U.S. patent application number 15/875547 was filed with the patent office on 2018-05-24 for methods for controlling towed marine sensor array geometry.
The applicant listed for this patent is PGS Geophysical AS. Invention is credited to Oyvind Hillesund, Nils Lunde.
Application Number | 20180143339 15/875547 |
Document ID | / |
Family ID | 41200998 |
Filed Date | 2018-05-24 |
United States Patent
Application |
20180143339 |
Kind Code |
A1 |
Hillesund; Oyvind ; et
al. |
May 24, 2018 |
METHODS FOR CONTROLLING TOWED MARINE SENSOR ARRAY GEOMETRY
Abstract
A method for towing a streamer array includes moving a vessel
along a body of water. Streamers are towed by vessel. A relative
position is determined at selected points along each streamer with
respect to the vessel. At least one of the streamers is deflected
at at least one longitudinal position along the streamer in
response to the determined positions to maintain the streamers in a
selected geometry. The selected geometry is related to one of
survey vessel heading, energy source trajectory, previously plotted
sensor trajectory and a lateral separation related to distance from
the towing vessel.
Inventors: |
Hillesund; Oyvind; (Nesbru,
NO) ; Lunde; Nils; (Houston, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
PGS Geophysical AS |
Oslo |
|
NO |
|
|
Family ID: |
41200998 |
Appl. No.: |
15/875547 |
Filed: |
January 19, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14605599 |
Jan 26, 2015 |
9903970 |
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15875547 |
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12409694 |
Mar 24, 2009 |
8976622 |
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14605599 |
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12148610 |
Apr 21, 2008 |
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12409694 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01V 1/3826 20130101;
G01V 1/3835 20130101; G01V 1/3817 20130101 |
International
Class: |
G01V 1/38 20060101
G01V001/38 |
Claims
1-18. (canceled)
19. A method, comprising: towing a plurality of sensor streamers
behind a survey vessel in a body of water; accessing information
that specifies a predetermined geodetic path; determining relative
positions at selected points along ones of the sensor streamers
with respect to one or more adjacent streamers; determining
geodetic position of one or more points along one or more of the
sensor streamers; and laterally deflecting at least one sensor
streamer at a plurality of positions based on the predetermined
geodetic path, the relative positions, and the determined geodetic
position of the one or more points, such that the at least one
sensor streamer follows the predetermined geodetic path.
20. The method of claim 19, wherein the predetermined geodetic path
is determined based on geodetic paths of one or more sensor
streamers in a prior survey pass.
21. The method of claim 19, wherein the determining relative
positions is performed using acoustic ranging via acoustic sensors
disposed on ones of the sensor streamers.
22. The method of claim 19, wherein the determining geodetic
position is performed based on global positioning system (GPS)
data.
23. The method of claim 19, further comprising: deflecting at least
one sensor streamer in a vertical direction based on the
predetermined geodetic path, the relative positions, and the
determined geodetic position of the one or more points such that
the at least one sensor streamer follows the predetermined geodetic
path.
24. The method of claim 19, further comprising: receiving sensor
data from the plurality of sensor streamers; and storing the sensor
data on a tangible, computer-readable medium.
25. The method of claim 19, wherein the laterally deflecting is
performed using selected wing angles for ones of a plurality of
streamer control devices.
26. A non-transitory computer-readable medium having instructions
stored thereon that in response to execution by a computing system,
cause the computing system to carry out operations comprising:
towing a plurality of sensor streamers behind a survey vessel in a
body of water; accessing information that specifies a predetermined
geodetic path; determining relative positions at selected points
along ones of the sensor streamers with respect to one or more
adjacent streamers; determining geodetic position of one or more
points along one or more of the sensor streamers; and laterally
deflecting at least one sensor streamer at a plurality of positions
based on the predetermined geodetic path, the relative positions,
and the determined geodetic position of the one or more points,
such that the at least one sensor streamer follows the
predetermined geodetic path.
27. The non-transitory computer-readable medium of claim 26,
wherein the predetermined geodetic path corresponds to
previously-recorded geodetic paths of one or more sensor streamers
in a prior survey pass.
28. The non-transitory computer-readable medium of claim 26,
wherein the determining relative positions is performed using
acoustic ranging.
29. The non-transitory computer-readable medium of claim 26,
wherein the determining geodetic position is performed based on
data from a satellite navigation system.
