Patent Publication Number: US-6671380-B2

Title: Acoustic transducer with spiral-shaped piezoelectric shell

Description:
TECHNICAL FIELD 
     The present invention relates to apparatus and methods for acoustic transducer technology for oil field and underwater applications, and more particularly to improvements in piezoelectric transmitters and receivers for oil field acoustic logging applications. 
     BACKGROUND OF THE INVENTION 
     Modern oil field acoustic logging involves sonic imaging of objects outside the borehole. This is accomplished by transmitting an acoustic signal along the borehole and detecting signals reflected back from objects outside the borehole. The reflected signal is subject to severe attenuation in this process and is typically very weak compared to the signal transmitted down the borehole. 
     Traditional sonic logging acquisition systems typically measure guided borehole waves that do not suffer such severe attenuation. Detecting the much weaker reflected signals from reflectors outside the borehole requires a more sensitive receiver, or a more powerful transmitter, or both. 
     Larger receivers or multiple receiving elements (e.g., stacked piezoelectric plates) of the prior art can be used to increase sensitivity and improve low-frequency response. However, for oil field logging application, particularly for acoustic receivers used in wireline and LWD acoustic logging, available space is limited. Available space is further limited by the need to place receivers in an azimuthal array for azimuthal resolution. 
     There is a large mismatch in acoustic impedance between borehole fluid and piezoelectric ceramics. Both the shape and the packaging of the piezoelectric ceramics affect the severity and frequency characteristics of the acoustic disturbance introduced by the mismatch. Receivers having larger surface area can be used to reduce the effects of mismatch. However, larger surface area in prior art receiver designs is only achievable at the expense of larger size. Also, receivers used for oil field logging must be designed to withstand the extremely high pressures experienced near the bottom of a borehole. 
     The prior art hydrophone best suitable for use as a receiver in wireline and LWD acoustic logging is the traditional cylindrical shape hydrophone disclosed in U.S. Pat. No. 3,327,023, “Piezoelectric Transducer Having Good Sensitivity Over A Wide Range Of Temperature And Pressure”, issued Jul. 30, 1974, to Henriquez, et al. Another cylindrical shape hydrophone is disclosed in U.S. Pat. No. 5,122,992, “Transducer Assembly”, issued Jun. 16, 1992, to Kompanek. 
     Other prior art acoustic receivers known as “benders” offer higher sensitivity, but lack the omni-directional capability of the hydrophone. 
     Available prior art acoustic transmitters most suitable for use in wireline and LWD acoustic logging are phased array transmitters, but these are inherently large for a given power output. More powerful transmitters of a given size would facilitate improvements in system sensitivity of wireline and LWD acoustic logging systems. In particular, there is a need for a high-power, pressure-balanced, acoustic transmitter small enough to fit in a logging tool. 
     There is a need to improve signal to noise ratio of downhole acoustic detection, and to improve low-frequency response. Thus, the need exists for more powerful transmitters and smaller, more sensitive, receivers with improved low-frequency response, both transmitters and receivers having higher capacitance and being better matched to the impedance of downhole borehole fluid. 
     SUMMARY OF THE INVENTION 
     The invention provides an acoustic transducer including a polarized piezoelectric shell having a spiral-shaped surface. The acoustic transducer may be used in a receiver or a transmitter. In one embodiment, the shell is a solid spiral having outer and inner spiral-shaped surfaces. In a preferred bender-type receiver embodiment, the shell defines an exterior, spiral-shaped, closed-loop surface and a spiral slot. The spiral slot defines a closed cavity with an interior, spiral-shaped, closed-loop surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of piezoelectric shell of a first hydrophone-type receiver embodiment of the present invention. 
     FIG. 2A is a cross-sectional, elevation view of a hydrophone-type receiver including the piezoelectric shell of FIG.  1 . 
     FIG. 2B is a cross-sectional, top view of the transducer assembly of FIG.  2 A. 
     FIGS. 3A,  3 B, and  3 C show the pieces produced in the process of cutting a spiral piezoelectric shell from a PZT disk. 
     FIG. 4A is a cross-sectional, elevation view of a second, preferred, bender-type receiver embodiment, including a closed, spiral-shaped piezoelectric shell. 
     FIG. 4B is a partially cut-away, cross-sectional top view of the preferred second receiver embodiment, showing exterior and interior spiral surfaces with conductive coatings. 
