Abstract:
A monopole acoustic transmitter with at least one disc assembly with a plurality of piezoelectric discs 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 or “slot” 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 discs in each of at least one disc assembly are connected electrically in parallel and fired simultaneously. The polarity of the discs and the wiring arrangement are such that each disc 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 discs within the one or more disc assemblies. Each disc assembly is preferably disposed within an oil filled pressure housing that is pressure and temperature compensated. The axis of the at least one disc assembly can be parallel or perpendicular to the major axis of the logging tool.

Description:
FIELD OF THE INVENTION 
       [0001]    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 
       [0002]    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. 
         [0003]    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. 
         [0004]    Frequencies used in monopole acoustic LWD tools are typically in the 5 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 output frequency. 
         [0005]    Logging-while-drilling (LWD) and measurement-while-drilling (MWD) tools impose severe limitations that affect the energy and frequency output of a monopole 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. 
         [0006]    If the transmitter comprises piezoelectric transducers, the acoustic pressure output of an acoustic transmitter is proportional to the dimensions and the configuration of the transmitter. 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 “port” or “pocket” in the outer surface of the tool housing wall. A LWD tool housing is typically a drill collar. For structural strength reasons, it is necessary to restrict the radial depth and azimuthal width of a recess to a minimum. The axial length of a recess is not as tightly constrained from a structural strength perspective. These dimensional restrictions of the recess therefore govern the maximum radial, azimuthal and axial dimensions of a transmitter that can be disposed within the wall of an LWD tool. These restrictions, in turn, affect the acoustic pressure and frequency output of the transmitter. 
         [0007]    The frequency output and the acoustic energy output of a piezoelectric element are both a function of the geometry and dimensions of the piezoelectric element. It is often difficult to obtain sufficient energy at the desired frequency from a transmitter consisting of a piezoelectric element. Moreover, space required to fasten, seal, and mechanically and electrically isolate a transmitter in the wall of a logging tool of any type adds additional limiting factors to the transmitter dimensions and therefore to the transmitter outputs. In addition, the transmitter must be covered to protect it from mechanical damage during drilling operations that include handling, drilling and tripping of the drill string. 
         [0008]    In summary, a piezoelectric transmitter must be dimensioned and geometrically configured to operate within a tool in harsh borehole conditions. The structure required to operationally dispose the transmitter within the wall of the tool (such as a drill collar) imposes additional transmitter dimensional restrictions that, in turn, affect energy and frequency output of the transmitter. 
         [0009]    In view of the brief background discussion, there is a need for a monopole 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 
       [0010]    The invention is a monopole acoustic transmitter comprising a plurality of piezoelectric discs. More specifically, the transmitter comprises a stack disc assembly comprising a plurality of thin piezoelectric discs each with a hole in the center. The discs are held together under constant compressional force by a clamping apparatus, such as a bolt, passing through the hole in each disc and terminated at each end by a first and a second end cap, respectively. The discs are electrically connected using either conductive epoxy or copper shims between the major surfaces of adjacent discs. Polarity and electrical wiring of the disc assembly is such that all discs expand or contract simultaneously upon application of a voltage to the disc assembly. End caps made of different materials are installed at each end of the stack to control the pressure output and frequency of the transmitter. 
         [0011]    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”. Embodied as a LWD acoustic logging system, the logging tool housing is typically a steel drill collar. Deposition of the acoustic transmitter within the wall of the drill collar does not interfere with drilling fluid flow through the interior of the drill collar. 
         [0012]    The transmitter can be disposed with the major axis of the transmitter either axially (parallel to) or radially (perpendicular to) to the major axis of the drill collar within a recess “port” or “pocket” in the outer surface of the tool housing. If the transmitter is disposed axially in the tool housing, a flextensional member is installed to transfer the energy from the axial to the radial direction. If the transmitter is disposed radially in the drill collar, the pressure output of the transmitter is along transmitter axis, but radial to the major axis of the drill collar. Radial deposition is the preferred embodiment. The end caps attached to the two ends of the transmitter can be configured to meet previously mentioned structural restrictions on the radial depth and azimuthal width of the recess while obtaining the suitable output at a desired frequency. Another transmitter configuration consists of two stacks of discs axially aligned but spaced radially apart from each other and connected with an arch-shaped mass. The two disc assemblies comprise first and second end caps that can be configured to control the frequency, output, and signal directionality. 
