Abstract:
An acoustic transmitter with preferably a plurality of piezoelectric plates configured to optimized acoustic pressure output within a desired frequency range while meeting physical restrictions of LWD and MWD logging systems. The transmitter is disposed in a recess in the perimeter of a logging tool housing to reduce acoustic pressure waves transmitted directly along the tool housing and to optimize acoustic energy transmission into the borehole environs. In order to increase acoustic pressure output at a desired logging frequency range, the plurality of piezoelectric plates are connected electrically in parallel and fired simultaneously. The polarity of the plates and the wiring arrangement are such that each plate expands or contracts in a common direction during simultaneous firing by an applied voltage. The desired output frequency is obtained by selectively polarizing and dimensioning the plates within the plate assembly. The plate assembly is preferably disposed within an oil filled pressure housing that is pressure and temperature compensated.

Description:
FIELD OF THE INVENTION 
     This invention is related to systems for measuring an acoustic property of material penetrated by a well borehole. More particularly, the invention is related to improved acoustic transmitters for use with acoustic logging-while-drilling (LWD) or measurement-while-drilling (MWD) borehole assemblies. 
     BACKGROUND 
     Acoustic logging systems are routinely used in the oil and gas industry to measure formation acoustic properties of earth formation penetrated by a well borehole. These properties include the compressional and shear velocities of the formation, which are subsequently used to determine a variety of formation parameters of interest such as porosity and pore pressure. Additionally, acoustic logging systems are used to produce acoustic images of the borehole from which well conditions and other geological features can be investigated. Other applications of acoustic logging measurements include seismic correlation and rock mechanic determination. 
     The downhole instrument or borehole “tool” of an acoustic logging system typically comprises one or more sources of acoustic pressure or “transmitters”, and one or more acoustic receivers. The transmitters and receivers are typically spaced axially on the body of the tool. Multiple transmitters and/or receivers can also be disposed at different radial positions around the tool. A portion of the energy emitted by the one or more transmitters propagates through formation material surrounding the borehole, and is subsequently detected by the one or more receivers. Receiver response is then used to determine properties and parameters of interest. 
     Frequencies used in acoustic LWD tools are typically within the 2 to 20 kiloHertz (KHz) range. In order to improve accuracy and precision of measured acoustic information; it is desirable to employ one or more transmitters that have the highest acoustic pressure output at the desired frequency. 
     Logging-while-drilling (LWD) and measurement-while-drilling (MWD) tools impose severe limitations that affect the energy and frequency output of an acoustic transmitter disposed within the wall of the tool and operating at a desired frequency. Some of these limitations are discussed briefly in the following paragraphs. 
     If the transmitter comprises piezoelectric crystals, the acoustic pressure output of an acoustic transmitter is proportional to the surface area of the transmitting element. In order to maximize the amount of energy reaching the borehole environs and minimize the propagation of acoustic energy along the tool, it is preferred to dispose the transmitter as near as possible to the outer periphery of the tool. It is, therefore, desirable to dispose the transmitter within a recess in the outer surface of the tool housing wall. An LWD tool housing is typically a drill collar. For structural reasons, it is necessary to restrict the depth, azimuthal and axial dimensions of any recess in the tool wall. These structural recess restrictions therefore govern the maximum dimensions of a transmitter that can disposed within the wall of an LWD tool and, therefore, restrict the acoustic energy output of the transmitter. 
     The frequency of a piezoelectric crystal element is a function of the geometry of the crystal element. Stated another way, the dimensions of a piezoelectric crystal transmitter determine the frequency output of the transmitter. Considering the discussion in the previous paragraph, a transmitter configured to optimize acoustic energy output within tool structural restrictions may not be configured to obtain the desired frequency output requirements. Conversely, a piezoelectric crystal dimensioned to achieve the desired frequency output may limit the acoustic energy output of the transmitter. 
     In summary, the transmitter must be configured to operate within a tool in harsh borehole conditions. Structure required to operationally dispose the transmitter within the wall of the tool imposes additional transmitter dimensional restrictions that, in turn, affect energy and frequency output of the transmitter. 
     In view of the brief background discussion, there is a need for a transmitter with optimized acoustic pressure output, with output frequency optimized to fall within a desired frequency range, and with a physical configuration suitable to meet structural restrictions of LWD and MWD logging systems. 
