Hydroacoustic ranging method using bottom reflections

What is claimed is:

1. A method for estimating a direct underwater acoustic range between a first and a second hydroacoustic device, for which a direct acoustic path therebetween is occluded, comprising the steps of:

a) deploying a first and a second hydroacoustic device at different underwater locations, each of said hydroacoustic devices having timing clocks and storage means, said first device being operable to transmit acoustic pulses having a predetermined range of characteristics through the water, said first device and said second device being operable to receive acoustic pulses having said predetermined range of characteristics;

b) establishing a set of pulse transmission characteristics, including transmission times and carrier frequencies, and storing the set of transmission characteristics in said first device to compose a schedule of pulse transmission times of pulses having the established transmission characteristics;

c) establishing a first set and a second set of pulse reception characteristics, including carrier frequencies and reception windows open during predetermined time periods for reception of pulses having the reception characteristics transmitted by said first device, and storing said first set and said second set of reception characteristics in the storage means of said first device and said second device, respectively, to compose a respective schedule of pulse reception windows in said first device and said second device;

d) synchronizing the timing clocks in the transceivers periodically at predetermined times;

e) transmitting a pulse from said first device according to the schedule of pulse transmission times stored in the storage means of said first device at one of said transmission times;

f) enabling detection by said first device of the pulse transmitted by said first device during the opening of a corresponding one of said pulse reception windows according to the schedule of pulse reception windows of said first set of pulse reception characteristics stored in said first device and assigning a first time of arrival coincident with pulse detection;

g) enabling detection by said second device of the pulse transmitted by said first device during the opening of a corresponding one of said pulse reception windows according to the schedule of pulse reception windows of said second set of pulse reception characteristics stored in said second device and assigning a second time of arrival coincident with pulse detection; and

h) determining an acoustic range R.sub.B from said first device to the sea bottom by comparing the first time of arrival of the pulse to the transmission time of the pulse transmitted by said first device;

i) determining a sea-bottom reflected range R.sub.B between said first device and said second device by comparing the second time of arrival to the transmission time; and

j) computing an estimate R.sub.D ', of the direct acoustic range R.sub.D between said first device and said second device from P.sub.B and R.sub..sub.R.

2. The method of claim 1, wherein:

in step h), R.sub.B is proportional to half the difference between the first time of arrival and the transmission time;

in step i), R.sub.R is proportional to the difference between the second time of arrival and the transmission time; and

in step j), the direct acoustic range is estimated according to R.sub.D '=(R.sub.R.sup.2 -4R.sub.B.sup.2).sup.1/2.

3. A method for estimating a direct underwater acoustic range between a first and a second hydroacoustic device, for which a direct acoustic path therebetween is occluded, comprising the steps of:

a) deploying a first and a second hydroacoustic device at different underwater locations, each of said hydroacoustic devices having timing clocks and storage means, said first device and said second device being operable to transmit acoustic pulses having a predetermined range of characteristics through the water, said first device and said second device being further operable to receive acoustic pulses having said predetermined range of characteristics;

b) establishing pulse characteristics, including pulse transmission times, carrier frequencies, and reception windows open during predetermined time periods for reception of pulses, and storing a first set and a second set of the pulse characteristics in said first device and said second device, respectively, to compose a schedule of pulse transmission times and of pulse reception windows in said first device and said second device;

c) transmitting a pulse from said first device at a transmission time t.sub.1x according to the schedule of pulse transmission times stored in the storage means of said first device and transmitting a pulse from said second device at a transmission time t.sub.2x according to the schedule of pulse transmission times stored in the storage means of said second device;

d) enabling detection by said first device of the pulse transmitted by said first device during the opening of a corresponding one of said pulse reception windows according to the schedule of pulse reception windows stored in said first device and assigning a time of arrival t.sub.1r1 coincident with pulse detection;

e) enabling detection by said second device of the pulse transmitted by said second device during the opening of a corresponding one of said pulse reception windows according to the schedule of pulse reception windows stored in said second device and assigning a time of arrival t.sub.2r2 coincident with pulse detection;

