Abstract:
A dipole source for borehole acoustic logging is provided, including a cylindrical shell, a center beam coupled to the cylindrical shell and a movable projector inside the cylindrical shell to impact the center beam. The dipole source includes a cavity acoustically insulating the interior of the cylindrical shell from the exterior of the cylindrical shell. An acoustic logging tool for making measurements of a substrate surrounding a borehole with a body insertable in the borehole is also provided. The body includes an acoustic detector and a dipole source as above, along the axial length. The acoustic logging tool may include a control unit to process data collected from the acoustic detector and obtain information about the substrate surrounding the borehole. According to embodiments disclosed herein, a method for operating a dipole source and an acoustic logging tool as above is provided.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    Embodiments disclosed herein relate generally to the field of borehole acoustic logging and more particularly to the field of dipole sources for acoustic logging in geophysical applications. 
         [0003]    2. Description of Related Art 
         [0004]    In the field of borehole acoustic logging, dipole sources generate borehole shear waves. Acoustic shear waves provide information related to the composition of a substrate surrounding a borehole. Acoustic dipole sources used currently include piezo-electric (PZ) plate bender bars, piston radiators, and shakers. Bender bars typically emit acoustic waves in a narrow frequency band between 1 kHz to 3 kHz (kilo-Hertz), limiting the measurement range of a logging device. Piston radiators provide a wider bandwidth, but have poor mode purity, typically including monopole, quadrupole and higher order acoustic modes associated with acoustic radiation. In addition, piston radiators lack very low frequency response, which is the soft formation spectral range of shear waves. Some attempts to create acoustic dipole sources have used shaker sources that produce shear waves transferred through the liquid in a borehole. However, shaker sources tend to induce interference of the driving mechanism into the logging tool body, thus requiring a good acoustic isolator to prevent interference. In addition, shaker sources can prove inadequate in providing strong dipole radiation, due to their mass. 
         [0005]    In some applications, an explosive force increases the strength of an acoustic wave source. However, explosive forces are difficult to control and may result in multimode contamination of the acoustic signal. 
         [0006]    What is needed is a dipole source for acoustic logging applications that has a wide bandwidth of operation and a high modal purity. In addition, what is needed is a dipole source for shear acoustic waves having low frequency and high signal strength. 
       SUMMARY 
       [0007]    According to embodiments disclosed herein, there is provided a dipole source for borehole acoustic logging may include a cylindrical shell, a center beam coupled to the cylindrical shell, and a movable projector disposed inside the cylindrical shell to impact the center beam. The dipole source may further include a cavity acoustically insulating the interior of the cylindrical shell from the exterior of the cylindrical shell. 
         [0008]    According to embodiments disclosed herein, there is also provided an acoustic logging tool for making measurements of a substrate surrounding a borehole that may include a body insertable in the borehole and an acoustic detector and a dipole source each disposed along an axial length of the body. In some embodiments the dipole source includes a cylindrical shell, a center beam coupled to the cylindrical shell, a movable projector disposed inside the cylindrical shell to impact the center beam in a direction along a dipole axis of the dipole source, and a cavity acoustically insulating the interior of the cylindrical shell from the exterior of the cylindrical shell. Further, in some embodiments an acoustic logging tool as disclosed herein may include a control unit operatively coupled to the acoustic detector and the dipole source to provide power thereto and to process data collected from the acoustic detector to obtain information about the substrate surrounding the borehole. 
         [0009]    According to embodiments disclosed herein, there is also provided a method for generating an acoustic wave in a spatial pattern having a dipole symmetry axis. The method includes providing a current to a coil for a first selected time interval, and propelling a projector to impact a center beam using a magneto-motive force generated by the current during the first selected time interval, the impact of the projector and the center beam comprising a force and a change in time of the force. In some embodiments the method may include providing a restoring force to the projector for a second selected time interval; wherein a direction of the force and a direction of the change in time of the force are substantially the same, along a dipole symmetry axis. 
         [0010]    Further according to some embodiments disclosed herein, there is provided a method for measuring properties of a substrate using an acoustic logging tool having a dipole source. The method may include generating an acoustic wave in a pattern substantially symmetric about an axis of the dipole source, detecting the acoustic wave using an acoustic detector placed along an axial length of the acoustic logging tool, and measuring a velocity of the acoustic wave through the substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  illustrates an acoustic logging tool including a dipole source inside a borehole, according to some embodiments. 
           [0012]      FIG. 2  is a cross-sectional view of a wide bandwidth, borehole dipole source, according to some embodiments. 
           [0013]      FIG. 3A  illustrates a cross-sectional view of a wide bandwidth, borehole dipole source, according to some embodiments. 
           [0014]      FIG. 3B  illustrates a cross-sectional view of a wide bandwidth, borehole dipole source, according to some embodiments. 
           [0015]      FIG. 4  illustrates a cross-sectional view of a wide bandwidth, borehole dipole source, according to some embodiments. 
           [0016]      FIG. 5  illustrates a cross-sectional view of a wide bandwidth, borehole dipole source, according to some embodiments. 
           [0017]      FIG. 6  illustrates a current waveform provided to a wide bandwidth, borehole dipole source, according to some embodiments. 
           [0018]      FIG. 7  shows a flowchart of a method for generating an acoustic wave using a wide bandwidth, borehole dipole source, according to some embodiments. 
           [0019]      FIG. 8  shows a flowchart of a method for measuring properties of a substrate using an acoustic logging tool having a wide bandwidth, borehole dipole source and an acoustic detector, according to some embodiments. 
       
    
    
       [0020]    Wherever possible, like reference numbers refer to the same or like elements throughout the drawings. 
       DETAILED DESCRIPTION 
       [0021]    In the field of oil extraction and prospection, acoustical logging tools provide information as to the hydrocarbon content of earth formations at locations of interest. In order to obtain accurate information it is desirable to have acoustic sources that provide good modal purity and a strong signal. According to embodiments disclosed herein, a wide bandwidth, borehole dipole source uses a strong impact mechanism inside a cylindrical shell. Thus, a dipole source as disclosed herein may generate a strong dipole signal, in some cases including a low frequency. Such acoustic dipole signal results in accurate shear speed measurements. The use of an impact mechanism such as the one disclosed herein may also provide a wide bandwidth of acoustic waves due to the frequency content of the impact pulse shape. Further, some embodiments of the present disclosure use materials in the dipole source that optimize the strength and bandwidth of the generated dipole acoustic waves. This in turn generates a higher signal-to-noise ratio (SNR) in the measurements obtained by the acoustic logging tool. 
         [0022]    A measure of the acoustic wave strength is the pressure output of the wave at a given distance from the source of the wave. In some embodiments a strong acoustic wave at about 500 Hertz (Hz) may produce a pressure output of about 80 Pa at about one meter (1 m) distance from the source. In some embodiments, a strong acoustic wave at about 5 kHz (kilo-Hz) may produce a pressure output of about 1000 Pa, at a bout 1 m distance from the source. Some embodiments may include a good modal purity such that the ratio of measured strength of a dipole component of the acoustic wave to the measured strength of a quadrupole component of the acoustic wave is greater than 20 decibels (dB). Thus, some embodiments provide an acoustic source that provides an acoustic wave with a strength that is about 99% in a dipole component and about 1% or less in a quadrupole or higher order component. The modal purity of an acoustic wave may depend on the frequency of the acoustic wave. According to some embodiments of dipole sources, a modal purity of the acoustic waves generated by the source may be about 20 dB or greater for a frequency range between 0.3 kHz and 9 kHz. 