     SUMMARY OF THE INVENTION 
     The invention is an acoustic transmitter comprising one or more piezoelectric plates. Details of the invention are presented with the transmitter embodied as a monopole transmitter. The basic concepts of the invention can also be embodied as a monopole, dipole, or a quadrupole transmitter by changing the polarity and sequence of firing of the transmitter. 
     The transmitter comprising a plate assembly having preferably a plurality of plates that are configured to optimized acoustic pressure output within a predetermined center frequency range while meeting physical restrictions of LWD and MWD logging systems. The transmitter is disposed on the perimeter of a logging tool housing to reduce acoustic pressure waves transmitted directly along the tool housing and to optimize acoustic energy transmission into the borehole environs. Acoustic energy that propagates from the transmitter and directly along the logging tool housing is typically referred to as the “tool mode signal”. The logging tool housing is typically a steel drill collar. 
     For purposes of discussion, it is assumed that the piezoelectric plates are rectangular with the largest “length” and “width” dimensions defining the “major” surface of the plate, and the “thickness” being the smallest dimension. A piezoelectric plate has three natural frequencies that are proportional to the three dimensions of the plate. A frequency in the desired range for acoustic logging typically corresponds to the longest dimension or “length” of the plate. This dimension is adjusted to obtain the desired transmitter frequency output. The configuration of any LWD or MWD logging tool housing is structurally restricted, as mentioned briefly in the BACKGROUND section of this disclosure. The dimensions of the plate must, therefore, be sufficiently restricted in size as not to jeopardize the mechanical strength of an acoustic LWD or MWD logging tool housing. The acoustic pressure output of a size restricted single plate in the desired frequency range is relatively low. In order to increase acoustic pressure output at a desired logging frequency range, the preferred transmitter comprises a plurality of piezoelectric plates connected electrically in parallel and fired simultaneously. Polarity of the plates and the wiring is such that each plate expands or contracts in the same direction during simultaneous firing. The plates can be embodied in one or more plate assemblies thereby forming the transmitter. Within a plate assembly, the plates can be placed side by side or alternately bonded together. In either configuration, the major surfaces of all plates in a plate assembly are parallel. The bonded plates provide the same acoustic output with a thinner plate assembly. By increasing acoustic pressure output at a desired frequency, multiple plate transmitter assembly maximizes the precision of measured acoustic parameters of interest. The one or more plate assemblies comprising a transmitter are each disposed in a pressure compensated housing preferably filled with oil. 
     The piezoelectric plates can be polarized along the thickness or the length of the plate. If polarized along the thickness, the desired length mode emission is obtained through the shrinkage in length as the thickness expands, which is not efficient. If the plate is polarized in the length mode, the desired frequency is obtained through the primary length mode. However, since it is difficult to polarize a long plate along its length, a striped plate may be used to obtain greater output efficiency than that of a thickness mode plate. As with the single piezoelectric crystal solid plates, the striped plates can be placed side by side, or alternately bonded together to form a plate assembly. Although offering advantages, the striped plates have some disadvantages compared to solid thickness mode plates. First, the output of a thickness mode plate is inversely proportional to its thickness while the output of a striped plate is a function of the stripe thickness and number of stripes. If the thickness of the plate is minimized to utilize more plates in a single plate assembly, the thickness mode plates may produce higher output than striped plates for a given assembly thickness. Second, it is easier to bond solid plates along their faces than to bond striped plates at their piezoelectric elements. Third, the signal directionality of the thickness mode plates is perpendicular to the face of the plate, which is more suitable for LWD logging applications. 
     In summary, thickness mode and striped plates both offer certain advantages and disadvantages that must be weighed considering the overall performance objectives of the acoustic transmitter. 
     Using any of the geometric embodiments briefly described above, a transmitter comprising the pressure chamber housing a plate assembly is disposed in a recess in the outer surface of the tool housing. Plates comprising a transmitter, whether comprising a single plate assembly or multiple plate assemblies, are oriented so that a normal to the major surface of each plate is essentially coincident with the radius of the tool housing. The one or more plate assemblies are acoustically coupled to fluid within the well borehole. In order to further increase acoustic pressure output of a transmitter, two or more plate assemblies and cooperating pressure housings can be disposed in two or more recesses in the outer surface of the tool housing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The manner in which the above recited features and advantages, briefly summarized above, are obtained can be understood in detail by reference to the embodiments illustrated in the appended drawings. 
