SE410126B - METHOD AND DEVICE FOR MARINE SEISMIC EXAMINATION - Google Patents

Description

771329991 longer, and formed by 2Ä - 96 spaced apart geophone groups, and so that the geophone group closest to the boat is 200 - 500 m behind the towed, seismic source. This arrangement gives a fairly large in-line shift between the source and the first receiver. When recording reflection data, this offset can create a certain uncertainty in the determined values of the zero position of the time measurements on the time scale, which is used in speed calculations. Furthermore, by towing the seismic source and the geophone cable near the boat, unwanted noise generated at the source is obtained, as well as noise from the boat in the seismic data, especially at the critically placed, for short-range geophone groups.

A method for marine seismic exploration is described in U.S. Pat. No. 1,7üU021 and involves the simultaneous scanning of a depth reflection profile and a ground reflection profile without significant interference to one another. Two weighting devices are disclosed in U.S. Pat. No. 3,1878,831, which are useful for placing a row of geophones on a cable. One method of seismic exploration is described in U.S. Pat. No. 3,310,1050, which uses screens which are of certain depths. A first object of the invention is to provide a system with a towed marine seismic source, in which an underwater, seismic air cannon is arranged with a transverse, controlled and predetermined distance to a submerged, towed geophone cable, whereby it is possible to provide more accurate seismic velocity measurements.

This and other objects, together with other advantages and objects which appear from the following description, are obtained according to the invention by a method which exhibits the features stated in claim 1.

When applying the method, it is suitable to use a device in accordance with the side requirement. The invention will now be described by way of example with reference to the drawings. Fig. 1 shows a schematic plan view of an exemplary embodiment of a device for marine seismic survey. Fig. 2 is a side view of the same embodiment as that shown in Fig. 1.

Fig. 3 is a schematic plan view of the tugboat used in Fig. 1. Fig. M is a schematic plan view of a steerable screen according to Fig. 1. Fig. 5 is a schematic side view of a geophone cable according to Fig. 1. Fig. 6 schematically shows a tugboat according to a second embodiment. Fig. 7 shows a steerable screen according to the modified embodiment shown in Fig. 6. Fig. 8 shows a geophone cable according to the embodiment shown in Fig. 6. Fig. 1 shows from above and in Fig. 2 from the side a boat 10, which tows an underwater geophone cable 11, which has groups of geophones 12-17 attached thereto. Two tow lines 18 and 19 are attached to each of the screens 20 and 21. The screens each have one or more air guns suspended, such as the three air guns 22, which are shown hanging from the screen 21. Any suitable seismic energy source or gun 22 can be used, and which examples but without limitation may be mentioned the energy sources shown in U.S. Pat. No. 3,923,122.

The marine seismic towing system shown includes means for keeping the air guns at a distance from the geophone groups, so as to maintain a substantially constant, predetermined distance, thereby obtaining more accurate seismic velocity measurements. These means essentially comprise a transmitter, a receiver and electrical connections on the boat 10 for continuously measuring the lateral or transverse distance between the air guns 22 or the paravans 20 and 21 on the one hand and the geophone groups 12-17 or the streamer cable 11 on the other hand. on the other hand, and control signals were sent to the controllable screens to keep them at the predetermined separation distance. The geophone groups are kept at a predetermined depth by being towed through the water at a predetermined speed, the geophones themselves being fairly heavy and balanced, so that they have the same specific gravity as the water they are immersed and towed in. Furthermore, the geophone cable can be kept at the predetermined depth by a conventional, underwater paravan, which maintains a predetermined depth. g 'g This solved problem is to keep the lateral distance between the air guns and the geophones at a desired, predetermined value in order to achieve accurate seismic velocity feeds. Fig. 3 shows how an existing and conventional frequency timer 24 for deep seismic surveys at the stern of the boat 10 is so connected that it emits input signals to a clock and countdown circuit 25. The circuit 25 is arranged to emit a clock signal at 1 kHz to a two-stage BCD counter (binary coded decimal) 26, a 50 Hz signal to a DC pulse width converter 27 and a 2 Hz trigger pulse to a 1 ms delay pulse generator 28, to the reset input of the BOD counter 26 and to a reset input 31 and to the reset input of another flip-flop 30. The output signal from the delay pulse generator 28 is connected to an acoustic transmitter or signal drive circuit 32 and to the start input of the BCD counter 26. The drive circuit 32 has an electrical connection through the streamer or geophone cable II to an acoustic transmitter 33, arranged therewith and which is shown in Fig. 5. The acoustic sensor 33 is acoustically connected through the water to the n acoustic receiver sensor 3H, which is shown in Fig. H and which may, for example, be mounted on the left panel 20.

