Differential Charge Amplifier for Marine Seismic Applications

The present invention relates in general to charge amplifier circuitry for use with capacitive transducers arranged for example in a towed hydrophone array as in a marine seismic streamer or an anti-submarine warfare streamer or the like, and more particularly to a differential charge amplifier for use in marine seismic streamer or anti-submarine warfare streamer applications involving towed hydrophone arrays for passive underwater detection, wherein the hydrophones drive long twisted pairs of leads or conductors for transmission through the streamer to shipboard signal processing stations.

Heretofore, a number of marine seismic detection cables or streamers, the terms being used interchangeably, have been devised for making seismic surveys of terrestrial sub-surface structures disposed beneath sea water. In general, the marine seismic streamers have included a lead-in cable and a long series of serially connected active streamer sections each usually formed of an oil filled plastic tube surrounding an array of hydrophones, strain cables, structural spacers, transformers, and mechanical and electrical leads or connectors, such as, for example the marine seismic streamer sections of the type disclosed in U.S. Pat. No. 2,465,696 issued Mar. 29, 1949 to Leroy C. Paslay or U.S. Pat. No. 3,371,739 issued Mar. 5, 1968 to Raymond H. Pearson. Inactive streamer sections are also often interspersed in the string of serially connected so called "active" sections having the sound pressure responsive hydrophones. Such marine seismic streamers or cables may in many cases be a mile or more in length, a typical streamer system being about 7,000 with each section being typical a hundred feet or more in length. During seismic survey or prospecting operations, such streamers are towed by the seismic survey vessel at a selected depth below the surface of the sea, by any of several conventional means for maintaining the seismic cable at the desired underwater depth. Typically, the cables may be provided with a plurality of weights at spaced intervals to make them negatively buoyant and flotation means or ring boom means may be associated with the cable to assist in maintaining it at the desired depth. Alternatively, regulation of the buoyancy of the streamer may be achieved by introducing into the streamer or withdrawing from it a fluid which will vary the buoyancy of the streamer, or paravane type structures may be used having adjustable diving planes which maintain the streamer at the appropriate depth.

Similarly, long towed hydrophone arrays have been employed as underwater listening devices for detecting submarines or underwater vessels used in warfare, such towed hydrophone arrays being employed as passive underwater detection systems in what are referred to as ASW streamers.

In towed underwater hydrophone arrays of either of the types described above, piezoelectric ceramic capacitance transducers are customarily used as hydrophones to respond to the underwater sound pressure waves or phenomena to be detected by the hydrophones and convert such phenomena to electrical information. Such transducers produce an output voltage that is proportional to the applied acoustic pressure and present day streamer applications typically employ up to one hundred groups of hydrophones towed in a single streamer, spaced from 500 to 10,000 feet behind a marine geophysical boat. In order to transmit the hydrophone output signals or output voltages to the signal processing equipment on the geophysical boat or the towing vessel, it has been the practice to connect a transformer to each hydrophone group as means of overcoming the change of signal loss over such a long line with variations in the cable length. In such a streamer systems involving transformers connected to the hydrophone groups, the transformer basically lowered the source impedance allowing the hydrophone group to drive a long twisted pair of cables for coupling the output signals from the hydrophone group to the processing equipment on the towing vessel. Such a system, however, has well recognized disadvantages, in that the output voltage from the hydrophone group is substantially reduced, in some cases by as much as 10 to 1. While a typical multidyne hydrophone may have an output of about 56 uv/ubar, at the transformer output in such a system this may be reduced to about 5-10 uv/ubar depending on the number of phones and transformer design, thus resulting in a serious loss of sensitivity of the hydrophone system and rendering it extremely difficult to obtain adequate signal-to-noise ratios where long lines are required to conduct the signals from a streamer to the towing vessel.

Charge amplifiers have been previously employed for certain types of transducers, such as capacitance microphones and some types of accelerometers, where the transducers operate on the principle of conversion of some mechanical, thermal, chemical, etc. phenomenon to an equivalent electrical charge. To complete the charge amplifier circuit, an Operational Amplifier is connected with its inverting input or negative input connected to the transducer, and with a feedback circuit including a capacitor and a resistor in parallel connected between the output and the negative input of the Operational Amplifier. It has been found that such a circuit has the desirable property of being virtually independent of shunt capacitance across the input of the Operational Amplifier. When using negative feedback, the Operational Amplifier will work in such a manner as to keep the two inputs at the same potential, and since the + input is connected to ground, the - input is also kept at ground potential by the negative feedback. Therefore, adding shunt capacitance across the input has no effect because no voltage is developed across the shunt capacitance.

