U.S. patent application number 11/122646 was filed with the patent office on 2006-11-16 for forward looking systems and methods for positioning marine seismic equipment.
Invention is credited to Jens Olav Paulsen, Rune Toennessen, Kenneth Welker.
Application Number | 20060256653 11/122646 |
Document ID | / |
Family ID | 37418966 |
Filed Date | 2006-11-16 |
United States Patent
Application |
20060256653 |
Kind Code |
A1 |
Toennessen; Rune ; et
al. |
November 16, 2006 |
Forward looking systems and methods for positioning marine seismic
equipment
Abstract
Systems and methods for positioning one or more spread elements
of a marine seismic spread are described. One system comprises a
seismic vessel-mounted acoustic Doppler current meter adapted to
ascertain at least the horizontal component of the current velocity
vector at a point ahead of the seismic vessel, and one or more
controllers adapted to use the ascertained current velocity vector
to control position of one or more seismic spread elements. It is
emphasized that this abstract is provided to comply with the rules
requiring an abstract, which will allow a searcher or other reader
to quickly ascertain the subject matter of the technical
disclosure. It is submitted with the understanding that it will not
be used to interpret or limit the scope or meaning of the claims.
37 CFR 1.72(b).
Inventors: |
Toennessen; Rune; (Oslo,
NO) ; Paulsen; Jens Olav; (Reistad, NO) ;
Welker; Kenneth; (Nesoya, NO) |
Correspondence
Address: |
WESTERNGECO L.L.C.
10001 RICHMOND AVENUE
(P.O. BOX 2469, HOUSTON, TX 77252-2469, U.S.A.)
HOUSTON
TX
77042
US
|
Family ID: |
37418966 |
Appl. No.: |
11/122646 |
Filed: |
May 5, 2005 |
Current U.S.
Class: |
367/16 |
Current CPC
Class: |
G01V 1/3826
20130101 |
Class at
Publication: |
367/016 |
International
Class: |
G01V 1/38 20060101
G01V001/38 |
Claims
1. A system comprising: a marine seismic spread, the spread
comprising spread elements including a vessel-mounted acoustic
Doppler current meter adapted to measure at least a horizontal
component of a current velocity vector at least at one location
generally ahead of the seismic spread elements; and a controller
adapted to use at least the horizontal component of the measured
current velocity vector to control position of a seismic spread
element.
2. The system of claim 1 wherein the current velocity vector is one
of a plurality of parameters used to control position of the spread
element.
3. The system of claim 1 wherein the acoustic Doppler current meter
is mounted near a front of the vessel.
4. The system of claim 1 wherein the acoustic Doppler current meter
is mounted near a center of gravity of the vessel.
5. The system of claim 1 wherein the acoustic Doppler current meter
comprises two acoustic sources.
6. The system of claim 1 wherein the acoustic Doppler current meter
comprises three or more acoustic sources.
7. The system of claim 1 wherein the acoustic Doppler current meter
has a single acoustic transducer that is adapted to be aimed and
sample at different points and vary its angle of projection in
time.
8. The system of claim 1 wherein the controller maintains position
of the spread element using the measured horizontal component of
the velocity vector as the spread element encounters the
current.
9. The system of claim 1 wherein the controller is adapted to steer
the spread element using the measured horizontal component of the
velocity vector to return the spread element to a defined path.
10. The system of claim 1 wherein the acoustic Doppler current
meter comprises a motion compensation sub-system selected from a
mechanical motion compensation sub-system, a computational motion
compensation sub-system, and combinations thereof.
11. The system of claim 10 wherein the motion compensation
sub-system is selected from a gimbaling system, a beam weighting
system, a motion filtering system, an orientation controller, a
local heave compensation system, and combinations thereof.
12. The system of claim 8 wherein the spread element is the seismic
vessel.
13. The system of claim 9 wherein the spread element is the seismic
vessel.
14. The system of claim 1 wherein the spread element includes the
seismic vessel, a seismic source, and a plurality of seismic
streamers.
15. A system for acquiring marine seismic data comprising: (a) a
seismic spread comprising a towing vessel, a seismic source, and
optionally a plurality of seismic streamers towed by the towing
vessel; (b) a acoustic Doppler current meter mounted on and adapted
to measure at least a horizontal component of a current velocity
vector at a point ahead of the towing vessel; (c) a controller
adapted to use the measured horizontal component of the current
velocity vector to control position of the towing vessel, the
seismic source, and optionally the plurality of seismic streamers;
and (d) a plurality of spread control elements associated with the
towing vessel, the seismic source, and optionally the plurality of
streamers, and controlled by the controller.
16. The system of claim 15 wherein the current velocity vector is
one of a plurality of parameters used to control the spread control
elements.
17. The system of claim 15 wherein the acoustic Doppler current
meter is mounted near a front of the towing vessel.
18. The system of claim 15 wherein the acoustic Doppler current
meter is mounted near a center of gravity of the towing vessel.
19. The system of claim 15 wherein the acoustic Doppler current
meter comprises a motion compensation sub-system selected from a
mechanical motion compensation sub-system, a computational motion
compensation sub-system, and combinations thereof.
20. The system of claim 19 wherein the motion compensation
sub-system is selected from a gimbaling system, a beam weighting
system, a motion filtering system, an orientation controller, a
local heave compensation system, and combinations thereof.
21. A method comprising: measuring at least a horizontal component
of a current velocity vector at least at one location generally
ahead of a seismic spread element using a vessel-mounted acoustic
Doppler current meter; and using the horizontal component of the
current velocity vector to control position of the seismic spread
element.
22. The method of claim 21 wherein the horizontal component of the
current velocity vector is used in conjunction with a plurality of
parameters to control position of the seismic spread element.
