Patent Publication Number: US-9885796-B2

Title: Monopole acoustic transmitter ring comprising piezoelectric material

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of U.S. application Ser. No. 12/943,171, filed Nov. 10, 2010, which is a continuation application of U.S. application Ser. No. 12/179,175, filed Jul. 24, 2008, both of which are incorporated by reference in their entirety, and to which priority is claimed. 
    
    
     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. 
     Typical frequencies used for monopole acoustic tools are between 5 to 20 kiloHertz (KHz). It is desirable to have a transmitter that has the highest possible output at the desired frequency. There are many limitations of achieving this in down-hole tools. The more significant limitations are discussed briefly as follows. 
     The output of an acoustic transmitter is a function of the physical dimensions of the transmitting element such as piezoelectric material. A typical (LWD) tool, which is typically a drill collar, has a wall thickness of less than 3 inches (7.6 centimeters) and an outer diameter of about 7 inches (17.8 centimeters). If the transmitter is disposed within the wall of the tool, less than half of the wall thickness and a maximum of 2 inches (5.0 centimeters) of the perimeter of the collar can be due to structural restrictions. These restrictions set the maximum dimensions of a transmitter that can be used in an LWD tool. 
     The frequency of a transmitter comprising a piezoelectric crystal is a function of the physical dimensions of the transmitter. The size required to achieve the desired frequency determines the dimensions of a transmitter and hence limits its output. 
     Space required to fasten, seal, and mechanically and electrically isolate a transmitter in 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. 
     In summary, a transmitter comprising one or more piezoelectric crystal elements, or 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 tool (such as a drill collar) imposes additional transmitter dimensional restrictions that, in turn, affect energy and frequency output of the transmitter. There is, therefore, 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 
     The invention is a monopole acoustic transmitter consisting of a ring that comprises one or more piezoelectric arc segments. The ring is oriented in a plane whose normal is essentially coincident with the major axis of a logging tool in which it is disposed. Piezoelectric rings have been used in transmitter assemblies of wireline acoustic tools. First, a ring shaped transmitter lends itself to the wireline tool geometry. A typical wireline tool has a relatively thin housing and a transmitter ring can be easily installed in the center of the tool. Second, the diameter of a wireline acoustic tool is typically the same in wireline logging operations regardless of the borehole diameter. Since the frequency of a pressure pulse emitted by the piezoelectric ring is proportional to its diameter, the size of the ring chosen in a wireline tool provides the same frequency regardless of the borehole size. There are several restrictive challenges in disposing a piezoelectric ring transmitter in a LWD tool. In LWD systems, the tool is part of the drill collar. Typical diameters of commonly used LWD tools are 4.75, 6.75, 8.25, and 9.5 inches (12.0, 17.1, 21.0, and 23.5 centimeters), respectively. The ring element of the transmitter must be sized according to the tool diameter. Since the pressure output frequency of a ring is proportional to its diameter, rings used in different LWD tool sizes could have different output frequencies. There is also a drilling fluid or “mud” column within an LWD tool in a conduit that allows the drilling fluid to flow through the drill collar. This conduit further limits the space needed to mount a ring transmitter element in a plane perpendicular to the major axis of the tool. Another limitation is the relationship between the ring diameter and frequency of the output acoustic pressure pulse. For most piezoelectric materials, the ring diameter required to provide the desired frequency of 10 to 15 KHz is on the order of 3 to 4 inches (7.6 to 10.1 centimeters). All of the above restrictive challenges are addressed by the disclosed piezoelectric ring acoustic transmitter for LWD tools. The piezoelectric ring acoustic transmitter in this disclosure is disposed within a recess on the outer surface of a short, cylindrical insert. The insert is inserted into the collar, rather than into the wall of the collar, from the “downhole” end. The insert further comprises electronics required to operate and control the transmitter. The collar serves as a pressure housing for the tool. In addition, the insert has first and second external electronic connectors. The first connectors are oriented toward a receiver section of the LWD tool, and the second connectors are oriented toward the bottom or “downhole” end of the collar. Standard wiring passing through the collar passes through the transmitter insert to the bottom of the collar. In addition, at least one wire to control the transmitter is passed from the receiver section to the transmitter. The transmitter insert is mounted to the collar using O-rings or other pressure sealing structures to pressure seal elements of the transmitter from the borehole environs and to further hold the insert securely within the collar. 
     One or more openings in the wall of the collar provide a path of pressure pulses from the transmitter to pass into the borehole environs. A tradeoff between the number, dimensions, and location of the openings is made to obtain the best measurement and the highest output possible while still maintaining mechanical integrity of the collar structure. For example, having four large openings spaced azimuthally at 90 degrees from each other maximizes the pressure signal amplitude but can result in signal distortion at a receiver array due to the different paths acoustic pressure waves take to arrive at a receiver array aligned azimuthally and disposed on the side of the receiver section of the collar. This effect can be minimized by matching the number and azimuthal location of the receiver elements to the number and azimuthal location of the openings. 
     An alternative embodiment uses a segmented piezoelectric ring instead of a continuous piezoelectric ring. A segmented ring consists of segments of piezoelectric ceramic bonded to segments of other materials such as alumina to increase the frequency or heavy metals such as tungsten to reduce the frequency. The material and dimensions of the material used between the piezoelectric segments is chosen to alter the frequency of the ring. All piezoelectric segments of only a selected number of segments can be activated depending upon the application. Another embodiment uses a continuous piezoelectric ring on which certain arc segments are polarized or polled. This is accomplished by applying, to the surfaces of the ring, bands or “stripes” of electrode material. The entire striped ring is activated simultaneously. 
     One of the major advantages of using a piezoelectric ring transmitter oriented as previously defined is the optimization of acoustic pressure pulse output. Even with some of the ring output blocked by the tool wall or deactivated in a segmented ring comprising non piezoelectric material, the formation signal from a ring transmitter is still higher than other types of transmitters that can be mounted in the wall of a drill collar. 
     The piezoelectric ring, oriented with its normal essentially coincident with the major axis of the logging tool excited in the hoop mode, emits an acoustic pressure signal along the diameter of the ring. The signal from the ring is along the transverse direction perpendicular to the normal of the ring. This signal directionality is optimal for acoustic logging measurements. For logging tools varying in diameter from 4.75 inches (12.1 centimeters) to 7.0 inches (17.8 centimeters), output frequencies between 8 to 12 KHz are obtained. This frequency range is adequate for essentially all borehole diameters logged with LWD acoustic systems. 
     The fact that a ring is mounted inside the drill collar and the sound waves are emitted only through openings in the collar causes part of the signal to transmit directly into the collar body. This can cause an increase in tool mode signal, which is undesirable. Several methods can be used to reduce this effect. 
     One of the major advantages of using a ring transmitter is its optimization of signal output. Even with some of the ring signal blocked by the tool body or deactivated in a segmented ring, the pressure signal reaching the borehole environs is still typically greater than other types of transmitters that are be mounted within or in the wall of a drill collar. 
    
