Abstract:
An acoustic logging apparatus includes a tool body and a housing. A transducer operating in the bending mode is mounted in the housing. The transducer operates such that it is excited by or emits acoustic energy in only one of the two directions substantially perpendicular to the face of the transducer. The housing is mounted substantially removed from the axis of the body. An acoustic receiver includes an outer sleeve having a flange and a hat slidably mounted within the outer sleeve. The sliding of the hat compensates for variations in pressure and temperature. An acoustic transmitter includes a main housing and a hat slidably supported within the main housing. The sliding of the hat compensates for variations in pressure and temperature. Combinations of air gaps and o-rings in the transducer acoustically isolate a piezoelectric crystal from its housing and the housing from its enclosure. The acoustic receiver and acoustic transmitter are configured to be replaced in the field.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to a method and apparatus utilized in hydrocarbon exploration. More specifically, the invention relates to the utilization of acoustic sources and receivers to determine acoustic properties of geologic formations as a logging tool traverses them, be it a wireline logging tool or a logging while drilling tool. More particularly, the present invention is directed to methods of and apparatus for converting between acoustic energy and electrical signals. 
     BACKGROUND OF THE INVENTION 
     Geologists and geophysicists are interested in the characteristics of the formations encountered by a drill bit as it is drilling a well for the production of hydrocarbons from the earth. Such information is useful in determining the correctness of the geophysical data used to choose the drilling location and in choosing subsequent drilling locations. In horizontal drilling, such information can be useful in determining the location of the drill bit and the direction that drilling should follow. 
     Such information can be derived in a number of ways. For example, cuttings from the mud returned from the drill bit location can be analyzed or a core can be bored along the entire length of the borehole. Alternatively, the drill bit can be withdrawn from the borehole and a “wireline logging tool” can be lowered into the borehole to take measurements. In still another approach, called “measurement while drilling” (“MWD”) or “logging while drilling” (“LWD”) tools make measurements in the borehole while the drill bit is working. There are a wide variety of logging tools, including resistivity tools, density tools, sonic or acoustic tools, and imaging tools. 
     An acoustic logging tool collects acoustic data regarding underground formations. The purpose of such a tool is to measure the “interval transit time” or the amount of time required for acoustic energy to travel a unit distance in a formation. In simple terms, this is accomplished by transmitting acoustic energy into the formation at one location and measuring the time that it takes for the acoustic energy to travel to a second location or past several locations. The measurement is complicated by the fact that the tool is roughly in the middle of a borehole of unknown diameter and is surrounded by mud. Further, the formation along the borehole may have been disturbed by the action of the drill bit and may no longer have the same acoustic characteristics as the undisturbed formation. 
     SUMMARY OF THE INVENTION 
     In general, in one aspect the invention features an acoustic logging apparatus comprising a tool body and a housing. A transducer operating in the bending mode is mounted in the housing. The transducer operates such that it is excited by or emits acoustic energy in only one of the two directions substantially perpendicular to the face of the transducer. 
     Implementations of the invention may include one or more of the following. The transducer may be a unimorph. The transducer may be a bimorph. The transducer may be utilized as an acoustic transmitter. The transducer may be utilized as an acoustic receiver. 
     In general, in another aspect, the invention features an acoustic logging apparatus comprising a tool body, a housing, a transducer operating in the bending mode mounted in the housing. The housing is mounted substantially removed from the axis of the body. 
     In general, in another aspect, the invention features an acoustic transmitter comprising a piezoelectric crystal for use in an acoustic logging tool configured to generate acoustic energy in response to an electric signal, the acoustic energy generated in a preferred direction being at least 3 dB larger than the acoustic energy generated in a direction substantially perpendicular to the preferred direction. 
     In general, in another aspect, the invention features an acoustic receiver comprising a piezoelectric crystal for use in an acoustic logging tool configured to generate an electrical signal in response to acoustic energy, the signal for acoustic energy of a magnitude received from a preferred direction being at least 3 dB larger than signals for acoustic energy of the magnitude received from a direction substantially perpendicular to the preferred direction. 