f) enabling detection by said second device of the pulse transmitted by said first device during the opening of a corresponding one of said pulse reception windows according to the schedule of pulse reception windows stored in said second device and assigning a time of arrival t'.sub.2r1 coincident with pulse detection;

g) enabling detection by said first device of the pulse transmitted by said second device during the opening of a corresponding one of said pulse reception windows according to the schedule of pulse reception windows stored in said first device and assigning a time of arrival t'.sub.1r2 coincident with pulse detection;

h) determining an acoustic range R.sub.B to the sea bottom as a function of the difference between the time of arrival t.sub.1r1 at said first device and the transmission time t.sub.1x of the pulse transmitted by said first device and of the difference between the time of arrival t.sub.2r2 at said second device and the transmission time t.sub.2x of the pulse transmitted by said second device;

i) determining a sea-bottom reflected range R.sub.R between said first device and said second device as a function of the difference between the time of arrival t'.sub.1r2 at said first device of the pulse transmitted by said second device and the transmission time t.sub.1x of the pulse transmitted by said first device and of the difference between the time of arrival t'.sub.2r1 at said second device of the pulse transmitted by said first device and the transmission time t.sub.2x of the pulse transmitted by said second device; and

j) determining an estimate R.sub.D ' of the direct acoustic range R.sub.D between said first device and said second device from R.sub.B and R.sub.R.

4. The method of claim 3, wherein, in step j), the direct acoustic range is estimated according to R.sub.D '=(R.sub.R.sup.2 -4R.sub.B.sup.2).sup.1/2.

5. A method of determining ranges between pairs of hydroacoustic devices, in which direct acoustic paths between individual pairs of the devices are intermittently occluded, comprising the steps of:

a) deploying a plurality of hydroacoustic devices at different underwater locations, said devices having timing clocks and storage means and being operable to transmit and receive acoustic pulses on a plurality of frequency channels;

b) establishing an overall schedule for said plurality of devices, said overall schedule comprising subschedules for each of said plurality of devices, each subschedule including pulse transmission times and pulse reception windows open for receiving pulses on individual channels, and storing each of said subschedules in the storage means of a corresponding one of said devices;

c) sequencing through each subschedule according to the timing clock in each said device;

d) determining a range between a first and a second said device according to the following substeps:

1) transmitting a pulse from said first device at a pulse transmission time t.sub.1x according to said subschedule stored therein, and transmitting a pulse from said second device at a pulse transmission time t.sub.2x according to said subschedule stored therein;

2) opening a reception window W.sub.1A in said first device according to said subschedule stored therein, detecting in said first device the pulse transmitted from said first device and reflected off the sea bottom, and assigning a time of arrival t.sub.1r1 coincident with pulse detection;

3) opening a reception window W.sub.2A in said second device according to said subschedule stored therein, detecting in said second device the pulse transmitted from said second device and reflected off the sea bottom, and assigning a time of arrival t.sub.2r2 coincident with pulse detection;

4) opening a reception window W.sub.1B in said first device according to said subschedule stored therein, detecting in said first device the pulse transmitted from said second device to said first device over at least one path, and assigning a time of arrival t'.sub.1r2 coincident with the last-occurring pulse detection in window W.sub.1B and a time of arrival t.sub.1r2 with any earlier-occurring pulse detection in window W.sub.1B ;

5) opening a reception window W.sub.2B in said second device according to said subschedule stored therein, detecting in said second device the pulse transmitted from said first device to said second device over at least one path, and assigning a time of arrival t'.sub.2r1 coincident with the last-occurring pulse detection in window W.sub.2B and a time of arrival t.sub.2r1 with any earlier-occurring pulse detection in window W.sub.2B ;

6) determining the sea depth R.sub.B according to a function of (t.sub.1r1 -t.sub.1x) and (t.sub.2r2 -t.sub.2x);

7) determining the sea-bottom reflected range R.sub.R between said first and second devices according to an average of (t'.sub.1r2 -t.sub.1x) and (t'.sub.2r1 -t.sub.2x);

8) if earlier pulses were detected in substeps 4) and 5), determining a direct range R.sub.D between said first and second devices according to an average of (t.sub.1r2 -t.sub.1x) and (t.sub.2r1 -t.sub.2x);

9) determining an estimate R'.sub.D of the direct range R.sub.D as a function of R.sub.B and R.sub.R ;

e) repeating step d) for preselected pairs of said plurality of devices.