30. The non-transitory computer-readable medium of claim 26,
wherein the operations further comprise: deflecting at least one
sensor streamer in a vertical direction based on the predetermined
geodetic path, the relative positions, and the determined geodetic
position of the one or more points such that the at least one
sensor streamer follows the predetermined geodetic path.
31. The non-transitory computer-readable medium of claim 26,
wherein the operations further comprise: receiving sensor data from
the plurality of sensor streamers; and storing the sensor data on a
tangible, computer-readable medium.
32. The non-transitory computer-readable medium of claim 26,
wherein the operations further comprise selecting wing angles for
ones of a plurality of streamer control devices to perform the
laterally deflecting.
33. A system, comprising: a plurality of sensor streamers
configured to be towed behind the survey vessel in a body of water;
one or more storage element configured to store information that
specifies a predetermined geodetic path; survey equipment
configured to: determine relative positions at selected points
along ones of the sensor streamers with respect to one or more
adjacent streamers; determine geodetic position of one or more
points along one or more of the sensor streamers; and laterally
deflect at least one sensor streamer at a plurality of positions
based on the predetermined geodetic path, the relative positions,
and the determined geodetic position of the one or more points,
such that the at least one sensor streamer follows the
predetermined geodetic path.
34. The system of claim 33, wherein the predetermined geodetic path
corresponds to geodetic paths of one or more sensor streamers in a
prior survey pass.
35. The system of claim 33, wherein the survey equipment is
configured to determine geodetic position based on global
positioning system (GPS) data.
36. The system of claim 33, wherein the survey equipment is
configured to determine relative positions using acoustic ranging
by acoustic sensors disposed on ones of the sensor streamers.
37. The system of claim 33, wherein the survey equipment is
configured to: receive sensor data from the plurality of sensor
streamers; and store the sensor data on a tangible,
computer-readable medium.
38. The system of claim 33, wherein the survey equipment is
configured to: selecting angles for ones of a plurality of streamer
control devices to laterally deflect the at least one sensor
streamer.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a Continuation-In-Part of U.S.
patent application Ser. No. 12/148,610 filed on Apr. 21, 2008.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
BACKGROUND OF THE INVENTION
Field of the Invention
[0003] The invention relates generally to the field of marine
geophysical surveying. More particularly, the invention relates to
methods for controlling the spatial distribution or geometry of an
array of geophysical sensor streamers towed behind a survey
vessel.
Background Art
[0004] Marine geophysical surveying systems such as seismic
acquisition systems and electromagnetic survey systems are used to
acquire geophysical data from formations disposed below the bottom
of a body of water, such as a lake or the ocean. Marine seismic
surveying systems, for example, typically include a seismic survey
vessel having onboard navigation, seismic energy source control,
and geophysical data recording equipment. The seismic survey vessel
is typically configured to tow one or more (typically a plurality)
of laterally spaced apart sensor streamers through the water. At
selected times, the seismic energy source control equipment causes
one or more seismic energy sources (which may be towed in the water
by the seismic vessel or by another vessel) to actuate. Signals
generated by various sensors on the one or more streamers in
response to detected seismic energy are ultimately conducted to the
recording equipment. A record with respect to time is made in the
recording system of the signals generated by each sensor (or groups
of such sensors). The recorded signals are later interpreted to
infer the structure and composition of the formations below the
bottom of the body of water. Corresponding components for inducing
electromagnetic fields and detecting electromagnetic phenomena
originating in the subsurface in response to such imparted fields
may be used in marine electromagnetic geophysical survey
systems.
[0005] The one or more sensor streamers are in the most general
sense long cables that have geophysical sensors disposed at spaced
apart positions along the length of the cables. A typical streamer
can extend behind the geophysical survey vessel for several
kilometers. Because of the great length of the typical streamer,
the streamer may not travel entirely in a straight line behind the
survey vessel at every point along its length due to interaction of
the streamer with the water and currents in the water, among other
factors.
[0006] Streamers towed by a vessel configured for towing multiple
streamers are associated with equipment that maintain the forward
ends of the streamers at selected lateral distances from each other
and from the centerline of the survey vessel as they are towed
through the water. Such multiple streamer systems are used in what
are known as three dimensional and four dimensional geophysical
surveys. A four dimensional seismic survey is a three dimensional
survey over a same area of the Earth's subsurface repeated at
selected times. The individual streamers in such systems are
affected by the same forces that affect a single streamer.