    
     
       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 piezoelectric ring transmitter within a LWD acoustic system disposed in a borehole drilling environment; 
         FIG. 2 a    is a perspective view of a “continuous” ring embodiment of the acoustic transmitter ring comprising a single loop of piezoelectric material; 
         FIG. 2 b    is a perspective view of a “segmented” ring embodiment of the acoustic transmitter ring comprising a plurality of arc segments of piezoelectric material bonded to intervening arc segments of non piezoelectric material; 
         FIG. 2 c    is a cross sectional view of the segmented ring embodiment; 
         FIG. 2 d    is a perspective view of a “striped” ring embodiment showing stripes of electrode material applied to the surface of a continuous piezoelectric ring; 
         FIG. 3  shows a transmitter ring assembly comprising a continuous or segmented piezoelectric ring that is disposed within a ring pressure compensation housing; 
         FIG. 4  shows an acoustic transmitter comprising the transmitter ring assembly disposed on a short insert. 
         FIG. 5  illustrates the acoustic transmitter within a drill collar; 
         FIG. 6 a    is a cross sectional view of the drill collar at the transmitter assembly ring with four openings spaced at 90 degrees and through which acoustic pulses from the transmitter enter the borehole environs; 
         FIG. 6 b    is a cross sectional view of a drill collar at the transmitter assembly ring with two larger openings spaced at 180 degrees and through which acoustic pulses from the transmitter enter the borehole environs; and 
         FIG. 7  illustrates the pressure beam pattern of the piezoelectric ring transmitter. 
     