A differential drive amplifier 35, See Fig. 3, on the boat, sends a signal to a paravan-mounted differential drive amplifier 36, see Fig. 4. The acoustic receiver sensor 3U is connected to a preamplifier 37, which is electrically connected to a bandpass filter 38. A later filter 38 is connected to an amplifier detector 39, which in turn is connected to a 5 ms pulse generator 40. The pulse generator 40 is connected to a differential drive amplifier 41, which in turn is connected via the paravan tug line 18 to a differential receiver amplifier arranged in the tugboat. .ü2. D The latter differential receiver amplifier H2, see Fig. 3, is connected to a 5 ms pulse generator 43, which is connected to both the stop input of the counter 26 and to one input of an OR gate HU. The latter gate has its output connected to a setting input on the flip-flop 31. The two-stage counter 26 in Fig. 3 has its unit and tens outputs, 1-8 and 1H, respectively, connected to seven resistors, the values of which are 1R, 2R, IS, 8R, respectively. , 163, 32R and 643. An 8-output from the counter 26 is connected to a setting input on the flip-flop 30, and to one input on the OR gate THEN. The flip-flop 30 has both an output Ö, which is connected to a control input on an electronic switch H5, and a Q output, which is connected to a control input on the electronic switch H6. The Q output of the flip-flop 31 is connected to a control input on the electronic switch H7. The seven resistors in the two-stage BCD counter 26 are connected in parallel to an input of the electronic switch 45, while the output of this switch H5 is connected to both an input of the electronic switch H6 and to an input of a voltage comparator 48. Out the passage from the electronic switch 46 is connected both to an input of the voltage comparator H8 and to a junction point between seven resistors H9, the resistances of which are respectively large R1a, R2a, Rßa, R8a, R16a, R32a and Röüa. These seven resistors H9 in Fig. 3 are connected. a switch 50 corresponding to the "distance" from the sensor ". This later switch is connected to an electrical voltage source 51. The output of the voltage comparator H8, see Fig. 3, is connected to an input of an electronic switch H7, and the output thereof later switch H7 is connected both to a capacitor C or 52 and to a control input on the direct current pulse width converter 27. The second connection line to the capacitor 52 is connected to earth or a logic earth potential. The output of the DC pulse width converter 27 is connected to an input of the differential drive amplifier (DDA) 35.

The output of amplifier 35 in Fig. 3 is connected via the tow line or cable 18 of the paravan to the input of differential receiver amplifier (DBA) 36 in Fig. N. The output of amplifier 36 is connected both to a control input of a variable pulse generator 53 and to one of the inputs. the aisles of a pulse width comparator 5H. The output of the variable pulse generator 53 is connected to the second input of the pulse width comparator 5%. The pulse width control input of the variable pulse generator 53 is connected to a feedback potentiometer 55. This potentiometer 55 is mechanically linked to a servomotor 57. The output signal from the pulse width comparator 54 is connected to a servomotor control circuit 56, which in turn is connected to the servo 5 just mentioned. the motor 57 is thus mechanically linked to the feedback potentiometer 55, as well as to the rudder mechanism 58 of the paravan for controlling its lateral movement in the right direction.