[0007] The quality of geophysical images of the Earth's subsurface
produced from three dimensional surveys is affected by how well the
positions of the individual sensors on the streamers are
controlled. The quality of images generated from the detected
signals also depends to an extent on the relative positions of the
sensors being maintained throughout the geophysical survey. Various
devices are known in the art for positioning streamers laterally
and/or at a selected depth below the water surface. U.S. Pat. No.
5,443,027 issued to Owsley et al., for example, describes a lateral
force device for displacing a towed underwater acoustic cable that
provides displacement in the horizontal and vertical directions.
The device has a hollow spool and a rotationally mounted winged
fuselage. The hollow spool is mounted on a cable with cable
elements passing therethrough. The winged fuselage is made with the
top half relatively positively buoyant and the bottom half
relatively negatively buoyant. The winged fuselage is mounted about
the hollow spool with clearance to allow rotation of the winged
fuselage. The difference in buoyancy between the upper and lower
fuselage maintains the device in the correct operating position.
Wings on the fuselage are angled to provide lift in the desired
direction as the winged fuselage is towed through the water. The
device disclosed in the Owsley et al. patent provides no active
control of direction or depth of the streamer, however.
[0008] U.S. Pat. No. 6,011,752 issued to Ambs et al. describes a
seismic streamer position control module having a body with a first
end and a second end and a bore therethrough from the first end to
the second end for receiving a seismic streamer therethrough. The
module has at least one control surface, and at least one recess in
which is initially disposed the at least one control surface. The
at least one control surface is movably connected to the body for
movement from and into the at least one recess and for movement,
when extended from the body, for attitude adjustment. Generally,
the device described in the Ambs et al. patent is somewhat larger
diameter, even when closed, than the streamer to which it is
affixed, and such diameter may become an issue when deploying and
retrieving streamers from the water.
[0009] U.S. Pat. No. 6,144,342 issued to Bertheas et al. describes
a method for controlling the navigation of a towed seismic streamer
using "birds" affixable to the exterior of the streamer. The birds
are equipped with variable-incidence wings and are rotatably fixed
onto the streamer. Through a differential action, the wings allow
the birds to be turned about the longitudinal axis of the streamer
so that a hydrodynamic force oriented in any given direction about
the longitudinal axis of the streamer is obtained. Power and
control signals are transmitted between the streamer and the bird
by rotary transformers. The bird is fixed to the streamer by a bore
closed by a cover. The bird can be detached automatically as the
streamer is raised so that the streamer can be wound freely onto a
drum. The disclosed method purportedly allows the full control of
the deformation, immersion and heading of the streamer.
[0010] UK Patent 2,364,388, by Canter et al. discloses a method of
repeating a marine seismic survey of a sub-surface area that has
been previously surveyed.
[0011] U.S. Pat. Nos. 6,932,017; 7,080,607; 7,222,579 and 7,293,520
(all based directly or indirectly on PCT Application
PCT/IB99/01590) describe various aspects of a control system for
positioning of marine seismic streamers.
SUMMARY OF THE INVENTION
[0012] A method for towing a sensor streamer array in a body of
water according to one aspect of the invention includes towing a
plurality of sensor streamers behind a survey vessel in the water.
A relative position is determined at selected points along each
streamer with respect to the vessel. At least one streamer is
deflected at at least one longitudinal position therealong in
response to the determined positions such that the streamers are
maintained in a selected geometry in the water, in which positions
along each streamer substantially follow a geodetic heading of the
survey vessel.
[0013] A method for towing a sensor streamer array in a body of
water according to another aspect of the invention includes towing
a plurality of sensor streamers behind a survey vessel. A relative
position is determined at selected points along each streamer with
respect to the vessel. At least one streamer is deflected at at
least one longitudinal position therealong in response to the
determined positions such that the streamers are maintained in a
selected geometry in the water, in which determined positions along
each streamer substantially follow a geodetic trajectory of a
geophysical energy source towed in the water.