         FIG. 1  illustrates the invention embodied as an LWD acoustic system disposed in a borehole drilling environment; 
         FIG. 2   a  is a cross-sectional view of a plate assembly comprising five piezoelectric plates  50  in a side-by-side configuration; 
         FIG. 2   b  is a perspective view of a single plate; 
         FIG. 3  illustrates a plate assembly  56  again comprising five bonded piezoelectric plates; 
         FIG. 4  illustrates a striped plate comprising a plurality of piezoelectric segments; 
         FIG. 5  is a cross-sectional view of a logging tool transmitter section showing a transmitter disposed within the tool section wall; 
         FIG. 6  is a cross-sectional view of a transmitter section showing two transmitters disposed in two axially aligned recesses in the wall of the transmitter section; 
         FIG. 7  conceptually illustrates the axial intensity distribution of the pressure output signal from the transmitter depicted in  FIG. 5 ; 
         FIG. 8  is a cross-sectional illustration of a plate assembly with axially staggered plates; and 
         FIG. 9  conceptually illustrates the axial intensity distribution of the pressure output signal from the transmitter using the staggered plate assembly depicted in  FIG. 8   
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An acoustic LWD logging tool typically comprises one or more acoustic transmitters and one or more acoustic receivers. 
       FIG. 1  illustrates a single acoustic transmitter embodied as an LWD acoustic system disposed in a borehole drilling environment. Details of the transmitter are presented with the transmitter embodied as a monopole transmitter. The basic concepts of the used in the transmitter can also be embodied as a dipole or a quadrupole transmitter. The LWD borehole instrument or “tool” component of the borehole assembly is designated as a whole by the numeral  10 , and comprises a pressure housing  11  which is typically a drill collar. The tool  10  is disposed within a well borehole  44  defined by borehole walls  43  and penetrating earth formation  34 . A drill bit  12  terminates a lower end of the tool  10 , and a connector  30  terminates an upper end of the tool. The connector  30  operationally connects the tool  10  to a lower end of a drill string  32 . The upper end of the drill string terminates at a rotary drilling rig  36 , which is known in the art and is illustrated conceptually at  36 . 
     Again referring to  FIG. 1 , the tool  10  comprises a transmitter section  16  and a receiver section  20 . An acoustic isolation section  18 , which reduces the tool mode signal, separates the transmitter section  16  from the receiver section  20 . The receiver section  20  comprises a plurality of receivers  22  axially spaced from the transmitter section  16 . Six receivers are illustrated, although more or fewer receivers can be used. The receivers  22  are also shown axially aligned, although axial alignment is not required as will be discussed in a subsequent section of this disclosure. 
     Still referring to  FIG. 1 , the tool  10  can comprise other elements that can be used to complement measurements made with the acoustic transmitter section  16  and the receiver section  20 . In the embodiment shown in  FIG. 1 , the tool comprises an optional directional section  24  that provides a real time measure of azimuthal angle therefore provides azimuthal orientation of the tool  10  within the borehole  44 . The tool  10  can optionally comprise an auxiliary sensor section  14  with one or more auxiliary sensors responsive to a variety of borehole environs parameters. It should be understood that operation of the acoustic transmitter disclosed herein does not require measurements from the directional section  24  or from the auxiliary sensor section  14 . 
     Once again referring to  FIG. 1 , an electronics section  26  provides power and control circuitry for the acoustic transmitter section  16 , receiver section  20 , the optional directional section  24 , and any optional auxiliary sensors in the auxiliary sensor section  14 . Power is typically supplied by batteries, but may be supplied by a mud powered turbine generator (not shown). The electronics section  26  is operationally connected to a down-hole telemetry unit  28 . Data from elements within the tool  10 , whether processed downhole as parameters of interest or in the form of “raw” data, are telemetered to the surface  46  of the earth by means of a suitable telemetry system. Suitable telemetry systems include a mud pulse system, and electromagnetic telemetry system, or an acoustic telemetry system that uses the drill string  32  as a data conduit. The telemetered data are received by an up-hole telemetry element (not shown) preferably disposed in a surface equipment module  38 . As the borehole assembly comprising the logging tool  10  is conveyed along the borehole  44  by the drill string  32 , one or more parameter of interest, or alternately raw data, are input to a recorder  40 . The recorder  40  tabulates the data as a function of depth within the borehole  44  at which they are measured. The recorder output  42  is typically a “log” of the data as a function of borehole depth. The data can alternately be recorded in down-hole processor memory (not shown), and subsequently downloaded to the surface equipment module  38  when the tool  10  is returned to the surface  46  during or after the drilling operation is completed. The downloaded data are typically processed further within the surface equipment module  38  to obtain additional parameters of interest that cannot be determined in the down-hole processor unit. 