         [0013]    The stacked piezoelectric disc acoustic transmitter acts as a spring mass system. The frequency of the disc assembly is largely a function of the length of the disc assembly and the mass of the end caps attached at the ends. The length of the disc assembly can be adjusted within reasonable limits as long as the overall length does not introducing serious tool housing structural problems. The acoustic output of the disc assembly is a function of the number of disc comprising the disc assembly, the thickness and diameter of the discs, and the force applied on the stack. Furthermore, output frequency and directionality of the acoustic output can be adjusted by varying the mass and geometry of the first and second end cap masses. By maximizing acoustic pressure output at a desired frequency, a disc assembly can be configured to maximize the precision of measured acoustic parameters of interest. 
         [0014]    The disc assembly is disposed in a pressure-compensated housing filled with oil thereby forming the transmitting element of a monopole acoustic transmitter suitable for borehole applications. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    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. 
           [0016]      FIG. 1  illustrates the invention embodied as an LWD acoustic system disposed in a borehole drilling environment; 
           [0017]      FIG. 2  is a cross sectional view of a disc assembly comprising a plurality of discs; 
           [0018]      FIG. 3  is a perspective view of a single piezoelectric disc; 
           [0019]      FIG. 4  illustrates in detail the polarization and electrical wiring of discs in a disc assembly; 
           [0020]      FIG. 5  is a cross sectional view of a logging tool transmitter section showing a single transmitter comprising a single disc assembly disposed radially within the tool section wall; 
           [0021]      FIG. 6  is a sectional view at A-A of the transmitter section comprising a single disc assembly and depicted in  FIG. 5 ; 
           [0022]      FIG. 7  is a cross sectional view of a logging tool transmitter section showing a single transmitter comprising a dual disc assembly disposed radially within the tool section wall and connected with an arch-shaped front mass; 
           [0023]      FIG. 8  is a sectional view at A-A of the transmitter section comprising the dual disc assembly and depicted in  FIG. 7 ; 
           [0024]      FIG. 9  conceptually illustrates the axial intensity distribution of the pressure output signal from the acoustic transmitters comprising both single and dual radial disc assemblies; 
           [0025]      FIG. 10  is a cross sectional view of a logging tool transmitter section showing a single transmitter comprising a single disc assembly disposed axially within the tool section wall; 
           [0026]      FIG. 11  is a sectional view at A-A of the transmitter section comprising a single disc assembly and depicted in  FIG. 10 ; 
           [0027]      FIG. 12  conceptually illustrates the axial intensity distribution of the pressure output signal from the single disc assembly transmitter shown in  FIGS. 10 and 11 ; and. 
           [0028]      FIG. 13  is a cross sectional view of the transmitter section comprising dual axially disposed acoustic transmitters that are not acoustically coupled. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]    An acoustic LWD logging tool typically comprises one or more acoustic transmitters and one or more acoustic receivers. 
         [0030]      FIG. 1  illustrates a single acoustic transmitter embodied as an LWD acoustic system disposed in a borehole drilling environment. The LWD borehole instrument or “tool” component of the borehole assembly is designated as a whole by the numeral  10 , and comprises a tool 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 . 
         [0031]    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. 
         [0032]    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 monopole acoustic transmitter disclosed herein does not require measurements from the directional section  24  or from the auxiliary sensor section  14 . 
         [0033]    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. 
         [0034]    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 . 
         [0035]    The monopole acoustic transmitter(s) disclosed herein comprise a disc assembly comprising a plurality or “stack” of piezoelectric disc elements. Pressure output of the disc assembly is greater than pressure output from a single disc comprising the assembly. Dimensions of individual discs, the disc assembly, and end caps cooperating with the stack of discs are adjusted to obtain maximum acoustic pressure output at a frequency suitable for acoustic logging. These dimensions must, however, meet structural strength and other environmental requirements necessitated by a borehole acoustic logging embodiment. 