Such a conventional charge amplifier, if employed in a towed hydrophone array application of the type described, wherein piezoelectric ceramic capacitance transducers are used as hydrophones, would seem to offer hope of eliminating the use of transformers associated with the hydrophone groups and the resultant loss of sensitivity, because of the independence of the charge amplifier of shunt capacitance across the input. However, if a charge amplifier were connected to the hydrophone transducers in conventional fashion, a number of problems are presented rendering the conventional charge amplifier apparently unsuitable for such an application, including particularly the property that, because of the unbalanced input present in the basic conventional operational charge amplifier circuit, the circuit has no common mode signal rejection capability.

An object of the present invention is the provision of a differential charge amplifier circuit to be connected to a group of hydrophones in a towed hydrophone array employing long twisted pairs of cable leads or conductors for conducting the electrical signals to the towing vessel, which circuit allows the array of hydrophones to drive the long twisted pairs of leads without the need for a transformer and the resulting loss of sensitivity.

Another object of the present invention is the provision of the novel differential charge amplifier circuit for use with hydrophones of a towed array of hydrophones of the piezoelectric ceramic capacitance transducer type, arranged for example in a towed underwater marine streamer for seismic or ASW applications and the like, which provides a balanced input to the differential amplifier providing common mode signal rejection capability, and which provides good frequency response characteristics over the band of frequencies appropriate for seismic prospecting and ASW detection applications.

Another object of the present invention is the provision of a differential charge amplifier circuit as described in the immediately preceding paragraph, which minimizes changes in frequency response or phase shift resulting from electrical leakage in the streamer sections where the hydrophones are located or resulting from sea water intrusion and the like, minimizing the need for removal and replacement of the hydrophone sections.

Other objects, advantages and capabilities of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings illustrating a preferred embodiment of the invention.

FIG. 1 is a diagrammatic view of a marine seismic streamer having a towed hydrophone array, showing a type of towed streamer application in which the differential charge amplifier of the present invention may be used;

FIG. 2 is a block diagram of a conventional charge amplifier using an operational amplifier connected to the equivalent circuit of a capacitance transducer;

FIG. 3 is a block diagram of a differential charge amplifier connected to an equivalent circuit representing a hydrophone or hydrophone group driving a long twisted pair of leads or conductors, embodying the present invention;

FIG. 4 is a schematic diagram of a differential charge amplifier connected to the equivalent circuit of a piezoelectric ceramic hydrophone and a long twisted-pair line to the hydrophone, showing a satisfactory embodiment of an arrangement of transistors amplifiers, and associated components;

FIGS. 5A and 5B are graphic representations of measurements of frequency response characteristics plotted against frequency and phase shift plotted against frequency, respectively, for a conventional hydrophone array employing a conventional transformer circuit for driving long twisted leads;

FIGS. 6A and 6B are graphic representations plotting frequency response characteristics and phase shift against frequency, respectively, for a hydrophone array employing a differential charge amplifier in accordance with the present invention; and

FIG. 7 is a block diagram of an alternative differential charge amplifier which may be used instead of the circuit of FIG. 3.