6. The method of claim 5, further comprising the substep of:

10) comparing the direct range estimate R'.sub.D with the direct range R.sub.D and, if R.sub.D and R'.sub.D differ by more than a preselected value, using R'.sub.D as the direct range.

7. The method of claim 5, wherein, in substep 9), the direct acoustic range is estimated according to R'.sub.D =(R.sub.R.sup.2 -4R.sub.B.sup.2).sup.1/2.

8. The method of claim 5, wherein, in substep 6), the sea depth R.sub.B is computed according to

R.sub.B =c[(t.sub.1r1 -t.sub.1x)+(t.sub.2r2 =t.sub.2x)]/4,

wherein c is the local speed of sound.

9. The method of claim 5, wherein, in substep 6), the sea depth R.sub.B is computed according to ##EQU2## wherein c is the local speed of sound.

10. The method of claim 5, wherein, in substep 7), the sea-bottom reflected range R.sub.R is computed according to

R.sub.R =c[(t'.sub.1r2 -t.sub.1x)+(t'.sub.2r1 -t.sub.2x)]/2,

and wherein, in substep 8), the direct range R.sub.D is computed according to

R.sub.D =c[(t.sub.1r2 -t.sub.1x)+(t.sub.2r1 -t.sub.2x)]/2,

wherein c is the local speed of sound.

11. A hydroacoustic ranging system for estimating a direct underwater acoustic path between a first and a second hydroacoustic device, for which a direct acoustic path is occluded, the system comprising:

a controller;

a plurality hydroacoustic devices deployed at different underwater locations;

communications means for transferring data between the controller and the plurality of devices;

wherein each device is enabled to transmit underwater acoustic pulses having preselected characteristics and to receive specified ones of the acoustic pulses transmitted by the plurality of devices according to a synchronized schedule in each device and to assign a time of arrival to each specified pulse received, and wherein a first device transmits a pulse at a transmission time t.sub.1x and a second device transmits a pulse at a transmission time t.sub.2x, and the first device is enabled to receive the pulse transmitted by the first device and the pulse transmitted by the second device and to assign times of arrival t.sub.1r1 and t'.sub.1r2 respectively thereto, and the second device is enabled to receive the pulse transmitted by the second device and the pulse transmitted by the first device and to assign times of arrival t.sub.2r2 and t'.sub.2r1 respectively thereto, and wherein the first device and the second device transfer data including the times of arrival t.sub.1r1, t'.sub.1r2 and t.sub.2r2, t'.sub.2r1, respectively, and the transmission times t.sub.1x and t.sub.2x, respectively, over the communications means to the controller, and wherein the controller determines an acoustic range R.sub.B to the sea bottom from the devices as a function of the different between the time of arrival t.sub.1r1 and the transmission time t.sub.1x and of the difference between the time of arrival t.sub.2r2 and the transmission time t.sub.2x, and further determines a sea-bottom reflected range R.sub.R between the first and the second device as a function of the difference between the time of arrival t'.sub.1r2 and the transmission time t.sub.1x and the difference between the time of arrival t'.sub.2r1 and the transmission time t.sub.2x, and determines an estimate of the occluded direct range R'.sub.D as a function of R.sub.B and R.sub.R.

12. The hydroacoustic ranging system of claim 11, wherein the direct range is estimated according to R'.sub.D =(R.sub.R.sup.2 -4R.sub.B.sup.2).sup.1/2.

13. The hydroacoustic ranging system of claim 11, wherein the range R.sub.B to the sea bottom is estimated by R.sub.B =c[(t.sub.1r1 -t.sub.1x)+(t.sub.2r2 -t.sub.2x)]/4, wherein c is the local speed of sound.