     FIG. 5A is a cross-sectional, view of a portion of the shell of the preferred second bender-type receiver embodiment of FIG. 4A, also showing the polarization of the shell, and the electrical connections for parallel bender configuration. 
     FIG. 5B is the same cross-sectional view as FIG. 5A, but showing polarization reversed. 
     FIGS. 5C and 5D show the receiver of FIGS. 5A and 5B with polarization and electrical connections for serial bender configuration. 
     FIGS. 5E-5H show the receiver of FIGS. 5A and 5B with polarization and electrical connections for hydrophone configuration. 
     FIGS. 6A and 6B show, respectively, a spiral piezoelectric shell of a piezoelectric receiver, and a spiral piezoelectric shell of a piezoelectric transmitter, illustrating the relative size of the two shells. 
     FIG. 7 is a graph comparing the spectral response of the receiver of the first embodiment to the spectral response of a cylinder hydrophone and a stacked plates hydrophone. 
    
    
     DETAILED DESCRIPTION 
     General 
     The present invention provides an acoustic transducer having a spiral-shaped piezoelectric shell, the transducer being of a type suitable for use in a transmitter or in a receiver for oil field logging and other applications. 
     A hydrophone-type receiver embodiment provides a small, sensitive, acoustic receiver having a spiral-shaped piezoelectric shell. 
     A preferred bender-type receiver embodiment provides a small, sensitive, acoustic receiver having a spiral-shaped, closed-loop, piezoelectric shell. 
     A hydrophone-type transmitter embodiment provides a powerful acoustic transmitter having a spiral-shaped piezoelectric shell. 
     A bender-type acoustic transmitter embodiment provides a powerful transmitter having a spiral-shaped, closed-loop, piezoelectric shell. 
     First Hydrophone-Type Receiver Embodiment 
     FIG. 1 shows spiral-shaped piezoelectric shell  20 . Shell  20  is used in a first hydrophone-type embodiment of a transducer of the present invention. This first hydrophone-type embodiment, receiver  50 , is configured for use as a small, sensitive, high-capacitance receiver. Receiver  50  is illustrated generally in FIGS. 2A and 2B. Receiver  50  is responsive to low-energy impinging acoustic energy to provide representative electrical signals. 
     FIG. 1 shows the geometry of shell  20 . Receiver  50  includes an outer electrically conductive coating  27  deposited on outer spiral-shaped surface  21  and a separate, inner electrically conductive coating  28  deposited on inner spiral-shaped surface  22 . Outer conductive coating  27  is deposited on outer spiral-shaped surface  21  of shell  20  to provide an electrical connection covering essentially the whole surface of outer spiral-shaped surface  21 . Inner electrically conductive coating  28  is deposited on inner spiral-shaped surface  22  of shell  20  to provide an electrical connection covering essentially the whole surface of inner spiral-shaped surface  22 . First axial end surface  23 , second axial end surface  24 , inner end  25 , and outer end  26  have no metallic coating, so as to maintain electrical isolation between outer coating  27  and inner coating  28 . Spiral-shaped surfaces  21  and  22  have a linear axial cross section, as illustrated in FIG. 1 by the longer edge of outer end  26 . 
     Shell  20  is radially polarized in the manufacturing process by applying a strong electric field between outer coating  27  and inner coating  28 . 
     Shell  20 , in a first receiver embodiment, is approximately 2 cm in maximum diameter. Its spiral-shaped strip is approximately 6 mm wide and 2 mm thick. The gap between spiral layers is approximately 3 mm. Shell  20  has approximately 1.5 turns, and preferably a number of turns between 1.1 and 3.0. The maximum diameter, the width and thickness of the spiral strip, the gap, and number of turns can be selected to meet design requirement specifications for bandwidth, sensitivity, and electric noise. 
     Hydrophone-type receiver  50  is shown in cross-sectional elevation view in FIG. 2A, and in cross-sectional top view in FIG.  2 B. FIG. 2A shows shell  20  clamped between end plates  51  and  52 . End plates  51  and  52  are preferably made of steel. The end plates serve as protective end caps, and provide mechanical support to the shell. Teflon plates  53  and  54 , located between the plates and the shell, provide electrical insulation between the steel plates and the conducting surfaces of the shell. The Teflon also prevents acoustic waves from passing directly in fill-fluid from one side of the spiral strip to the other. The two plates are clamped together by bolt  55 , nuts  56  and  58 , and lock nuts  57  and  59 , to form transducer assembly  40 . 