     In general, in another aspect, the invention features an acoustic transponder comprising an outer sleeve and an inner assembly coupled to the outer sleeve, the inner assembly being substantially acoustically isolated from the outer sleeve. 
     In general, in another aspect, the invention features an acoustic logging tool comprising an acoustic transmitter and an acoustic receiver. The acoustic receiver has a different electrical ground from the acoustic transmitter. 
     In general, in another aspect, the invention features an acoustic receiver for converting acoustic energy to an electronic signal comprising a hat, a piezoelectric crystal mounted within the hat, and a first compliant element separating the crystal from the hat. 
     Implementations of the invention may include one or more of the following. The hat may comprise a thermoplastic. The thermoplastic may comprise polyetheretherketone. The hat may comprise a metal. The acoustic receiver may include an excluder separated from the crystal by a second compliant element. The excluder may comprise a thermoplastic. The excluder may comprise a metal. The acoustic receiver may include a connector, a wire coupled to the connector and to the piezoelectric crystal, a portion of the wire being supported by the excluder. 
     In general, in another aspect, the invention features an acoustic receiver comprising an outer sleeve having a flange, a hat being slidably mounted within the outer sleeve. 
     Implementations of the invention may include one or more of the following. The hat may have a flange. The flange of the hat may move toward the flange of the outer sleeve as the hat slides into the outer sleeve. A first compliant element may be placed between the flange of the hat and the flange of the outer sleeve. The acoustic receiver may further comprise a piston, and a second compliant element between the piston and the hat. The acoustic receiver may further comprise a securing element configured to secure the piston relative to the outer sleeve. The securing element may comprise a snap ring. The acoustic receiver may comprise a lower housing coupled to the outer sleeve, a port through the lower housing for communicating with a chamber formed by the outer sleeve, the hat and the lower housing, pressure compensating fluid for filling the chamber, and a seal for preventing the pressure compensating fluid from escaping the chamber. The pressure compensating fluid may comprise oil. The seal may comprise one or more o-rings. The seal may separate the lower housing from the outer sleeve, and the hat from the outer sleeve. The compensation fluid, the hat, and the lower housing may compensate for pressure and temperature variations. A piezoelectric crystal may be secured within the assembly comprising the hat and the lower housing. The hat and crystal assembly may be configured to move relative to the lower housing. The movement may compensate for pressure and temperature variations. 
     In general, in another aspect, the invention features an acoustic transmitter comprising a main housing, and a hat slidably supported within the main housing. 
     Implementations of the invention may include one or more of the following. A compliant element may separate the hat from the main housing. A support element may rigidly separate the transducer housing from the compliant element. The support element may comprise metal. The support element may comprise a thermoplastic. The thermoplastic may comprise polyetheretherketone. The hat may comprise thermoplastic. The hat may comprise a metal. 
     The acoustic transmitter may comprise a piston engaging the main housing and a first compliant element separating the upper side of the hat from the piston. The acoustic transmitter may comprise a second compliant element configured to bias the hat against the first compliant element. The acoustic transmitter may comprise a connector coupled to the main housing, and a wire coupled to the connector, a portion of the wire being supported by the hat. 
     The acoustic transmitter may comprise a port through the main housing for communicating with a chamber formed by the main housing and the hat, pressure compensating fluid for filling the chamber, and a seal system for preventing the pressure compensating fluid from escaping the chamber. The pressure compensating fluid may comprise oil. The seal system may comprise one or more o-rings. The compensation fluid, hat, and lower housing may compensate for pressure and temperature variations. A piezoelectric crystal may be adhesively secured within the assembly comprising the hat and the lower housing. The hat and crystal assembly may be configured to move relative to the lower housing. The movement may compensate for pressure and temperature variations. 
     In general, in another aspect, the invention features an acoustic logging tool comprising an elongated body and an acoustic transducer mated to the body in such a way that the acoustic transducer can be replaced in the field. 