14. The hydroacoustic ranging system of claim 11, wherein the sea-bottom reflected range R.sub.R is estimated by R.sub.R =c[(t'.sub.1r2 -t.sub.1x)+(t'.sub.2r1 -t.sub.2x)]/2, wherein c is the local speed of sound.

15. A hydroacoustic ranging system used in marine seismic exploration, comprising a plurality of hydroacoustic transceivers attached at different locations to underwater seismic exploration apparatus, each of the transceivers including:

a) a hydroacoustic energy to electric energy transducer adapted for underwater transmission and reception of hydroacoustic pulses;

b) an electronic system coupled to the transducer for transmitting and receiving hydroacoustic pulse energy including means for processing pulses within a predetermined range of characteristics;

c) a pulse detector in said electronic system for detecting pulses received by the transducer;

d) means for assigning times of arrival to the pulses having characteristics within a predetermined range of characteristics received by the transducer;

e) a pulse synthesizer in the electronic system for producing a waveform of predetermined characteristics for transmission from the transducer at a time different from the arrival of pulses;

f) a clock for producing periodic time counts;

g) means for synchronizing the clocks of the plurality of hydroacoustic transceivers to each other; and

h) scheduling means in each transceiver for scheduling a sequence of pulse transmission and reception events and for initiating the sequence of events in response to a synchronizing signal from the means for synchronizing;

and wherein the scheduling means in a first one of the transceivers includes in its scheduled sequence of events transmission of a pulse at a time t.sub.1x and self-reception of the bottom-reflected pulse returning at a later time t.sub.1r1.

16. The hydroacoustic ranging system of claim 15, further comprising means for computing the sea depth R.sub.B1 below the first one of the transceivers according to

R.sub.B1 =c(t.sub.1r1 -t.sub.1x)/2,

wherein c is the local speed of sound.

17. The hydroacoustic ranging system of claim 15, further comprising a controller remote from the plurality of transceivers and communications means for transferring data between the controller and the plurality of transceivers, the data including the time of arrival of the pulses and the pulse transmission times from each of the transceivers, the controller computing the ranges between pairs of the transceivers and the sea depth below individual transceivers from the data.

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BACKGROUND

This invention relates to apparatus for transmitting and receiving hydroacoustic pulses used to determine the spatial separation between pairs of such apparatus. More particularly, the invention in a preferred embodiment relates to high-frequency hydroacoustic transceivers using digital-signal-processing devices and deployed at known positions along the lengths of towed hydrophone streamers for the purpose of determining their shapes and geodetic positions.

In search of geologic formations likely to trap oil or gas, the offshore seismic exploration industry surveys the outer layers of the earth's crust beneath the ocean by towing an array of hydrophones behind a boat, periodically firing a source of seismic energy, recording the responses of the hydrophones to reflections of the acoustic energy from geologic formations, and processing the seismic hydrophone data. The hydrophone array is linearly arranged in a streamer whose depth is controlled. The streamer, which may be a few kilometers long, may also include a head buoy tethered to the head end of the streamer and a tail buoy to the tail end as surface references.

Historically, only one streamer containing the hydrophone array was deployed from the exploration boat during a survey. The accuracy of the survey depended on, among other things, the accuracy of the estimate of the shape of the hydrophone streamer and the accuracy of the positioning of a known point on the streamer.

One way the shape can be estimated is by mechanically modeling the streamer and computing its dynamic performance under various towing speeds and ambient conditions. The accuracy of the estimation is, of course, only as good as the model. Placing magnetic compasses and depth sensors along the streamer represented an improvement in streamer shape estimation. Data representing the depth and magnetic heading of sections of the streamer are sent from the distributed compasses and depth sensors to a controller on board the tow boat for immediate computation of streamer shape and for storage of the raw data for later detailed processing. Accurate shape estimation is achieved in this way.