Clock 4 and the countdown circuit 25 in Fig. 3 provide timing for all the electronics in the system. The circuit 25 gives a clock signal of 1 kHz to the BCD counter 26, and a clock signal of 50 Hz to the direct voltage pulse width converter 27. The circuit 25 also gives a 2 Hz trigger pulse, which is arranged to reset the BCD counter 26 to 0, together with resetting the flip-flops 30 and 31 at the beginning of the signal or transmitter cycle. The rocker 30 then sets an electronic switch 45 to the closed position and the electronic switch H6 to the open position. Rocker 31 sets the electronic switch H7 to the open position. Capacitor C or 52 will maintain the current charge for at least 100 ms.

The trigger pulse of 2 Hz is inhibited during deep seismic recording by the sequence timer of this system, which can be done, for example, by the apparatus specified in U.S. Pat. No. 2 BU4 211.

The trigger pulse from the clock circuit 25, see Fig. 3, also activates the delay pulse generator 28, which sends a 1 ms pulse to start the BCD counter 26 to count in 1 ms steps. The signaling or transmitter drive circuit 32 is also activated, and in turn transmits a burst of 50 kHz signals to the signal transmitter 33, which is located on the geophone cable 11.

The acoustic signal transmitter 33 in Fig. 5 emits acoustic energy to the water, and this is received by the receiver sensor 32 in Fig. H, which is mounted on one of the screens. The receiver sensor may be of any of the many crystal types available in the trade. The short burst of acoustic energy is converted by the receiver 3H into an electrical signal and passed on to the preamplifier 37. The preamplifier increases the signal level sufficiently and passes the signal on to the bandpass filter 38, where all unwanted frequencies above and below the acoustic signal frequency is eliminated. From here the signal goes to the amplifier and the detector circuit 39, where the high-frequency bursts of signals are converted into a DC voltage pulse, corresponding to the burst envelope.

The pulse generator H0 converts this DC pulse to a 5 ms pulse, which is sent to the differential drive amplifier (DDA) 41. Such amplifiers are used in this system to provide a high degree of filtering out of uniform signals 'common mode rejection' over the long lines in question. if. The 5 ms signal is transmitted from the differential drive amplifier H1 mounted on the paravan, see Fig. M, to the differential receiver amplifier H2, see Fig. 3, via the paragon's tow cable 18. The differential receiver amplifier H2 in Fig. 3 is part of the DC rate eliminating system used here. From this amplifier the signal is taken and converted by the 5 ms pulse generator 43 and led to the stop input of the BCD counter 26. Now that the BCD counter is stopped, a voltage proportional to the distance between the receiver and the signal transmitter will be present at the junction point. for the step-shaped resistance network connected to the unit and tens outputs of the counter 26. This voltage is used at one side of the voltage comparator H8 to determine whether the paravane is in the correct position. The 5 ms pulse generator M3 also sets the flip-flop 31 via the OR gate ÄH. g This in turn closes the electronic switch H7.

Under normal operating conditions, the voltage comparator 48 compares the step voltage from the BCD counter with the voltage set through the selection switch 50. The selection switch 50 has a similar step network, and the switch has BCD outputs, which are similar to those of the BOD counter. The switch can suitably be calibrated in feet or meters rather than in time units.

If the acoustic pulse from the paravan is not received, the count value 8 from the tens output (128 ms) from the BCD counter 26 in Fig. 3 will set the flip-flops 30 and 31. The flip-flop 30 in turn opens the electronic switch H5, the ladder network disconnected from the input of the voltage comparator 48. The rocker 30 also closes the electronic switch 46, whereby the two inputs of the voltage comparator are connected. When the inputs are interconnected, the voltage comparator 48 will operate as if the paravane were in the correct position and control accordingly. In f7713299 ~ 1 the output signal from the voltage comparator H8 in Fig. 3 goes to the electronic switch H7, which is now closed. The output voltage is applied to the capacitor C or 52, and to the input of the direct current pulse width converter 27. The capacitor C acts as a filter and brakes the changes in voltage, while the electronic switch H7 is momentarily opened between the reset and the reception of a pulse from the paravan electronics. The DC-pulse-width converter 27 operates as its name implies and is clocked at 50 Hz by the clock 25. The differential drive amplifier 35 then transmits the pulse train via the pair's tow cable 18 according to Fig. 1, to the in-pair differential receiver amplifier 36 according to Fig. H. The amplifier 36 on the pair amplifier The pulse is further transmitted to the variable pulse generator 53 and to the pulse width comparator 50. The variable pulse generator 53 has its pulse width controlled by the feedback potentiometer 55, which is mechanically linked to the servomotor 57, which controls the rudder function. The pulse width comparator 5U now compares the two pulse widths and emits a correction signal to the servomotor control circuit 56. This control circuit in turn controls the servomotor 57.