     In general, in another aspect, the invention features a method for transforming acoustic energy to an electrical signal comprising configuring a piezoelectric receiver in an acoustic logging tool to generate a signal in response to the acoustic energy, the signal for acoustic energy of a magnitude received from a preferred direction being at least 3 dB larger than signals for acoustic energy of the magnitude received from a direction substantially perpendicular to the preferred direction. 
     In general, in another aspect, the invention features a method for transforming an electrical signal to acoustic energy comprising configuring a piezoelectric transmitter in an acoustic logging tool to generate acoustic energy in response to the electric signal, the acoustic energy generated in a preferred direction being at least 3 dB larger than the acoustic energy generated in a direction substantially perpendicular to the preferred direction. 
     In general, in another aspect, the invention features a method for transforming between acoustic energy and an electrical signal comprising mounting a piezoelectric transducer in a hat, the hat being slidably mounted within a housing such that the hat slides into and out of the housing depending on the difference in pressure between the inside and the outside of the housing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a logging while drilling system incorporating an acoustic logging while drilling tool of the present invention. 
     FIG. 2 is a representation of an acoustic logging tool. 
     FIG. 3 is a plan view of an acoustic logging tool incorporating the present invention. 
     FIG. 4 is a representation of the forces generated by a piezoelectric crystal stack. 
     FIGS. 5,  6 , and  7  are representations of a bimorph transducer and the forces it generates or that act upon it. 
     FIGS. 8,  9 , and  10  are representations of a unimorph transducer and the forces it generates or that act upon it. 
     FIG. 11 is a perspective view of an acoustic receiver according to the present invention. 
     FIG. 12 is a top view of an acoustic receiver according to the present invention. 
     FIGS. 13 and 15 are a section views of an acoustic receiver according to the present invention along lines XIII on FIG.  12 . 
     FIG. 14 is a section view of an acoustic receiver according to the present invention along lines XIV on FIG.  12 . 
     FIG. 16 is a perspective view of an acoustic transmitter according to the present invention. 
     FIG. 17 is a top view of an acoustic transmitter according to the present invention. 
     FIGS. 18 and 20 are section views of an acoustic transmitter according to the present invention along lines XVIII on FIG.  12 . 
     FIG. 19 is a section view of an acoustic transmitter according to the present invention along lines XIX on FIG.  12 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, a drilling rig  10  (simplified to exclude items not important to this application) comprises a derrick  12 , derrick floor  14 , draw works  16 , hook  18 , swivel  20 , kelly joint  22 , rotary table  24 , drillstring  26 , drill collar  28 , LWD tool  30 , LWD acoustic logging tool  32  and drill bit  34 . Mud is injected into the swivel by a mud supply line  36 . The mud travels through the kelly joint  22 , drillstring  26 , drill collars  28 , and LWD tools  30  and  32  and exits through ports in the drill bit  34 . The mud then flows up the borehole  38 . A mud return line  40  returns mud from the borehole  38  and circulates it to a mud pit (not shown) and back to the mud supply line  36 . 
     The data collected by the LWD tools  30  and  32  is returned to the surface for analysis by telemetry transmitted through the drilling mud. A telemetry transmitter  42  located in a drill collar or in one of the LWD tools collects data from the LWD tools and modulates the data to transmit it through the mud. A telemetry sensor  44  on the surface detects the telemetry and returns it to a demodulator  46 . The demodulator  46  demodulates the data and provides it to computing equipment  48  where the data is analyzed to extract useful geological information. 
     In abstract, the acoustic logging tool  50  has an acoustic transmitter  52  and an acoustic receiver  54  supported by a body  56 , as shown in FIG.  2 . The acoustic logging tool  50  is part of a drill string (not shown) inserted in a borehole  58  through a formation  60 . The acoustic transmitter  52  emits acoustic energy  62  into the formation  60 . The acoustic energy  62  is refracted and travels through the formation  60  along the borehole  58 . As it travels along the borehole  58 , a portion of the acoustic energy  62  is emitted back into the borehole  58  where it is detected by the acoustic receiver  54 . By measuring the elapsed time from the transmission of the acoustic energy  62  by the acoustic transmitter  52  to the receipt of the acoustic energy by the acoustic receiver  54 , and processing the measurement, the “slowness” of the formation can be derived. Using the derived slowness of the formation and formation measurements made by other tools, a variety of formation characteristics can be derived. 