         [0023]    Wireline logging applications may use acoustic dipole sources consistent with the present disclosure. Wireline logging performs acoustic logging in a previously drilled borehole. Logging-while-drilling (LWD) applications may also use an acoustic dipole source according to embodiments consistent with the present disclosure. An LWD configuration logs acoustic data while drilling a borehole. 
         [0024]    An acoustic perturbation may be produced by a pressure wave p(R,t) propagating through a medium such as a fluid, a solid, or a colloid. The behavior with time, t, of pressure perturbation, p({right arrow over (R)},t), located at a point {right arrow over (R)} relative to a point dipole source driven by an impulsive force {right arrow over (F)}(t), is given by 
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         [0000]    where ê R  is a unit vector oriented along the direction of receiving position {right arrow over (R)}, and c is the speed of sound in the medium. The vector product in Eq. (1) is a ‘dot’ product, leading to a scalar quantity, p({right arrow over (R)},t). The dipole source may be located in the origin of the coordinate system in Eq. (1). According to some embodiments consistent with the present disclosure, the force {right arrow over (F)} and its time derivative are aligned in the same direction. The direction of alignment of the force {right arrow over (F)} and its time derivative forms the axis of the dipole source, or dipole axis. In such configuration, the spatial pattern of an acoustic perturbation as described in Eq. (1) is substantially symmetric about the dipole axis. That is, the value of the perturbation p({right arrow over (R)},t) is substantially the same for all points in a circle centered on the dipole axis, the circle being in a plane perpendicular to the dipole axis. 
         [0025]    From Eq. (1), an increase in the amplitude of the acoustic perturbation may be obtained by increasing the magnitude of the driving force {right arrow over (F)}, by increasing the time derivative of the driving force {right arrow over (F)}, or a combination of both. According to embodiments disclosed herein, an impact force enhances both the magnitude and the time derivative of driving force {right arrow over (F)}. 
         [0026]    Use of an impact force {right arrow over (F)}({right arrow over (R)},t) having a sharp temporal profile increases the magnitude and the time derivative of driving force {right arrow over (F)} and, thus, the amplitude of the dipole perturbation p({right arrow over (R)},t) (cf. Eq. (1)). Thus, an acoustic source according to embodiments disclosed herein provides a wide bandwidth dipole acoustic perturbation. 
         [0027]    Also from Eq. (1) it is seen that the acoustic perturbation ‘p’ is greater in a direction {right arrow over (R)} aligned with driving force {right arrow over (F)} and its time derivative. The acoustic perturbation, p, has a positive value in a parallel orientation to driving force {right arrow over (F)} and its time derivative, and an equal but negative value in the opposite “anti-parallel” direction. Aligning the driving force {right arrow over (F)} and its time derivative in the same direction enhances the magnitude of the acoustic perturbation, p. Thus, a pressure wave having a maximum positive value in a given direction and a minimum negative value in the opposite direction may be a pure dipole emitter as described by Eq. (1). Note that, in general, an acoustic wave may have multiple components. Some of these components may not be of a pure dipole nature as described in Eq. (1) above. For example, a pressure wave radiating from a point outward symmetrically in all directions may be associated to a monopole radiation source. Other types of multi-pole radiation are feasible, such as a combination of two anti-parallel dipole emitters placed close to one another, and one with a single dipole alternating sign. Such combinations may produce a quadrupole wave. A quadrupole wave may have two perturbation maxima alternating with two perturbation minima along a circumference surrounding an axis of symmetry including the source point. 
         [0028]    According to embodiments disclosed herein, an acoustic dipole source uses electrical coils, permanent magnets, and magnetically susceptible materials to deliver a strong impact force consistent with Eq. (1). Thus, embodiments disclosed herein implement a dipole acoustic source having a wide frequency band. In embodiments consistent with the present disclosure, use of a dipole source enables an efficient generation of shear waves in a substrate.  FIG. 1  illustrates in detail the use of a dipole source to generate shear waves in a substrate. 
         [0029]      FIG. 1  illustrates an acoustic logging tool  10  including a dipole source  100  inside a borehole  15 , according to some embodiments. Some applications insert acoustic logging tool  10  inside borehole  15 , having a longitudinal axis, Z, as illustrated in  FIG. 1 . A controller  20  provides power and data exchange channels with logging tool  10 , through a connector  25 . Controller  20  may include a processor circuit  21  and a memory circuit  22  for processing data and control commands. Connector  25  may include digital and analog data lines, electrical wiring, and mechanical couplings between logging tool  10  and controller  20 . According to some embodiments, controller  20  is located on a surface  30 , outside of borehole  15 . In some embodiments, controller  20  or a portion of it may be included inside logging tool  10 . Dimensions of elements and components in  FIG. 1  may vary according to the specific application and should not be limiting of the embodiments disclosed herein. The specific diameter of the logging tool  10  is not limiting and may vary depending on the conditions and tools used for creating the borehole  15 . In some embodiments, the logging tool  10  may have a diameter of a few inches, for example less than about 4 inches. In some embodiments, the logging tool  10  may have a diameter of about 3 inches, or less. 
         [0030]      FIG. 1  illustrates a Z-axis in the vertical direction and an X-axis in a horizontal plane. This configuration is not limiting and one of ordinary skill will recognize that some applications drill borehole  15  in a substrate  50  in any orientation relative to the vertical. Thus, Z-axis may be a longitudinal direction along borehole  15 , oriented in any direction relative to a vertical direction. Likewise, the X-axis is orthogonal to the Z-axis, in a cross-sectional plane perpendicular to the longitudinal direction. 
         [0031]    In order to provide a wide band of acoustic frequencies, the design and materials chosen for logging tool  10  and dipole source  100  are such that the resonance frequency of dipole source  100  and the logging tool  10  is outside of the desired acoustic bandwidth range. Thus, logging tool  10  or dipole source  100  absorb and scatter little or no acoustic energy generated by dipole source  100 . Instead, it is desirable to transmit most of the generated acoustic energy directly to fluids surrounding logging tool  10  in borehole  15 . 
         [0032]    Consistent with some embodiments, borehole  15  is drilled through substrate  50  prior to introducing logging tool  10  inside borehole  15 . Consistent with such embodiments, once borehole  15  has been drilled logging tool  10  is slowly displaced along an axis Z of borehole  15 , to create an acoustic log of substrate  50 . This may be the case for a wireline application of tool  10 , as disclosed herein. For example, some applications may place logging tool  10  at the bottom of borehole  15  and slowly move it up, while collecting data. Some embodiments may collect data while displacing logging tool  10  down into borehole  15 , along the Z-axis. Some embodiments drill borehole  15  at the same time as displacing tool  10  along the Z-axis to provide acoustic logging of substrate  50 . This is the case of LWD or measure-while-drilling (MWD) applications of logging tool  10 , as disclosed herein. Substrate  50  may include earth formations such as sand gap  51  and porous rock  52 . According to some embodiments, borehole  15  may include a material such as a fluid, a colloid, or mud between tool  10  and the inner walls of borehole  15 . 