23. The method of claim 21 wherein the spread element is a seismic
towing vessel.
24. The method of claim 21 including using the horizontal component
of the current velocity vector to maintain a position of the spread
element.
25. The method of claim 24 including using the measured horizontal
component of the velocity vector to return the spread element to a
defined path by steering the spread element.
26. The method of claim 21 including motion compensating the
acoustic Doppler current meter by a method selected from mechanical
motion compensation, computational motion compensation, and
combinations thereof.
27. The method of claim 26 wherein the motion compensating is
selected from gimbaling, beam weighting, motion filtering,
controlling orientation, compensating for local heave, and
combinations thereof.
28. A method for acquiring marine seismic data comprising: (a)
towing a seismic spread comprising a towing vessel, a seismic
source, and optionally a plurality of seismic streamers; (b)
measuring at least a horizontal component of a current velocity
vector at a point ahead of the towing vessel using an acoustic
Doppler current meter mounted on the towing vessel; (c) adjusting
the meter to compensate for motion of the towing vessel while
measuring the horizontal component of the current velocity vector
to form a motion-compensated horizontal component of the current
velocity vector; and (d) using the motion-compensated horizontal
component of the current velocity vector to control position of the
towing vessel, the seismic source, and the plurality of seismic
streamers.
29. The method of claim 28 wherein the horizontal component of the
current velocity vector is used in conjunction with a plurality of
parameters during to control position.
30. The method of claim 28 wherein the adjusting the meter is
selected from mechanical motion compensation, computational motion
compensation, and combinations thereof.
31. The method of claim 30 wherein the adjusting the meter is
selected from gimbaling, beam weighting, motion filtering,
controlling orientation, compensating for local heave, and
combinations thereof.
32. A method comprising: (a) creating a current profile between an
acoustic Doppler current meter mounted on a seismic towing vessel
and a point distant from the meter and generally in front of the
seismic towing vessel during a time period, the vessel moving
generally toward the point during the time period; and (b) using
the current profile to continuously estimate at least a horizontal
component of a current velocity vector at the point during the time
period.
33. The method of claim 32 including motion-compensating the
acoustic Doppler current meter.
34. The method of claim 32 wherein a distance between the acoustic
Doppler current meter and the point is continuously decreasing.
35. The method of claim 32 wherein the continuously estimating
comprises continuously calculating the current velocity vector at
the point using a plurality of cell pairs between two acoustic
beams of the acoustic Doppler current meter.
36. The method of claim 35 wherein the continuously calculating
comprises using two or more high-frequency acoustic beams.
37. The method of claim 36 wherein all acoustic beams are in one
plane and all but two acoustic beams are used for quality
control.
38. The method of claim 35 wherein the acoustic Doppler current
meter comprises three acoustic beams, two beams in a horizontal
plane and a third beam not in the horizontal plane.
39. The method of claim 32 including relating the estimated current
velocity vector to an earth-fixed coordinate system by measuring
the current velocity over ground using a positioning system
relating to an earth-fixed coordinate system.
40. A method comprising estimating a vertical current profile at a
predefined distance ahead of a spread element using a
vessel-mounted acoustic Doppler current meter mounted in a mounting
and having one or more acoustic transducers sampling current
vertical component data at a defined data sampling rate, the data
sampling rate being at a frequency higher than a frequency of
movement the beam transmitter.
41. The method of claim 40 comprising fixing at least the acoustic
transducers in a single position relative to the vessel.
42. The method of claim 40 comprising forcing at least the acoustic
transducers to move relative to the vessel.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of Invention
[0002] The present invention relates to the field of marine seismic
instrumentation and methods of using same. More specifically, the
invention relates to systems and methods for positioning one or
more spread elements of a marine seismic spread using information
about the ocean current ahead of the towing vessel.
[0003] 2. Related Art
[0004] Marine seismic exploration investigates and maps the
structure and character of subsurface geological formations
underlying a body of water. For large survey areas, seismic vessels
tow one or more seismic sources and multiple seismic streamer
cables through the water. The entire system is typically referred
to as a spread, and the elements making up the spread are referred
to as spread elements. The seismic sources typically comprise
compressed air guns for generating acoustic pulses in the water.
The energy from these pulses propagates downwardly into the
geological formations and is reflected upwardly from the interfaces
between subsurface geological formations. The reflected energy is
sensed with hydrophones attached to the seismic streamers, and data
representing such energy is recorded and processed to provide
information about the underlying geological features.
[0005] While there have been some efforts to use information
regarding environmental conditions, including ocean currents,
previous attempts have not provided the desired precision in
positioning marine seismic spread elements.
SUMMARY OF THE INVENTION
[0006] In accordance with the present invention, systems and
methods are described for positioning one or more marine seismic
spread elements, which may include a towing vessel, seismic source,
and streamers. The systems and methods of the invention, which
employ vessel-mounted acoustic Doppler current meters, reduce or
overcome problems with previous systems and methods employing
acoustic Doppler current meters. Systems and methods of the
invention may be used during seismic data collection, including 3-D
and 4-D seismic surveying.
[0007] A first aspect of the invention are systems comprising:
[0008] (a) a marine seismic spread, the spread comprising spread
elements including a vessel-mounted acoustic Doppler current meter
adapted to measure at least a horizontal component of a current
velocity vector at least at one location generally ahead of the
seismic spread elements; and [0009] (b) a controller adapted to use
at least the horizontal component of the measured current velocity
vector to control position of a seismic spread element.
[0010] The controller may control position either before the spread
element encounters the measured current ahead of the vessel, or
when the spread element passes by the point or location where the
current was measured. Systems of the invention may include a
seismic spread comprising one or more vessels such as towing
vessels, a chase vessel, a work vessel, one or more a seismic
sources, and one or more seismic streamers towed by towing vessels.