     Some acoustic energy  64  emitted by the acoustic transmitter  52  is transmitted into the body  56  of the acoustic logging tool  50  and travels along the body  56  to the acoustic receiver  54 . The receipt of the acoustic energy  64  that travels along the body interferes with the acoustic energy  64  transmitted through the formation  60 , making the “slowness” calculation more difficult or even impossible to perform. 
     An acoustic transducer according to the invention reduces the amount of acoustic energy that travels along the body by directing most of the acoustic energy it generates in a direction that is generally perpendicular to the body. Further, the acoustic transducer is configured to reduce the coupling between the element generating the transducer&#39;s acoustic energy and the body. The acoustic transducer is also configured to maintain the acoustic decoupling under the pressures and temperatures experienced during down hole operations. The acoustic transducer is further configured to operate under the conditions pertaining to down hole operations, i.e. pressure, temperature, shock, vibration, interaction with drilling fluid, interaction with formation fluid/gas. 
     As shown in FIG. 3, a logging-while-drilling acoustic logging tool  66  comprises an array of transmitters  68  and an array of receivers  70 . The illustrated acoustic logging tool  66  shows a transmitter array  68  comprising a pair of transmitters and a receiver array  70  comprising seven pairs of receivers. The invention is not limited to such an arrangement and can include any number of transmitters and receivers arranged in any orientations. The words “transponder”, “transducer”, “acoustic transponder” and “acoustic transducer” will be understood to apply to both acoustic receivers and acoustic transmitters. It is understood that the words “unimorph” and “bimorph” mean piezoelectric elements (crystals) operating in the “bending mode”. 
     As illustrated in FIG. 4, some existing acoustic transducers comprise a stack of piezoelectric crystals  72 . The crystals are oriented such that when operating as a transmitter an electric signal is applied to each crystal in the stack, causing the crystals to contract (expand) across the crystal face and expand (contract) in thickness thereby emitting acoustic energy. Consequently, the stack of crystals exerts an expanding force  74  perpendicular to the faces of the crystals and a contracting force  76  parallel to the faces of the crystals. The expanding force  74  is directed away from the tool and into the surrounding mud and formation. The contracting force, however, is directed into the tool parallel to the tool body and is likely to interfere with the acoustic velocity measurements unless it is controlled or confined. Conversely when operating as a receiver an acoustic energy field interacts with the crystals causing them to contract (expand) across the crystal face and expand (contract) in thickness, causing an electric signal to be generated that is substantially proportional to the acoustic energy in the field. The acoustic energy field interaction with the crystal face contains all the useful information required to perform acoustic measurements in acoustic logging. It is then preferred that the receiver embodiment minimizes or eliminates the sensitivity of the receiver to acoustic energy delivered to the transducer from a direction substantially parallel to the face of the crystal. 
     The invention uses a bimorph or unimorph transducer to generate acoustic energy (acoustic transmitter). It is well known to those skilled in the art that bimorphs and unimorphs are commercially available items. As shown in FIGS. 5 and 6, a bimorph comprises two piezoelectric crystals  78  and  80  bonded to each other. In an alternate embodiment the bimorph is constructed as two piezoelectric crystals bonded to each other through a metal disk. For ease of discussion the arrangement of FIG. 6 will be referenced, but those skilled in the art will appreciate that substantially the same discussion is valid for the bimorph constructed in the alternate way. The two crystals are oriented in such a way that upon an application of an electrical signal to them, crystal  78  expands across its face and crystal  80  contracts across its face. The combination of the expansion of crystal  78  and the contraction of crystal  80  causes the two crystals to arch, as shown in FIG.  7 . This arching causes forces  82  to be generated perpendicular to the faces of the crystals. Forces  84  are also generated parallel to the faces of the crystals but they are generally substantially smaller than the corresponding forces  76  (shown in FIG. 4) generated by the crystal stack used in existing acoustic transmitters. The bimorph is oriented so that forces  82  are directed outward from the body of the tool. Consequently, most of the acoustic energy generated by the bimorph is directed out of the body of the tool and into the surrounding mud and formation. 