[0014] A method for towing a sensor streamer array in a body of
water according to another aspect of the invention includes towing
a plurality of sensor streamers behind a survey vessel. A relative
position is determined at selected points along each streamer with
respect to the vessel. At least one streamer is deflected at at
least one longitudinal position therealong in response to the
determined positions such that the streamers are maintained in a
selected geometry in the water, in which determined positions along
each streamer substantially follow a selected geodetic path.
[0015] A method for towing a sensor streamer array in a body of
water according to another aspect of the invention includes towing
a plurality of sensor streamers behind a survey vessel. A lateral
deflecting force is applied at selected positions along each
streamer. The lateral deflecting force at each selected position is
proportional to a lateral distance of a forward end of each
streamer from a center line of the survey vessel.
[0016] Other aspects and advantages of the invention will be
apparent from the following description and the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 shows an array of seismic streamers each including
lateral force and depth control devices for adjusting geometry of
the respective streamer.
[0018] FIGS. 2 through 4 schematically show various examples of
controlling array geometry according to the invention.
DETAILED DESCRIPTION
[0019] FIG. 1 shows a typical marine geophysical survey system that
can include a plurality of sensor streamers. Each of the sensor
streamers can be guided through the water by one or more lateral
force and depth ("LFD") control devices cooperatively engaged with
each of the streamers. As will be explained further below, the use
of LFD devices, which provide depth control capability in addition
to horizontal position control, is a matter of choice. It is only
necessary for purposes of the invention that the devices associated
with the sensor streamers provide directional control, that is, to
affect the direction of the streamer parallel to the plane of the
water surface as it moves through the water. The geophysical survey
system includes a survey vessel 10 that moves along the surface of
a body of water 11 such as a lake or the ocean. The survey vessel
10 may include thereon equipment, shown generally at 12 and for
convenience collectively referred to as a "recording system." The
recording system 12 typically includes devices such as a data
recording unit (not shown separately) for making a record with
respect to time of signals generated by various sensors in the
acquisition system. The recording system 12 also typically includes
navigation equipment (not shown separately) to determine and
record, at selected times, the geodetic position of the vessel 10,
and using other devices to be explained below, each of a plurality
of geophysical sensors 22 disposed at spaced apart locations on
streamers 20 towed by the survey vessel 10.
[0020] In one example, the device for determining the geodetic
position may be a geodetic position signal receiver 12A such as a
global positioning system ("GPS") receiver, shown schematically at
12A. Other geodetic position determination devices are known in the
art. The foregoing elements of the recording system 12 are familiar
to those skilled in the art, and with the exception of the geodetic
position detecting receiver 12A, are not shown separately in the
figures herein for clarity of the illustration.
[0021] The geophysical sensors 22 can be any type of geophysical
sensor known in the art. Non-limiting examples of such sensors may
include particle motion-responsive seismic sensors such as
geophones and accelerometers, pressure-responsive seismic sensors,
pressure time gradient-responsive seismic sensors, electrodes,
magnetometers, temperature sensors or combinations of the
foregoing. In various implementations of the invention, the
geophysical sensors 22 may measure, for example, seismic or
electromagnetic field energy primarily reflected from or refracted
by various structures in the Earth's subsurface below the bottom of
the water 11 in response to energy imparted into the subsurface by
an energy source 17. Seismic energy, for example, may originate
from a seismic energy source, or an array of such sources, deployed
in the water 11 and towed by the survey vessel 10 or by another
vessel. Electromagnetic energy may be provided by passing electric
current through a wire loop or electrode pair (not shown for
clarity). The energy source (not shown) may be towed in the water
11 by the survey vessel 10 or a different vessel (not shown). The
recording system 12 may also include energy source control
equipment (not shown separately) for selectively operating the
energy source 17.
[0022] In the survey system shown in FIG. 1, there are four sensor
streamers 20 towed by the survey vessel 10. The number of sensor
streamers shown in FIG. 1, however, is only for purposes of
explaining the invention and is not a limitation on the number of
streamers that may be used in any particular geophysical survey
system according to the invention. As explained in the Background
section herein, in marine geophysical acquisition systems such as
shown in FIG. 1 that include a plurality of laterally spaced apart
streamers, the streamers 20 are typically coupled to towing
equipment that secures the forward end of each of the streamers 20
at a selected lateral position with respect to adjacent streamers
and with respect to the seismic vessel 10. As shown in FIG. 1, the
towing equipment can include two paravane tow ropes 8 each coupled
to the vessel 10 at one end through a winch 19 or similar spooling
device that enables changing the deployed length of each paravane
tow rope 8. The distal end of each paravane tow rope 8 is
functionally coupled to a paravane 14. The paravanes 14 are each
shaped to provide a lateral component of motion to the various
towing components deployed in the water 11 when the paravanes 14
are moved through the water 11. "Lateral" in the present context
means transverse to the direction of motion of the survey vessel 10
in the water 11. The lateral motion component of each paravane 14
is opposed to that of the other paravane 14. The combined lateral
motion component of the paravanes 14 separates the paravanes 14
from each other until they put into tension one or more spreader
ropes or cables 24, functionally coupled end to end between the
paravanes 14.
