Device for Determining the Instant of Reception of an Acoustic Wave

This invention concerns a device for determining the instant of reception of an acoustic wave received at a sensor intermittently connected to a recording or a processing system.

More particularly, the device is adapted to determine the instant of reception of acoustic waves directly received from a seismic source after triggering thereof, by means of a sensor arranged in a seismic streamer comprising a system for the sequential transmission of the information received at each of the sensors or sensor groups thereof.

The recording of the instant of arrival of the first acoustic wave received at the sensor of the seismic streamer, after the triggering of the seismic source, hereinafter designated as the reference instant of time, allows, by means of a comparison, to the determination of the propagation time of the echoes received after the reference time and, consequently, the depth of the subterranean layers producing these echoes can be determined.

In the reception systems of the prior art type, the reference time is determined by making use of a special sensor generally located at the head of the seismic streamer and permanently connected to the signal recording or processing system arranged on a ship, in the case of marine seismic prospecting, by means of a high-pass filter, i.e., an element adapted to detect the envelope of the filtered signal and a Schmitt trigger adapted to be triggered when the level of the detected signal is higher than an adjustable threshold value. The reference time is known with accuracy due to the permanent connection between the sensor and the recording or the processing system. This permanent connection of the special sensor is not only present in the conventional prior art seismic streamers where each sensor or sensor group is connected through conductors to the recording or processing system, but also in the seismic streamers where the transmission of information is achieved through a sequential transmission system called multiplexing system, and is provided in order to avoid an unacceptable inaccuracy in the detection of the reference instant, a disadvantage inherent in the very principle of the discontinuous connection mode of operation which characterizes multiplexing transmission, as it will become clearly apparent from the following description.

The device according to the invention has the advantage of allowing omission this permanent connection, and of making use of the same sequential transmission system for all the sensors arranged in the streamer, including the sensor which transmits information representing the time of reception of the first received acoustic wave (reference time). For this purpose, the invention comprises means adapted for generating, starting from an initial time coinciding with the time of reception, and for a longer time than the time interval between two successive connections of the sensor to the recording or the processing system, through the sequential transmission system, a voltage varying over time in a known manner, means for selecting at least two discrete values of said voltage, produced at of time separated by a time interval .DELTA.T higher than the time interval separating two successive transmissions of information elements issuing from said sensor, through said sequential transmission means, the latter being used for the transmission to the recording or the processing system of the discrete values constituting the information representing the time of reception of the first acoustic wave at said sensor.

The voltage varying over time in a known manner may vary a linearly or non linear, and increasing or decreasing variation according to a known equation. The equation in nature may, for example, be of a parabola hyperbola or exponential but, for reasons of simplicity, a linearly increasing variation is generally preferred.

The recording or processing system to which are transmitted the discrete values of the voltage comprises means for determining the initial time of reception of the first acoustic wave of the discrete values.

The device is designed that, in the absence of a reception of acoustic waves, the means for generating the variable voltage, referred to as integrating means, has a zero output voltage. These means comprise a switch controlled by means for shaping signal generated by the sensor in response to the acoustic wave it has received.

Other characteristics and advantages of the invention will be made apparent from the following description of a non limitative particular embodiment of the invention relating to the application of marine seismic prospecting, given with reference to the accompanying drawings wherein:

FIG. 1 is a schematic illustration showing a seismic streamer and a seismic source towed from a ship;

FIG. 2 shows a voltage square wave produced at initial time instant t.sub.0 in the interval between two successive sampling time instants, in response to the reception of an acoustic wave;

FIG. 3 is a schematic diagram illustrating the circuitry employed in the device of the invention, and

FIG. 4 shows a portion of a curve representing the variable voltage generated by the integrating means.

In FIG. 1, the seismic streamer 1, towed from the ship, comprises a sensor or hydrophone H, generally located at the head and used for determining the time of reception of the acoustic waves directly issued from a seismic source S, periodically triggered. The hydrophone H is connected to a recording or a processing system arranged on the ship, either by means of a continuous connection or in a discontinuous manner when the transmission of the signal detected therewith is also performed by means of a multiplexing transmission system.

In such a system, the sensors or sensor groups, or the data acquisition devices, arranged spaced inside the seismic streamer for gathering and digitizing the seismic signals sensed by a specific number of transducers distributed all along a section, are cyclically and consecutively interrogated. In the case where hydrophone H is also connected to a multiplexing transmission system, it will be interrogated at a predetermined recurrence frequency .DELTA.t. The time t.sub.0 when hydrophone H detects the arrival of the direct acoustic wave is generally intermediate between two successive sampling times t and t+.DELTA.t (FIG. 2). Accordingly, it is necessary to wait until instant t+.DELTA.t, at which the hydrophone H will be again connected in order to transmit to the recording or processing system a indication of the change of level of the Schmitt trigger memorizing the reception of the wave. But this indication will also include a systematic error resulting from the failure to detect the exact instant of reception of the wave within the time interval .DELTA.t of the recurrence period. This systematic error, which can be as high as .DELTA.t, is unacceptable since the required accuracy for the determination of t.sub.0 is generally of the order of 0.1 ms, whereas the sampling period is comparatively much longer.