     Alternatively, the transducer may comprise a unimorph, as shown in FIG. 8. A unimorph comprises a layer of metal  86  with a piezoelectric crystal  88  bonded to it, as shown in FIGS. 8 and 9. When an electric signal is applied to the piezoelectric crystal  88  it contracts (or expands) and the metal layer  86  does not, as shown in FIG.  10 . Consequently, the combination of the crystal  88  and the metal layer  86  bends and generates forces  90  generally perpendicular to the face of the crystal  88  and forces  92  generally parallel to the face of the crystal  88 . Forces  92 , which are generated parallel to the faces of the crystal, are generally substantially smaller than the forces generated by the crystal stack used in existing acoustic transmitters. The unimorph is oriented so that forces  90  are directed outward from the body of the tool. Consequently, most of the acoustic energy generated by the unimorph is directed out of the body of the tool and into the surrounding mud and formation. 
     The invention uses a bimorph or unimorph to transform acoustic energy into electrical energy (acoustic receiver). A piezoelectric crystal generates an electric signal when it is changed in one of its dimensions. For example, if a mechanical force is applied to a bimorph causing it to arch (bend) as shown in FIG. 7, crystal  78  is expanded across its face and crystal  80  is contracted across its face. If the two crystals are properly oriented, the electrical signals produced by their respective expansion and contraction add and provide an indication of the amount of bending (force) being exerted on the bimorph. A force applied parallel to the face of the crystals will have much less effect than a force applied perpendicular to the crystals. Consequently, an acoustic transducer comprising a bimorph will be more sensitive to acoustic energy acting perpendicular to its face than parallel to its face. A similar discussion applies to unimorph transducers in acoustic receivers. 
     Further, bimorphs and unimorphs have effective acoustic impedances that more closely match the acoustic impedance of the surrounding mud than single crystals or stacks of crystals, as described above. Consequently, more energy generated by a bimorph or unimorph will be transferred to/from the mud than with the single crystals or stacks of crystals used in existing transducers. 
     Consequently, because of the use of bimorphs and unimorphs, and because of the acoustic isolation techniques described below, an acoustic transmitter according to the invention, upon application of an electrical signal, generally generates between 3 dB and 100 dB more acoustic energy in a preferred direction than in a direction substantially perpendicular to the preferred direction. Preferably, the acoustic transmitter generates between 5 dB and 50 dB more acoustic energy in a preferred direction than in a direction substantially perpendicular to the preferred direction. More preferably, the acoustic transmitter generates between 5 dB and 20 dB more acoustic energy in a preferred direction than in a direction substantially perpendicular to the preferred direction. Preferably, the preferred direction is perpendicular to the face of the bimorph or the unimorph. 
     Further, because of the use of bimorphs and unimorphs, and because of the acoustic isolation techniques described below, an acoustic receiver according to the invention will generate an electrical signal having a magnitude between 3 dB and 100 dB greater for acoustic energy received from a preferred direction than for acoustic energy received from a direction substantially perpendicular to the preferred direction. Preferably, the invention will generate an electrical signal having a magnitude between 5 dB and 50 dB greater for acoustic energy received from a preferred direction than for acoustic energy received from a direction substantially perpendicular to the preferred direction. More preferably, the invention will generate an electrical signal having a magnitude between 5 dB and 20 dB greater for acoustic energy received from a preferred direction than for acoustic energy received from a direction substantially perpendicular to the preferred direction. Preferably, the preferred direction is perpendicular to the face of the bimorph or the unimorph. 