As important as estimating the streamer's shape is tying its position to a geodetic reference. Typically, radiopositioning receivers aboard the boat are used to tie a spot on the boat to a geodetic reference. Accurate optical positioning systems, such as a laser, are then used to tie the front buoy to the geodetic reference. It is also common to have a radiopositioning receiver aboard the tail buoy to fix its position. The positions of the distributed compasses and depth sensors with respect to the buoys is then estimated based on a model of the streamer and the buoy tethers. Inaccuracies in the model result in absolute errors in transferring the geodetic reference from the buoys to the streamer. Furthermore, the performance of optical positioning systems degrades with inclement weather.

An important advance in the exploration for oil and gas is the development of the three-dimensional seismic survey, often using more than one hydrophone streamer. With multiple streamers towed behind one or more boats, more seismic hydrophone data can be gathered in much less time than with a single streamer, resulting in a significant reduction in exploration costs. With multiple streamers, accurate estimations of the positions of the hydrophone streamers with respect to each other and to the acoustic source are essential. Fortunately, multiple streamers towed more or less in parallel provide a geometry favorable for determining the positions of the streamers with respect to each other, to the boat, to the acoustic source, or gun, and to the buoys by means of acoustic ranging. With individual hydroacoustic transceivers positioned along the streamers, on the acoustic source, on the boat or boats, and on the buoys, acoustic transit times of pulses transmitted by the transceivers and received by neighboring transceivers can be telemetered to the controller on the boat where a position solution can be performed and the raw data stored for further processing. Using the speed of sound through the water, the controller converts the transit times into spatial separations between pairs of transceivers in developing the position solution. With information from a radiopositioning system and from depth sensors and compasses positioned along the array, the position solution is complete.

In a typical three-dimensional survey run using more than one streamer, the towing boat or boats follow a more or less constant heading at a more or less constant speed through the survey field. Waves, wind, current, and inevitable variations in boat speed and heading continuously affect the shapes of the streamers. Periodically, for example, every ten seconds, the acoustic source, or gun, is fired. An impulse of compressed air is forced into the water creating a bubble. The collapse of the bubble causes an acoustic pulse that radiates through the water and into the earth. Reflections of the pulse off geologic structures are picked up by the hydrophones and data representing these reflections are sent to the controller on the boat. Each firing of the gun and the associated interval during which the acoustic echoes are detected is known as a shot point. It is important that data sufficient to perform a complete position solution for each shot point be available. For a group of long streamers with acoustic transceivers distributed along each, many acoustic ranges must be measured. In theory, it would be best if all of the ranges to be measured could be determined simultaneously before the streamer has a chance to change its shape and position. Unfortunately, that is not possible in practice. The idea, then, is to measure all the acoustic ranges in as little time as possible, which requires a high throughput for each transceiver.

The separation between a pair of transceivers is generally measured by either one-way or two-way ranging. In one-way ranging, the first transceiver transmits a hydroacoustic pulse at time t.sub.s. The pulse propagates through the water where it is received by the other transceiver at time t.sub.r. The time difference t.sub.r -t.sub.s is proportional to the spatial separation of the two transceivers. For an accurate one-way ranging measurement, the timers of both transceivers must be closely synchronized because the value t.sub.s is determined by the transmitting transceiver while the value t.sub.r is determined by the receiving transceiver. In two-way ranging, each transceiver transmits a pulse, the first at time t.sub.1s and the second at t.sub.2s. The first receives the second's pulse at time t.sub.1r, and the second receives the first's pulse at time t.sub.2r. Even if the timers of both transceivers are not synchronized, the spatial separation is proportional to [(t.sub.1r -t.sub.1s)+(t.sub.2r -t.sub.2s)]/2, because the offset between the timers is removed by the subtraction. Consequently, the precise synchronization required for one-way ranging is not needed in two-way ranging systems.

Although a two-way ranging system avoids the synchronization problem in one-way ranging, each transceiver in a two-way ranging scheme must do more processing, that is, each transceiver must receive a pulse for each range it is involved in measuring. The times of arrival of the received pulses and time of transmission of the transmitted pulse or their differences must be telemetered to the controller aboard the boat for each shot point. For a transceiver involved in the measurement of many ranges, a lot of data must be processed. Consequently, only a transceiver with a high throughput can be used effectively in a two-way ranging system.