     As stated previously, the tool housing  11  is typically a steel drill collar with a conduit through which drilling fluid flows. Elements of the tool  10  illustrated conceptually in  FIG. 1  are typically disposed within the wall of the drill collar pressure housing  11 . 
     Embodiments of acoustic transmitters disclosed herein all comprise piezoelectric plate elements. A piezoelectric plate has three natural frequencies that are proportional to the three dimensions of the plate. A frequency in the desired range for acoustic logging typically corresponds to the longest dimension or “length” of the plate. This dimension is adjusted to obtain the desired frequency output for acoustic logging. Energy output in the form of a pressure pulse is proportional to the major surface of the piezoelectric plate. Frequency and acoustic energy outputs of a piezoelectric plate are, therefore, a function of plate dimensions, polarization and orientation. In order to obtain optimum frequency and acoustic energy outputs and still meet structural dimensional restrictions of a logging tool, it is desirable to combine or “stack” a plurality of piezoelectric plate elements thereby forming a plate assembly within a single acoustic transmitter. The elements of the plate assembly are activated simultaneously to emit energy in a common direction. 
       FIG. 2   a  is a cross-sectional view of a plate assembly  51  having piezoelectric plates  50  polarized in a “thickness mode” and in a side-by-side configuration. The assembly can contain more or fewer plates, but a plate assembly having an odd number of plates is preferred for simplicity in wiring. The plates  50  are axially and azimuthally aligned. Polarization of each plate is indicated at the top of the plate. The thickness mode plates are polarization and physically oriented and electrically connected so that they respond simultaneously to a voltage applied to the plate assembly and emit acoustic pressure in a common direction. This is accomplished by connecting all positive and negative polarization surfaces with common electrical busses  54  and  52 , respectively. The length and thickness of the plate assembly  51  are indicated by the dimensions  55  and  53 , respectively. 
       FIG. 2   b  is a perspective view of a single thickness mode plate  50  of length  55 , width  55   b  and thickness  55   c . A normal vector  50   e  to the major surface  50   d  is illustrated for future reference. Polarization is indicated by the appropriate arrows. Within the context of this disclosure, the major surface of a plate is parallel to the major surface of a plate assembly that contains the plate. 
       FIG. 3  illustrates a plate assembly  56  again comprising five bonded piezoelectric plates  50 . Plate polarization is in the thickness mode and is again indicated at the top of each plate. In this plate configuration, the plates  50  are bonded with intervening layers of electrical conducting material  60 . As in the side-by-side embodiment shown in  FIG. 2   a , the plates  50  are oriented and electrically connected so that they respond to an applied voltage and simultaneously emit acoustic energy in a common direction. The plates  50  are also axially and azimuthally aligned. This again is accomplished by connecting all positive and negative polarization surfaces with common electrical busses  54  and  52 , respectively. The length  55  of the plate assembly  56  is the same as that for the assembly  51  shown in  FIG. 2   a . The thickness  53   a  of the assembly  56  has, however, been reduced using the bonding embodiment. The reduced thickness is advantageous in meeting the previously mentioned physical restrictions imposed by LWD applications. 
     It is preferred to operate the transmitter in a predetermined center frequency range suitable for acoustic logging applications. A plate can be polarized along the thickness or along the length, with “length” and “thickness” being defined in  FIG. 2   b . If polarized along the thickness  55   c , the desired length mode frequency is obtained through shrinkage in plate length  55  as the thickness expands. This is not an efficient methodology for obtaining length mode output. If the plate is polarized in the length mode, the desired frequency is obtained through the primary length mode. However, since it is relatively difficult to polarize a piezoelectric plate long plate along its length  55 , a “striped” plate may be used to obtain higher efficiency than that obtainable with a thickness mode plate. 