         [0036]      FIG. 2  is a cross sectional view of a disc assembly  51  comprising a plurality of discs  50 . Preferably 10 to 20 discs  50  comprise the disc assembly  51 . The number of discs that can be used is determined somewhat by the assembly embodiment, as will be discussed in subsequent sections of this disclosure. The assembly is held together under constant compressional force by a clamping mechanism comprising a rod  54 , such as a metal bolt, passing through a hole in each disc (see  FIG. 3 ). The rod  54  is electrically insulated from the discs and from the electrical conducting material separating the disc. The ends of the rod  54  are terminated by a first mass end cap  52   a  and a second mass end cap  52   b , respectively. The length of the disc assembly  51  is designated at  55 . 
         [0037]      FIG. 3  is a perspective view A-A of the disc assembly  51  shown in cross section in  FIG. 2 . The outside diameter of the disc  50  is shown at  56   a . The disc has a center hole of inside diameter  56   b . The thickness of the disc is shown at  56   c . The rod  54  passes through and preferably fits tightly in the hole in center of the disc. It is noted that the normal of the illustrated disc surface  56   d  is parallel to the axis of the rod  54 . By definition, the major axis of the disc assembly  51  is coincident with the major axis of the rod  54 . 
         [0038]    There are alternate techniques for applying clamping force to the disc elements of the disc assembly. As an example, an alternate clamping mechanism comprises a cylindrical housing in which a plurality of hollow or solid discs is disposed. A threaded ring is then used to apply an equivalent claming force to the disc assembly. It should be understood that other clamping mechanisms can be used to apply clamping force to the disc assembly. 
         [0039]    Considering both  FIGS. 2 and 3 , the acoustic output of the disc assembly  51  is a function of the length  55 , number of discs  50  comprising the disc assembly, the diameter  56   a  and thickness  56   c  of the discs, and the force applied on the stack by the rod  54  cooperating with the end caps  52   a  and  52   b . Furthermore, output frequency and directionality of the acoustic output can be adjusted by varying the mass and geometry of the first and second end caps  52   a  and  52   b . As an example, the force and end plate masses can be selected so that a disc assembly of length of 3.0 inches (7.62 centimeters) comprising 10 discs of diameter 1.0 inches (2.54 centimeters) yields a monopole frequency output in the range of 10 to 12 KHz at 2 to 3 KiloPascals (KPa) at 39.4 inches (1.0 meter) and at an input voltage of 1000 volts. These output parameters are desirable for acoustic logging applications. 
         [0040]      FIG. 4  shows in more detail a portion of the disc assembly  51 , and illustrates the polarization and electrical wiring of the disc assembly. The discs  50  are electrically connected using either conductive epoxy or copper shims  60  between the major surfaces  56   d  (see  FIG. 3 ) of each adjacent disc. The polarizations of the individual discs  50  are indicated by “+” and “−”. Electrical connections to the disc assembly  51  are such that all discs  50  expand or contract simultaneously upon application of a voltage to the assembly  51 . As an example, a positive voltage applied to the electrical bus  61  expands the assembly in the axial direction, while a negative voltage applied to the electrical bus  62  contracts the assembly in the axial direction, as illustrated conceptually by the arrow  63 . 
       Radial Transmitter Configurations 
       [0041]      FIG. 5  is a cross sectional view of the transmitter section  16  (see  FIG. 1 ) illustrating an acoustic monopole transmitter  151  disposed radially therein. The transmitter  151  comprises a disc assembly  51  of the type shown in  FIGS. 2 through 4 . The disc assembly  51  is housed in a pressure housing  72  that is preferably filled with oil and is pressure and temperature balanced. By definition, the major axis of the transmitter  151  is coincident with the major axis of the disc assembly  51 . The transmitter  151  comprising the disc assembly  51  and pressure housing  72  is disposed within a recess or essentially cylindrical “slot” (defined by the surfaces  83 ) in the outer surface of the wall  89  of the transmitter section  16  of the tool  10 . Material  70  mechanically and acoustically isolates the transmitter  151  from the wall  89  of the transmitter section  16 . A cover  91  protects the pressure housing  72  from abrasion encountered in the borehole environment. The cover is illustrated as a hatch, but desired protection could also be obtained with a circumferential sleeve. The transmitter  151  is oriented so that the major axis of the transmitter is essentially perpendicular to the major axis of the transmitter section  16  and thus perpendicular to the major axis of the tool  10 . In this illustration, the disc assembly 
         [0042]    Still referring to  FIG. 5 , the cover  91  is configured so that it acoustically couples the transmitter  151  to the borehole environs. The axial centerline of the transmitter  151  is shown at  80 , and will be referenced in a subsequent discussion. 