         [0033]    According to some embodiments, dipole source  100  may be oriented in a direction perpendicular to an axis of borehole  15 . For example, as shown in  FIG. 1 , dipole source  100  is oriented along in a direction X (i.e. the “X-axis”), perpendicular to the borehole axis Z, i.e. the “Z-axis”. Further consistent with some embodiments, the acoustic wave pattern generated by dipole source  100  may be symmetric with respect to orientation direction, which is the X-axis in  FIG. 1 . Note that the choice of X-Z axes as in  FIG. 1  is arbitrary and bears no limitation on the general concept illustrated in  FIG. 1 . The acoustic perturbation, p, produced by dipole source  100  travels in a plane perpendicular to the Z-axis through the material (mud) in borehole  15  impinging substrate  50 , at a point P o . The acoustic perturbation, p, is then transformed in substrate  50  into a shear/flexural wave S that travels axially back and forth along the Z-axis, parallel to borehole  15 . Wave S has a frequency spectrum influenced by the frequency spectrum of acoustic perturbation p. Wave S has a given propagation speed, v s , at a low frequency and is dispersive at high frequencies. The dispersion properties of wave S include the propagation speed of wave S as a function of its frequency. An acoustic detector  35  in logging tool  10  may detect wave S at a known separation along the Z-axis from dipole source  100 . The nature of the earth formations traversed by wave S, such as sand gap  51  or porous rock  52 , may influence the speed v s . Consequently, by measuring the time of travel of wave S through substrate  50 , a measure of speed v s  may be determined using the known separation between source  100  and detector  35 . As a result, this measurement of speed v s  provides information about earth formations encountered by wave S in its travel, such as sand gap  51  or porous rock  52 . 
         [0034]      FIG. 1  shows acoustic detector  35  above dipole source  100 . One of ordinary skill will recognize that the relative location of acoustic detector  35  and dipole source  100  is not limiting. In some embodiments, an acoustic detector  35  may be located below dipole source  100 . Further, some embodiments may include a plurality of acoustic detectors  35  located above, below, or both above and below dipole source  100 , with knowledge of the distances between dipole source  100  and acoustic detectors  35 . 
         [0035]    Dipole source  100  may be a “pure” dipole source. A “pure dipole source” produces a pressure perturbation having a spatial pattern given by Eq. (1) above. Use of a pure dipole source in tool  10  is desirable due to the directionality discussed in relation to Eq. (1), above. A pure dipole source more efficiently transmits the acoustic perturbation energy in the direction of the X-axis into the mud of borehole  15  and substrate  50 . When acting as a pure dipole, source  100  provides a maximum perturbation, p, at point P o  in substrate  50  closest to source  10 , along the dipole axis (cf.  FIG. 1 ). Once the acoustic perturbation interacts with the substrate at point P o  it becomes shear/flexural wave S, traveling through substrate  50 . The dispersion properties of wave S can provide information as to the nature of formations in the substrate  50 , such as sand gap  51  and porous rock  52 . 
         [0036]      FIG. 2  is a cross-sectional view of a wide bandwidth, borehole dipole source  200 , according to some embodiments. Logging tool  10  may use dipole source  200  as source  100 , consistent with embodiments disclosed herein (cf.  FIG. 1 ).  FIG. 2  depicts X-Z directions as being oriented in a horizontal-vertical configuration, as in  FIG. 1 . However, the selection of X-Z coordinates as in  FIG. 2  is arbitrary and not limiting of the general concept illustrated in the figure. Dipole source  200  includes a cylindrical shell  255 . According to embodiments consistent with the present disclosure, cylindrical shell  255  has an axis oriented along the Z-axis. 
         [0037]    Dipole source  200  may include a center beam  210  having a bushing  215  to allow shaft  230  to move back and forth along an axis of shaft  230 , which is the X-axis in  FIG. 2 . In some embodiments, frame  250  couples center beam  210  to cylindrical shell  255  mechanically. Shaft  230  can have a cylindrical shape and an axis that is perpendicular to the axis of cylindrical shell  255 , according to some embodiments. According to some embodiments, a projector  231  is contained inside cylindrical shell  255 . In some embodiments, projector  231  may be a moveable projector configured to move along a direction of motion. Projector  231  includes shaft  230 , a first portion  235 - 1 , a second portion  235 - 2 , stoppers  220 - 1 ,  220 - 2  and springs  240 - 1  through  240 - 4 . Stoppers  220 - 1  and  220 - 2  provide a point of contact between first portion  235 - 1  and second portion  235 - 2  with either side of center beam  210 , respectively. Springs  240 - 1  through  240 - 4  couple projector  231  to center beam  210 . Springs  240 - 1  through  240 - 4  center the travel of projector  231  back and forth along the X-axis. Springs  240 - 1  through  240 - 4  also provide a restoring force to projector  231 . 
         [0038]    According to some embodiments, projector  231  may also include two or more magnets  260 - j  (six are shown  260 - 1  through  260 - 6 ) and two or more coils  270 - j  (six are shown  270 - 1  through  270 - 6 ). Some embodiments include placing magnets  260 - 1  through  260 - 6  symmetrically distributed on projector  231  about center beam  210 . Further, some embodiments include placing magnets  260  symmetrically about the axis of shaft  230  (X-axis), although only partially shown in  FIG. 2 . For example, embodiments consistent with the present disclosure include magnets  260 - 1  through  260 - 3  attached to first portion  235 - 1  of projector  231 , and magnets  260 - 4  through  260 - 6  to second portion  235 - 2  of projector  231 . Each of the conductive coils  270 - 1  through  270 - 6  surrounds one of the magnets  260 - 1  through  260 - 6 . Some embodiments include attaching conductive coils  270 - 1  through  270 - 6  to cylindrical shell  255 , as illustrated in  FIG. 2 . Each of conductive coils  270 - j  may be electrically coupled to a power source outside of dipole source  200  to receive a current, I  270 - j.    
         [0039]    Embodiments of a dipole source as disclosed herein provide a magneto-motive force to projector  231  along the X-axis. The interaction of each of magnets  260 - 1  through  260 - 6  and a temporary magnetic field established through each of coils  270 - 1  through  270 - 6  produces the magneto-motive force. Each pair of magnet  260 - j  and coil  270 - j  contributes with a force F x -j on projector  231 . The net force on projector  231  is the sum of all forces F x -1 through F x -6, according to embodiments disclosed herein. For example, when current I  270 - j  is transmitted through coil  270 - j , a magnetic field M  270 - j  is generated along the X-axis, centered on coil  270 - j . Magnetic field M  270 - j  interacts with magnetic field M  260 - j  of magnet  260 - j . The interaction between M  270 - j  and M  260 - j  may be attractive, if field M  270 - j  and M  260 - j  are anti-parallel; or repulsive, if field M  270 - j  and M  260 - j  are parallel. While the orientation of magnetic field M  260 - j  is fixed by permanent magnet  260 - j , the orientation of field M  270 - j  depends on the direction of current I  270 - j , relative to the X-axis (clockwise or counter-clockwise). The direction of current I  270 - j  relative to the X-axis is determined by the sign of current I  270 - j  and by the orientation of the windings in coil  270 - j . Thus, for a given configuration of magnet  260 - j  and coil  270 - j , the magneto-motive force F x -j on projector  231  may be reversed by reversing the direction of current I  270 - j.    