[0023] The sensor streamers 20 can each be coupled, at the axial
end thereof nearest the vessel 10 (the "forward end"), to a
respective lead-in cable termination 20A. The lead-in cable
terminations 20A can be coupled to or associated with the spreader
ropes or cables 24 so as to fix the lateral positions of the
streamers 20 with respect to each other and with respect to the
centerline of the vessel 10. Electrical and/or optical connection
between the appropriate components in the recording system 12 and,
ultimately, the geophysical sensors 22 (and/or other circuitry) in
the ones of the streamers 20 inward of the lateral edges of the
system may be made using inner lead-in cables 18, each of which
terminates in a respective lead-in cable termination 20A. A lead-in
termination 20A is disposed at the forward end of each streamer 20.
Corresponding electrical and/or optical connection between the
appropriate components of the recording system 12 and the sensors
22 in the laterally outermost streamers 20 may be made through
respective lead-in terminations 20A, using outermost lead-in cables
16. Each of the inner lead-in cables 18 and outermost lead-in
cables 16 may be deployed by a respective winch 19 or similar
spooling device such that the deployed length of each cable 16, 18
can be changed. The type of towing equipment coupled to the forward
end of each streamer shown in FIG. 1 is only intended to illustrate
a type of equipment that can tow an array of laterally spaced apart
streamers in the water. Other towing structures may be used in
other examples of geophysical acquisition system according to the
invention.
[0024] The acquisition system shown in FIG. 1 can also include a
plurality of lateral force and depth ("LFD") control devices 26
cooperatively engaged with each of the streamers 20 at selected
positions along each streamer 20. Each LFD control device 26 can
include one or more rotatable control surfaces (not shown
separately) that when moved to a selected rotary orientation with
respect to the direction of movement of such surfaces through the
water 11 creates a hydrodynamic lift in a selected direction to
urge the streamer 20 in any selected direction upward or downward
in the water 11 or laterally along the water surface with respect
to the direction of motion of the vessel 10. Thus, such LFD control
devices 26 can be used to maintain the streamers in a selected
geometric arrangement. A non-limiting example of LFD device that
may be used in some examples is described in U.S. Patent
Application Publication No. 2008/0008033 by Fossum et al. The
particular configuration of the LFD devices 26, however, is not a
limit on the scope of the present invention. As previously
explained, for purposes of the present invention it is only
necessary for any devices used as the LFD devices 26 be able to
apply a selectable lateral force to the associated streamers 20.
Depth control of the streamers 20 may be provided passively, such
as by providing the streamers 20 with a selected overall specific
gravity, or by separate depth control devices (not shown).
Therefore, any reference to "depth" control as provided by the LFD
devices 26 is only intended to cover the present example
implementation, such as using the device shown in the Fossum et al.
'033 patent application publication referred to above. Any
reference to active depth control of the streamers 20 is not a
limit on the scope of the present invention. For purposes of
defining the scope of the invention, therefore, the LFD devices 26
need only perform the function of "lateral force" control devices,
and the inclusion of depth control as a part of the function of the
LFD devices 26 explained herein is intended to ensure that those of
ordinary skill in the art understand that the use of the example
LFD devices 26 disclosed herein, and any other similar examples,
are within the scope of the present invention.