The streamers and sources may be separately towed or towed by the
same vessel. The acoustic Doppler current meter may be mounted on a
chase vessel, a work vessel, an AUV, or a tow vessel, as long as it
is able to provide the desired data, and may comprise a transducer
that produces at least one beam that is horizontal and forward
looking, or has a useable forward-looking horizontal component, and
may be adapted to measure a current velocity vector at a point
ahead of the towing vessel. The controller may control position of
all or some of the spread elements through commands given to spread
control elements, such as deflectors, steerable birds, and the
like. Optionally, the vessel-mounted acoustic Doppler current meter
may be motion-compensated, as explained more fully herein.
[0011] As used herein the phrases "passes by the location" and
"passes by the point" mean that the spread element need not
actually pass through the location or point, but that the spread
element in question may be smartly positioned relative to the
location or point as the spread approaches or moves away from the
location or point. "Location" is meant to be a broader term than
"point", which implies a specific spatial coordinate; by location
we mean a set of (or range of) coordinates that a point may be
within. Both points and locations may be in an arc defined by a
specific distance in a direction generally ahead of the vessel and
at a specific depth. An exact description of where the current
measurement applies is not required. In general, the controller may
implement control commands based on what the horizontal current
measurement system reports as the current the spread elements will
encounter. Although there may be some degree of error in the
reported current due to a variety of error sources, including
errors in the model of current in space and time, and instrument
measurement error, even with the errors, the spread elements may be
better controlled with the horizontal current measurement input the
majority of the time. Systems and methods of the invention may also
be used to estimate variation in current velocity in a vertical
plane that is a defined distance from and generally ahead of the
vessel.
[0012] The current velocity vector may be one of a plurality of
parameters used to control position of the spread element or
elements. The acoustic Doppler current meter may be mounted near a
front of the vessel, near a center of gravity of the vessel, or
some other location, as long as the transducers producing the
acoustic beams are allowed to travel forward of the vessel and in
the water at least a substantial amount of time. The meter may
comprise one, two, three, or more than three acoustic transducers
producing acoustic "beams". Two or three measurements offset by
just enough in space to give a non-singular estimate of a 2D or 3D
current will suffice. In embodiments using one acoustic beam, the
beam may be transmitted first in one direction, then a second
direction, and switched back and forth in high frequency and used
to calculate a 2D current velocity vector at an average point or
location between the two directions. If the meter produces more
than two acoustic beams, two beams may be used to calculate a 2D
current velocity vector at an average point or location between the
two selected beams, while the third and perhaps other beams may be
used for quality control and/or improvement of the current vector
estimation. Alternatively, three non-coplanar beams may be used to
calculate a 3D current velocity vector. The measured current field
may be all around a triangle formed by the three beams, unless the
beam lengths are extremely long. Mathematical techniques, for
example those described herein, may be used to calculate current
velocity vector at specific points ahead of the vessel.
[0013] Systems and methods of the invention may optionally be used
in conjunction with other systems and methods. For example, since
the position of spread elements is known from acoustic ranging
networks, GPS, and other position sensors, and since the seismic
team knows the paths the spread elements are supposed to follow
based on the survey specifications, the controller may use at least
the horizontal current velocity vector component to calculate an
optimum track for a spread element, either to steer it back to the
survey-specified path, or ensure that the survey-specified path is
adhered to.
[0014] The acoustic Doppler current meter may be motion-compensated
by including a motion compensation sub-system. The motion
compensation sub-system functions to correct for expected and
unexpected movements of the seismic vessel, such as heave, pitch,
and roll. The motion compensation sub-system may be mechanical,
computational, and combinations thereof. As non-limiting examples,
the motion compensation sub-system may be a gimbaling system, a
beam weighting system, a motion filtering system, an orientation
controller, a local heave compensation system, and combinations
thereof, as will become evident.
[0015] Another aspect of the invention comprises methods of
measuring at least a horizontal component of a current velocity
vector at least at one location generally ahead of the seismic
spread elements using a vessel-mountedacoustic Doppler current
meter, and using at least the horizontal component of the current
velocity vector to control position of a seismic spread element
before the spread element passes by the location.
[0016] Methods of the invention may comprise towing a seismic
spread comprising a towing vessel, a seismic source, and optionally
a plurality of seismic streamers, which may be towed in over/under
configuration, "V" configuration, "W" configuration, or some other
configuration; measuring a current velocity vector at a point ahead
of the towing vessel using a horizontal acoustic Doppler current
meter mounted on the towing vessel; adjusting the meter to
compensate for motion of the towing vessel while measuring the
current velocity vector to form a motion-compensated current
velocity vector; and controlling position of the towing vessel, the
seismic source, and the plurality of seismic streamers before they
pass by the point using the motion-compensated current velocity
vector.
[0017] Another aspect of the invention is a method comprising:
[0018] (a) creating a current profile between an acoustic Doppler
current meter mounted on a vessel and a point or location distant
from the meter and generally in front of seismic spread elements
during a time period, the vessel moving generally toward the point
or location during the time period; and [0019] (b) continuously
estimating a current velocity vector at the point or location
during the time period using the current profile.
[0020] Another method of the invention comprises estimating a
vertical current profile at a predefined distance ahead of a spread
element by using a vessel-mounted acoustic Doppler current meter
mounted in a mounting and sampling current vertical component data
at a defined rate, the mounting either fixing the meter in a single
position relative to the vessel or enabling at least the acoustic
beam transmitter or transmitters (sometimes referred to herein as
"eyes") to move relative to the vessel, the data sampling rate
being at a frequency higher than a frequency of movement of meter
or the beam transmitter. If a fixed position mounting is used, the
meter will be useful when the vessel pitches. If the vessel is
riding in a calm body of water, a pitching motion may be enforced.
