CA1089075A - Methods for accurately positioning a seismic energy source while recording seismic data - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Methods for maintaining a steerable paravane supported seismic energy source, as an air gun, positioned laterally at a precise constant predetermined distance from a geophone mounted on a submerged geophone streamer cable while recording offshore marine seismic reflection data from a seismic tow system including electrical controls on a recording boat for towing the paravane and geophone streamer cable are disclosed.
Two modifications of an offshore maring seismic energy source tow system are also disclosed. Both systems com-prise transmitters and receivers therein for continuously measuring the distance between a vessel towed submerged geophone streamer cable and a towed seismic air gun support-ing paravane on each side of the streamer cable for generating and transmitting corrective steering signals to the paravane for maintaining each seismic air gun spaced from the towed submerged geophones at an exact predetermined distance for producing more accurate seismic velocity measurements. One embodiment has the acoustic transmitter on the geophone streamer cable and an accoustic receiver on each paravane in line with the transmitter, and a second embodiment has an acoustic transmitter on each paravane and a corresponding accoustic receiver for each paravane on the geophone streamer cable transversely of the transmitter. Methods for assem-bling the above two systems are disclosed.

Description

3~3's~ t;9 B KGROUND OF THE INVENTION
The search for hydrocarbons is being pursued on a worldwide scale which includes most of the po-tentially prospective, water-covered, sedimentary basins. Many of these water covered areas represent relatively new, unexplored basins where little is now known about the sedimentary section. The present state of seismic technology permits extraction of stacking velocities from seismic reflection data which, wh~n converted to interval velocities, can under certain conditions be related in a gross way to rock types. In other words, there are occasions when it is possible to associate seismic velocities with the rock types which make up the sedimentary section. However, the accuracy of these velocity measurements is critically dependent upon a number of factors, some of which are listed as follows:
1. Seismic reflection data quality.

2. High signal-to-noise ratio.

3. Know water bottom geometry topography.

4. Subsurface or below bottom structural geometry.

5. Statics or maintaining the geophones in a horizontal plane.

6. Accuracy o To i~e. the instant the acoustical energy was released measurements derived from fitting hyperbolae to the reflection data.
The present procedure for recording offshore marine seismic reflection data includes a seismic source towed along side of or immediately behind the recording boat and a streamer geophone`cable which is usually a mile or more in length, made up of 24 to 96 spaced geophone groups and with the geophone group located nearest the boat -~
positioned 200 to 500 meters behind the towed seismic source. This arrangement introduces a significant in-line offset between source -~
to first receiver. When recording reflection data this offset can ;`~

produce a degree of uncertainty in -the determined To's used in the velocity calculations. Furthermore, towing the seismic source -' :.
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and geophone cable close to the boat introduces undesirable source-generat~d noise and boat-generated noise in the seismic data, particularly on those critically located, short range geophone groups.
An offshore seismic exploration method is disclosed in U.S. Patent ~o. 3,774,021, involving simultaneous running of a dPep-reflection profile and a shallow-reflection profile without substantial interference of one with the other. Two weighting devices are illustrated for positioning a string of geophones on a cable in U~S. Patent No. 3,187,831. A
method of seismic exploration utilizing paravanes which will hold a preset depth is disclosed in U.S. Patent No. 3,331,050.
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A primary object of this invention is to provide a marine seismic source tow system that maintains a submerged seismic air gun spaced transversely of a submerged geophone streamer cable at a controlled predetermined distance, to permit production of more accurate seismic velocity measure-ments.
- According to the present invention there is provided a method of obtaining marine seismic data comprising towing behind a tow vessel both a submerged streamer cable and a steerable paravane laterally spaced from said cable, said streamer cable having at least one geophone thereon and said paravane supporting a seismic energy source, said source and said geophone being operable to obtain said data, generating ,::
a signal representati~e of the distance between the paravane ::
and a transducer associated with said cable by transmitting ~ -and receiving a signal between one and the other, generating .
from said distance signal a steering control signal, and :
. 30 applying said steering control signal to a steering element : ~
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of said paravane to tend to maintain said lateral distance at a predetermined value.
In another aspect, the invention provides apparatus for ohtaining marinP seismic data comprising a tow vessel, a streamer cable having at least one geophone thereon and adap~ed for towing by said vessel, a steerable paravane sup-porting a se~smic energy source and adapted for towing by said vessel laterally spaced from said cable, said source and said geophone being operative to obtain said data, means for generating a signal representative of the distance between said paravane and a transducer associated with said cable by transmitting and receiving a signàl between one and the other, means for generating from said distance signal a steering control signal, and means for applying said steering control signal to a steering element of said paravane to tend to maintain said lateral distance at a predetermined value.
Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Fig. 1 is a schematic plan view of one embodiment of the offshore marine seismic source tow system;
Fig. 2 is a side view of the embodiment of Fig. l;
Fig. 3 is a schematic plan view of the tow boat in the em~odiment of Fig. l;
Fig. 4 Ls a schematic plan ~iew of the steerable para- ;~
vane of Flg. 1:
Fig. 5 is a schematic side vie~ of the geophone streamer cable of Fig. l;
Fig. 6 is a schematic plan view of a second embodiment of the offshore marine seismic source tow system;

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Fig. 7 is a schematic plan view of the steerable para-vane of the modification of Fig. 6; and Fig. 8 is a schematic side view of geophone streamer cable of the modification of Fig. 6.
Referring to the drawings, Fig. 1 is a top view and Fig. 2 is a side view of a boat 10 pulling a s~bmerged geo-phone streamer cable 11 having groups o geophones 12 to 17 thereon. Two paravane tow lines 18 and 19 each has a steer-able paravane 20 and 21, respectively, attached thereto~
Each paravane has one or more air guns suspended therefrom, such as the three air guns 22 illustrated as being suspended from paravane 21. Any suitable seismic energy source or air gun 22 may be utilized, such as but not limited to one of those disclosed in U.S. Patent No. 3,923,122.
The illustrat~d marine seismic source tow system includes means for maintaining the air guns spaced from the geophone groups at a substantially constant predetermined distance for producing more accurate seismic velocity measure-ments. This means comprises basically a transmitter, a receiver, and electrical interconnections on the boat 10 for continually measuring the lateral or transverse distance of separation between the air guns 22 or the paravanes 20 and 21 and the geophone groups 12-17 or streamer cable 11 and send-ing steering signals to the steerable paravanes to maintain each para~ane at its predetermined distance of separation. -~
The geophone groups are maintained at a predetermined depth by towing them through the water at a predetermined speed, the geophones themselves being weighted and balanced to have the same speci~ic gravity as the water they are immersed and towed in. Further, the geophone cable may be maintained at , , , .: .. ,; ., . , :

the predetermined depth with a conventional submerged para-vane that maintains a preset depth.
The problem solved here is to maintain the lateral distance between the air guns and geophones at a desired predetermined value for producing accurate seismic velocity measurements.
Referring now to Fig. 3, an existing conventional deep seismic system sequence timer 24 at the back of the boat 10 is connected to transmit input signals to a clock and count-down circuit 25. The circuit 25 is connected to trans-mit a 1 Khz (kilo-hertz-) clock siynal to a two-stage BCD
(binary coded decimal) counter 26, a 50 Hz signal to a D.C.
to pulse width converter 27, and a 2 Hz trigger pulse to a 1 ms (millisecond) delay pulse generator 28, to the reset . :.
input of the two-stage BCD counter 26, and to the reset input of flip-flop switch 31, and the reset input of flip-flop 30. :
The output of the 1 ms delay pulse generator 28 connects to ~ .
an acoustical transmitter or pinger dr~ver circuit 32 and to the start input of the BCD counter 26. The transmitter 20 . driver circuit 32 has an electrical connection through the streamer or geophone lead cable 11 to an acoustical trans-mitter 33 thereon, Fig. 5. The acoustical txansmitter 33 is acoustically coupled through the water to the acoustic receiver transducer 34, Fig. 4, mounted on the left paravane ::
20 for examplff.
A differential dri~er amplifier 35, Fig. 3, on the boat sends a signal to a paravane mounted differential driver amplifier 36, Fig. 4. The acoustical receiver transducer 34 . is connected to a pre-amplifier 37 which is electrically connected ~o a band pass filter 38. The latter filter 38 ., . ;.

is connected to an amplifier/detector 39 which is connected to a 5 ms pulse generator 40. The pulse generator 40 is con-nected to a differential driver amplifier 41 which in turn is connerted through the paravane tow line 18 to a shipboard differential receiver amplifier 42.
The latter differential receiver amplifier 42, Fig. 3, is connected to a 5 ms pulse generator 43 which is connected to both the stop-input of the counter 26 and one input of an . OR gate 44. The latter gate 44 is connected to a set input of flip-flop 31. The two-stage counter 26, Fig. 3, connects its units and tens outputs, 1-8 and 1-4, respectively, to seven resistors, the valves of which are lR (resistance), 2R, 4R, 8R, 16R, 32R and 64R. An 8 output o counter 26 is con- .:
nected to a set input of flip-flop 30, and to one input of .:.
the OR gate 44. Flip-flop 30 has both a Q output connected to a control input of electronic switch 45 and a Q output connected to a control input of ele~tronic switch 46. Flip-flop 31 Q output is connected to a control input of elec-: tronic switch 47. The seven resistors of two-stage BCD
counter 26 are connect.ed in parallel to an input of the electronic switch 45, while the output of latter switch 45 is connected to both an input of the electronic switch 46 .;
and to one input of a voltage comparator 48. The output of ~ .
electronic switch 46 is connected both to one input of the voltaga comparator 48 and to a junction o~ seven resistors 49~ ~le values o which are RIa, R2a, R4a~ R8a, R16a, R32a.
and R64a. These seven resistors 49, Fig. 3, are connected to a "distance-from-transmitter" switch 50. This latter switch 50 is connected to an electrical voltage source 51.
The output of voltage comparator 48, Fig. 3, is connected ~8~'7~

to an input of electronic switch 47, and the output of the latt~r switch 4~ is connected to both a capacitor C or 52 and to a control input of the D.C.-to-pulse width converter 27. The other lead of capacitor 52 is connected to a ground or logic common~ The output of the D,C.-to-pulse width converter 27 is connected to an input of differential driver amplifier (D.D.A.) 35.
The output from D.D.A. 35, Fig. 3, i5 connected through the paravane tow line or cable 18 to the input of the differ-ential receiver amplifier (D.R.A.) 36, Fig. 4. The output of the D.R.A. 3~ is connected to both a control input of a variable pulse generator 53 and to one of the inputs of a pulse width comparator 54. The output of the ~ariable pulse -generator 53 is connected to the other input of the pulse width comparator 54. The pulse width control input of the variable pulse generator 53 is connected to a feed back potentiometer 55. This potentiometer 55 is mechanically : .
linked to a servo motor 57. The output of the pulse width compara.tor 54 is connected to a servo motor control circuit 56 which in turn is also connected to the servo motor 57.
The servo motor 57 is mechanically linked to the feed back potentiometer 55 and paravane steering vane mechanism 58 for controlling lateral movement of the paravane in the correct direction. . :.
The clock and count-down circuit tC C C.) 25~ Fig. 3, provides the timing for the entire electronics of this system. The C.C.C. provides a 1 kHz clock signal for the ::.
BCD counter 26 and a S0 Hz cloc~ signal for the D.C. to ; pulse width converter 27. The C.C.C. 25 also provides a 2 30. Hz trigger pulse to reset the BCD counter 26 to zero, and a

-8-reset for flip-flops 30 and 31, respectively, at the ~eginn-ing of the pinging or transmitting cycle,. Flip-flop 30 then sets electronic switch 45 closed and electronic switch 46 open. Flip-flop 31 sets electronic switch 47 open.
Capacitor C or 52 will retain its present change for approx- ', imately 100 ms or so. The 2 Hz trigger pulse is inhibited during a deep seismic record by that system's sequence timer, such as, but not limited to the sequence timer disclosed in ~ U.S. Patent No. Z,849~211.
The trigger pulse from clock circuit 25, Fig. 3, also activates the delay pulse ~enerator 28 which sends a 1 ms pulse ~o start the BCD counter 26 counting in 1 ms steps.
Also, the pinger or transmitter driver circuit 32 is activated which in turn sends a burst of 50 kHz signal to the pinger transmitter 33 mounted on the geophone streamer cable 11.
The acoustical or pinger transmitter 33, Fig. 5, emits acoustical energy into the water which is picked up by the receiver transducer 34, Fig. 4, mounted on one of the para~
vanes 20. The receiver transducer may be one of many crystalline types available commercially. The short burst of acoustic energy is converted by the receiver transmitter 34 into an electrical signal and passed on to the pre-amplifier 37. The pxe-amplifier increases the signal level sufficiently and then passes it on to the band pass filter 38 where all unwanted frequencies above and below the acous-tical or pinger frequency are rejected. From here the signal , goes to the amplifier and detector circuit 39 where high , frequency bursts of signal is convexted into a D.C. pulse representing the envelope of the burst. From here this D.C.
pulse is converted to a 5 ms pulse by the pulse generator 40 and then sent to the differential driver amplifier (D. D .A. ) 41. D.D.A.'s are used in this system to give high common mode rejection over the long wire lengths involved. The 5 ms signal is sent from the paravane mounted D.D.A. 41, Fig. 4, to the shipboard D.R.A. (differential receiver amplifier) 42, Fig. 3, via the paravane tow cable 18.
The D.R.A. 42, FigO 3, is part of the common mode rejection scheme used here. From the D.R.A. the pulse is reshaped by the 5 ms pulse generator 43 and applied to the stop input of the BCD counter 26. With the BCD counter now stopped a voltage proportional to the distance between the receiver transducer and the pinger transmitter will appear at the junction of the ladder resistor network connected to the units and tens outputs of the counter 26. This voltage will be used as one side of the voltage comparator 48 to determine whether or not the paravane is in the correct position. The 5 ms pulse generator 43 also sets flip-flop 31 through the -OR gate 44. This in turn sets electronic switch 47 closed.
Under normal operating conditions the voltage compar- , -ator 48 compares the BCD counter ladder ~oltage with that set in by the "distance from pinger" selection switch 50.
The selection switch 50 has a comparable ladder network and the switch has BCD outputs similar to the BCD counter. The switch would be calibrated in feet'rather than in time.
' In the event the,acoustical pulse from the paravane is not received, the 8 count from the tens output (128 ms) of the BCD counter 26, Fig. 3, will set ~lip-flops 30 and 31.
Flip-flop 30 in turn opens electronic switch 45y removing the ladder network from the voltage comparator 48 input. - ' Flip-flop 30 also closes electronic switch 46, thereby ~ ., "',:

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connecting the two inputs of the voltage comparator together.
With the inputs connected together the voltage comparator 48 functions as if the paravane is in the correct position and controls it accordingly.
The output of the voltage comparator 48, Fig. 3, goes to electronic switch 47 which is now closed. The output volt-age is impressed on capacitor C or 52 and the input of the D.C. to pulse width converter 27. The capacitor C serves as a filter and holds up the voltage while electronic switch 47 is opened momentarily between reset and the recei~ing of a pulse from the paravane electronics. The D.C. to pulse width converter 27 does just that and is clocked at a 50 Hz `
rate by the C.C.C. 25. The D.D.A. 35 then transfers the pu}se train down the paxavane tow cable assembly 18, Fig. 1, to the paravane mounted D.R.A. 36, Fig. 4~ The D.R.A. 36 on paravane 20 sends the pulse on to the variable pulse generator 53 and the pulse width comparator 54. The variable pulse generator 53 pulse width is controlled by the feed back potentiometer 55 which is mechanically linked to the steering servo motor 57. The pulse width comparator 54 now compares the two pulse widths and provides a correction signal to the servo motor control circuit 56. $he servo motor control cir-cuit in turn controls the servo motor 57.
It may be noted that the two-stage BCD counter 26, Fig. 5, could be a three-stage unit, i.e. tenths, units, and tens with a clock frequency of 10 kHzo The "distance from pinger" selection switch 50 would likewise need expanding.
This change would increase resolution from 5 feet to 0.5 feet.
Also thexe will be of necessity a second paravane ~
control circuit (not shown) if two paravanes are used. The -':".

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2 Hz, 50 Hz and l k~z signals from the C.C.C. 25, Fig. 3, will bP used by this second control circuit. This second control circuit will consist of everything contained in Figs.
3-5 with the deletion of the C . C . C ., the 1 ms delay pulse generator, the driver circuit, and the pinger transmitter.
SECOND EMBODI~ENT OF FIGS. 6 8 Figs. 6-8 de~ict a modified offshore marine seismic source tow system wherein the acoustical pinger transmitter 33a, Fig. 7, for carrying out the above described methods is mounted on the paravane 20a and the receiver 34a, Fig. 8, and its associated electronics would be mounted on or in the streamer cable lla. The connections and the operation in this case would be similar to that of the embodiment of Figs.
1-6 with the exception of the second 2 Hz trigger pulse from the clock count down circuit 25a, Fig. 6. This trigger pulse for the modified paravane control circuit 25a is 180 out of phase with the first modification control circuit. This allows the two transmitters 33a, Fig. 8 and 33b (not shown) on the two respective paravanes (only the left paravane 20a being shown) to alternately operate and use the same receiver transducer 34a to stop their respective control circuits.
For a two paravane operation everything in Figs. 5 and 7 is duplicated for the second paravane with the exception of the clock count down circuit (C.C.C.) 25a, Fig. 6, all the streamer mounted components on Fiy. 8, the shipboard differ-ential receiver amplifier ~D.R.A.) 42a, Fig. 6, and the shipboard 5 ms pulse generator 43a, Fig. 6. -;
Accordingly, in the embodiment of Figs. 6-8, the acoustical transmitters 33 are mounted in the para~anes 20, 21 instead of in the geophone streamer cable 11, as in the '''' : ~ '

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first embodiment of Figs. 1-5. Two advantages of this particular mounting are (1) the paravanes provide more room for ease of mounting of the acoustical transmitters therein and (2) there is less detrimental coupling of the trans-mitted electrical signal into the geophone streamer cable by moving the acoustical transmitters from the geophone streamer cable to the paravanPs~
Obviously other methods may be utilized for forming the embodiments of either Figs. 1-5 or Figs. 6-8 than those listed above, depending on the particular design parameters desired or the different rock types which make up the sedimentary section.
A feature of both embodiments is that the geophones 12-17 may be towed or trailed far behind to thereby avoid interference generated by the boat.

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Claims (21)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A method of obtaining marine seismic data compris-ing towing behind a tow vessel both a submerged streamer cable and a paravane laterally spaced from said cable, said streamer cable having at least one geophone thereon and said paravane supporting a seismic energy source, said source and said geophone being operable to obtain said data, character-ised by: generating a signal representative of the distance between the paravane and a transducer associated with said cable by transmitting and receiving a signal between one and the other, generating from said distance signal a steering control signal, said paravane having a controllable steering element, and applying said steering control signal to said steering element to tend to maintain said lateral distance at a predetermined value.

2. A method as claimed in claim 1, characterised in that said distance signal is generated by transmitting and receiving an acoustic signal between one and the other of said paravane and said transducer.

3. A method as claimed in claim 1 or claim 2, characterised in that said cable has a plurality of groups of said geophones.

4. A method as claimed in any one of claims 1 to 3, characterised in that said seismic energy source comprises a seismic air gun.

5. A method as claimed in any one of claims 1 to 4, characterised in that said step of generating a steering con-trol signal is effected on said tow vessel, and said steering control signal is then transmitted through a cable to said paravane.

6. A method as claimed in any one of claims 1 to 5, characterised in that said distance signal is generated by.
transmitting a signal from said transducer and receiving said signal at said paravane.

7. A method as claimed in claim 6, characterised in that said distance signal is generated by timing a signal pulse transmitted from control means in said tow vessel through said cable to said transducer, through the water to said paravane and back through a paravane cable to said con-trol means.

- 8. A method as claimed in any one of claims 1 to 5, characterised in that said distance signal is generated by transmitting a signal from said paravane and receiving said signal at said transducer.

9. A method as claimed in claim 8, characterised in that said distance signal is generated by timing a signal pulse transmitted from control means in said tow vessel through a paravane cable to said paravane, through the water to said transducer and back through said streamer cable to said control means.

10. A method as claimed in any one of claims 1 to 9, characterised by inhibiting said distance signal generation during operation of said source and said geophone to obtain said data.

11. Apparatus for obtaining marine seismic data com-prising a tow vessel, a streamer cable having at least one geophone thereon and adapted for towing by said vessel a paravane supporting a seismic energy source and adapted for towing by said vessel laterally spaced from said cable, said source and said geophone being operative to obtain said data, characterised by: means (26,32,33,34,41,42) for generating a signal representative of the distance between said paravane (20) and a transducer (33) associated with said cable (11) by transmitting and receiving a signal between one and the other, means (48) for generating from said distance signal a steering control signal, said paravane having a controll-able steering element (58), and means (35,36,57) for apply-ing said steering control signal to said steering element (58) to tend to maintain said lateral distance at a pre-determined value.

12. Apparatus as claimed in claim 11, characterised in that said distance signal generating means (26,32,33,34, 41,42) comprises means (33,34;33a;34a) for transmitting and receiving an acoustic signal between one and the other of said paravane and said transducer.

13. Apparatus as claimed in claim 11 or claim 12, characterised in that said cable has a plurality of groups (12-17) of said geophones.

14. Apparatus as claimed in any one of claims 11 to 13, characterised in that said seismic energy source comprises a seismic air gun (22).

15. Apparatus as claimed in any one of claims 11 to 14, characterised in that said steering control signal generating means (48) is disposed in said tow vessel (10), and including means (35,36) for transmitting said steering control signal from said vessel to said paravane.

16. Apparatus as claimed in any one of claims 11 to 15, characterised in that said distance signal generating means (26,32,33,34,41,42) comprises means (32) for trans-mitting a signal from said transducer (33) and means (34) for receiving said signal carried by said paravane (20).

17. Apparatus as claimed in claim 16, characterised in that said distance signal generating means includes a signal pulse timer (26) on said vessel (10) for timing a signal pulse passing from said timer (26) through said cable (11) to said transducer (33), through the water to said paravane (20), and back through a paravane cable (18) to the pulse timer (26).

18. Apparatus as claimed in any one of claims 11 to 15, characterised in that said distance signal generating means (26,32,33,34,41,42) comprises means (33a) for trans-mitting a signal carried by said paravane (20) and means (34a) for receiving said signal at said transducer (34a).

19. Apparatus as claimed in claim 18, characterised in that said distance signal generating means includes a signal pulse timer (26) on said vessel (10) for timing a signal pulse passing from said timer (26) through a paravane cable (18a) to said paravane transmitting means (33a), through the water to said transducer receiving means (34a), and back through said streamer cable (11a) to the pulse timer (26).

20. Apparatus as claimed in any one of claims 11 to 19, characterised by including means (24,25) for inhibiting said distance signal generating means during operation of said source (22) and said geophone (12-17) to obtain said data.

21. Apparatus for controlling the lateral position of a paravane relative to a submerged streamer cable, said para-vane supporting a seismic energy source and said cable having at least one geophone, said paravane and said cable both being towed by a vessel, said apparatus comprising means (26, 32,33,34,41,42) for generating a signal representative of the distance between said paravane (20) and a transducer (33) associated with said cable (11) by transmitting and receiving a signal between one and the other, means (48) for generating from said distance signal a steering control signal, said paravane having a controllable steering element (58), and means (35,36,57) for applying said steering control signal to said steering element (58) to tend to maintain said lateral distance at a predetermined value.