It can be noticed that the two-stage BOD counter 26 in Fig. 5 could be replaced by a three-stage unit, ie with tenths, units and tens with a clock frequency of 10 kHz. The switch 50 provided for selecting a distance may also need to be increased. This change would improve the resolution from 5 feet to 0.5 feet.

It may also be necessary to have a second paravan control circuit (which is not shown), if you have two paravanes. The signals at 2 Hz, 50 Hz and 1 kHz from the clock circuit 25 in Fig. 3 are used by this other 2 control circuit. This second control circuit is equipped with all the devices shown in Figs. 3-5, with the exception of the clock circuit, the 1 ms delay pulse generator, the drive circuit and the signal transmitter.

Figs. 6-8 show a modified towing system, in which the acoustic signal transmitter 33a, see Fig. 7, for carrying out the method described above, is mounted on the screen 20a, while the receiver 34a according to Fig. 8 is mounted together with associated electronics. on or in the streamer cable lla. The switching method and the function in this case are the same as in the exemplary embodiment shown in Figs. 1-6, with the exception of the second 2 Hz trigger pulse from the clock and countdown circuit 25a according to Fig. 6. This trigger pulse for the modified paravan control circuit 25a is 18o ° out of phase in relation to what applies to the first described control circuit. Thus, the two sensors 33a according to Figs. 8 and 33b (not shown) on the two paravans (of which only the left para-. 77-13299-1 I vvanen 20a is shown) can use the same receiver sensors Süa to stop their respective control circuits. For a function with two paravanes, everything in Figs. 5 and 7 in the second paravane is doubled, with the exception of the clock and countdown circuit 25a in Fig. 6, all components mounted in the cable in Fig. 8, the differential resistor present on the boat the take-up amplifier Ä2a according to Fig. 6 and the 5ms pulse generator ßša present on the ship according to Fig. 6.

Thus, in the embodiment shown in Figs. 6-8, the acoustic sensors 33 are mounted in the screens 20, 21 instead of in the geophone cable 11, according to the first embodiment in Figs. 1-5. Two advantages of this particular mounting method is that (1) the screens have more space to mount the acoustic sensors and (2) you get less harmful connection between the transmitted electrical signals to the geophone cable, by moving the acoustic transmitters from the geophone cable to the screens. It is clear that other methods can be used to provide designs either according to Figs. 1-5 or Figs. 6-8, than those listed above, depending on the desired construction parameters for the different mineral types which form the sediment section.

* In both embodiments, the geophones 12-17 can be towed or towed far behind the vessel, whereby interference generated by this can be avoided.

Claims (20)