    
    
     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 piezoelectric ring acoustic transmitter disposed within a LWD logging system operating 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 . 
     Again referring to  FIG. 1 , the tool  10  comprises a transmitter section  16  and a receiver section  20 . The transmitter section is an insert that is mounted within the drill collar  11 . The receiver section  20  comprises a plurality of receivers  22  disposed in the wall of the drill collar  11  and 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. Since the transmitter section  16  is mounted inside the drill collar  11 , a portion of the transmitter pressure pulse signals that are emitted through one or more openings in the collar into the borehole environs. Furthermore, another portion of this signal to transmit directly into the body of the drill collar  11 . This results in an increase in tool mode signal at the receivers  22 , which is undesirable. Several isolation apparatus and methods can be used to isolate the tool mode. An isolation structure is illustrated conceptually at  18 , and various isolation options 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 monopole 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 and optionally stores 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. 
     The monopole acoustic transmitter disclosed herein comprises a transmitter ring comprising one or more piezoelectric elements. 
     A first embodiment of the transmitter ring  52  is illustrated in perspective in  FIG. 2 a    and comprises a single loop of piezoelectric material  54 . This embodiment of the transmitter ring will be referred to as a “continuous” ring embodiment. The polarization of the ring is indicated by “+” and “−”. Electrical connections to the piezoelectric material  54  (see  FIG. 3 ) are such that the ring  52  expands or contracts upon application of a voltage. As an example, a positive voltage applied the outer and inner surfaces of the ring  52  expands the ring outward in the radial direction, while a negative voltage contracts the ring in the axial direction. This expansion and contraction is illustrated conceptually by the arrows  56 . The normal of the transmitter ring, in this and other disclosed embodiments, is illustrated by the arrow  53 . 
     A second embodiment of the transmitter ring  58  is illustrated in perspective in  FIG. 2 b   , and comprises a plurality of arc segments  60  of piezoelectric material with intervening arc segments  62  of material. This embodiment will be referred to as a “segmented” ring. For a given ring dimension, intervening arc segments  62  of relatively light material, such as alumina, increase output frequency. Conversely, intervening arc segments of relatively heavy materials, such as tungsten, decrease output frequency. The polarization of each segment  60  of each piezoelectric segment is again indicated by “+” and “−”. Electrical connections are such that the same voltage is applied simultaneously to each piezoelectric segment  60 . Each segment  60  expands and contracts simultaneously in an azimuthal direction illustrated conceptually by the arrows  64 . 
       FIG. 2 c    is a cross sectional view of the segmented ring embodiment  58 . Since all segments are rigidly bound to one another, the azimuthal expansions and contractions (see arrows  64 ) of the piezoelectric segments  60  result in a radial expansion and contraction of the segmented ring  58 . The ring expansion and contraction is illustrated conceptually by the arrows  68 . 
       FIG. 2 d    illustrates the “striped” ring embodiment  59 . The embodiment comprises continuous ring piezoelectric ring  63  on which active arc segments are polarized or polled. This is accomplished by applying, to the surfaces of the ring  63 , bands  61  or “stripes” of electrode material  61  thereby defining active arc segments. The active arc segments of piezoelectric material are polarized by the bands of electrode material  61  as indicated by “+” and “−” annotations. The entire striped ring  59  is activated simultaneously, as opposed to the segmented ring embodiment  58  in which certain segments of piezoelectric material can be activated independently. The acoustic pressure signal in the hoop mode, indicated conceptually by the arrows  69 , is greater than a continuous ring  52  of identical dimensions and applied voltages. 
       FIG. 3  shows a cross sectional view of a transmitter ring assembly  81 . A continuous  52 , striped, or segmented piezoelectric ring  58 , or a striped piezoelectric ring  59  is disposed within a ring pressure compensation housing  80 , as shown in  FIG. 3 . The outer wall  84  of the ring pressure compensation housing  80  is preferably thinner than the side and inner walls  82 . This optimizes radial acoustic pressure transmission into the borehole environs. High pressure connector  85  electrically connects the piezoelectric ring to the transmitter electronics (see  FIGS. 4 and 5 ) which connect electronically to the electronics section  26  of the tool  10  (see  FIG. 1 ). The ring pressure compensation housing  80  is filled with high dielectric oil  87  to balance the borehole pressure through a pressure compensator  86  such as a piston, bellows, diaphragm or the like. The pressure compensation housing  81  is sealed so that drilling fluids cannot enter the chamber and short circuit poles connected to the piezoelectric ceramic in the electrical connector  85 . 
       FIG. 4  is a cross sectional view of the acoustic transmitter  16  comprises the transmitter ring assembly  81  disposed on a short cylindrical insert  92 . The transmitter ring assembly  81  is disposed within a recess in an outer surface of the insert  92  with the normal  53  of the transmitter ring assembly essentially coincident with the major insert axis of the insert. The insert is fabricated from stainless steel or other material suitable for operation in harsh borehole conditions. Drilling fluid flows through a conduit  94  that is essentially coincident with the major axis of the insert  92 . The insert  92  further comprises a transmitter electronics element  93  that cooperates with the electronics section  26  (see  FIG. 1 ) to operate and control the transmitter  16 . In addition, the insert  92  has first and second external electronic connectors. The first connectors  95  are oriented toward the receiver section  20  of the LWD tool  10 , and the second connectors  97  are oriented toward the bottom or “downhole” end of the drill collar  11 . 
