of the shear wave. The method of claim 5 wherein the detecting step comprises: detecting, with a plurality of acoustic receivers, each of which has a dipole radiation detection pattern oriented along predetermined directions, acoustic radiation attributable to the first and second dipole sources and producing respectively first and second waveforms signals representative thereof wherein the combining step comprises combining first and second waveforms signals from each receiver to produce a plurality of composite dipole waveform signals representative of the azimuthally steered acoustic investigation. The method of claim 9 further comprising the steps of: deriving portions in the composite dipole waveform signals representative of a common acoustic wave; and determining a characteristic of said acoustic waves from said derived portions as a function of azimuthal directions. The method of claim 10 wherein said common acoustic wave is a shear wave and said characteristic is the shear wave velocity. The method of claim 5 wherein the first and second dipole source radiation orientations are substantially perpendicular. The method of claim 6 wherein the first and second dipole source radiation orientations are substantially perpendicular and wherein the first and second directions of the first and second acoustic receivers dipole radiation patterns are substantially perpendicular. A method of acoustically investigating a subsurface earth formation that is traversed by a borehole through which an elongate sonde (10) is adapted to move, where the sonde has an acoustic dipole source (44) for generating dipole shaped acoustic pulses and further having dipole receivers (82) located along the sonde to detect acoustic energy with predetermined dipole patterns relative to that of the dipole source, comprising the steps of: energizing an acoustic dipole source (44a,44b) with a first dipole radiation orientation; detecting, with a first acoustic receiver (84a,84b) having a first dipole radiation pattern oriented along a first direction, acoustic radiation attributable to the energizing of the dipole source and producing a first waveform signal indicative thereof; detecting, with a second acoustic receiver (84c,84d) having a second dipole radiation pattern that is oriented along a second direction that bears a predetermined azimuthal angle relative to the first direction, acoustic radiation attributable to the energizing of the dipole source and producing a second waveform signal indicative thereof; and combining the first and second waveform signals with relatively adjusted amplitudes so as to effectively azimuthally steer the acoustic investigation of the subsurface earth formation. The method of claim 14 wherein the first and second dipole radiation patterns are substantially perpendicular to each other. The method of claim 14 further including the steps of: energizing a second acoustic dipole source (44c,44d) with a second dipole radiation orientation that forms a predetermined azimuthal angle with the first dipole radiation orientation; detecting, with said first acoustic receiver (84a,84b), acoustic radiation attributable to the energizing of the second acoustic dipole source and producing a third waveform signal indicative thereof; detecting, with said second acoustic receiver (84c,84d), acoustic radiation attributable to the energizing of the second acoustic dipole source and producing a fourth waveform signal indicative thereof; and combining the third and fourth waveform signals with relatively adjusted amplitudes so as to effectively azimuthally steer the acoustic investigation of the subsurface earth formation.

Inventors: CHANG, SHU-KONG, FROELICH, BENOIT G., VARGA, GUYLA
Applicants: SCHLUMBERGER LIMITED, SOCIETE DE PROSPECTION ELECTRIQUE  SCHLUMBERGER
Application year: 1987
Main classification: G01V1
IPC codes: G01V1