         [0040]    The magneto-motive force F x  on projector  231  produces an accelerated motion of projector  231  in either direction along the X-axis. Eventually, projector  231  impacts center beam  210  through stoppers  220 - 1  and  220 - 2 . Thus, when projector  231  moves in the +X direction (due to a positive F x ), stopper  220 - 1  impacts a left face of center beam  210  (cf.  FIG. 2 ). Likewise, when projector  231  moves in the −X direction (due to a negative F x ), stopper  220 - 2  impacts a right face of center beam  210  (cf.  FIG. 2 ). Frame  250  transfers the impact force of projector  231  on center beam  210  to cylindrical shell  255 . A conversion of the force of projector  231  on center beam  210  to acoustic energy occurs through the elastic properties of the material in center beam  210 . Furthermore, the acoustic energy thus produced contains a wide frequency bandwidth because of the rise time and fall time of the impact force by projector  231 . 
         [0041]    Note that, according to embodiments consistent with the present disclosure, frame  250  transfers acoustic energy to cylindrical shell  255  along the X-direction. For example, most of the acoustic energy produced by the impact of projector  231  on center beam  210  can be transferred to the edges of cylindrical shell  255 , located in the left (−X) and right (+X) faces (cf.  FIG. 2 ). Moreover, due to the rigidity of frame  250  and the directionality of the impact produced by projector  231 , some embodiments may produce a positive pressure wave on one side of cylindrical shell  255  (say, the +X face of shell  255 ). At the same time, some embodiments produce a negative pressure wave on the opposite side of cylindrical shell  255  (the −X face of shell  255 ). This may be the case when a positive force +F x  moves projector  231  in the +X direction. When a negative force −F x  moves projector  231  in the −X direction, a positive pressure wave is produced on the −X face of shell  255  and a negative pressure wave is produced on the +X face of shell  255 . According to embodiments consistent with the present disclosure, the impulsive force produced by projector  231  has a time derivative in a direction substantially parallel to the force itself (in the X-direction). Thus, the impulse force of projector  231  on center beam  210  may act as a “pure dipole source” and produce a pressure perturbation having a spatial pattern given by Eq. (1) above. Such a pure dipole source provides advantages such as having low contamination from other modes, such as a monopole, a quadrupole, or higher order modes. 
         [0042]    The detailed temporal profile of an impact force iF x (t) exerted by projector  231  on center beam  210  is determined in part by stoppers  220 - 1  and  220 - 2 . The elastic properties of the material selected for stoppers  220 - 1  and  220 - 2  may result in force iF x (t) having different temporal profiles. For example, a soft, resilient material in stoppers  220 - 1  and  220 - 2  may result in a longer contact period having a slower rising time. Some embodiments use soft materials such as rubber or plastic for stoppers  220 - 1  and  220 - 2  consistent with the present disclosure. A hard material in stoppers  220  may result in a narrow profile having a faster rise time and a shorter contact period. Some examples of hard materials used in stoppers  220  consistent with the present disclosure include copper and stainless steel. Balance springs  240 - 1  through  240 - 4  maintain the motion of projector  231  along the X-axis. Springs  240 - 1  through  240 - 4  also provide a restoring force rFx to projector  231  back to its original position after impact with center beam  210 . 
         [0043]    To generate a dipole mode having low higher order contamination it is desirable that projector  231  strike a well-supported and balanced center beam  210 . Vibrations or displacements of projector  231  outside of the X-axis direction may alter the direction of force {right arrow over (F)} and its time derivative, reducing the dipole strength (cf. Eq. (1)). It is also desirable to transmit the impact force efficiently to cylindrical shell  255 . The space inside cylindrical shell  255  is limited, thus a large driving force F x  may accelerate projector  231  to a desirable speed in order to provide a strong impact. For example, in some embodiments a speed of about 10 m/s or up to about 20 m/s may be reached within a cylindrical shell  255  having about 4 inches (˜0.10 m) in length (along the X-direction). In some embodiments consistent with the present disclosure the total mass of projector  231  may be increased to produce a stronger impact force iF x (t). Consistent with some embodiments, the plurality of magnetic coils  270  is used to drive the plurality of magnets  260  in push and pull modes to enhance the driving force F x  In some embodiments the elements shown inside cylindrical shell  255  in  FIG. 2 , including projector  231 , are not in direct contact with borehole mud. Preventing direct contact of mechanical elements with borehole mud enhances the conversion efficiency of acoustic perturbation p into a shear wave S, as described above. 
         [0044]    Dipole source  200  may also include cavity  280  surrounding frame  250 , providing sound isolation between center beam  210  and cylindrical shell  255 . For example, air or any other fluid having a large impedance mismatch with the material of shell  255  may fill cavity  280 . Thus, cavity  280  may form an ‘air cushion’ surrounding center beam  210  and frame  255 . Shell  255  may be made of a material having a high Young modulus, such as metal. Thus, sound produced by the impact of projector  231 , i.e. the “impact sound,” on center beam  210  may propagate through the fluid in cavity  280 , but be decoupled from shell  255 . Embodiments disclosed herein prevent impact sound occurring inside shell  255  from interfering with the detection of shear acoustic wave S propagating through substrate  50  (cf.  FIG. 1 ). 
         [0045]      FIG. 3A  illustrates a wide bandwidth, borehole dipole source  300 , according to some embodiments. The operation and components in dipole source  300  may be similar to the operation and components of source  200  (cf.  FIG. 2 ). Dipole source  300  may include a center beam  310  having a bushing  315  to allow shaft  330  to move back and forth along an axis of shaft  330 , shown in  FIG. 3A  as the X-axis. Frame  350  couples center beam  310  to a cylindrical shell  355 . Shaft  330  has a cylindrical shape and has an axis along the X-axis. According to some embodiments, a projector  331  is contained inside cylindrical shell  355 . Projector  331  has sub-parts including shaft  330 , a first portion  335 - 1 , and a second portion  335 - 2 . Stoppers  320 - 1  and  320 - 2  provide a point of contact between first portion  335 - 1  and second portion  335 - 2  with either side of center beam  310 , respectively. Springs  340 - 1  and  340 - 2  provide a restoring force to projector  331 . 
         [0046]    According to some embodiments, projector  331  may also include two or more magnets ( FIG. 3A  illustrates two:  360 - 1  and  360 - 2 ). Some embodiments include distributing magnets  360 - 1  and  360 - 2  symmetrically around projector  331 , about center beam  310 , and about the X-axis. For example, according to some embodiments consistent with the present disclosure magnet  360 - 1  may be contained within first portion  335 - 1  of projector  331 , and magnet  360 - 2  may be contained within second portion  335 - 2  of projector  331 . Conductive coils  370 - 1  and  370 - 2  may surround magnets  360 - 1  and  360 - 2 , respectively. Some embodiments include attaching conductive coils  370 - 1  and  370 - 2  to center beam  310 , as illustrated in  FIG. 3A . Some embodiments include connecting each of conductive coils  370 - 1  and  370 - 2  to a power source or a current source outside of dipole source  300  to receive a current, I  370 - 1 , and I  370 - 2 , respectively. 