A mounting having a combination of features may be used, wherein
the meter may be locked in a fixed position when the vessel is
pitching, and when the seas are calm the locking mechanism released
and a controlled pitching motion imposed, for example using a
sensor/controller/actuator arrangement. During a pitch cycle the
aiming point will the scan through the vertical water column with a
depth variation determined by the pitch amplitudes and the distance
ahead. The spread elements are located at different depths with the
vessel from 0 to maximum draft of the vessel, and the source
generally shallower than the streamers, which again are deeper than
the vessel, and particularly in over-under streamer configuration
and during handling when the streamers are stacked on top of each
other. Knowing that the current often changes both in strength and
direction in the water column, and in particular close (within 10's
of meters) to the sea surface, then it is clear that a good picture
of the vertical water column may be helpful for controlling the
positioning of the spread elements accurately.
[0021] Systems and methods of the invention will become more
apparent upon review of the brief description of the drawings, the
detailed description, and the claims that follow.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The manner in which the objectives of the invention and
other desirable characteristics can be obtained is explained in the
following description and attached drawings in which:
[0023] FIG. 1 is a plan or overhead view of a first system of the
invention;
[0024] FIG. 2 is a schematic plan view, with portions cut away, of
the acoustic Doppler current meter used in FIG. 1;
[0025] FIG. 3 is a schematic perspective view of a system of the
invention and also showing the six possible movements of the towing
vessel in the X-Y-Z coordinate system;
[0026] FIGS. 4A and 4B illustrate schematic side elevation views,
with portions cut away, of one embodiment of a vessel-mounted,
motion-compensated current meter;
[0027] FIGS. 5A and 5B illustrate schematic side elevation views,
with portions cut away, of another embodiment of a vessel-mounted,
motion-compensated current meter;
[0028] FIGS. 6A and 6B illustrate schematic side elevation views,
with portions cut away, of another embodiment of a vessel-mounted,
motion-compensated current meter;
[0029] FIG. 7 illustrates a side elevation view of another system
of the invention; and
[0030] FIG. 8 is a schematic block diagram of a control scheme that
may be utilized to control the motion compensation systems and
other parameters useful with current meters in the systems of the
invention.
[0031] It is to be noted, however, that the appended drawings are
not to scale and illustrate only typical embodiments of this
invention, and are therefore not to be considered limiting of its
scope, for the invention may admit to other equally effective
embodiments.
DETAILED DESCRIPTION
[0032] In the following description, numerous details are set forth
to provide an understanding of the present invention. However, it
will be understood by those skilled in the art that the present
invention may be practiced without these details and that numerous
variations or modifications from the described embodiments may be
possible. For example, in the discussion herein, aspects of the
invention are developed within the general context of controlled
positioning of seismic spread elements, which may employ
computer-executable instructions, such as program modules, being
executed by one or more conventional computers. Generally, program
modules include routines, programs, objects, components, data
structures, etc. that perform particular tasks or implement
particular abstract data types. Moreover, those skilled in the art
will appreciate that the invention may be practiced in whole or in
part with other computer system configurations, including hand-held
devices, personal digital assistants, multiprocessor systems,
microprocessor-based or programmable electronics, network PCs,
minicomputers, mainframe computers, and the like. In a distributed
computer environment, program modules may be located in both local
and remote memory storage devices. It is noted, however, that
modification to the systems and methods described herein may well
be made without deviating from the scope of the present invention.
Moreover, although developed within the context of controlling
position of seismic spread elements, those skilled in the art will
appreciate, from the discussion to follow, that the principles of
the invention may well be applied to other aspects of seismic data
acquisition. Thus, the systems and method described below are but
illustrative implementations of a broader inventive concept.
[0033] All phrases, derivations, collocations and multiword
expressions used herein, in particular in the claims that follow,
are expressly not limited to nouns and verbs. It is apparent that
meanings are not just expressed by nouns and verbs or single words.
Languages use a variety of ways to express content. The existence
of inventive concepts and the ways in which these are expressed
varies in language-cultures. For example, many lexicalized
compounds in Germanic languages are often expressed as
adjective-noun combinations, noun-preposition-noun combinations or
derivations in Romanic languages. The possibility to include
phrases, derivations and collocations in the claims is essential
for high-quality patents, making it possible to reduce expressions
to their conceptual content, and all possible conceptual
combinations of words that are compatible with such content (either
within a language or across languages) are intended to be included
in the used phrases.
[0034] The present invention relates to various systems and methods
for controlling position of one or more marine seismic spread
elements. One aspect of the present invention relates to systems
including a vessel-mounted acoustic Doppler current meter. Another
aspect of the invention comprises methods of using a system of the
invention to measure at least the horizontal component of a current
velocity vector at least at one location generally ahead of a
seismic spread element using a vessel-mounted acoustic Doppler
current meter, and using at least the horizontal component of the
current velocity vector to control position of a seismic spread
element before the spread element passes by the location.
[0035] The phrase "acoustic Doppler current meter", or ADCM, means
a device capable of transmitting two or more high frequency
acoustic beams in different directions (or switching one beam
between two directions at a given frequency) and generally at an
angle to each other, and that is capable of receiving acoustic
echoes from particles in the paths of the beams in order to
calculate the velocity of a fluid at a point or location of
interest that is an average distance between the beams. A
"horizontal acoustic Doppler current meter", or H-ADCM, means an
ADCM that is capable of transmitting at least two acoustic beams in
a horizontal plane (or switching one beam between two directions)
and receiving acoustic echoes in that plane. The phrase "acoustic
Doppler current profiler", or ADCP, means an ADCM that calculates
velocity vectors between a plurality of cells pairs of the
beams.