[0025] In a particular implementation of the invention, a position
determination device may be associated with each LFD device 26. In
one example, the position determination device may be an acoustic
range sensing device ("ARD") 26A. Such ARDs typically include an
ultrasonic transceiver or transmitter and electronic circuitry
configured to cause the transceiver to emit pulses of acoustic
energy. Travel time of the acoustic energy between a transmitter
and a receiver disposed at a spaced apart position such as along
the same streamer and/or on a different streamer, is related to the
distance between the transmitter and a receiver, and the acoustic
velocity of the water. The acoustic velocity can be assumed to not
change substantially during a survey, or it can be measured by a
device such as a water velocity test cell. Alternatively or
additionally, acoustic range sensing devices ("ARDs") may be
disposed at selected positions along each one of the streamers not
collocated with the LFD devices 26. Such ARDs are shown at 23 in
FIG. 1. Each of the ARDs 26A, 23 may be in signal communication
with the recording system 12 such that at any moment in time the
distance between any two ARDs 26A, 23 on any of the streamers 20 is
determinable. One or more ARDs may be placed at selected positions
proximate the aft end of the vessel 10 so that relative distances
between the selected positions on the vessel 10 and any of the ARDs
on the streamers may also be determined. A non-limiting example of
an ARD and system used with such ARDs is described in U.S. Pat. No.
7,376,045 issued to Falkenberg et al. and assigned to the assignee
of the present invention.
[0026] The streamers 20 may additionally or alternatively include a
plurality of heading sensors 29 disposed at spaced apart positions
along each streamer 20. The heading sensors 29 may be geomagnetic
direction sensors such as magnetic compass devices affixed to the
exterior of the streamer 20. One type of compass device is
described in U.S. Pat. No. 4,481,611 issued to Burrage and
incorporated herein by reference. The heading sensors 29 provide a
signal indicative of the heading (direction with respect to
magnetic north) of the streamer 20 at the axial position of the
heading sensor 29 along the respective streamer. Measurements of
such heading at spaced apart locations along each streamer may be
used to interpolate the geometry (spatial distribution) of each
streamer.
[0027] Each streamer 20 may include at the distal end thereof a
tail buoy 25. The tail buoy 25 may include, among other sensing
devices, a geodetic position signal receiver 25A such as a GPS
receiver that can determine the geodetic position of each tail buoy
25. The geodetic position receiver 25A in each tail buoy 25 may be
in signal communication with the recording system 12.
[0028] By determining the distance between ARDs 26A, 23, including
the one or more ARDs on the vessel 10, and/or by interpolating the
spatial distribution of the streamers from the heading sensor 29
measurements, an estimate of the geometry of each streamer 20 may
be made. Collectively, the geometry of the streamers 20 may be
referred to as the "array geometry." For purposes of defining the
scope of the present invention, the various position measurement
components described above, including those from the heading
sensors 29, from the ARDs 26A, 23, and, if used, from the
additional geodetic position receivers 25A in the tail buoys 25,
may be used individually or in any combination. The ARDs and
heading sensors may be referred to for convenience in defining the
invention as "relative position determination" sensors. By
determining relative positions at each point along each streamer
with reference to a selected point on the survey vessel or the
energy source, is it possible to determine the geodetic position of
each such streamer point if the geodetic position of the vessel or
the energy source is determined. As explained above, the navigation
portion of the recording system 12 may include a GPS receiver or
any other geodetic location receiver 12A. In some examples, the
energy source 17 may also include a geodetic position location
receiver 17A such as a GPS receiver. A particular example of a
system for determining relative positions of the streamers using
acoustic signals is described in the Falkenberg et al. patent
referred to above.
[0029] During operation of the geophysical acquisition system shown
in FIG. 1, it may be desirable to adjust portions of the streamers
20 laterally in order to maintain a predetermined array geometry
during geophysical surveying. The recording system 12 may be
configured to send suitable control signals to each of the LFD
devices 26 to move associated portions of each streamer 20
laterally. Such lateral motion may be selected so that each point
along each streamer is located at a predetermined relative position
at any moment in time. The relative positions may be referenced to
the position of either the survey vessel 10 or the energy source
17. Examples of various array geometry control modes according to
the invention are provided below.
[0030] During operation of the acquisition system shown in FIG. 1
when used for seismic surveying, for example, it is desirable for
the streamers 20 to be arranged as evenly as practicable behind the
vessel 10 to avoid "holes" in the survey coverage. "Evenly" or
"even" in the present context means that the streamers 20 are
substantially parallel to each other along their entire length,
that there is substantially equal lateral distance between adjacent
streamers, and that the streamers extend substantially parallel to
a selected direction. Deviation from such an even arrangement, as
is known in the art, may be caused by rip currents in the body of
water 11 and propeller wash from the survey vessel 10, among other
causes. "Holes" in the coverage is a condition known in the art
wherein seismic sensors are disposed more sparsely than would be
the case if the geometry of the array were "even" as defined
above.