         [0047]    Embodiments of a dipole source as disclosed herein provide a magneto-motive force to projector  331  along the X-axis. The interaction of magnets  360 - 1  and  360 - 2 , and a temporary magnetic field established through coils  370 - 1  and  370 - 2  produces the magneto-motive force. Each pair of magnets  360 - 1 ,  2  and coils  370 - 1 ,  2  contributes with a force F x -1, 2 on projector  331 . The net force on projector  331  is the sum of forces F x -1 and F x -2. For example, when current I  370 - 1  is provided through coil  370 - 1 , a magnetic field M  370 - 1  is generated along the X-axis, at the center of coil  370 - 1 . Magnetic field M  370 - 1  interacts with magnetic field M  360 - 1  of magnet  360 - 1 . The interaction between M  370 - 1  and M  360 - 1  may be attractive, if M  370 - 1  and M  360 - 1  are anti-parallel; or repulsive, if M  370 - 1  and M  360 - 1  are parallel. While the orientation of magnetic field M  360 - 1  is fixed by permanent magnet  360 - 1 , the orientation of field M  370 - 1  depends on the direction of current I  370 - 1 , relative to the X-axis (clockwise or counter-clockwise). The direction of current I  370 - 1  relative to the X-axis is determined by the sign of current I  370 - 1  and by the orientation of the windings in coil  370 - 1 . Thus, for a given configuration of magnet  360 - 1  and coil  370 - 1 , the magneto-motive force F x -1 on projector  331  may be reversed by reversing the direction of current I  370 - 1 . Likewise, magneto-motive force F x -2 may be applied on projector  331  and reversed using current I  370 - 2 . 
         [0048]    The magneto-motive force F x  on projector  331  produces an accelerated motion of projector  331  in either direction along the X-axis. Eventually, projector  331  impacts center beam  310  through stoppers  320 . Thus, when projector  331  moves in the +X direction (positive F x ), second portion  335 - 2  impacts stopper  320 - 2  on the right face of center beam  310 . Likewise, when projector  331  moves in the −X direction (negative F x ), first portion  335 - 1  impacts stopper  320 - 1  on the left face of center beam  310 . Frame  350  transfers the impact force of projector  331  on center beam  310  to cylindrical shell  355 . The elastic properties of the material in center beam  310  convert the force of projector  331  on center beam  310  to acoustic energy. Furthermore, the acoustic energy thus produced contains a wide frequency bandwidth because of the fast rise time and fall time of the impact force by projector  331 . 
         [0049]    Springs  340 - 1  and  340 - 2  provide a restoring force rFx to projector  331 . In some embodiments springs  340 - 1  and  340 - 2  may be coiled around shaft  330  and such that stoppers  320 - 1 ,  320 - 2  and shaft  330  form cavities  325 - 1  and  325 - 2  to compress springs  340 - 1  and  340 - 2 , respectively. As projector  331  moves in the −X direction (to the right in  FIG. 3A ), spring  340 - 1  is compressed inside cavity  325 - 1 . In some embodiments, the depth of cavity  325 - 1  is greater than the minimum length of spring  340 - 1 . Thus, compression of spring  340 - 1  inside cavity  325 - 1  does not damage spring  340 - 1 . Once magneto-motive force on projector  331  is turned off, spring  340 - 1  provides restoring force rFx on projector  331  to move it back to its equilibrium position, in the +X direction. Spring  340 - 2  performs a similar action a magneto-motive force moves projector  331  in the +X direction. In this case, spring  340 - 2  provides restoring force rFx on projector  331 , in the −X direction. 
         [0050]    Dipole source  300  may also include cavity  380  surrounding frame  350 , to provide sound isolation between center beam  310  and cylindrical shell  355 . In some embodiments, air fills cavity  380  forming an air cushion around center beam  310  and frame  350 . Sound is then prevented from interfering with the detection of shear acoustic wave S propagating through substrate  50  (cf.  FIG. 1 ), in a similar manner as described in detail in relation to cavity  280  in dipole source  200  (cf.  FIG. 2 ). 
         [0051]    Note that, according to some embodiments consistent with the present disclosure, frame  350  transfers acoustic energy to cylindrical shell  355  preferentially along the X-direction. Thus, most of the acoustic energy produced by the impact of projector  331  on center beam  310  will be transferred to cylindrical shell  355 , in the left (−X) and right (+X) directions. Moreover, due to the rigidity of frame  350  and the directionality of the impact produced by projector  331 , some embodiments may produce a positive pressure wave on one side of cylindrical shell  355  (e.g., the +X side of shell  355 ). At the same time, some embodiments produce a negative pressure wave on the opposite side of cylindrical shell  355  (the −X side of shell  355 ). This may be the case when a positive force +F x  moves projector  331  in the +X direction. When a negative force −F x  moves projector  331  in the −X direction, a positive pressure wave is produced on the −X side of shell  355  and a negative pressure wave is produced on the +X side of shell  355 . According to embodiments consistent with the present disclosure, the impulsive force produced by projector  331  has a time derivative oriented in a direction substantially parallel to the force itself (in the ±X-direction). Thus, the impulse force of projector  331  on center beam  310  may act as a “pure dipole source” and produce a pressure perturbation having a spatial pattern given by Eq. (1) above. Such a pure dipole source provides advantages such as having a low contamination from other acoustic modes, including as a monopole, a quadrupole, or higher order modes. 
         [0052]      FIG. 3B  illustrates wide bandwidth borehole dipole source  300 , according to some embodiments.  FIG. 3B  is a different view of wide bandwidth borehole dipole source  300  as shown in  FIG. 3A . Dipole source  300  may include a first current feed-through  313  to provide current I  370 - 1  to coil  370 - 1  and a second current feed-through  314  to provide current I  370 - 2  to coil  370 - 2 . According to embodiments consistent with the present disclosure, dipole source  300  may include connector  315  coupling feed-through  313  with coil  370 - 1 . Also included is connector  316  coupling feed-through  314  with coil  370 - 2 . Center beam  310 , cylindrical shell  355 , magnet  360 - 1 , and coils  370 - 1 ,  2  are as described above in relation to  FIG. 3A . 
         [0053]      FIG. 4  illustrates a cross-sectional view of a wide bandwidth, borehole dipole source  400  according to some embodiments. Dipole source  400  may include current feed-through  413  and current feed-through  414 , as described in detail above with respect to elements  313  and  314  in  FIG. 3B . According to embodiments consistent with the present disclosure, dipole source  400  includes coils  470 - 1  and  470 - 2 , each coupled to either feed-through  413 , or feed-through  414 . Contact portion  432  separates a ring stopper  430  from a cylindrical shell  455 , and fixes ring stopper  430  to a shell  455 . Ring stopper  430  has a circularly symmetric shape about the axis of motion of projector  431 , i.e. the X-axis. Consistent with some embodiments, using ring stopper  430  separated from cylindrical shell  455  reduces low resonance bending modes of the structure forming dipole source  400 . This may be desirable for generation of low frequency acoustic waves decoupled from dipole source  400 . Projector  431  includes magnets  460 - 1  and  460 - 2 . Projector  431  also has an extrusion  432  in the middle portion of a shaft  433  forming projector  431 . Extrusion  432  has a disk shape, centered along the shaft  433  of projector  331 . As projector  431  is accelerated in the +X direction (to the left in  FIG. 4 ), extrusion portion  432  impacts ring stopper  430 . 
         [0054]    Contact portion  434  transmits the impact force of projector  431  from ring stopper  430  to cylindrical shell  455 . Thus, the impact force generates acoustic waves in a dipole pattern from source  400 . After impact, projector  431  returns to its original position, moving along the −X direction by the decompression of spring  440 , similarly to embodiments shown in  FIGS. 2 ,  3 A, and  3 B. Sleeves  445 - 1  and  445 - 2  separate projector  431  from coils  470 - 1  and  470 - 2 , respectively. Sleeves  445 - 1  and  445 - 2  reduce the friction at the surface of magnets  460 - 1  and  460 - 2 , as projector  431  moves along the X-axis. Embodiments consistent with the present disclosure de-couple coils  470 - 1  and  470 - 2  from ring stopper  430 , to avoid acoustic transmission from ring stopper  430  through coils  470 - 1  and  470 - 2 . Also shown in  FIG. 4 , cavity  480  provides acoustic isolation between both projector  431  and ring stopper  430  inside cylindrical shell  455 , and the exterior of cylindrical shell  455 . 