[0036] The phrase "current profile" means a plurality of current
velocity vectors calculated between cell pairs of two acoustic
beams emitted by an ADCP.
[0037] The term "vessel-mounted" means any device or component that
is at least temporarily attached to a vessel, wherein the vessel
may be either the seismic tow vessel, a chase vessel, a work
vessel, an ROV, and the like.
[0038] The term "spread" and the phrase "seismic spread" are used
interchangeably herein and mean the total number of components,
including vessels, vehicles, and towed objects including cables,
that are used together to conduct a marine seismic data acquisition
survey.
[0039] The term "control", used as a transitive verb, means to
verify or regulate by comparing with a standard or desired value.
Control may be open loop, closed loop, feedback, feed-forward,
cascade, adaptive, heuristic and combinations thereof.
[0040] The term "controller" means a device at least capable of
accepting input from sensors and meters (including an ADCM) in real
time or near-real time, and sending commands directly to spread
control elements, and/or to local devices associated with spread
control elements able to accept commands. A controller may also be
capable of accepting input from human operators; accessing
databases, such as relational databases; sending data to accessing
data in databases, data warehouses or data marts; and sending
information to and accepting input from a display device readable
by a human. A controller may also interface with or have integrated
therewith one or more software application modules, and may
supervise interaction between databases and one or more software
application modules.
[0041] The phrase "spread control element" means a spread component
that is controllable and is capable of causing a spread component
to change coordinates, either vertically, horizontally, or both,
and may or may not be remotely controlled.
[0042] The terms "control position", "position controllable",
"remotely controlling position" and "steering" are generally used
interchangeably herein, although it will be recognized by those of
skill in the art that "steering" usually refers to following a
defined path, while "control position", "position controllable",
and "remotely controlling position" could mean steering, but also
could mean merely maintaining position, for example when current
hits an element. In the context of the present invention, "control
position" means we use at least the horizontal component of current
ahead of the seismic spread elements in order to give steering
commands to steering elements in order for them to return to a
desired pre-defined path, or to be able to maintain present
position when the new current hits the steering elements. The
current data may also be used to calculate a preferred path of for
instance the vessel that, with a minimum discrepancy relative to
the desired pre-defined path, brings the seismic elements back
without conflicting the steering limitations by the system. As
"position controllable" and "control position" are somewhat broader
terms than "steering", these terms are used herein, except when
specific instances demand using more specific words.
[0043] "Real-time" means dataflow that occurs without any delay
added beyond the minimum required for generation of the dataflow
components. It implies that there is no major gap between the
storage of information in the dataflow and the retrieval of that
information. There may be a further requirement that the dataflow
components are generated sufficiently rapidly to allow control
decisions using them to be made sufficiently early to be effective.
"Near-real-time" means dataflow that has been delayed in some way,
such as to allow the calculation of results using symmetrical
filters. Typically, decisions made with this type of dataflow are
for the enhancement of real-time decisions. Both real-time and
near-real-time dataflows are used immediately after the next
process in the decision line receives them.
[0044] The term "position", when used as a noun, is broader than
"depth" or lateral (horizontal) movement alone, and is intended to
be synonymous with "spatial relation." Thus "vertical position"
includes depth, but also distance from the seabed or distance above
or below a submerged or semi-submerged object, or an object having
portions submerged. When used as a verb, "position" means cause to
be in a desired place, state, or spatial relation.
[0045] The term "adjusting" means changing one or more parameters
or characteristics in real-time or near-real-time. The phrase
"adjusting the meter" includes one or both of changing position of
the meter and correcting the data gathered by the meter to
compensate for motion of the vessel to which it is mounted while
measuring a current velocity vector to form a motion-compensated
current velocity vector using an ADCP-type current meter.
[0046] FIG. 1 is a plan or overhead view of a first system of the
invention, illustrating a seismic towing vessel 2 pulling a seismic
source 4 as well as four seismic streamers, 6. In the arrangement
illustrated, which is but one of a great variety within the
invention, seismic streamers 6 are pulled laterally by deflectors 8
and 10, which may be of the type known under the trade designation
MONOWING.TM., available from WesternGeco, LLC, Houston, Tex. It is
understood that source 4 and seismic streamers 6 are towed at some
depth below the water surface. Sources are typically towed at
depths ranging from 0 to 10 meters, while seismic streamers may be
towed at multiple depths, but are typically at depths ranging form
3 to 50 meters, depending on the survey specifications. The four
seismic streamers 6 are illustrated in FIG. 1 towed by respective
four tow members 12 as indicated, with separation members 14
provided between adjacent seismic streamers. Passive or active tow
members (not shown) may connect source 4 with one or more seismic
streamer tow members 12. One or more seismic streamers 6 may have a
companion seismic streamer 6' (not illustrated) where the
companions are towed in over/under fashion. The vertical distance
between seismic streamers 6, 6' in a seismic streamer pair may
range from 1 meter to 50 meters, and may be about 5 meters. From
hereon we discuss seismic streamers all positioned at the same
vertical position, it being understood that the principles of the
invention are applicable to over-under streamer arrangements. A
selected number of hydrophones, either mounted within the seismic
streamer or in/on equipment mounted onto the seismic streamer, may
be used as receivers in an acoustic ranging system and thereby
provide knowledge of the horizontal and vertical position of
seismic streamers 6.