     A striped plate  62  is shown in  FIG. 4 , and comprises a plurality of piezoelectric segments  64  or piezoelectric “stripes” disposed on a solid piece of piezoelectric plate  67  and separated by electrical conducting material  65 . Polarization of each segment  64  is illustrated. The plates  50  are oriented and electrically connected so that they respond simultaneously to a voltage applied to the plate assembly and emit acoustic energy in a common direction. This again is accomplished by connecting all positive and all negative polarization surfaces with common electrical busses  68  and  70 , respectively. Output frequency can be adjusted by varying the dimensions of the piezoelectric segments  64 . 
     As discussed previously, a striped plate  62  as shown in  FIG. 4  may be used to obtain greater output efficiency than that of a thickness mode plate of the type illustrated in  FIG. 2   b . Although offering this advantage, the striped plate  62  has some disadvantages compared to a solid thickness mode plate. The output of a thickness mode plate  50  is inversely proportional to its thickness while the output of a striped plate  62  is a function of the stripe thickness and number of stripes. If the thickness of each plate is minimized to utilize more plates in a single plate assembly, the thickness mode plates may produce higher output than striped plates for a given assembly thickness. Furthermore, it is easier to bond solid plates, of the type shown in  FIGS. 2   b  and  3 , along their faces than bonding striped plates  62  along the piezoelectric stripes  64  (see  FIG. 4 ). Finally, the signal directionality of a thickness mode plate  50  is perpendicular to the face  50   d  of the plate, which may be more suitable for LWD applications. To summarize, plate geometries and polarizations offer certain advantages and disadvantages that must be weighed in considering the overall performance objectives of the acoustic transmitter. 
       FIG. 5  is a cross-sectional view of the transmitter section  16  (see  FIG. 1 ) illustrating a transmitter  86  disposed therein. The transmitter  86  comprises a plate assembly  88  having preferably a plurality of piezoelectric plates in the side-by-side, bonded, or striped embodiments previously discussed. The piezoelectric plates within the illustrated plate assembly  88  are axially and azimuthally aligned. The plate assembly  88  is housed in a pressure housing  84  that is preferably filled with oil and is pressure and temperature balanced. The plurality of plates contained in the transmitter  86  is disposed within a recess (defined by the surfaces  83 ) in the outer surface of the wall  89  of the transmitter section  16  of the tool  10 . A cover  90  protects the pressure housing from abrasion encountered in the borehole environment. The transmitter  86  is oriented so that the radius of the tool  10  is coincident with the normal of the plate assembly  88 , which is the same as the normal of a given plate (see  50   e  in  FIG. 2   b ). Openings  85   a  and  85   b  are typically filled with borehole fluid, which acoustically couples the transmitter  86  to the borehole environs. The axial centerline of the transmitter is shown at  96 , and will be referenced in a subsequent discussion. 
     Again referring to  FIG. 5 , the transmitter section  16  of a typical LWD tool  10  is typically a steel drill collar comprising a wall  89  and a conduit  87  through which drilling fluid flows. The transmitter section  16  has an outer diameter of about 7 inches (17.8 centimeters) and a wall thickness of less than 3 inches (7.6 centimeters). For structural reasons, it is necessary to restrict the depth of the recess housing the transmitter  86  to a depth of less than half of the wall thickness, to restrict the azimuthal arc of the recess to 2 inches (5.1 centimeters) or less, and to restrict the axial length of the recess to 6 inches (15.2 centimeters) or less. These structural based recess restrictions set the maximum dimensions of a transmitter that can disposed within the wall of an LWD tool and, therefore, affect the energy and frequency outputs of the transmitter  86 . Stated another way, physical restrictions on the recess housing the transmitter  86  affect the frequency and energy outputs of the transmitter. 
     The plate assembly  88  is disposed in the recess in an outer surface of the logging tool such that the radius of the logging tool is essentially normal to a major surface of the plate assembly. As discussed previously, individual plates are polarized and configured to emit a pressure pulse in a common direction normal to major surface of the plate assembly upon application of a voltage to the plate assembly. 