Referring to the drawings, wherein like reference characters designate corresponding parts throughout the several figures, there is illustrated in FIG. 1 in diagrammatic form a seismic detection streamer cable 10 coupled to a drum or similar device diagrammatically indicated at 10A on a towing vessel 11, so that the seismic detection streamer can be towed in normal fashion at a preselected depth through fresh water or sea water by the towing vessel 11. It is to be understood that the streamer may be either a seismic detection streamer to be employed in seismic prospecting, or may be a passive detection streamer for detecting sounds from underwater equipment or vessels as used in anti-submarine warefare detection systems and the like. The detection streamer 10 includes a lead-in cable 12 which may have fairings and contains, typically, the signal wires leading from hydrophones in what are termined active sections of the streamer, as well as strain cables and other connection elements required for operation. The lead-in cable typically is followed by, for example, an elastic section 13 and the active sections 14, separated at desired locations in a conventional streamer by inactive streamer sections 15 interspersed between the active sections 14. Conventional depth control means are associated with the streamer, which may take the form of paravane depth control devices or pressure sensing means for regulating the buoyancy of the streamer in a known manner to maintain the desired depth. The general method of operation of such detection streamers when employed in seismic exploration is described in the prior Paslay U.S. Pat. No. 2,465,696 and a typical construction of the streamer sections may be similar to that illustrated in the prior Pearson U.S. Pat. No. 3,371,739. The streamer sections 14, many of which are arranged in serial relation along the streamer cable and are coupled together by various types of coupling devices, typically may form a streamer of about 1 mile or more in length, with each of the active streamer sections containing an array or group of hydrophones spaced in a selected arrangement along the associated active streamer sections. Heretofore, the outputs of the hydrophones in a single section have been typically connected in parallel to an impedance matching transformer to couple the electrical voltage signals from the hydrophones produced responsive to the acoustic pressure variations sensed by the hydrophones to signal processing equipment, indicated diagrammatically by the reference character 16, on the marine geophysical towing vessel 11. The streamer formed by the elastic section 13, the active sections 14, and the inactive sections 15, may be spaced from about 500 to 10,000 feet behind the geophysical vessel 11, employing a very long lead-in cable 12, housing the signal wires through which the electrical voltage signals from the hydrophones must be conducted to the processing equipment 16, as well as the strain cables and other connection elements.

A conventional prior art charge amplifier as used heretofore in connection with capacitive transducers such as piezoelectric accelerometers and the like is shown in block diagram form in FIG. 2. The transducer is represented by the broken line block, indicated by the reference character t. The equivalent circuit of the transducer t can be represented by a low impedance voltage source e.sub.1 and capacitor C.sub.1 in series. The charge on this circuit is Q = C.sub.1 e.sub.1, and either a change of capacitance or a change or voltage will cause a change in charge. The resistance R.sub.1 represents the line resistance from the transducer t to the operational amplifier A.sub.1, the capacitance C.sub.x shown in broken lines represents the shunt capacitance along the lines from the transducer to the amplifier input. An analysis of the circuit will reveal that because i.sub.1 = i.sub.2, since no current is assumed to flow into the negative input terminal of the amplifier A.sub.1, the following response equation applies: ##EQU1##

This response equation relates the output voltage both to a change in capacitance and to a change in the equivalent series voltage. Note that either or both, a change in C.sub.1, or a change in e.sub.1, will cause a change in output voltage e.sub.o, because either will change in charge . . . hence, the name "charge amplifier".

The gain in the charge amplifiers circuit is the ratio of the transducer capacitance C.sub.1 to the amplifier feedback capacitance C.sub.2. This circuit has the desirable feature of being independent of shunt capacitance across the input of the amplifier. When using negative feedback, the operational amplifier will work in such a manner as to keep the two inputs at the same potential. Since the + input is connected to ground, the - input is also kept at ground potential by the negative feedback. Therefore, adding shunt capacitance across the input has no effect because no voltage is developed across the shunt C.sub.x.

The resistance R.sub.1 limits the high frequency response of the amplifier to ##EQU2## The feedback resistance R.sub.2 is required to produce a DC input bias current path for the negative input to the amplifier A.sub.1. The parallel combination of R.sub.2 and C.sub.2 limits the low frequency response of the circuit to ##EQU3## Because a large value is required for the resistance R.sub.2 (typically in the megohm range,) for low frequency operation, amplifiers with low noise and bias currents need to be used. Also, because of the unbalanced input, this circuit has no common mode signal rejection capability. The differential charge amplifier circuit of the present invention was developed to overcome these limitations and allow the use of a charge type amplifier on long unshielded twisted pair cables.