Therefore, one object of this invention is to provide a hydroacoustic transceiver capable of the high throughput rates required for two-way acoustic ranging without the need for accurate time synchronization.

If all the transceivers on a ranging system transmit on only one frequency, the only way to measure the various ranges is by time-division multiplexing, i.e., staggering the transmissions in such a way that no two pulses transmitted by different transceivers can arrive at any receiver simultaneously. Such a requirement, in addition to causing a transmit scheduling nightmare, results in a long time to measure many ranges, which causes errors in the position solution.

Another object of the invention is to provide a transceiver capable of transmitting and receiving hydroacoustic pulses having selected characteristics.

A further problem with acoustic ranging is errors caused by multipath interference. The straight-line path from transmitting transceiver to receiving transceiver is the direct path, which is the path defining the actual spatial separation. Other paths are due to reflections of the transmitted pulse off the ocean surface or floor. Depending on the differences in the lengths of the reflected paths with respect to the direct path, the reflected pulses may interfere with the direct pulse. Such interference can be destructive, preventing or distorting the detection of the pulse, resulting in an error in determining the time of arrival of the direct pulse. In addition, the shorter the transmitted pulse the less susceptible it is to multipath interference and the greater is its spatial resolution. It is well known in the art that the narrower the pulse, the wider the transmitter and receiver bandwidths must be. In other words, there is a tradeoff between resolution (pulsewidth) and bandwidth.

Wider bandwidths for each pulse of a given carrier frequency require that each channel in a frequency-division-multiplexed system be separated further. Accommodating a wide range of carrier frequencies is difficult in typical hydroacoustic transducers.

One way of squeezing more channels in a given transducer's bandwidth is by synthesizing narrow transmit pulses and detecting them using a matched-filter receiver. With a matched-filter receiver, it is possible to achieve a lower pulsewidth-bandwidth product than with ordinary receivers. A true matched-filter receiver, however, cannot be realized in the linear analog transceivers typically used. Consequently, analog transceivers must sacrifice resolution to enjoy the flexibility afforded by more channels or must sacrifice frequency flexibility to improve resolution.

One technique used with analog transceivers to avoid the multipath problem is to sequentially transmit pulses on different channels and analyze the transit times measured on each channel. The idea is that, for the same reflected paths, the interference between the direct and reflected pulses is different at different frequencies and that, at one of the frequencies, the interference will not be destructive and the range measurement can be made. This use of frequency diversity to solve the multipath problem takes more time, because more than one pulse must be transmitted by each transceiver to get a valid range measurement.

Therefore, it is a further object of this invention to provide a hydroacoustic transceiver operating on a number of efficiently packed channels and transmitting hydroacoustic pulses sufficiently narrow to minimize multipath interference.

In some operating environments, the direct acoustic path between a pair of transceivers is occluded completely or intermittently. For example, a bubble curtain produced by the seismic source often exists directly between a pair of transceivers. The bubble curtain attenuates the acoustic ranging signal along the direct path, making it unavailable for ranging.

Therefore, yet another object of the invention is to provide a method for estimating a range between two underwater locations when the direct acoustic path is occluded.

SUMMARY

These and other objects and advantages will be obvious and will in part appear hereinafter, and will be accomplished by the present invention which provides apparatus for transmitting narrow hydroacoustic pulses and for determining the times of arrival of received hydroacoustic pulses for the purpose of measuring the spatial separations between pairs of such apparatus. An example of such an apparatus is a hydroacoustic transceiver used as part of an acoustic-ranging system for estimating the positions and shapes of hydrophone streamers to improve the accuracy of a seismic survey. In such a ranging system, individual transceivers may be attached at various points along hydrophone streamers, on the gun, on the head buoy, on the tail buoy, on submerged towfish trailed from the buoys, or on the hull of the tow boat. A controller, some sort of processing device, on board the boat controls the operation of the system and collects data from the transceivers over communications links.

The invention teaches a transceiver having a transducer for converting hydroacoustic energy into electrical energy and vice versa. In a preferred embodiment, the transducer is a ceramic sphere having a bandwidth ranging from about 50 kHz to about 100 kHz. The transducer is alternately connected to either the transceiver's electrical transmission path or its reception path by means of a transmit/receive switch.