     In the acoustic receiver the unimorph or bimorph transducer is preferably not rigidly held at its edges. Consequently, the transducer in the acoustic receiver is damped and can act as a transducer for broadband acoustic energy. In contrast, the transducer in the acoustic transmitter is preferably only lightly damped. 
     It will be appreciated by those skilled in the art that a unimorph or bimorph transducer can be optimized to operate in different frequency ranges depending on the size of the crystal or crystals, and the size and type of the metal disk that is used. For the preferred embodiment the frequency ranges selected are those of interest in acoustic logging, i.e. from 2 kHz to 30 kHz. 
     As shown in FIG. 11, an acoustic receiver  94  comprises an outer sleeve  96 . An external static seal system, comprising cavities  98  and  100  into which o-rings (not shown) may be seated, seals against hydrostatic pressure. The combination of the cavity (air gap) and o-rings seated in cavities  98  and  100  provide acoustic isolation between the acoustic receiver and the body of the acoustic logging tool. A threaded area  102  of the acoustic receiver  94  couples to an insert (not shown) within the acoustic logging tool. Thus, there is no contact between the acoustic receiver  94  and the body of the acoustic logging tool except through the insert and through the o-rings that seat in cavities  98  and  100 . In the preferred embodiment the threaded area  102  of the acoustic receiver  94  couples to a ring (not shown) floating on an insert (not shown) within the acoustic logging tool. This method provides even better acoustic isolation between the receiver and the acoustic logging tool. A coaxial connector  104  provides a connection for the electrical signal generated by the acoustic receiver. The coaxial connector  104  has its own electrical ground that is separate from the electrical ground used by the acoustic transmitters. This feature limits the amount of electrical noise coupled from the acoustic transmitter to the acoustic receiver. 
     The acoustic receiver has a hexagonal socket  106 , shown in FIG. 12, that allows the acoustic receiver to be gripped by a tool or by hand. This feature, along with the threaded connection to the insert provided by threaded area  102 , allows the acoustic receiver to be changed in the field. 
     As shown in FIG. 13, an acoustic receiver is comprised of an outer sleeve  96 , a lower housing  108 , a pressure compensating piston  110 , and a piezoelectric crystal/hat sub-assembly  112 . 
     The outer sleeve  96  is made from suitable material to withstand the extreme temperature and pressure conditions of the downhole environment. The static seal system, discussed above, is used to withstand the high differential pressure between the well bore and the atmospheric conditions inside the tool. The air gap created on the outside diameter and the inside diameter of the outer sleeve  96  aid in reducing the level of the unwanted sonic waves travelling through the tool body. 
     The piezoelectric crystal/hat subassembly  112  is comprised of two housings  114  and  116  made of a suitable thermoplastic housings such as “peek” (Polyetheretherketon) or such similar material. In the preferred embodiment the upper housing (hat) is constructed from a metal. Piezoelectric ceramics  118 , configured either as bimorphs or unimorphs, are sandwiched between two rubber washers  120  and  122 , placed into the upper housing. The lower peek housing  116  preloads (through spring  124 ) the piezoelectric ceramics  118  inside the upper housing  114  using a peek nut  126 . The lead wires  128  and  130  from the piezoelectric ceramics  118  are routed through and adhered into the grooves  132  and  134  inside the lower peek housing  116 . This is done to prevent the wires  128  and  130  from moving under downhole shock and vibration and inducing unwanted signals into the piezoelectric ceramics. A hermetically sealed connector  136  terminates the lead-wires  128  and  130 . 
     Two springs  138  and  140  (constructed as wave springs in the preferred embodiment) are placed on either side of a flange  142  of the upper peek housing  114  or “hat” in order to position the piezoelectric crystal/hat sub-assembly  112  in a null position and also in order to allow the assembly to be preloaded so that it can withstand handling, tripping in and out of hole and drilling conditions. 