In accordance with the device of the invention, it is possible to avoid this disadvantage. The device generally comprises (FIG. 3) a band-pass filter 2 limits the signal generated by the hydrophone H, to a frequency band comprised for example between 200 Hz and 800 Hz, an envelope detector 3 and a Schmitt trigger 4 which compares the level of the detected signal issued from detector 3 with a threshold voltage supplied by an integrator element 5. As soon as the level of the detected signal exceeds the value of the threshold voltage, the output of trigger 4 passes to the upper level, in a manner well known to those skilled in the art. The integrator element 5 is adapted for integrating either a positive voltage +V or the symmetrically equivalent negative voltage -V. One or the other of these two voltages is applied to the input of the integrator 5 by means of a switch 6 which is actuated by a logic actuating assembly 12 which is in turn controlled by the recording or the processing system, during the time interval required for the level of the signal issued from the integrator to reach the predetermined value. The elements 5, 6 and, 12 make it possible to adjust at will the threshold value in order to adapt it to the amplitude of the signal issued from the hydrophone H. The nature and the position of the hydrophone H inside the seismic streamer is not critical. The output of the Schmitt trigger, is connected to the input of a second trigger or flip-flop of the monostable type 7. The square wave generated by the latter is used to control the opening of a switch 8, preferably of the electronic type, which directly connects, in the on position, the inverting input of an amplifier-integrator 9 forming the integrating means to its output. The inverting input of the amplifier 9 is connected, conventionally, on the one hand, to a source of negative voltage -V through a resistor R and, on the other hand, to the output of the amplifier 9 through a condenser C, when the switch 8 is in off position. The output of integrator 9 is connected to one input E.sub.1 of an auxiliary multiplexer 10 having four inputs, for example. To two other inputs E.sub.2 and E.sub.3 l of this multiplexer are applied respectively two voltages V.sub.1 and V.sub.3 supplied by auxiliary means and to the input E.sub.4 is applied the threshold voltage generated by the integrator element 5. The output of the auxiliary multiplexer 10 is connected to one of the inputs a.sub.n of a general multiplexer 11 having n inputs, included in a local device for acquisition of the seismic data collected in the streamer section wherein the hydrophone H is located. The output of the general multiplexer 11 is connected to the input of a sample amplifier of a known type. The sampling period of each line of the general multiplexer 11 being equal to .DELTA.t, the sampling period of the auxiliary multiplexer 10 is selected so that its output is successively connected to its four inputs, at the successive times when line a.sub.n is sampled. The sampling period of each line of the auxiliary multiplexer by the general multiplexer is designated by .DELTA.T.

Having described the device of the invention, its operation will now be discussed.

The detection by hydrophone H of the arrival of the directly transmitted wave produces the triggering of the monostable flip-flop 7, and actuates the switch 8 to its "off" position. Since the switch was previously in the "on" position, the output voltage of integrator 9 was during that time equal to zero. The input of the integrator has a voltage -V applied thereto, and the capacitor 14 begins to charged through resistor 13 as of the initial time where the switch is actuated to its off position, and the output voltage of the integrator linearly increases over time. At instant t.sub.1, subsequent to instant t.sub.0 of reception of the direct wave, a first sample of the output voltage of integrator 9, having for example a value a.sub.1, is picked up and transmitted to the recording or processing system. At a subsequent instant, separated from the preceding one by a time interval .DELTA.T, equal to the sampling period by the general multiplexer 11 of the voltage issued from integrator 9, the output voltage of the integrator 9 is again sampled. The value a.sub.2 of the new sample is also transmitted to the recording or processing system. Since the value of .DELTA.T is known with great accuracy, the value of the slope of the straight line representing the variations of the output voltage in the integrator, and the successive transmitted values a.sub.1 and a.sub.2, the instant t.sub.0 at which the previously zero voltage began to increase, may be easily determined from the following relationship: ##EQU1##

This determination may be performed by a special analog computer of a known type, or by a digital computer incorporated into the recording or processing system.

As soon as the square wave generated by the flip-flop 7 is discontinued, the switch 8 returns to its "on" position which resets to zero the output voltage of the integrator 9.

The operation time of integrator 9 between two successive time of zero reset, which corresponds to the length of the square wave generated by the flip-flop 7, must be sufficient to allow the transmission of two successive output voltages.

However, the integration time may be lengthened in order to transmit a greater number of samples of the integrator output voltage.

The threshold voltage used as reference by the Schmitt trigger being also transmitted intermittently to the recording or processing system, the recording or processing system may control its actual amplitude and, optionally, modify it before any subsequent triggering of the seismic source by actuating the switch 6 by means of the logic assembly 12.