     Transducer assembly  40  is enclosed within bellows assembly  71  and protective butyl rubber housing  78  to make hydrophone-type receiver  50 . 
     Shell  20  is mounted between the flat surfaces of Teflon plates  53  and  54 , the flat surfaces providing a sealing contact with flat axial end surfaces  23  and  24  of the shell. The enclosure in which transducer  40  is mounted is filled with fill-fluid, the fill-fluid occupying all spaces between the coils of the shell. Note that the open spiral acoustic path through fill-fluid between outer electrically conductive coating  27  and inner electrically conductive coating  28  is a narrow, elongated path. The longer and narrower the path, the less low frequency performance is degraded. 
     Bellows assembly  71  comprises thin cylindrical metal bellows  72 , bellows base plate  73 , and bellows cover plate  74 . Cover plate  74  is attached to thin cylindrical metal bellows  72  after transducer assembly  40  has been installed and fastened within cylindrical bellows  72  using nut  58  and lock nut  59 . Cover plate  74  is attached to cylindrical bellows  72  and sealed with a gasket (not shown) by screws  75  and  76 , after the bellows cavity is filled with a suitable fill-fluid  77 . The fill-fluid is preferably castor oil. 
     Electrical connection to outer spiral-shaped surface  21  of shell  20  is made by wire  31  which is welded to outer spiral-shaped surface  21  by weld  33 . Likewise, electrical connection to inner spiral-shaped surface  22  of shell  20  is made by wire  32 . Wire  32  is welded to inner spiral-shaped surface  22  by weld  34 . Alternatively, metal end caps are used to make these electrical connections. 
     Wires  31  and  32  are electrically connected through the bellows cavity, through a seal in bellows cover plate  74 , and through housing  78 , to first and second electrical output terminals  41  and  42 , respectively. 
     Damping layers (not shown) may be provided to further protect the hydrophone or to increase the bandwidth. 
     Making the Spiral Piezoelectric Shell 
     One method of making a spiral piezoelectric shell is to cut a solid disk of piezoelectric material, preferably PZT, using the high-pressure water jet cutting method. A disadvantage of using this cutting technique is that the spread of the high-pressure jet beam produces a gentle tapering of thickness along the cutting direction, and the tapering angle tends to increase as the thickness of the sample increases. Therefore, the maximum height of the hydrophone that stays within the machining tolerance is limited. FIGS. 3A,  3 B, and  3 C show the pieces produced by cutting a spiral piezoelectric shell from a PZT disk using the high-pressure water jet cutting method. 
     The preferred method of making a spiral piezoelectric shell is to cut a solid disk of piezoelectric material using a diamond-impregnated wire. This method does not introduce thickness taper along the cutting direction and is expected to produce less surface damage. 
     Second, Preferred, Bender-Type Receiver Embodiment 
     A second, preferred, bender-type receiver embodiment of a spiral piezoelectric transducer of the present invention, configured for use as a small, sensitive, high-capacitance receiver, is shown in FIGS. 4A and 4B. Bender-type receiver  100  is responsive to low-energy impinging acoustic energy to provide representative electrical signals. 
     FIGS. 4A and 4B show receiver  100  including piezoelectric shell  120 . FIG. 4B shows piezoelectric shell  120  having an elongated spiral slot  130 . Slot  130  dividing the spiral shell into outer spiral portion  121  and inner spiral portion  122 . 
     Outer spiral portion  121  has an outer, exterior, spiral-shaped, closed-loop surface  123 , and an outer interior, spiral-shaped, closed-loop surface  124 , as indicated in FIG.  4 B. Inner spiral portion  122  has an inner, interior, spiral-shaped, closed-loop surface  125 , and an inner, exterior, spiral-shaped, closed-loop surface  126 , also indicated in FIG.  4 B. On each of these surfaces, is deposited a conductive coating, preferably metallic. Thus, surfaces  123 - 126  are coated with conductive coatings  133 - 136 , respectively. To maintain electrical isolation between the four conductive coatings, coatings  133 - 136  do not cover either the outer end  127  or the inner end  128  of the shell. Thus we have four electrically isolated conductive coatings: outer, exterior conductive coating  133 , outer, interior conductive coating  134 , inner, interior conductive coating  135 , and inner, exterior conductive coating  136 . 