[0047] Attached to towing vessel 2 is a housing 18 for one
vessel-mounted acoustic Doppler current meter useful in the
invention, illustrated in more detail in FIG. 2. The ADCM in this
embodiment transmits two acoustic signals, or beams, indicated by
lines 20 and 22, which may be equally spaced from a centerline 24.
Also illustrated are four current velocity vectors V.sub.C1,
V.sub.C2, V.sub.C3, and V.sub.C4, represented by respective arrows,
where the length of the arrow indicates the magnitude (speed of the
current) and the direction of the arrow indicates the general
direction of the current in the vicinity of the arrow. Note that
FIG. 1 illustrates a situation wherein the current is faster close
to vessel 2, decreases from V.sub.C1 to V.sub.C2, and again from
V.sub.C2 to V.sub.C3, and then actually changes direction at
V.sub.C4. This information is of course useful in positioning
spread elements.
[0048] FIG. 2 is a schematic plan view, with portions cut away, of
the ADCM used in FIG. 1, showing a body element 19 that may contain
electronics and processing equipment, and in FIG. 2, two transducer
elements arranged in horizontal or near-horizontal fashion.
Transducers 21, 23 of the ADCM each produce acoustic beams 22 and
20, respectively, having an angle .alpha. between them ranging from
about 5 to about 30 degrees. The acoustic beams will also spread to
give a larger footprint with increased distance, as indicated by
angle .beta., which may range from about 0 to 5 degrees. When using
a single beam switching back and forth between two directions, or
two or more beams in a horizontal plane, the device is sometimes
referred to as an H-ADCM. In any case, transducers 21 and 23 send
out high frequency acoustic signals, receive echoes, and record
Doppler shifts returned from different cells along the beams.
Meters of this type are a sub-set of a larger group of current
meters known as ADCMs, and those having two beams pointing with an
angle between them are capable of constructing a water velocity
vector in the plane of these two beams. The beams are divided into
elements or so-called cells. Data are returned from each cell and
from this data the water velocity vectors may be calculated from
each cell pair along the beam. A current vector is calculated at an
average position between the beams of each cell. ADCMs that return
values from more than one cell pair on each sampling are referred
to as "current profilers" or simply "profilers" as they calculate
the current vectors from the head of the device and to a predefined
set point or location ahead. The profile may be used for quality
control purposes, for example, to improve the accuracy of the
current velocity recorded at the desired point or location ahead of
the vessel as the vessel moves forward. To be more specific, as the
vessel moves forward the water velocity at a point or location in
space located between the ADCM and the specified point furthest
ahead may be continuously estimated with an increased accuracy
based on the continuously increased number of measurement samples
of the point in space as the vessel moves ahead. Many ADCMs are
constructed with three or more beams. If at least three of these
beams are out of plane, the ADCM is capable of constructing a 3D
water velocity vector. If there are more than three beams, or all
beams are in one plane, then the extra beam(s) may be used to
improve the accuracy of the result. In this invention, ADCMs whose
beams are in plane are denoted as 2D-ADCMs, and ADCMs whose three
or more beams are out of plane are referred to as 3D-ADCMs. Both
2D-ADCMs and 3D-ADCMs may be an H-ADCM.
[0049] FIG. 2 illustrates a 2D-ADCM, with beams 20 and 22 divided
into cells indicated by the dashed lines. In other words, a cell
pair is formed between lines d.sub.0 and d.sub.1, between lines
d.sub.1 and d.sub.2, between lines d.sub.2 and d.sub.3, and between
d.sub.3 and d.sub.4, and a corresponding current vector calculated
for a position midway between beams of the cells, as indicated by
the arrows in FIG. 2. It will be understood that these lines are
imaginary and the placement of the lines and definition of the
cells will necessarily vary with the transducers, electronics, and
computational power available.
[0050] FIG. 3 is a schematic perspective view of a system of the
invention and also showing the six possible movements of the towing
vessel in the X-Y-Z coordinate system. There is a challenge when it
comes to acquiring reliable current velocity data from the
predefined point or location ahead of the vessel when the current
meter, such as an ADCM or H-ADCM, is mounted on the vessel. In
particular, the actual motion of the vessel (and sources,
streamers, and all other equipment) may be any combination of the
motions illustrated on the X-Y-Z coordinates in FIG. 3. By
convention in the marine seismic industry, the X direction is
generally used to denote the direction of travel of the towing
vessel, and FIG. 3 illustrates towing vessel 2 having a
vessel-mounted acoustic Doppler current meter 18 attached thereto.
The embodiment of FIG. 3 depicts a dual source arrangement, in
other words two seismic sources 4 are used. The vessel is actually
moving with six degrees of freedom, where three are linear (surge,
sway, and heave) and three are rotational (roll, pitch and yaw).
Since the current ahead of the vessel is important, regardless of
where the current meter is pointing, it may not be necessary to
compensate for all six degrees of freedom. For example, in many
embodiments it may not be necessary to compensate for sway motion,
and in most cases surge and yaw motions. However, the vessel's
average forward speed must be compensated for, and even unsteady
surge motion in waves may sometimes be noticeable. Crab, a wind
driven steady state yaw motion, may be corrected for in order to
measure the current in the direction of the bearing rather than in
the direction the vessel is pointing. Also yaw may in certain sea
conditions be noticeable and may be corrected for. When the vessel
is heaving, the ADCM, unless mechanically motion-compensated for in
accordance with some embodiments of the present invention, will
move up and down with the vessel and it will move up and down
relative to the pre-specified target depth where current data is
desired. The local heave motion is minimum at or close to the
center or gravity (COG) of vessel 2. The local heave motion is
usually largest at the bow of the vessel due to the coupling with
the pitch motion. Local heave increases also at positions offset to
the centerline due to coupling with roll. All this means that the
local heave motion is dependent on where the ADCM is mounted on the
vessel.