[0031] 1. Vessel Heading Follow Mode: Referring to FIG. 2, an
automatic technique to maintain a substantially even array geometry
uses the vessel heading (geodetic direction) as a control parameter
for adjustment of the array geometry. A sensor (not shown), such as
a magnetic compass, in or associated with the recording system (12
in FIG. 1) measures the geodetic heading 42 of the survey vessel 10
and communicates such position to the recording system (12 in FIG.
1) at selected times according to operational programming of the
recording system (12 in FIG. 1). The measurements of geodetic
heading 42 may be filtered.
[0032] A method according to the Vessel Heading Follow Mode
includes communicating control signals to each LFD device (26 in
FIG. 1) such that the lateral distance of each streamer from each
adjacent streamer as measured by the ARDs (23 and 26A in FIG. 1) is
maintained at a selected value, and the streamers 20 are maintained
substantially in a selected geometry with respect to the vessel
heading direction. The selected geometry may be a straight line
that is parallel to the direction of the vessel heading. However,
the selected geometry may be a substantially straight line at an
angle to the vessel heading direction. The selected geometry may be
a streamer configuration in which the streamers are maintained in a
configuration other than a straight line.
[0033] In the present example, the recording system may generate
control signals for each LFD device such that the lateral distance
30A from the centerline 48 of the vessel measured by each of the
ARDs is maintained at a selected value along the length of each
streamer 20 and the geodetic position of any location along the
streamers 20 follows the vessel heading trajectory.
[0034] The vessel heading 42 may differ from the vessel trajectory
40 because of the direction and magnitude of current flow 44 in the
water 11. It is believed the vessel heading follow mode may have
advantages under particular environmental conditions, such as when
a current flow 44 in the water transverse to the towing direction
varies in magnitude along the towing direction. An example of such
transverse current would be found in an ocean lake or bay proximate
the mouth of a river. Other examples will occur to those skilled in
the art.
[0035] 2. Source Trajectory Follow Mode: In another example, the
trajectory 46 of the energy source (17 in FIG. 1) may be used as
the reference for controlling the array geometry. Referring to FIG.
3, an automatic technique to maintain a substantially even array
geometry uses the source trajectory (geodetic direction) 46 as a
control parameter for adjustment of the array geometry. A filtered
value of the source trajectory 46 is used as a reference to define
the selected travel path for each streamer 20. The source
trajectory 46 may be calculated in the navigation portion of the
recording system. Such calculation may include making a record with
respect to time of the geodetic position of the source 17 within
selected time intervals. The calculation programming instructions
may include a smoothing filter for the record of source position
with respect to time. In the present example, the recording system
may generate control signals for each LFD device such that the
lateral distance 30 with respect to the source trajectory 46
measured by each of the ARDs is maintained at a selected value
and/or the geodetic trajectory of any location along the streamers
20 follows the source trajectory.
[0036] A method according to the Source Trajectory Follow Mode
includes communicating control signals to each LFD device (26 in
FIG. 1) such that the lateral distance of each streamer from each
adjacent streamer as measured by the ARDs (23 and 26A in FIG. 1) is
maintained in a selected geometry with respect to the source
trajectory direction. The selected geometry may be substantially a
straight line that is parallel to the vessel trajectory direction.
However, the selected geometry may be a substantially straight line
at an angle to the vessel trajectory direction. The selected
geometry may be a streamer configuration in which the streamers are
maintained in a configuration other than a straight line.
[0037] 3. Preplot Follow Mode: A seismic survey "preplot" line (a
geodetic path defined by the subsurface area being investigated by
the seismic survey) can also be used as a control variable for the
selected travel path of each streamer. In the preplot follow mode,
reference to the geodetic position and/or heading of the vessel or
the source may be omitted. The array geometry may be configured to
move the geophysical sensors along a path based only on the
geodetic path intended to be followed by the sensors in making
measurements of the subsurface. In one example, a preplot survey
path may be predetermined for a geophysical survey of an area of
the subsurface, and for subsequent surveys conducted over the same
area, the same preplot survey path may be utilized for conducting
the subsequent geophysical surveys. In another example, subsequent
geophysical surveys may be conducted over substantially the same
area of the subsurface as a previous survey, and the recorded
locations of the sensors from the previous survey are utilized for
guidance in selecting the travel path for each streamer.