         [0055]      FIG. 5  illustrates a wide bandwidth, borehole dipole source  500  according to some embodiments. Dipole source  500  may include a current feed-through  513  and a current feed-through  514 , as described in detail above with respect to  FIGS. 3B and 4 . According to embodiments consistent with the present disclosure, dipole source  500  includes a single coil  570  coupled to both current feed-through  513  and to current feed-through  514 . A centerpiece  536  is separate from cylindrical shell  555  and fixed to shell  555  through contact portion  534 . According to some embodiments, a projector  531  has a cylindrical shape, with a symmetry axis along the X-direction in  FIG. 5 . In some embodiments, projector  531  is made of a ferromagnetic material, such as ferrite. Thus, a current passing through coil  570  magnetizes projector  531 . The magnetic field induced in projector  531  interacts with the magnetic field in coil  570 , producing a force along the X-direction in  FIG. 5 . The force accelerates projector  531  in the +X-direction towards centerpiece  536 . Projector  531  has a female indentation  537  that fits into a male extrusion  538  in centerpiece  536 . 
         [0056]    According to some embodiments, female indentation  537  in projector  531 , and male extrusion  538  in centerpiece  536 , have a frustoconical shape. The frustoconical shape formed by female indentation  537  in projector  531  and by male extrusion  538  in centerpiece  536  has a symmetry axis along the X-direction in  FIG. 5 . A space  539  forms between female indentation  537  of projector  531  and male extrusion  538  in centerpiece  536 . In some embodiments, air or other fluid fills space  539 . Elastomeric pads  560 - 1  and  560 - 2  dampen any vibration produced by contact with centerpiece  536  and with projector  531 , de-coupling these vibrations from cylindrical shell  555 . Thus, contact portion  534  transmits vibrations produced by the impact of projector  531  onto centerpiece  536  to cylindrical shell  555 , in the middle part of dipole source  500 . The symmetry of contact portion  534  forms a ring centered on the X-axis such that the acoustic wave produced by source  500  has a substantially dipolar pattern. 
         [0057]    As projector  531  accelerates in the +X direction (to the left in  FIG. 5 ), female portion  537  impacts male portion  538 . Contact portion  534  transmits the impact force of projector  531  from centerpiece  536  to cylindrical shell  555 . Thus, source  500  generates acoustic waves in a dipole pattern. After impact, projector  531  returns to its original position, moving along the −X-direction by the decompression of spring  541  similarly to embodiments shown in  FIGS. 2 ,  3 A,  3 B, and  4 . Projector  531  is separated from coil  570  by a sleeve  545  formed from centerpiece  536 . Sleeve  545  in centerpiece  536  reduces friction at the surface of projector  531  and avoids contact between projector  531  and coil  570 . Cavity  580  is disposed between centerpiece  536  and cylindrical shell  555 . Cavity  580  provides acoustic isolation between both projector  531  and centerpiece  536  inside cylindrical shell  555 , and the exterior of cylindrical shell  555 . This is analogous to the acoustic isolation provided by cavities  280  and  380  (cf.  FIGS. 2 and 3A , above). 
         [0058]      FIG. 5  illustrates springs  571  holding dipole source  500  inside a logging tool, such as logging tool  10  (cf.  FIG. 1 ). The outer ends of springs  571  attach the springs to logging tool  10 . Springs  571  reduce the acoustic coupling between dipole source  500  and the logging tool. Springs  571  allow dipole source  500  to move along the X-direction, which is the direction of the impact force generated by dipole source  500 . Springs  571  restrict the motion of projector  531  in a direction perpendicular to the impact force (along either the Z-axis, or the Y-axis). This configuration enhances the efficiency of acoustic generation of dipole source  500 , eliminating inertial effects that deflect the impact force from the dipole direction along the X-axis. The precise value of the spring constant in springs  541  and  571  is not limiting, the value depends on the specific application. In some embodiments, springs  571  have a higher spring constant than restoring spring  541 . For example, in some embodiments restoring spring  541  may have a spring constant of about 10 lb/in (pounds per inch), and springs  571  may have a spring constant of about 50 lb/in. It is desirable to have spring constants for springs  541  and spring  571  such that no spring is resonant with the acoustic waves generated by dipole source  500 . 
         [0059]    Embodiments of a dipole source such as disclosed herein produce acoustic waves in a bandwidth of interest from a lower limit at about 300 Hz to a higher limit at about 9 kHz. Dipole sources according to embodiments herein have materials and designs including mass and spring constants such that the dipole source structure has intrinsic natural frequencies outside the desired acoustic bandwidth. Thus, a low intrinsic frequency of the structure formed by the dipole source and a logging tool may be less than the lower limit desired for acoustic generation, e.g. less than about 300 Hz. The high intrinsic frequencies of the structure may be greater than the higher limit for acoustic generation, at about 9 kHz. With the natural frequencies of the structure out of the target frequency range, the output frequency spectrum is smooth and flat in the region of interest. Thus, in the acoustic range of interest (e.g. 300 Hz-9 kHz) no single frequency dominates the output. 
         [0060]    Another consideration to include in embodiments consistent with the present disclosure is the material used in cylindrical shell  255 ,  355 ,  455 , or  555 . A light material may be desirable such that most of the energy produced by the impact of the projector on the cylindrical shell is transferred into kinetic energy of vibration. In addition, a material having high tensile strength may be desirable in order to provide efficient acoustic coupling in the higher end of the desired frequency band. Furthermore, embodiments consistent with the present disclosure may include a cylindrical shell made of a material having low magnetic permeability and susceptibility. Having a material with low magnetic susceptibility, the surrounding dipole source structure does not distort the magnetic field produced by coils inside the dipole generator. A material with a low magnetic susceptibility is ‘transparent’ to the magnetic field. 
         [0061]    According to the above considerations, some embodiments may use a titanium alloy for a cylindrical shell consistent with the present disclosure. For example, materials for a cylindrical shell as used in some embodiments may have a density of less than about 5 grams per cubic centimeter (g/cc). In some embodiments, a material with a high tensile strength may have a tensile strength greater than 800 Mega-Pascals (MPa, 1 Mega Pascal=10 6  Pa). For example, some materials for a cylindrical shell as used in some embodiments may have a tensile strength of about 860 MPa, 930 MPa, or even higher than 1000 MPa. Furthermore, materials for a cylindrical shell as used in some embodiments may have a magnetic susceptibility of about 4×10 −6  cubic centimeters per gram (cc/g, in CGS, mass units), or less. 
         [0062]    Further according to some embodiments, a material desired for projector  531  ( FIG. 5 ) may have high magnetic susceptibility and permeability. In some embodiments, a ferrite material having a relatively large density forms projector  531  (e.g. compared to the titanium alloy in cylindrical shell  355 ). According to some embodiments, a material used in projector  531  may have a magnetic susceptibility up to one thousand (1000) times greater than the magnetic susceptibility of the material used in the cylindrical shell. Further, according to some embodiments a material used in projector  531  may have a density of about eight (8) g/cc, or more. Increasing the projector mass in embodiments consistent with the present disclosure tends to increase the activation time of the device.  FIG. 6  below depicts the activation time, as discussed in more detail below. 