     The piezoelectric crystal/hat sub-assembly  112 , is inserted into the lower housing  108 . This combination is inserted into and keyed to the outer sleeve  96 . Two set screws  144  and  146  are installed into the lower housing  108  to hold the connector  136  in place relative to the lower housing  108 . 
     The piston  110  is placed on the top of the upper spring  138  and held against the outer sleeve  96  using a snap ring  148 . The piston  110  preloads the piezoelectric crystal/hat sub-assembly  112  inside the outer sleeve  96 . 
     An interior cavity formed by the outer sleeve  96 , the lower housing  108  and the piezoelectric crystal/hat sub-assembly  112  is evacuated and filled through a pair of lower ports  150  and  152  with compensation fluid, as shown in FIG.  14 . The ports  150  and  152  are fitted with plugs  154  and  156  having high pressure o-rings  158  and  160  and back-up rings  162  and  164 . 
     A sealed system retains the compensating fluid inside the assembly and prevents the borehole fluid from reaching the internal cavity of the electronic insert assembly. The sealed system comprises o-rings  158 ,  160 ,  166 ,  168 ,  170 , and  172  and backup rings  162 ,  164 ,  174  and  176 . It will be appreciated by those skilled in the art that the system air gap/o-ring  166 ,  168  and  170  also acts as an acoustic isolator between the crystal/hat subassembly  112  and the outer sleeve  96 . This acoustic isolation further isolates the crystal from acoustic energy imparted on the crystal from a direction parallel to its face. 
     The oil volume inside the annular cavity will expand and contract with the changes in ambient pressure and temperature. Upon any increase of the oil volume due to temperature, the piezoelectric crystal/hat sub-assembly  112  will act as a piston and move upward, as shown in FIG. 15 (which is outward toward the bore hole wall when the acoustic receiver is installed in the tool body) compressing spring  138  and unloading spring  140 . Consequently, the oil volume inside the cavity will expand. If however the oil volume contracts due to increase in the hydrostatic pressure, the piston  110  and the piezoelectric crystal/hat sub-assembly  112  can both move downward as separate compensating pistons to reduce the oil volume. When the piezoelectric crystal/hat subassembly moves downward, it compresses spring  140 . When the piston  110  moves downward, it compresses spring  138 . An adequate amount of lead wire  128  and  130  length and strain relief is provided to allow for the movement of the piezoelectric crystal/hat sub-assembly for temperature and pressure compensation relative to the electrical connector. When the piezoelectric crystal/hat sub-assembly is in the position shown in FIG.  15  and in the position shown in FIGS. 13 and 14 and in any position between those two positions, it has no direct contact with the outer sleeve  96  because the two parts are separated by o-rings  166  and  170 . This separation provides some acoustic isolation between the piezoelectric crystal/hat sub-assembly and the outer sleeve. 
     As shown in FIG. 16, the acoustic transmitter  178  comprises an outer shell  180  having two cavities  182  and  184  into which o-rings (not shown) may be seated and seal against hydrostatic pressure. The combination of the cavity (air-gap) and o-rings seated in cavities  182  and  184  provide acoustic isolation between the acoustic transmitter and the body of the acoustic logging tool. A threaded area  186  of the acoustic transmitter  178  couples to an insert (not shown) within the acoustic logging tool. Thus, there is no contact between the acoustic transmitter  178  and the body of the acoustic logging tool except through the o-rings that seat in cavities  182  and  184  and through the insert. In the preferred embodiment the threaded area  186  of the acoustic transmitter  178  couples to a ring (not shown) floating on an insert (not shown) within the acoustic logging tool. This method provides even better acoustic isolation between the transmitter and the acoustic logging tool. 
     The acoustic transmitter has a hexagonal coupling  188 , as shown in FIG. 17, which allows the acoustic transmitter to be gripped by a tool or by hand. This feature, along with the threaded connection to the insert provided by threaded area  186 , allows the acoustic transmitter to be changed in the field. 