         [0043]    Again referring to  FIG. 5 , the transmitter section  16  of a LWD tool  10  is typically a steel drill collar comprising a wall  89  and a conduit  87  through which drilling fluid flows. For a transmitter section  16  that has an outer diameter of about 7 inches (17.8 centimeters) and a wall thickness of less than 3 inches (7.6 centimeters), it is necessary to restrict the dimensions of the recess slot defined by the surfaces  83 . More specifically, it is desirable to restrict the depth of slot to less than half of the wall thickness, to restrict the azimuthal arc of the slot to 2 inches (5.1 centimeters) or less, and to restrict the axial length of the slot to 6 inches (15.2 centimeters) or less. These structural restrictions set the maximum dimensions of the transmitter  151  (and on other embodiments of the transmitter) that can be disposed within the wall of an LWD tool and, therefore, affect the energy and frequency outputs of the transmitter. Stated another way, physical restrictions on the recess slot housing the transmitter  86  affect the frequency and energy outputs of the transmitter. 
         [0044]    As discussed previously, individual discs  50  comprising the disc assembly  51  are polarized, configured and electrically connected to emit a pressure pulse in a common direction upon application of a voltage to the transmitter assembly. 
         [0045]      FIG. 6  is a sectional view at A-A of the transmitter section  16  depicted in  FIG. 5 . The acoustic monopole transmitter  151  disposed therein is shown from this perspective with all elements and their identifying numbers being defined in the discussion of  FIG. 5 . 
         [0046]    Consider now both  FIGS. 5 and 6 . Recall that the output of the transmitter assembly  151  is a function of the piezoelectric material used in the discs  50 , the material and dimensions if the mass end caps  52   a  and  52   b , the material and dimensions of the rod  54 , and the force applied to the disc assembly  51  by the end caps  52   a  and  52   b  cooperating with the rod  54 . These parameters are adjusted so that a disc assembly is about 3 inches (7.6 centimeters) in length and has an operating frequency of 10-12 KHz. This frequency range is ideal for monopole acoustic logging applications. The thickness mode frequency of a 0.125 inches (0.32 centimeters) thick disc is in the megahertz range and does not penetrate far from the transmitter  151 . Therefore, in a LWD tool in a borehole environment, the thickness mode of the discs within a disc assembly has no effect on the measurement made using the length mode. A disc with a diameter of 1.5 inches (3.8 centimeters) has a frequency in the range of 40 to 50 KHz in the radial mode. In order for this frequency not to interfere with the length mode frequency, the diameters of discs within a disc assembly should be kept at a maximum of 1.5 inches (3.8 centimeters). 
         [0047]    A second embodiment of the transmitter comprises two disc assemblies joined by an arch-shaped mass structure. Attention is directed to  FIG. 7  which illustrates a cross sectional view of this dual disc radial assembly, hereafter referred to as the “dual radial assembly” transmitter and denoted as a whole by the numeral  152 . More specifically, the dual radial assembly transmitter comprises two disc assemblies  51   a  and  51   b , with each assembly being disposed in pressure housings  72   a  and  72   b , respectively. The construction of each disc assembly is essentially the same as that shown for the single disc assembly  51  illustrated in  FIGS. 5 and 6 , with the exception of the front mass cap. Some identifier numbers have, however, been omitted for clarity. The axes of the disc assemblies  51   a  and  51   b  are parallel, are each perpendicular to the major axis of the transmitter section  16 , and are axially aligned within the transmitter section. 
         [0048]    The arch-shaped mass structure  76  acoustically couples the two disc assemblies  51   a  and  51   b  such that the end caps  50   b , used in previously discussed disc assembly embodiments, are not used in the dual radial assembly. The arch-shaped mass structure  76  functions as one end cap for each of the two disc assemblies. The mass structure  76  is affixed to the stack assemblies  51   a  and  51   b  by the assembly rods  54  (see  FIG. 2 ) which preferably extend through the mass structure and are terminated by suitable fasteners such as nuts  54   a  and  54   b . The arch-shaped mass structure thereby physically and acoustically couples the stack assemblies  51   a  and  51   b . The dual radial assembly is disposed in a recess slot in the outer surface of the wall  89  of the transmitter section  16 . The recess slot is defined by the surfaces  83 . As in the previous transmitter embodiment, material  70  lining portions of the recess slot isolate the disc assemblies  51   a  and  51   b  mechanically and acoustically from the wall  80  of the transmitter section  16 . 