       FIG. 5  is a cross sectional illustration of the transmitter  16  disposed within the drill collar  11 . Standard wiring (not shown in  FIG. 5 ) passing through the collar  11  passes through the transmitter  16  to the bottom of the collar. In addition, at least one wire to control the transmitter  16  via the transmitter electronics element  93  and electronics section  26  is passed from the receiver section  20  to the transmitter  16 . Connectors  95  and  97  provide electrical connections between the electronics section  26  and additional tool sections, such as an auxiliary sensor section  14 , shown in  FIG. 1 . The transmitter insert  92  is mounted using O-rings or other mechanism to hold it securely within the drill collar  11 , as will be detailed subsequently. The drill collar  11  has at least one opening  106  exposing the transmitter ring assembly  81  to the borehole and thereby providing a radial path for pressure waves emitted by the transmitter ring assembly to enter the borehole environs. 
     As shown in axial cross section of  FIG. 5 , the “downhole” or drill bit end of the borehole assembly is to the left. O-rings  99 ,  100  and  102  hold the insert  92  securely within the collar  11 . O-rings  99  and  100  also seal the interior of the tool  10  from borehole environs to which the transmitter ring assembly  81  is exposed through the opening  106 . O-rings  102  likewise seal the “uphole” portion of the tool  10 , including the transmitter electronics element  93 , from the borehole environs. It should be understood that other sealing elements can be used to perform the sealing and position securing functions. 
       FIG. 5  shows the transmitter section  16  electrically and physically connected to the receiver section  20 . It is preferred to isolate the transmitter and receiver sections, as will be discussed in a subsequent section of this disclosure. 
     As mentioned previously, one or more openings  106  (see  FIG. 5 ) in the wall of the collar  11  and axially aligned with the transmitter ring assembly  81  provide a path for pressure pulses or waves from the transmitter ring assembly to pass into the borehole environs. A tradeoff between the number and location of the openings  106  is made to obtain the best acoustic measurement and the highest acoustic output possible into the borehole environs.  FIG. 6 a    is a cross sectional view of a collar  11  at the transmitter ring assembly  81  (not shown) with four openings  106  at 90 degrees from each other. This embodiment maximizes the output signal amplitude, but can result in received signal distortion due to the different paths acoustic pressure waves could take to arrive at receivers  22  if they are azimuthally aligned as shown in  FIG. 1 . 
     Recall that the receivers  22  are disposed in the wall of the drill collar  11 . Received signal distortion can, therefore, be minimized by azimuthally matching the number and location of the receivers  22  to the number and location of the openings  106 . Stated another way, the receiver array will comprise four receivers  22  (or four groups of receivers) disposed at 90 degree azimuthal spacings around the receiver section  20 , with each receiver (or receiver group) being aligned azimuthally with a corresponding opening.  FIG. 6 b    is a cross sectional view of a collar  11  with two openings  106  azimuthally spaced at 180 degrees. The opens are azimuthally larger than those shown in  FIG. 6 a   . Once again, received signal distortion is minimized by azimuthally matching the number and location of the receivers  22  to the number and location of the openings  106 . Although preferred, the dimensions and azimuthal spacings of multiple openings need not be equal. Finally, if a single receiver  22  embodiment is used, a single opening (not shown) in the collar is azimuthally aligned with the single receiver to minimize received signal distortion. 
     Tool Mode Considerations 
     As discussed previously, the transmitter ring assembly  81  is mounted inside the drill collar  11 . A portion of the emitted acoustic pressure signal is emitted radially through one or more openings  106  in the collar. Another portion of the emitted acoustic pressure signal is transmitted directly into the collar body. This can cause an increase in tool mode signal, which is undesirable. There are several ways of reducing this effect. 
     A tool mode isolator can be disposed between the transmitter section  16  and the receiver section  20 . The following techniques can be used reduce the tool mode signal. The transmitter ring assembly  81  can be acoustically isolated from the insert  92  using materials such as tungsten-loaded rubber. The transmitter insert  92  can be acoustically isolated from the collar  11  using various acoustic isolation materials. If a segmented transmitter ring  58  is used, the piezoelectric segments  60  can be made to correspond to the openings  106  in the collar  11 . Since the other segments  62  are not active, the acoustic signal emitted directly into the collar  11  will be greatly reduced. All of the above techniques are represented conceptually as an “acoustic isolator” at  18  of  FIG. 1 . 
     Output and Frequency Considerations 
     Since the pressure output frequency of a piezoelectric ring is proportional to its diameter, rings used in different LWD tool sizes can have different output frequencies. There is also the drilling fluid conduit  94  within the insert  92  with a major axis essentially coincident with the major axis of the tool  10 . The conduit  94  limits the space needed to mount the transmitter ring assembly  81  in a plane essentially perpendicular to the major axis of the tool  10 . Another limitation is the relationship between the ring diameter and output frequency. For most piezoelectric materials, the ring diameter required to provide the desired frequency of 10 to 15 KHz is on the order of 3 to 4 inches (7.6 to 10.2 centimeters). 
     Considering the previously described physical constraints, one of the major advantages of using a piezoelectric acoustic ring transmitter disposed inside of a drill collar  11  is pressure output optimization. Measurements show that a 4 inch (10.2 centimeter) outside diameter ring provides more than 3 KiloPascals (KPa) of pressure at 39.4 inches (1.0 meter) when excited with 1000 volt pulse. These output parameters are desirable for acoustic logging applications. Even with some of the transmitter ring output signal blocked by the collar  11  or deactivated in a segmented ring  58 , the formation signal from a ring transmitter  16  is still greater than other types of transmitters that must be mounted within the wall of a drill collar. 
     Depending on the piezoelectric material, a 12 KHz frequency, which is ideal for monopole logging measurements, is obtained from 3.0 to 3.5 inch (7.6 to 8.9 centimeters) outside diameter ring. The following table shows ring frequencies that can be obtained in different LWD tools, where the tool size dimension (in inches) is the outside diameter of the tool shown in  FIG. 1 . 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                 Tool Size 
                 Inner Ring 
                 Outer Ring 
                 Nominal 
               