Referring to FIG. 3, illustrating the differential charge amplifier circuit in block diagram form, the hydrophone or hydrophones of the towed hydrophone array, such as piezoelectric ceramic capacitance hydrophones in the towed underwater detection streamer, such as a seismic or ASW streamer, are represented by the broken line block T, and the equivalent circuit of the hydrophone is represented by the voltage source E1 and capacitor C1 in series. The line resistance from the hydrophone T to the operational amplifiers A1 and A2 is represented by the resistance R1, and the parallel combination of resistance and capacitance in the feedback circuits for the operational amplifiers A1 and A2 is indicated by R2 and C2 or R2' and C2' respectively. The input current I1 produced by the voltage source E1 of the hydrophone flows into node A and out of node B. Since this current is the same at both nodes A and B, the output voltage of operational amplifier A1 and A2 is the same magnitude but opposite in polarity. The gain of amplifiers A1 and A2 is ##EQU4## The differential amplifier A3 has a gain of ##EQU5## The total circuit gain equals ##EQU6##

If the feedback capacitors C2 and C2' are equal, common mode current signals appearing at the input nodes A and B are cancelled by the differential amplifier A3. The resistance R1 represents the round trip line resistance in the twisted pair of leads extending from the hydrophone to the differential amplifier circuit and the high frequency response of the circuit is determined by ##EQU7## The high frequency cutoff is also related to the distributed effect of the line resistance R1 and the shunt line capacitance Cx as with the previously described prior art charge amplifier. The shunt capacitance has no effect when there is zero voltage across the effective shunt capacitor Cx, which is true only, in a precise sense, at the amplifier input nodes or terminals A and B. In practical application, the round trip line resistance R1 and the shunt capacitance Cx are distributed along the total length of the cable, and therefore small increasing voltages are developed across the distributed shunt capacitance Cx by the current I1 and distributed line resistance R1. In a twisted pair cable 10,000 feet long, the cutoff frequency is still above the seismic band, and the low frequency cutoff is determined by the equation ##EQU8##

FIG. 4 illustrates in schematic form the detail schematic circuit for a practical embodiment of a differential charge amplifier used with a towed hydrophone array in accordance with the present invention. The hydrophone, or group of hydrophones, to be connected by the long twisted pair of leads to the differential charge amplifier is again indicated by the broken line block T and represented by the capacitor C1 and voltage source E1 as the equivalent circuit for the hydrophone. In FIG. 4, the transistors Q1 and Q2, the amplifier A1 and the associated components function as the block diagram operational amplifier A1 of FIG. 3. The transistors Q3 and Q4, amplifier A2, and the associated components function as the block diagram operational amplifier A2 in FIG. 3. The amplifier A3 and its associated components in FIG. 4 function as the block diagram amplifier A3 in FIG. 3. Since the operation of the circuitry associated with the amplifier A2 is identical to the function of the circuitry associated with amplifier A1, the circuitry of amplifier A2 will not be explained in detailed.

Resistors R1 and R2, appearing in upper and lower input circuits, represent resistance of wire in each half of the twistedpair line to the hydrophone T. The hydrophone capacitance is shown as C1. Capacitors C4 and C5 form a non-polarized capacitor which passes low-frequency AC signals, but block DC signals. The maximum input voltage is limited to approximately 0.6 volt peak by diodes CR1 and CR2 for protection of the circuit. The input signal is applied to the gate of field-effect transistor Q1A, which forms one-half of a differential amplifier. Resistors R5 and R6 are the load resistors for this amplifier, and bias current is supplied by transistor Q2 through resistors R8 and R9. Bias current is fixed by the constant voltage from resistor R17 and zener diode CR5 in the base circuit of transistor Q2, so transistor Q2 collector current is constant. As input signal voltage becomes more positive, source/drain current in transistor Q1A increases. Consequently, source/drain current in transistor Q1B decreases. The result is a rising voltage at the minus input of operational amplifier A1, and a decreasing voltage at the positive input of amplifier A1. Resistor R7 and capacitor C8 limit the output frequency of differential amplifier Q1. The increasing input signal causes reduced output from amplifier A1 and, at the same time, increased output from operational amplifier A2.

Amplifier A1 output is coupled back to the positive input through resistor R3 and capacitor C2. Amplifier A2 output is coupled back through to the negative input through resistor R4 and capacitor C3. The gain of amplifier A1 is set by the ratio of the hydrophone capacitance C1 to the feedback capacitor C2. Resistor R3 provides a DC bias current path for the gate of transistor Q1A. Capacitor C6 is for frequency compensation of operational amplifier A1. Use of field-effect transistor input stages for amplifiers A1 and A2 reduces the input bias current and noise such that resistors R3 and R4 can be made very large for low frequency response without increasing amplifier noise and DC temperature drift. Output from amplifiers A1 and A2 is routed to differential-input amplifier A3 through resistors R19 and R20.