     FIG. 4A shows first output terminal  141  and second output terminal  142 . In the preferred receiver embodiment, operating in bender mode, electrical connections are provided between conductive coatings  133 - 136  and output terminals  141  and  142  as shown in FIG.  5 A. FIG. 5A also shows the polarity of shell outer portion  121  and shell inner portion  122 . FIG. 5B shows the same electrical configuration as FIG. 5A but with the polarization of each shell portion reversed. This would simply reverse the polarity of the electrical output signals. 
     Connecting output terminals and conductive coatings as shown in FIG. 5C or  5 D would cause the receiver to operate on a hydrophone mode, with a less desirable low-frequency response. 
     FIG. 4A shows shell  120  clamped between end plates  151  and  152 . End plates  151  and  152  are preferably made of steel. The end plates serve as protective end caps, and provide mechanical support to the shell. Teflon plates  153  and  154 , located between the plates and the shell, provide electrical insulation between the steel plates and the conducting surfaces of the shell. The Teflon also prevents acoustic waves from passing directly in fill-fluid from one side of the spiral strip to the other. The two plates are clamped together by bolt  155 , nuts  156  and  158 , and lock nuts  157  and  159 , to form transducer assembly  140 . 
     Transducer assembly  140  is enclosed within bellows assembly  171  and protective butyl rubber housing  178  to make bender-type receiver  100 . 
     Shell  120  is mounted between the flat surfaces of Teflon plates  153  and  154 , the flat surfaces providing a sealing contact with the flat axial end surfaces of the shell. The enclosure in which transducer  140  is mounted is filled with fill-fluid, the fill-fluid occupying all spaces between the coils of the shell. Note that elongated spiral slot  130  and the Teflon plates define a closed cavity  131 , entirely filled with fill-fluid  177 . 
     Electrical connections are made to the several coatings by welds and wires or by conventional metallic caps as discussed above for the first embodiment. If welds and wires are used, pass-through seals (not shown) in an endplate are used to provide electrical connections between wires within closed cavity  131  and terminals  141  and  142  outside the cavity. 
     As in the first embodiment, flat axial end surfaces on both sides of the shell have no metallic coating and are in contact only with Teflon plate, so as to maintain electrical isolation between the several conductive coatings. 
     When a pair of end plates are attached to piezoelectric shell  120 , the plates cover open areas of the slot to form a closed cavity containing interior, spiral-shaped, closed-loop surface  132 . This cavity is filled with a fill-fluid. Note that after the end plates are attached, after elongated spiral cavity  131  is filled with a fill-fluid, and after exterior conductive coating  133  is surrounded by fill-fluid, there is no open acoustic path through fill-fluid between exterior conductive coating  133  and interior conductive coating  134 . The absence of such path (in contrast to the first receiver embodiment which has a narrow, elongated path) further improves low-frequency performance. 
     Outer and inner portions  121  and  122  of piezoelectric shell  120  are radially polarized in the manufacturing process by applying a strong electric field between conductive coatings  133  and  134  to polarize portion  121 , and between conductive coatings  135  and  136  to polarize portion  122 . Polarization directions are shown in FIG.  5 A. Polarization direction of shell outer spiral portion  121  is indicated by arrow  137 . Polarization direction of shell inner spiral portion  122  is indicated by arrow  138 . FIG. 5A produces a parallel bender configuration. Reversal of polarization, as shown in FIG. 5B, also a parallel bender configuration, would simply reverse the polarity of the output signal across first and second output terminals  141  and  142 . 
     FIGS. 5C and 5D show the receiver of FIGS. 5A and 5B with polarization and electrical connections for serial bender configuration. 
     FIGS. 5E-5H show the receiver of FIGS. 5A and 5B with polarization and electrical connections for hydrophone configuration. 
     Elongated spiral cavity  131  is filled with fill-fluid, preferably castor oil, before the shell is clamped between plates. Clamping the shell between the plates seals the fill-fluid in cavity  131  defined by slot  130 . 
     FIG. 5A also shows electrical wire  143  connecting via weld  144  to exterior conductive coating  133 . Likewise, electrical wire  145  connects via weld  146  to interior conductive coating  134 . 