With the switch in the receive position, the transceiver is listening for pulses from other transceivers. The reception path conducts the electrical energy representative of the hydroacoustic energy impinging on the transducer to conversion means such as a sampling analog-to-digital converter, which converts the electrical energy at its input into a sequence of digital words, or samples, at its output. In a preferred embodiment, the reception path includes a highpass filter for attenuating the low frequency noise that can be significant in a marine environment. From the sequence of digital samples, detection means detect the presence of pulses transmitted from other similar transceivers, the pulses having known characteristics. In a preferred embodiment, the known characteristics are the shape of the pulse and its carrier frequency and the detection means is a multiple-channel digital filter realized in a digital-signal-processing (DSP) integrated circuit. Coefficients of the digital filter, stored in memory means such as an EPROM or RAM, are configured to detect pulses of the known shape on one of five known carrier frequencies, or channels, from about 50 kHz to about 100 kHz. The digital filter detects pulses on each channel by correlating the sequence of digital samples with the filter's coefficients. Relative maximum correlation values from the filter represent detected pulses, the magnitudes of the correlation values indicating their signal strengths. The time count of timer means at the detection of a pulse, representing the time of arrival of the pulse, is saved in memory. The detection means similarly saves the signal strength of each of the received pulses. The times of arrival and signal strengths of up to eight pulses can be saved.

While the transducer is connected to the transceiver's electrical transmission path, the transceiver outputs a hydroacoustic pulse of known shape and carrier frequency. In a preferred embodiment, one pulse is transmitted for each shot point on one of five carrier frequencies. The pulse is digitally synthesized in synthesizer means at the shot point rate. Count comparison means in cooperation with the timer determine the transmission interval by comparing the timer count with the value stored in a register. In a preferred embodiment the count of the timer is reset to zero when its count matches the count in the register. The times of arrival of received pulses are referenced to the time of transmission. The digitally synthesized pulse is converted into an analog signal by a 12-bit digital-to-analog converter and conducted to the transducer through the switch over an electrical transmission path including a bandpass filter for increasing the pulse's power to a level sufficient to be detected by other transceivers. The pulse is coupled into the water by the transducer. In a preferred embodiment, the transmit/receive switch is in the transmit position for about 500 microseconds for each shot point interval. To save power, the power amplifier is turned on only during the brief transmit time.

In a preferred embodiment, the timer, comparison means, detection means, and synthesizer means less the digital-to-analog converter are realized by a DSP chip, its support circuitry, and its operating machine code. A DSP chip is used because it is capable of quickly performing the many arithmetic and logical operations, such as those required in implementing a multiple-channel digital matched filter. With the DSP chip, high-throughput, near-simultaneous two-way acoustic ranging with good multipath rejection on two or more streamers is possible.

In a typical application, the transceivers are attached to the streamer at various positions, to the head and tail buoys, to the gun, to towfish, and to the hull of the boat. Before deployment while the streamers are still on the deck of the boat, each transceiver is configured by the controller over the communications link. Parameters that can be configured include: a) the transmit frequency; b) the interrogate interval, i.e., the rate of transmission; c) the transmit time, i.e., the time to transmit relative to the resetting of the timer to zero, which occurs at the start of the interrogate interval or upon a master sync reset; d) the receive window open times and close times, i.e., the acceptance interval for each receive pulse relative to the start of the interrogate interval; e) the receive channel number (or carrier frequency); f) the receive detection threshold; and g) the receive time calibration value. The configurable parameters are stored in registers. Eight registers are reserved for each of items d-g, permitting the reception of up to eight pulses each shot point that meet the criteria defined by the corresponding settings of items d-g. In addition, each transceiver can be configured as a responder that transmits a pulse on a selected channel only upon reception of a pulse on a given channel. Responders are used in locations in the system, such as on the tail buoy, having no communications link with the controller on the boat. Communications over the link is between the controller and each transceiver over a party line. A microcontroller in each transceiver handles the communications and stores the config