Amplifier A3 and resistors R19, R20, R21, R22, R23 and R24 form a differential amplifier which responds only to a differential voltage as produced by the hydrophone. Common-mode voltage introduced into the twisted input wire are rejected by amplifier A3. Variable resistors R22 and R24 are used to set the gain and common-mode rejection of amplifier A3. Capacitor C12 provides frequency compensation for amplifier A3.

FIG. 7 illustrates in block diagram form another differential charge amplifier circuit which may be used and provides the desired common mode signal rejection. A standard prior art charge amplifier such as that shown in FIG. 2 differs significantly in operation from the circuit of FIG. 7 because the charge amplifier of FIG. 2 does not have common mode signal rejection. The FIG. 2 type circuit provides an output from the amplifier stage A1 which is proportional to the voltage e.sub.1 across the low impedance voltage source in the equivalent circuit of the transducer. The circuit of FIG. 7 comprises an operational amplifier, indicated by the triangle, like the operational amplifier A1 of FIG. 2, but includes resistors R.sub.2 and R'.sub.2 which provide a DC bias return path for the plus and minus inputs of the operational amplifier. These resistors provide functions similar to the resistors R4 and R4' of the circuit of FIG. 3. The two capacitors C2 and C'2 are equal, and thus common mode signals appearing at the two conductors are cancelled by the differential charge amplifier.

The use of the differential charge amplifier connected to the long twisted-pair of leads from the hydrophone or hydrophone group in the underwater detecting array in the streamer allows the hydrophones to drive the twisted-pair of leads and thereby transmit the electrical voltage signals from the hydrophones to the processing equipment on the towed vessel without the need for the transformer previously customarily employed in the seismic art. Also, additional significant advantages arise from the use of the differential charge amplifier in such an arrangement. It will be appreciated that accidental sea-water intrusion into the seismic streamer is a persistant problem. The shunting of the hydrophone or hydrophone group with reasonable values of resistance which would result from such sea-water intrusion severely reduces the frequency response in conventional seismic streamers with transformers between the hydrophones and the long twisted-pair of leads in the most critical frequency band for seismic devices. With a conventional transformer circuit, a loss of over 12 db at 10 Hz is possible with a 10 K ohm shunt resistance. The plot in FIG. 5A shows the significant change in frequency response which occurs with a conventional transformer circuit coupled to the hydrophones, for various values of resistance representing sea-water intrusion, as compared with the dramatic improvement in frequency response for similar leakage resistance across the hydrophone with the differential charge amplifier of the present invention, as plotted in FIG. 6A. Similarly, the dramatic improvement in phase shift with various leakage resistance across the hydrophone from sea-water intrusion or the like is shown by comparing the graphs of FIG. 5B for a conventional transformer circuit and FIG. 6B for a differential charge amplifier circuit of the present invention. It will be noted from FIG. 6A that the loss due to a 10K ohm shunt resistant across the hydrophone amounts to only 1.5 db and does not change the roll-off characteristics of the circuit. As shown in FIG. 6B, a 5K shunt resistance across the hydrophone produces no phase shift change below about 10 Hz and very little change above 10 Hz.

The upper curve in FIG. 5A with R.sub.s (the damping resistance) equal to 62K ohm is representative of a marine seismic towed array having a low frequency cutoff of 9 Hz (minus 3 db). The curve just below the upper curve on FIG. 5A shows the response through 10,000 feet of wire, and the other curves for leakage resistances of 10K and 5K ohms show the effect of extreme electrical leakage in the section where the hydrophones are located. In actual use this is not unusual, and when such leakages occur that hydrophone section must be removed and replaced. Comparing this with the data shown in FIG. 6A, using the differential charge amplifier, it would be noted that dramatic improvement in frequency response and phase shift with leakage resistance is realized, so that the change of frequency response or phase shift is not enough to require the expensive removal and replacement of the hydrophone section when such leakage resistances arise.