     In this second receiver embodiment, piezoelectric shell  120  is approximately 2 cm in maximum diameter, and is approximately 6 mm wide. The thickness of each of the shell outer and inner portions  121  and  122 , is approximately 1.2 mm, and gap  147  between these outer and inner portions is approximately 1 mm wide. Gap  148  between successive spiral coils of piezoelectric shell  120  is approximately 1.2 mm. In a preferred embodiment, the spiral-shaped strip has approximately 1.5 turns, and preferably a number of turns between 1.1 and 3.0. The maximum diameter and the width of piezoelectric shell  120 , the thickness of the elements, the gap between the elements, and the number of turns can be selected to meet design requirement specifications for bandwidth, sensitivity, and electric noise. 
     FIGS. 5B-5H show alternative polarization and wiring configuration. 
     In the second receiver embodiment, the plates can be made thinner. This is an advantage because the sensitivity of a bender-type piezoelectric sensor increases as the ratio of radius to thickness increases. 
     Making the Second Receiver Embodiment 
     The preferred method of making a spiral piezoelectric shell is to cut a solid disk of piezoelectric material using a diamond-impregnated wire. 
     Polarizing shell outer spiral portion  121  and shell inner spiral portion  122  requires applying the conductive coatings to each of outer and inner shell portions, and applying a high voltage across the coatings of each of outer and inner shell portions before the electrical connections in FIG. 5 are made. 
     First Transmitter Embodiment 
     The first transmitter embodiment includes a larger shell than the shell used in the first receiver embodiment. The relative size of the two shells is shown in FIGS. 6A and 6B. FIG. 6A shows the receiver shell. FIG. 6B shows the transmitter shell. Apart from being larger in size, the structure of the transmitter embodiment is similar to the structure of the first receiver embodiment shown in FIG.  2 A. 
     One difference is that resilient rubber gaskets are required between the shell and the end plates to provide a proper acoustic seal between fill-fluid outside and inside the transducer enclosure. 
     In the first transmitter embodiment, the shell is approximately 7.5 cm in maximum diameter, and the spiral-shaped strip is approximately 1.2 cm wide and 2.5 mm thick. The gap between spiral layers is approximately 3 mm. In a preferred embodiment, the spiral-shaped strip has approximately 2.5 turns, and preferably a number of turns between 1.5 and 3 turns. As in the first receiver embodiment, the maximum diameter, the width and thickness of the spiral strip, the gap, and number of turns can be selected to meet design requirement specifications for bandwidth, sensitivity, and electric noise. 
     Second Transmitter Embodiment 
     The second transmitter embodiment is similar in structure to the first transmitter embodiment, except that it uses a shell of the type shown in FIGS. 4A and 4B. 
     Test Results 
     FIG. 7 compares the spectral response to a 4 kHz center frequency pulse of the spiral receiver (SR) to the spectral response of a cylinder hydrophone (CH) and a stacked-plates hydrophone (SPH). 
     Benefits of the Invention 
     The invention, by virtue of using a spiral-shaped piezoelectric shell having more than one turn, provides an acoustic transducer having a larger surface area and a more flexible piezoelectric member than a cylindrical-shape transducer of similar size. The larger surface area provides a higher capacitance. In a receiver embodiment, when a charge amplifier is used, the larger surface area provides a sensitivity improvement, approximately in proportion to the increase in surface area. 
     The invention, by virtue of the spiral-shaped piezoelectric shell having a free inner end (i.e., an end that is not physically constrained), provides a piezoelectric shell that has more flexibility than a cylindrical shape hydrophone of similar size. In a receiver embodiment, this provides additional sensitivity improvement. 
     The invention provides an acoustic transducer having a higher electrical capacitance than a cylindrical transducer of similar size. This makes a receiver embodiment that is less affected by the electric load of the cable, and less sensitive to spurious electromagnetic energy. 
     The invention provides an acoustic transducer having a a spiral-shaped piezoelectric transducer that can be free-flooded to withstand the high ambient pressures encountered in underwater, marine seismic, and oil well applications. 
     The invention provides an acoustic transducer having a spiral-shaped piezoelectric shell operating in bender mode with a large radius/thickness ratio. In the receiver embodiment, this provides additional sensitivity improvement.