         [0063]      FIG. 6  illustrates a current waveform  600  provided to a wide bandwidth borehole dipole source, according to some embodiments. Waveform  600  is a time profile of current fed to conducting coils in a wide bandwidth dipole source, such as illustrated in any of  FIG. 2 ,  3 A,  3 B,  4 , or  5 . However, for the purpose of illustration, the following description of FIG. refers to  FIG. 2 , such that dipole source  200  receives waveform  600 , and current waveform  600  feeds coils  270 - 1  through  270 - 6 . Waveform  600  includes coil activation periods  610  and coil de-activation periods  615 . Activation periods  610  last for a time τ 1    601 , and de-activation periods  615  last for a time τ 2    602 . According to some embodiments, each activation period  610 - 1  through  610 - 5  may have the same profile and the same duration  601 . Likewise, in some embodiments each de-activation period  615 - 1  through  615 - 5  may have the same duration  602 . Further consistent with the present disclosure some activation periods  610 - i , and  610 - j  may have different time duration such that  601 - i ≠ 601 - j  (i≠j). Also consistent with the present disclosure some de-activation periods  615 - i , and  615 - j  may have different time duration such that  602 - i ≠ 602 - j  (i≠j). 
         [0064]    Activation periods  610  include the injection of a current I a    605  to a conducting coil in a dipole source (cf.  FIG. 6 ). According to some embodiments, period  610  in waveform  600  may include providing a constant current I a    605  to a conducting coil during time  601 . In some embodiments, the current I a    605  provided to a dipole source in periods  610 - i  and  610 - j  may have different values  605 - i ≠ 605 - j  (i≠j). According to embodiments consistent with the present disclosure, waveform  600  may be a square waveform. A square waveform such as illustrated in  FIG. 6  may result in an impact force having a wide bandwidth, which is desirable in many applications of a dipole source such as  200 ,  300 ,  400 , and  500  (cf.  FIGS. 2 ,  3 A,  4 , and  5 ). 
         [0065]    Consistent with at least some embodiments, the activation period may last for about 8 ms. In some embodiments, the activation period may last for about 3 ms. Other embodiments may have activation periods between about 3 ms and about 8 ms. Furthermore, activation periods longer than about 8 ms and shorter than about 3 ms are possible in embodiments consistent with the present disclosure. To enhance the impact force, some embodiments use a longer activation period relative to the previously mentioned range, creating a higher terminal velocity for the projector upon impact. The impact of the projector on the stopper determines the frequency bandwidth of the resulting acoustic wave. The frequency bandwidth of the resulting acoustic wave may include frequencies between about 300 Hz and about 9 kHz. 
         [0066]    According to some embodiments, the current provided to the conductive coil during an activation period causes an acceleration of a projector such as projector  231  (cf.  FIG. 2 ) to a velocity from about 10 m/s to about 20 m/s before impacting the center beam  210 . 
         [0067]    In some embodiments, the sign of current  605  provided by waveform  600  may alternate between positive and negative values. For example, in some embodiments using dipole source  200  a coil  270 - i  may receive a positive current I a    605  to produce a positive magneto-motive force +Fx-i during a period  501 - m . Coil  270 - i  may also receive a negative current I a    605  to produce a negative magneto-motive force −Fx-i during a period  601 - n  (m≠n). Likewise, in embodiments using dipole source  300  a coil  370 - i  may receive a positive current I a    605  to produce a positive magneto-motive force +Fx-i during a period  601 - m , and a negative current I a    605  to produce a negative magneto-motive force −Fx-i during a period  601 - n  (m≠n). 
         [0068]    Current I a    605  through coil  270 - i  (dipole source  200 ) or coil  370 - i  (dipole source  300 ) may produce a positive or negative horizontal force Fx on projector  231  (source  200 ) or  331  (source  300 ), depending on the sign of current I a    605 . The specific sign of the force Fx for a given value of current I a    605  depends on the orientation of conductive coil  270 - i  or  370 - i  relative to the orientation of magnet  260 - i  or  360 - i , respectively. The specific orientation of coils and magnets in a dipole source used with waveform  600  is not limiting of the general concept depicted in  FIG. 6 . In some embodiments consistent with the present disclosure, activation period  610  may activate all the coils in a dipole source at the same time. For example, activation period  610  may activate conductive coils  270 - 1  through  270 - 6  in source  200 , or coils  370 - 1  and  370 - 2  in source  300 . By arranging the orientation of each of the coils relative to the magnet interacting with it, a positive or negative value of current I a    605  may result in all coil-magnet pairs generating a magneto-motive force Fx-i in the same direction. 
         [0069]    In some embodiments, a selected group of coils may be activated during a period  610 - i , and a different group of coils may be activated during a different period  610 - j  (i≠j). For example, in some embodiments using dipole source  200  coils  270 - 1  through  270 - 3  may be activated during a period  610 - i , and coils  270 - 4  through  270 - 6  may be activated during a period  610 - j  (i≠j). Likewise, in embodiments using dipole source  300  coil  370 - 1  may be activated during a period  610 - i , and coil  370 - 2  may be activated during a period  610 - j  (i≠j). 
         [0070]    Further according to some embodiments, only the coils located on one side of a projector, such as  231  or  331  (cf.  FIGS. 2 and 3 ) may receive current I a    605  during activation intervals  610 . For example, some embodiments activate only coils  270 - 1  through  270 - 3  during each period  610  (cf.  FIG. 2 ). Likewise, in embodiments using source  300  (cf.  FIG. 3A ) only activate coil  370 - 1  during period  610 . 
         [0071]    The value of activation time interval  601  and de-activation time interval  602  may vary according to different applications of a dipole source using waveform  600 . In some embodiments, time interval  602  may be selected to allow for a projector such as  231  (source  200 ) or  331  (source  300 ) to return to an equilibrium position before the next activation period  610  occurs. Some embodiments of waveform  600  include de-activation time  602  such that a shear wave S produced in substrate  50  may travel through substrate  50 , parallel to borehole  15 , and reach detector  35  in logging tool  10  before the next activation period  610  occurs (cf.  FIG. 1 ). 
         [0072]    According to some embodiments, de-activation period  602  may be comparable to a delay of an acoustic wave propagating along a borehole axis from the dipole source to a sensor. 
         [0073]    Further, according to some embodiments the frequency of a shear acoustic wave generated is between about 500 Hz to about 6 kHz. In some embodiments, the impact of the projector on the stopper is such that the shear acoustic wave has a spectral bandwidth greater than about 6 kHz. For example, the frequency bandwidth may be from about 300 Hz up to about 9 kHz, according to some embodiments consistent with the present disclosure. 
         [0074]      FIG. 7  shows a flowchart of a method  700  for generating an acoustic wave using a wide bandwidth, borehole dipole source, according to some embodiments. Method  700  may be used in connection with a dipole source as disclosed herein, such as dipole source  100  (cf.  FIG. 1 ), dipole source  200  (cf.  FIG. 2 ), dipole source  300  (cf.  FIG. 3A ), dipole source  400  ( FIG. 4 ), and dipole source  500  ( FIG. 5 ). For example, in some embodiments consistent with the present disclosure, controller  20  may include instructions stored in memory  22  that, when executed by processor  21  cause a wide bandwidth, borehole dipole source to execute method  700 . Controller  20  may provide commands, instructions, and power to dipole source  100  through connector  25 . Dipole source  100  may be included within logging tool  10 , as depicted in  FIG. 1 . Further, according to some embodiments, controller  20  may perform method  700  at the command of a user. In some embodiments, logging tool  10  may provide instructions and commands for executing method  700  with a wide bandwidth, borehole dipole source. Some embodiments consistent with the present disclosure may perform method  700  by a combination of commands and instructions from a user, controller  20 , and logging tool  10 , as depicted in  FIG. 1 . 