7713299-1 Patent claims A method for marine seismic survey and data collection, wherein behind a ship both a streamer cable (ll) arranged below the water surface and a paravan (20,2l) are towed at a lateral distance from the cable (ll), the streamer cable being provided with at least a geophone (12-17) and the screen carries a seismic energy source (22), and the source and the geophone are adapted to collect said data, characterized in that a distance signal is generated, corresponding to the distance between the screen (20, 2l) and a sensor connected to the cable, in that a signal is transmitted and received between these two units, / that a maneuvering signal is calculated on the basis of distance signals, the paravane having a controllable maneuvering element, to which the maneuvering signal is guided to hold the said the side distance at a predetermined value. 2. A method according to claim 1, characterized in that said W signal is generated by an acoustic signal being transmitted and received in either direction between the paravane and the sensor. 3. A method according to claim 1 or 2, characterized in that the cable has several groups of the said geophones. 4. A method according to any one of claims 1-3, characterized in that said seismic energy source comprises a seismic air gun. 5. A method according to any one of claims 1-4, characterized in that said generation of an operating signal is performed on the towing vessel and then transmitted via a cable to the paravane. 6. A method according to any one of claims 1-5, characterized in that said distance signal is generated by transmitting a signal from the sensor and receiving at the paravane. A method according to claim 6, characterized in that the distance signal is generated by timing a signal pulse transmitted from control means in the towing vessel through said cable to the sensor, through the water to the paravane and back through a paravan cable. to said control means. x g 8. A method according to any one of claims 1-5, characterized in that said distance signal is generated by transmitting a signal from the paravane and receiving it at the sensor. 9. A method according to claim 8, characterized in that the distance signal is generated by timing a signal pulse transmitted from control means in the towing vessel through a paravan cable to the paravane, through the water to the sensor and back through said. f. n \ w 7713299-1 10 streamar cable to said control means. 10. A method according to any one of claims 1-9, characterized in that distance signal generation is prevented when the seismic energy source and the geophone are in use for data collection. e 11. ll. A device for marine seismic survey and data collection for applying the method according to claim 1, and comprising a tug, a streamer cable (II), which has at least one geophone and is arranged to be towed by the ship, a paravan, which carries a seismic energy source and is arranged for towing by means of the vessel at a lateral distance from the cable, the energy source and the geophone being arranged for collecting said data, characterized by means (26,32,33,5ü, Hl , Ä2) for generating a signal corresponding to the distance between the paravane (20) and a sensor (53), connected to the cable (II), by transmitting and receiving a signal between these two units, means (H8) for generating starting from the distance signal of an operating signal, the paravane being provided with a controllable operating element (58), together with means (} 5,36,57) for applying the operating signal to the operating element (58) to keep said lateral distance at a precursor true value. Device according to claim 11, characterized in that said distance signal generating means (26,32,33,34, fl 1, U2) comprises means (53,3H § 33a; 3Ha) for transmitting and receiving an acoustic signal from one to the other of said paravan and said sensor. Device according to claim 11 or 12; k ä non e t e c k n a d by the fact that said cable has several groups (12-17) of geophones. Device according to any one of claims ll-13, characterized in that said seismic energy source comprises a seismic air k & n0n. ' Device according to any one of claims 11, characterized in that said means (# 8) for generating the maneuvering signal are arranged in the tug (10) and comprise means (35, 36) for transmitting the maneuvering signal from the ship to paravanen. g i Device according to any one of claims 11-15, characterized in that said distance signal generating means (26,32,33,3ä, M1, ü2) 'comprises means (32) for transmitting a signal from the sensor (33). and means (34) for receiving said signal, and which means are arranged in the paravane (20). Device according to claim 16, characterized in that said distance signal generating means comprises a signal pulse timer (26) on the vessel (10) for timing a signal pulse passing from the timer (26) via the cable (11) ) to the sensor (33), through the water to the paravane (20) and again through a paravan cable (18) to the pulse timer (26) » Device according to any one of claims 11-15, characterized in that said distance signal generating means (26, 32,} 3,54,41,42) comprises means (jja) for transmitting a signal, which means are arranged in the paravane (20) together with means (34a) for receiving said signal at the sensor (jëa). ' An apparatus according to claim 18, characterized in that said distance signal generating means comprises a signal pulse transmitter (26) on the vessel (10) for timing a signal pulse passing from the timer (26) via a paravan cable (18a) to said, transmitter means (jja) arranged in the paravan, through the water to said sensor (34a) acting as a receiver, and back through said streamer cable (iia) to the pulse timer (26). Device according to any one of claims 11-19, characterized in that it comprises means (24,25) for inactivating said distance signal generating means using said energy source (22) and geophone (12-25). 17) for said data collection. IN THE ABOVE PUBLICATIONS: