Note: Descriptions are shown in the official language in which they were submitted.
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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
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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
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. 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
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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
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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
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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
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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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