[0038] The preplot follow mode may have advantages in "4D" seismic
surveying, wherein a survey is repeatedly conducted over a same
area of the subsurface at selected times. As is known in the art,
the accuracy of such 4D surveys may be increased by causing the
geophysical sensors to as closely as possible follow a same
geodetic path each time a survey is conducted. The preplot follow
mode may improve the capability of the sensor array to make such
repeated travel paths.
[0039] The preplot follow mode may be performed using geodetic
position signals detected, for example, using the geodetic position
signal receivers (25A in FIG. 1) on the tail buoys (25 in FIG. 1)
to determine geodetic position of at least one point in the array.
Relative positions between the sensors in the array may be
determined as explained above using ARDs (23 and 26A in FIG. 1).
Geodetic heading or direction of the streamers may be determined,
for example, using the heading sensors (29 in FIG. 1) as explained
above. The LFDs (26 in FIG. 1) may be operated such that the
streamers follow a selected geodetic path substantially
irrespective of the vessel position and the vessel heading. It will
be appreciated by those skilled in the art that the possible range
of array geometry may be limited by the lengths of the various
towing components and the offset of the vessel trajectory from the
centerline of the predetermined array travel path, however the
preplot follow mode may be used to provide more consistent survey
results between successive surveys of the same subsurface geodetic
area than methods that control array geometry only by reference to
the vessel trajectory or heading.
[0040] 4. Proportional Force Separation Mode: During deployment and
retrieval of the streamers from the vessel, and during periods of
severe weather, the focus of the steering of seismic streamers is
not on subsurface coverage, but on increasing the safety of the
streamers from entanglement with each other. One way to obtain a
high degree of safety is to apply a selected lateral force (or
constant LFD steering element deflection angles) on each streamer
in a direction outward from the centerline of the vessel, with the
largest lateral force being applied on the laterally outermost
streamers, and with a linear or other proportional relationship
between the lateral force being applied on each streamer and the
lateral distance from the centerline of the vessel of each
respective streamer. As an example, for a 10 streamer array, the
rightmost streamer (designated streamer 1) could apply a 400N (or,
for example, a 15 degree "wing angle" on the LFD device) lateral
force outward from the centerline of the vessel, and lateral
outward forces of 320N, 240N, 160N and 80N, respectively, on
streamers 2, 3, 4 and 5. The same lateral force values could be
applied for streamers 6 through 10, but in the opposite direction
from the centerline of the vessel. The foregoing steering principle
is illustrated in FIG. 4, where lateral force F1 and F3 are smaller
than corresponding lateral forces F2 and F4. The term
"proportional" as used herein is intended to mean a substantially
monotonically increasing force with respect to lateral distance
from the centerline of the vessel and is not limited to a linear or
other directly proportional relationship.
[0041] By providing such lateral outward forces on each streamer, a
sufficient lateral separation of the streamers can be achieved even
without the need for high quality position information, or even
with no positioning information, whether relative or absolute. To
further increase the safety of deployment and retrieval operations,
the depth of each streamer could be related to lateral distance
from the centerline of the vessel, such that separation of the
streamers may be maintained in two planes.
[0042] It might sometimes be desirable to steer all devices in one
direction. The principle then is as above, but with the outmost
streamer on one side applying a large force and with a reduction of
force for every streamer until zero or a small force is applied on
the outermost streamer on the other side. The main principle of
such mode is that an individually selected, constant force is
applied to the steering devices to optimize streamer control.
[0043] Methods for operating LFD devices and controlling geometry
of a sensor array according to the various aspects of the invention
may provide more even coverage in marine geophysical surveying, may
provide more accurate positioning of geophysical sensors, and may
improve safety of the array in hostile environments.
[0044] While the invention has been described with respect to a
limited number of embodiments, those skilled in the art, having
benefit of this disclosure, will appreciate that other embodiments
can be devised which do not depart from the scope of the invention
as disclosed herein. Accordingly, the scope of the invention should
be limited only by the attached claims.
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