[0051] Pitch may also be problematic, especially with a long-range
horizontal-looking ADCM. If the vessel is pitching only 2 degrees,
and the ADCM is rigidly mounted to the vessel, and the target point
is 200 meters ahead of the ADCM, then the ADCM aim point will
oscillate about the target depth with a depth variation of +/-7
meters. And this is due to pitch only. In certain methods and
systems in accordance with the present invention the ability to
sample current data and filter it so as to acquire the data for the
targeted depth is provided for. Another complicating factor is the
fact that, unless motion-compensated for, the beams will hit the
sea surface when the vessel pitches nose up and when large waves
are passing by. As seen in FIG. 1, in a two beam ADCM the beams are
angled with an angle between them. This means that when rolling,
one beam will point upwards and the other will point downwards.
That again means that the beams may collect data at different water
layers, and hence when combining the data from the two beams to
construct a vector, this will not represent the water velocity
vector at the mean water depth, particularly not if there is a
significant vertical current gradient as is often the case.
[0052] The present invention contemplates several system and method
embodiments including motion compensation to deal with the above
problems. We now describe several motion compensation options,
which may be combined if desirable. One embodiment comprises
gimbaling to compensate for rotational motions, and is described in
reference to FIGS. 4A and 4B. This system and method may be used
provided that the Eigen frequencies of the gimbaling mechanism are
far outside the range of the frequencies of vessel motions and
provided that no other forces such as fluid forces will tilt the
ADCM in either direction. To avoid or reduce fluid forces due to
ocean current, waves, and the like, the ADCM may be protected from
the fluid flow by partially enclosing it in a dome or equivalent
partially enclosed body 18 (hereinafter termed a housing) with
windows for transducers 21, 23 in the front, as illustrated in
FIGS. 4A and 4B. Housing 18, illustrated with portions cut away to
reveal the ADCM, 19, 21 inside, may be suspended beneath vessel 2
(housing 18 could be suspended at the hull side or at or on the
bulb in front of the bow on vessels having such a bulb) by a
connector 34 that may supply necessary power and communications to
ADCM body 19 and transducers 21 and 23. Alternatively, the ADCM may
receive power via batteries, and may communicate via wireless
transmission. Vessel 2 may or may not be the seismic towing vessel.
It could, for example, be a chase vessel or work vessel. A
gimbaling mechanism is depicted schematically as a ring 26.
Optional torque motors 28, 30, and 32 may be used for assisting one
or more of the gimbaling movements. FIG. 4A depicts a calm sea 3
and vessel 2 rides near horizontally in the sea, and in this case
beam 22 is able to point more or less directly at the target point
or location ahead of vessel 2 (not illustrated). This point or
location ahead of the vessel may be as far as the ADCM is able to
"shoot", but may be about 250 meters with presently known devices.
FIG. 4B depicts a pitch down situation, where vessel 2, and thus
connector 34 and housing 18, also pitch down due to a rough sea 3a.
However, gimbaling mechanism 26, assisted by any motors or other
actuators 28, 30, 32, compensates for the vessel's pitch, and beam
22 remains pointed more or less at the target. Note that gimbaling
will not be able to compensate for any translational motions such
as heave, but if the ADCM is mounted close to the vessel COG the
local heave motion may be limited. This system and method may be
combined with other motion compensation methods described herein
for heave compensation.
[0053] Gimbaling is a mechanical solution. Motion compensation may
also be performed through beam weighting, which is a computational
solution. In these methods, the ADCM may employ a 3D-ADCM rigidly
attached to the vessel. A 2D-ADCM will calculate a 2D velocity
vector at an average position between the two beams. A 3D-ADCM will
calculate a 3D velocity vector all around a triangle formed by the
three beams, unless the beam lengths are extremely long, in which
case the velocity vector is calculated at a location comprised by
the center of a triangle bound by at least three beams and the
distance ahead as predefined. If the target position is not in the
center of this triangle, then interpolation procedures between the
beams may be used to estimate the value of the velocity vector
offset from the center point. This means that the three beams may
be weighted differently. If the target point is outside the
triangle bound by the three beams, extrapolation techniques may be
employed. Software application programs may be used for these
functions. However, even the horizontal and vertical components of
the velocity vector at points and locations not at the center of
the triangle, or even outside of the triangle, may be useful and
better than no knowledge at all.
[0054] Another computational approach to motion compensation useful
in the invention may also be performed by continuous or
semi-continuous recording, motion sensing, picking and filtering of
the velocity data. In these methods, a 2D, or in-plane ADCM may be
rigidly attached to the vessel, as illustrated in FIGS. 5A and 5B.
In these methods, the actual motion and orientation of the ADCM is
sensed. Utilization of a so-called motion capture system,
illustrated at 36 and 37, respectively, may perform this function.
The term "motion capture system" is most often used in connection
with analysis of motion for sports medicine, and to simulate motion
for video games. This type of system can be used for short time
periods when the inertial instrument drift doesn't have time to
build up and when motion can be calibrated by visiting known points
during the motion. As shown in FIGS. 5A and 5B, as opposed to the
situation depicted in FIGS. 4A and 4B, when vessel 2 pitches down
due to a rough sea 3a, the ADCM 19, 21 also pitches down, and thus
acoustic beam 22 more or less follows the pitching motion of vessel
2. The solution in this embodiment is the use of a stationary
platform 36 and a motion capture system 37. "Motion capture system"
is a general term for an assembly of accelerometers and/or
gyroscopes that are able to follow motion in space with time.