     As illustrated in FIG. 18, the acoustic transmitter comprises crystal/hat assembly  190 , which comprises a piezoelectric crystal assembly  192  secured between upper peek housing  194  and lower peek housing  196 . In the preferred embodiment the upper housing (hat) is constructed from a metal. The upper and lower peek housings  194  and  196  are made of a suitable thermoplastic material such as peek or a similar material. The piezoelectric crystal assembly  192  is pre-wired and located inside a recess in the lower peek housing  196 . A hermetically sealed connector  198  is fitted into the outer shell  180  and fits within a recess in the opposite side of the lower peek housing  196  and terminates lead wires  200  and  202 . The upper peek housing  194  also has a mating recess to accept the piezoelectric crystal assembly  192 . The piezoelectric crystal assembly  192  is therefore sandwiched between an upper peek element  204  and a lower peek element  206  and is connected to the electrical connector  198  via the lead wires  200  and  202 . In the preferred embodiment the piezoelectric crystal assembly  192  is adhesively constrained between the upper peek element  204  and the lower peek element  206 . 
     Two springs  208  and  210  (constructed as wave springs in the preferred embodiment) are placed on either side of flanges  212  and  214  of the upper and lower peek housings  194  and  196 , respectively, when the piezoelectric crystal assembly  194  is inserted into the outer shell  180 . The springs position the crystal/hat assembly  190  in a null position and also allow the assembly to be preloaded so that it can withstand handling, tripping in and out of hole and drilling conditions. 
     A metal backing ring  216  between spring  210  and crystal/hat assembly  190  provides a solid backing for the generation of acoustic energy. 
     Two set screws  218  and  220  are installed into the outer shell  180  to hold the connector  198  assembly in place relative to the outer shell  180 . 
     A piston  222  is placed on the top of the upper spring  208  and held against the outer shell  180  using a snap ring  224 . The piston  222  preloads the piezoelectric crystal assembly  192  inside the outer shell  180 . 
     A chamber formed by the outer shell  180  and the crystal/hat assembly  190  is evacuated and filled through ports  226  and  228  with compensation fluid, as shown in FIG.  19 . The ports are fitted with plugs  230  and  232  having high pressure o-rings  234  and  236  and back-up rings  238  and  240 . 
     A sealed system, comprising o-rings  234 ,  236 ,  242 ,  244 , and  246  and backup rings  238 ,  240  and  248 , prevents the borehole fluid from reaching inside the electronic insert assembly and prevents the compensation fluid from escaping the chamber. It will be appreciated by those skilled in the art that the system air gap/o-ring  242  and  244  also acts as an acoustic isolator between the crystal assembly  192  and the body of the transmitter  180 . This acoustic isolation further limits the acoustic energy emitted by the crystal in a direction parallel to its face, from coupling into the body of the transmitter  180 , and through the body of the tool to the receivers. 
     The oil volume inside the annular cavity will expand and contract with the changes in ambient pressure and temperature conditions. Upon any increase of the oil volume due to temperature, the crystal/hat assembly  190  acts as a piston and moves upward, as shown in FIG. 20, compressing spring  208  and expanding the oil volume. If however the oil volume is contracted due to an increase in the hydrostatic pressure, the piston  222  and the crystal/hat assembly  190  can both move downward as separate compensating pistons to reduce the oil volume. Downward movement of the crystal/hat assembly  190  compresses spring  210  and downward movement of piston  222  compresses spring  208 . Adequate amount of lead wire  198  and  202  length and strain relief is provided to allow for the movement of the crystal/hat assembly  190  for temperature and pressure compensation relative to electrical connector  198 . When the crystal/hat assembly  190  is in the position shown in FIG.  19  and in the position shown in FIGS. 17 and 18 and in any position between those two positions, it has no direct contact with the outer shell  180  because the two parts are separated by o-ring  242 . This separation provides some acoustic isolation between the crystal/hat assembly and the outer shell. 
     The foregoing describes preferred embodiments of the invention and is given by way of example only. The invention is not limited to any of the specific features described herein, but includes all variations thereof within the scope of the appended claims.