         [0049]    Both transmitters are operated simultaneously so that all discs  50  in each disc assembly  51   a  and  51   b  emit acoustic pressure pulses of the same polarity into the surrounding borehole environs. The dual radial assembly transmitter  152  is acoustically coupled to the borehole environs through the arch-shaped mass structure  76 . Because of geometric constraints, the lengths of disc assemblies in the dual radial assembly transmitter  152  (see  FIG. 7 ) are greater than the length of the disc assembly used in the single radial disc assembly transmitter  151  (see  FIG. 5 ). The arch-shaped mass structure is also greater than the outer mass  50   b  of the single disc assembly. This results in the dual radial assembly transmitter operating at more than twice the pressure output of the single disc assembly transmitter and further operating at a more optimum frequency range. 
         [0050]    Referring to  FIG. 5 , two acoustically uncoupled single stack assemblies (not shown) could be disposed within the drill collar  87  similar to the deposition of the stack assemblies  51   a  and  51   b  shown in  FIG. 7 . This would allow the length of each single stack assembly to be increased thereby increasing acoustic output. Each uncoupled single stack assembly could not, however, be azimuthally aligned with the plurality of receivers  22  axially spaced from the transmitter section  16  and shown in  FIG. 1 . This would result in signal distortion at the receivers  22 . Distortion due to transmitter-receiver azimuthal misalignment is not a problem using the dual radial assembly transmitter  152 . The arch-shaped mass  76  azimuthally focused to a single point outputs from the two stack assemblies  51   a  and  51   b . This point is azimuthally aligned with the receivers  22 . 
         [0051]      FIG. 9  conceptually illustrates the axial distribution of the intensity of the pressure output signal, operating in the length mode 11-13 KHz frequency range, from both the single disc assembly acoustic transmitter  151  and dual radial assembly transmitter  152 . Curve  92  represents acoustic pressure intensity as a function of position along the Z axis. The axial centerlines  80  and  80   a  define the axial center of the transmitters  151  and  152  shown in  FIGS. 5-6  and  FIGS. 7-8 , respectfully. It can be seen that the output is essentially a Gaussian distribution with a peak at the centerline  80 . As mentioned previously, the output of the dual radial assembly transmitter  152  is more than twice the output of the single disc assembly acoustic transmitter  151 . 
       Axial Transmitter Configurations 
       [0052]      FIG. 10  is a cross sectional view of the transmitter section  16  (see  FIG. 1 ) illustrating an acoustic monopole transmitter  251  disposed axially therein. The transmitter  251  comprises a disc assembly  51  of the type shown in  FIGS. 2 through 4 . The disc assembly  88  is again housed in a pressure housing  72  that is preferably filled with oil and is pressure and temperature balanced. The transmitter  251  also comprises a flextensional member  200  that surrounds the pressure housing  72 . The flextensional member  200  is fabricated from spring type material such as metal leaves and functions to transfer the transmitted acoustic energy from the axial to the radial direction. By definition, the major axis of the transmitter  251  is coincident with the major axis of the disc assembly  51 . 
         [0053]    The transmitter  251  is disposed within a recess (defined by the surfaces  183 ) in the outer surface of the wall  89  of the transmitter section  16  of the tool  10 . As in previously discussed embodiments, this recess will be referred to as a “pocket” or “slot”. A cover  210  protects the transmitter  251  from abrasion encountered in the borehole environment. The transmitter  251  is oriented axially so that the major axis of the transmitter is parallel to the major axis of the transmitter section  16  and thus parallel to the major axis of the tool  10 . 
         [0054]    Still referring to  FIG. 10 , acoustic pulses generated by the transmitter  251  enter the borehole environments through the protective cover  210 , which acoustically couples the transmitter to the borehole environs. Acoustic pulses can also enter the borehole environs through openings shown at  185   a  and  185   b , which are typically filled with borehole fluid thereby acoustically coupling the transmitter  251  to the borehole environs. The axial centerline of the transmitter is shown at  196 , and will be referenced in a subsequent discussion. 