     Depending on the piezoelectric material, a plate assembly comprising a plurality of plates each with a length of 4.5 inch (11.4 centimeters) has a natural frequency of 11-13 KHz in the length mode, which is ideal for monopole acoustic logging measurements. The thickness mode frequency of a 0.125 inches (0.32 centimeters) thick plate is in the megahertz range and does not penetrate far from the transmitter  86 . Therefore, in a LWD tool in a borehole environment, the thickness mode of the plate within a plate assembly has no effect on the measurement made from the length mode. A plate with a width of 1.5 inches (3.8 centimeters) has a frequency in the range of 32 to 38 KHz in the width mode. In order for this frequency not to interfere with the length mode frequency, the width of plates within a plate assembly should be kept at a maximum of 1.5 inches (3.8 centimeters). 
     The pressure output of a single 4.5 inch (length)×1.25 inch (width)×0.125 inch (thick) piezoelectric plate has been found to be approximately 200 Pascal at an input voltage of +500 volts at 39.4 inches (1.0 meter). Again referring to  FIG. 5 , the desired pressure output of a transmitter  86  used in an acoustic logging tool  10  is in the range of 1000-2000 Pascal. Therefore, a plate assembly  88  of eight to ten plates should produce a pressure output in the desired range for logging applications. An odd number of nine plates would be preferred for previously mentioned ease of wiring. Considering the individual plate dimensions above required for the desired frequency output, a plate assembly  88  comprising a stack of nine plates requires a recess of depth that exceeds previously discussed physical restrictions for a typical drill collar. 
     The desired monopole pressure pulse output at the desired predetermined frequency can be obtained using two transmitters with each comprising plate assemblies of preferably five plates with dimensions defined above.  FIG. 6  is a cross-sectional view of the transmitter section  16  showing two transmitters  86   a  and  86   b  disposed in two axially aligned dedicated recesses  83   a  and  83   b , respectively, in the outer surface. Both transmitters are operated simultaneously so that all eight plates emit acoustic pressure pulses of the same polarity into the surrounding borehole environs. Since each transmitter preferably comprises only five plates, the depths of the recesses  83   a  and  83   b  do not exceed the physical restrictions of the drill collar. Both transmitters  86   a  and  86   b  are protected from abrasion by covers  90   a  and  90   b , respectively, and are acoustically coupled to the borehole environs through opening of the type depicted at  85   a  and  85   b  of  FIG. 5 . Given the closeness of the azimuthal separation between the two transmitters  88   a  and  88   b , the two transmitters respond as a monopole transmitter and the arrival of acoustic energy at the receiver section  20  is not excessively broadened or phase shifted. 
       FIG. 7  conceptually illustrates the axial distribution of the intensity of the pressure output signal from the transmitter  86  depicted in  FIG. 5 . The direction of the pressure signal is from the length mode of a plate assembly comprising a plurality of plates of 4 inches (10.2 centimeters) in length oriented along the Z axis, which is the major axis of the transmitter section  16  of the tool  10 . The plates are axially and azimuthally aligned. Curve  92  represents acoustic pressure intensity as a function of position along the Z axis. The axial centerline  96  defines the axial center of the transmitter  86  shown in  FIG. 5 . There are two pronounced lobes  94   a  and  94   b  corresponding to the top and bottom, respectively, of the transmitter  86 . This can lead to phase shift and signal distortion since waves emitted from each end of the transmitter  86  arrive at the receivers  22  of the receiver section  20  (see  FIG. 1 ) at slightly different times. In practice, however, the signal of the transmitter  86  is “smeared out” when it is mounted within the wall  89  of the transmitter section  16 . If the axial dimension of a plate assembly  88  that can be used in logging tools is a maximum length of 5 inches (12.7 centimeters), the effects of the two-lobe pressure emission patter are negligible in fast and medium velocity formations penetrated by the borehole. The two-lobe pressure emission could, however, be noticeable in slow formations. It is noted that the lobes are also approximately axially coincident with the opening  85   a  and  85   b , respectively, shown in  FIG. 5 . The two lobes can cause difficulties in that energy from each lobe arrives at a different time at the receiver section  20  thereby resulting in a phase shift (see  FIG. 1 ). The two lobed pressure beam pattern from the transmitter  86  is not as efficient as the radiation pattern from the hypothetical compression transmitter yielding a single lobe. The transmitter  86  comprising preferably a plurality of plates can, however, still be used as an acoustic source as long as the top and bottom of the transmitter is acoustically coupled through openings  85   a  and  85   b  to the borehole fluid thereby allowing the pressure wave to propagate into the formation. The axial extent of each opening  85   a  and  85   b  should be sufficiently long to encompass the majority of the signal in the two lobes  94   a  and  94   b . It has been determined that openings of axial extent of approximately 1.5 inches (3.8 centimeters) above and below the plate assembly  88  meet this criterion. 