         [0075]    Step  710  provides a current for a selected time interval. According to some embodiments, a waveform such as waveform  600  representing a value of current as a function of time (cf.  FIG. 6 ) may provide the current in step  710 . The waveform in step  710  may have activation periods  610  for a time τ 1    601 , and de-activation periods  615  for a time τ 2    602 . In some embodiments, step  710  provides the current during activation periods  610  of waveform  600 . According to some embodiments, step  710  may include providing a current to a conductive coil included in the dipole source during the selected time interval. In some embodiments, step  710  in method  700  provides a current to one or more conducting coils such as coils  270 - 1  through  270 - 6  (cf.  FIG. 2 ), or coils  370 - 1  and  370 - 2  (cf.  FIG. 3A ). Step  720  uses the current provided in step  710  to provide an impact force in the dipole source during the selected time interval. According to some embodiments, step  720  provides a magneto-motive force propelling a projector against a center beam during activation period  610  for a time τ 1    601  (cf.  FIG. 6 ). Some embodiments of method  700  use a dipole source including the projector and the center beam such as projector  231  and center beam  210  (cf.  FIG. 2 ). 
         [0076]    Providing an impact force to a dipole source in step  720  generates a dipole acoustic wave. Thus, step  720  may include generating an acoustic wave having a spatial pattern substantially symmetric about an axis of the dipole source, namely the dipole axis. The acoustic wave travels preferentially in a direction perpendicular to the Z-axis in the logging tool configuration and impinges on the substrate surrounding a borehole, such as substrate  50  and borehole  15  (cf. point Po in  FIG. 1 ). According to embodiments consistent with the present disclosure, step  720  provides an impact force including a force {right arrow over (F)} and a time derivative ∂{right arrow over (F)}/∂t applied substantially along a common direction. Consistent with embodiments disclosed herein, the common direction of the force {right arrow over (F)} and its time derivative ∂{right arrow over (F)}/∂t, is substantially parallel to the dipole axis. 
         [0077]    Step  730  provides a restoring force to the projector for a selected time interval. According to some embodiments,  730  may provide the restoring force during de-activation period  615  for a time τ 2    602  in waveform  600  (cf.  FIG. 6 ). In some embodiments, step  730  provides a restoring force to projector  231  by springs  240 - 1  through  240 - 4  (cf.  FIG. 2 ). In some embodiments, step  730  provides the restoring force to projector  331  by springs  340 - 1  and  340 - 2  (cf.  FIG. 3A ). 
         [0078]    In some embodiments a logging tool including a dipole source as disclosed herein and an acoustic detector may perform method  700  for measuring properties of a substrate.  FIG. 8  describes such method in detail, as follows. 
         [0079]      FIG. 8  shows a flowchart of a method  800  for measuring properties of a substrate using an acoustic logging tool having a wide bandwidth, borehole dipole source and an acoustic detector, according to some embodiments. Method  800  may be used in connection with a dipole source as disclosed herein, such as dipole source  100  (cf.  FIG. 1 ), dipole source  200  (cf.  FIG. 2 ), dipole source  300  (cf. FIG.  3 A), dipole source  400  (cf.  FIG. 4 ), and dipole source  500  (cf.  FIG. 5 ). For example, in some embodiments consistent with the present disclosure, controller  20  may include instructions stored in memory  22  that, when executed by processor  21  cause processor  21  to perform method  800 . Controller  20  may provide commands, instructions, and power to dipole source  100  through connector  25 . Dipole source  100  may be included within logging tool  10 , as depicted in  FIG. 1 . Further, according to some embodiments, controller  20  may perform method  800  upon receiving commands from a user. In some embodiments, logging tool  10  provides instructions and commands for executing method  800  with a dipole source as disclosed herein. Some embodiments consistent with the present disclosure may perform method  800  by a combination of commands and instructions from a user, controller  20 , and logging tool  10 , as depicted in  FIG. 1 . 
         [0080]    Step  810  provides a current for a selected time interval. In some embodiments consistent with the present disclosure, step  810  may be as step  710  in method  700  described above (cf.  FIG. 7 ). Step  820  uses the current in step  810  to provide an impact force to the dipole source. According to some embodiments, step  820  in method  800  may be as described in detail above in relation to step  720 , in method  700 . Step  830  detects an acoustic wave. According to some embodiments, an acoustic detector included in a logging tool such as logging tool  10  (cf.  FIG. 1 ) detects an acoustic wave in step  830 . The acoustic detector used in step  830  may be at a certain distance along the Z-axis, in logging tool  10 . In some embodiments, acoustic detector  35  may be placed above, below, or both above and below the dipole source  100  in logging tool  10 . 
         [0081]    In embodiments consistent with the present disclosure, an acoustic wave detected in step  830  may be a shear wave S propagating through substrate  50  along the Z-direction (cf.  FIG. 1 ). Shear wave S may be produced in substrate  50  at a point P o  by the acoustic dipole perturbation produced by dipole source  100  according to step  820  above. Step  840  measures the velocity of acoustic wave S through the substrate. In some embodiments, step  840  measures the velocity using the time at which step  820  generates the dipole perturbation and the time at which step  830  detects wave S. Furthermore, step  840  may be performed using knowledge of the distance between dipole source  100  and acoustic detector  35 , along logging tool  10  (Z-axis, cf.  FIG. 1 ). 
         [0082]    A velocity measurement in step  840  may further include measurements performed for a plurality of acoustic waves S having different frequencies. Thus, in embodiments of method  800  consistent with the present disclosure step  840  obtains a dispersion pattern. A dispersion pattern obtained in step  840  may include different acoustic frequencies and the corresponding velocity measurement for each acoustic frequency. The use of a wide bandwidth dipole source such as source  100  (cf.  FIG. 1 ),  200  (cf.  FIG. 2 ), or  300  (cf.  FIG. 3A ) enables the collection of detailed dispersion patterns in  840 , due to the high frequency content of the impulsive forces produced by such dipole sources. 
         [0083]    Step  850  determines geophysical properties of substrate  50  using the propagation properties of acoustic wave S as measured in step  840 . For example, in embodiments consistent with the present disclosure step  850  uses the speed measured in step  840  to determine whether the wave S has traversed through a substrate such as sand gap  51  or porous rock  52 . According to some embodiments, processor circuit  21  in controller  20  (cf.  FIG. 1 ) performs step  850 . For example, a computer in controller  20  may compare velocity measurements in step  840  with a previously collected database, stored in memory circuit  22 . In some embodiments, method  800  includes the use of a database having a plurality of dispersion patterns in step  850 . Each dispersion pattern stored in the database relates to a specific earth formation of known acoustic properties. Thus, step  850  selects an earth formation that more closely approximates the dispersion pattern obtained in step  840 . 
         [0084]    Throughout the specification, elements that are ‘coupled’ may be directly connected or indirectly connected to each other. 
         [0085]    Embodiments described herein are exemplary only. One skilled in the art may recognize various alternative embodiments from those specifically disclosed. Those alternative embodiments are within the scope of this disclosure. As such, only by the following claims limit the embodiments disclosed herein.