         [0055]    As stated previously, the transmitter section  16  of a typical LWD tool  10  is typically a steel drill collar comprising the wall  89  and the conduit  87  through which drilling fluid flows. Referring again to  FIG. 10 , consider a transmitter section  16  that has an outer diameter of about 7 inches (17.8 centimeters) and a wall thickness of less than 3 inches (7.6 centimeters). It is necessary for structural reasons to restrict the dimensions of the recess slot defined by the surfaces  183 . As stated previously, it is desirable to restrict the depth of slot to less than half of the wall thickness, to restrict the azimuthal arc of the slot to 2 inches (5.1 centimeters) or less, and to restrict the axial length of the slot to 6 inches (15.2 centimeters) or less. These structural based recess restrictions set the maximum dimensions of a transmitter  251  that can disposed within the wall of an LWD tool and, therefore, affect the energy and frequency outputs of the transmitter. As in previous embodiments, physical restrictions on the recess slot housing the transmitter  251  affect the frequency and energy outputs of the transmitter. 
         [0056]    As discussed previously, individual discs comprising the disc assembly  88  are polarized, configured and electrically connected to emit a pressure pulse in a common direction upon application of a voltage to the transmitter assembly. 
         [0057]    Details of the basic disc assembly  51  have been illustrated (see  FIGS. 2 and 3 ) and discussed in detail in a previous section of this disclosure. 
         [0058]      FIG. 11  is a cross a partial cross sectional view A-A of the transmitter section  16  shown in  FIG. 10 . The transmitter  251  comprising the flextensional member  200  that surrounds the disc assembly  51  is shown disposed in the slot defined by surfaces  183  in the wall  89  of the transmitter section  16 . The flextensional member  200  is fabricated from spring type material such as metal leaves and functions to transfer the transmitted acoustic energy from the axial to the radial direction. More specifically, the flextensional member  200  cooperates with the disc assembly  51  to direct a major component of acoustic energy radially into the borehole environs thereby minimizing an axial component which propagates along the borehole and the tool  10 . 
         [0059]      FIG. 12  conceptually illustrates the axial distribution of the intensity of the pressure output signal, operating in the length mode 11-13 KHz range, from the transmitter  251  depicted in  FIGS. 10 and 11 . Curve  192  represents acoustic pressure intensity as a function of position along the Z axis. The axial centerline  196  defines the axial center of the transmitter  251  shown in  FIG. 10 . There is a pronounced peak at  198  with two smaller excursions  197  and  195  on either side of the peak indicating that a major component of the output acoustic energy is directed radially into the borehole environs. 
         [0060]    Optionally, the dimensions of a single axially oriented transmitter can be varied and the same pressure and frequency output can be obtained by using two transmitters.  FIG. 13  is a cross sectional view of the transmitter section  16  showing two transmitters  251   a  and  251   b  disposed in two axially aligned slot recesses defined by the surfaces  251   a  and  251   b , respectively. Both transmitters are operated simultaneously so that all discs in each transmitter emit acoustic pressure pulses of the same polarity into the surrounding borehole environs. Unlike the dual transmitter discussed previously and shown in  FIGS. 7 and 8 , the transmitters are not mechanically and directly acoustically coupled to each other. Both transmitters  251   a  and  251   b  are protected from abrasion by covers  210   a  and  210   b , respectively, and are acoustically coupled to the borehole environs through corresponding flextensional members  200   a  and  200   b , openings of the type depicted at  185   a  and  185   b  in  FIG. 10 , and through the covers  210   a  and  210   b . In order to insure that the arrival of acoustic energy at the receiver section  20  (see  FIG. 1 ) is not excessively broadened or phase shifted, it is desirable to use two receiver arrays, with a plurality of receivers  22  in each array azimuthally aligned with the normal of each transmitter  251   a  and  251   b.    
         [0061]    One drawback of this design is that the pressure signal is emitted in all directions. The spring elements that are oriented inward toward the tool body will generate acoustic pressure that might increase the tool body mode. An alternate flextensional device can be structured to flex in all radial directions but to only generate significant pressure in a direction that is radially outward from the major axis of the logging tool. 
         [0062]    The above disclosure is to be regarded as illustrative and not restrictive, and the invention is limited only by the claims that follow.