     An ideal transmitter output would be a single lobe preferably centered at the axial center  96  of the transmitter. The adverse effects of the double lobed signal can be minimized by redistributing the signal of a transmitter  86  to approach a single lobe emission. Three techniques for signal redistribution are discussed below. 
     A first technique of redistributing the double lobed pressure output signal would be to dispose the entire transmitter  86  in a heavy mass thereby smearing pressure output pattern at the transmitter. Enclosing the transmitter  86  in a heavy mass body has been shown to smear out the signal even in the slowest formations. However, the housing may cause frequency changes and may attenuate the signal. Therefore, the material and thickness of the housing are chosen so that signal attenuation and frequency changes are kept at a minimum. 
     A second technique for essentially eliminating the double lobe pressure signal output is to use a transmitter comprising a plurality of azimuthally aligned but axially staggered piezoelectric plates. The staggered plate concept is illustrated in the cross-sectional view of  FIG. 8  showing a plate assembly  98  comprising six plates  50 , where each plate is 4 inch (10.2 centimeters) in length. The plates  50  are axially staggered in a pressure housing (not shown) in 0.5 inch (1.3 centimeters) increments. The axial centerline of the staggered plate assembly  98  is shown at  95 .  FIG. 9  conceptually illustrates the intensity of the axial pressure output signal from a transmitter using the staggered plate assembly  98 . Again, the direction of the pressure signal is from the length mode of the plate assembly oriented along the Z axis, which is the major axis of the transmitter section  16  of the tool  10 . Curve  99  represents acoustic pressure intensity as a function of position along the Z axis. The axial centerline  95  defines the axial center of the transmitter shown in  FIG. 8 . The response curve  99  of the staggered assembly  98  has a much smoother shape than that of the axially aligned plate assembly shown in  FIG. 7 . The response curve  99  assumes an exponential attenuation in the radial direction from each plate  50 . As a result, the center  95   a  of the response curve  99  falls slightly below the axial  95 . The absence of the double lobed response curve eliminates some to the related problem discussed above. Staggering of the plate has, however, increased the length of the transmitter by 2 inches (5.1 centimeters) thereby requiring an axially elongated recess to receive the transmitter and, in turn, imposing additional physical restrictions on the logging tool housing. As in many logging methodologies, a compromise between transmitter length and signal distribution must be made to achieve the best measurement for a certain tool design. 
     A third technique is to place shields over the two lobes, which would focus the signal at the center of the plate. Experimental measurements and numerical modeling show that shielding the top and bottom 1.0 inch (2.5 centimeters) of a 4 inch (5.1 centimeters) long plate results in focusing the pressure wave toward the center line of the plate. 
     As mentioned previously, the transmitter is disclosed as a monopole transmitter. It should be understood that two transmitter assemblies can be azimuthally disposed, at essentially 180 degree centers, in dedicated recesses in the outer surface of the wall  89  of the transmitter section  16 . The transmitter assemblies are preferably axially aligned and operated to generate dipole acoustic pulses within the borehole environs. Similarly, four transmitter assemblies can be azimuthally disposed, at essentially 90 degree centers, in dedicated recesses in the outer surface of the wall  89  of the transmitter section  16 . The transmitter assemblies are preferably axially aligned and operated to generate quadrupole acoustic pulses within the borehole environs. Embodied as a dipole or quadrupole transmitter section, the transmitter assemblies are preferably axially aligned. Furthermore, responses from pulses generated by each azimuthal transmitter assembly are preferably detected with multiple receivers  22  (see  FIG. 1 ) that are axially spaced from and azimuthally aligned with that transmitter assembly. 
     The above disclosure is to be regarded as illustrative and not restrictive, and the invention is limited only by the claims that follow.