               
                 (in) 
                 Diameter (in) 
                 Diameter (in) 
                 Frequency (KHz) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 4.75 
                 2.85 
                 3.25 
                 12 
               
               
                 6.75 
                 3.4 
                 3.9 
                 10 
               
               
                 8.25 
                 4.25 
                 4.75 
                 8 
               
               
                 9.5 
                 4.25 
                 4.75 
                 8 
               
               
                   
               
            
           
         
       
     
     For a 4.75 inch (12.07 centimeter) diameter of the tool  10 , the desired 12 KHz frequency can be easily obtained. For a 6.75 inch (17.15 centimeter) tool diameter, a frequency of 10 KHz can be obtained, which is still within the range of monopole measurements. For the 8.25 and 9.5 inch (20.1 and 24.1 centimeters) tool diameters, a maximum of 8 KHz can be obtained from a ring transmitter ring assembly  81  that is mounted inside the drill collar  11  as shown in this disclosure. Although this is less than optimum, these tools are typically used to drill 12 to 17 inch (30.5-43.2 centimeter) diameter boreholes, and a lower frequency transmitter may be desirable to provide deeper radial depth of investigation. 
     Signal Directionality 
     The pressure beam pattern of a piezoelectric ring transmitter  16  is shown in  FIG. 7 , which is a perspective illustration of pressure P (arbitrary units) output along the x and z axes. The z axis is coincident to the major axis of the logging tool  10 . A ring transmitter excited in the hoop mode emits a signal that is maximum perpendicular to the diameter of the ring. This direction is denoted as the x axis. The transmitter ring assembly  81  is located at coordinate (0,0) as indicted at  160 , with its normal coincident with the z axis. The magnitude of pressure output as a function of x and z is denoted by contours  162 ,  164 ,  166 ,  168  representing increasing values of P, with the maximum  170  of P at (0,0). The pressure output from the piezoelectric ring transmitter  16  is, therefore, optimal along the transverse direction (x axis), which is optimal for acoustic logging measurements. 
     The above disclosure is to be regarded as illustrative and not restrictive, and the invention is limited only by the claims that follow.