Motion capture system 37 captures the movements of ADCM, 19, 21. A
motion capture system is basically a sensor or collection of
sensors, typically optical or magnetic, that are capable of
transferring motion data to a controller containing software for
data processing, according the specifications of the seismic
survey. In addition an instrument may be used to re-set the motion
capture system since they tend to drift over time. In the aerospace
and maritime industries it is common to combine high precision
relative GPS signals between three antennas with a fixed baseline
between them with an inexpensive solid-state component inertial
system to sense 3D motion. GPS/inertial is an inexpensive and
smaller equipment set than typical vessel gyros. The inexpensive
inertial units have much higher drift rates than mechanical gyros
but the drift is bounded by the high recalibration rate available
from GPS. The motion measurement system may be synchronized with
the ADCM measurements system and time tagging may be used to link
motion events to measurement events. One may use a depth sensor for
this, such as a pressure sensor measuring the water depth. Other
calibration methods may be used as well and one may utilize the
fact that over time the orientation of the ADCM will be level. When
the motion and orientation of the ADCM is known, it is then known
where the ADCM is pointing. Data is then recorded through the
motion cycles and a picker picks the data that corresponds to the
ADCM actually pointing at the predefined point or location ahead.
Filtering may have to be applied in order to gather smooth data. An
advantage with this method is that the vessel's pitch motion may be
utilized in order to gather data for a variety of water depths.
These methods and systems may also take advantage of using
gimbaling in, for example, the roll degree of freedom so as to make
it easier to identify instances where the ADCM is actually pointing
at the target point and at the same time have both beams leveled to
gather data at the same water depth.
[0055] A variation of the previous motion compensation system and
method utilizes a moveable platform rather than a fixed platform
36, and this system and method are illustrated in FIGS. 6A and 6B.
Instead of just monitoring the motion and the orientation of the
ADCM, and leaving the ADCM fixed to the vessel, one may actually
actively correct for the motion so that the ADCM always points at
the pre-defined target ahead. This system and method utilizes the
same motion sensing and calibration system as the system and method
just mentioned in reference to FIGS. 5A and 5B. In addition, an
active control system comprised of electric or hydraulic actuators
controlling at least two motions, for example the pitch and roll of
the ADCM, is added in the form of a motion platform. A commercially
available hull-mounted motion-compensated acoustic Doppler
transducer is available from Reson A/S, Slangerup, Denmark, under
the trade designation "SeaBat 8101", with optional pitch
stabilization. This option provides the ability for the system
processor to receive an external vessel pitch value and in
real-time automatically maintain a vertical transmit beam.
[0056] Motion platforms are ubiquitous in the computer gaming
industry, for example, and in flight simulators and amusement
rides; many are home-built. Electric, hydraulic, or pneumatic
actuators may drive them. One motion platform useful in the
invention may be that described in U.S. Pat. No. 6,027,342. Another
may be that disclosed in U.S. Pat. No. RE 27,051, which employs a
classic "hexapod" or "Stewart" configuration of six hydraulic legs
to provide controlled motion in six degrees of freedom. Modern
versions may be controlled by a standard PC-type computer running
Microsoft Windows.TM. and equipped with suitable control software,
and may include a local controller connected by a USB connector.
The software may be manually or automatically controlled, and may
have the ability to store and replay motion profiles, and interface
to a supervisory controller for real time or near-real-time
control. Depending on the degrees of freedom of motion desired, the
motion platform may comprise two, three, four, five, or six
hydraulic or electronic actuators, one end of each fixed to a base,
the other end fixed to a moveable deck. The base and deck may be of
any configuration, such as rectangular, triangular, oval, circular,
and the like. By separately controlling the leg extensions of the
actuators, the motion platform may produce any combination of
surge, sway, heave, yaw, pitch and roll motions. High-bandwidth
servo valves may operate the actuators, and a hydraulic pumping
unit is included in hydraulic systems. Leg extension may be
controlled by servo controllers, and read by linear potentiometers
integrated into each leg. The USB interface may allow communication
with the supervisory controller, a host computer, or other
device.
[0057] In one basic configuration, the ADCM may be placed and fixed
on a two, three, four, five, or six-degrees-of-freedom motion
platform 36. Motion platform 36 may be moved by one or more
actuators 38, controlled locally by a local controller 40, or
directly by a supervisory controller (not shown), or combination
thereof. If desired, local controller 40 or the supervisory
controller may send new motion commands to the motion platform,
closing one cycle of data-loop. It is not necessary to control for
heave as long as one may correct for this with the pitch so as to
still aim at the pre-defined point ahead. Local controller 40 may
gather information from the operator and motion platform 36, and
based on this information give commands to the actuators 38. The
system and method of this embodiment may also be configured so that
the ADCP continuously scans a vertical water column with controlled
pitch motions and thereby continuously record the water velocity in
the pre-defined water column of interest. This system has the
advantage that, if implemented with control of all rotational
degrees of freedom, the aim point may always be hit if the
actuators are controlled in an optimum way.
[0058] FIG. 7 illustrates a side elevation view of another system
of the invention, comprising vertical pole 50 mounted somewhere on
seismic vessel 2 via brackets 52 and 53. The local heave motion at
that location is comprised of both the heave motion of the COG and
local heave components derived from pitch and roll. An ADCM 18 is
mounted to the pole in such a way that it is allowed to slide up
and down the pole and mounted to it is a buoyancy element 54 that
rides on the sea surface. Buoyancy element 54 may or may not be
connected to pole 50, but if it is it should be a sliding
connection in the same manner as for the ADCM. The dynamic system
comprised of buoyancy element 54 and ADCM 18 comprises a mass
dominated dynamic system so that it should be relatively
insensitive to the wave motion. This means that the volume of
buoyancy element 54 should be limited and its water plane area
should be as small as possible while still being able to sustain an
average depth of ADCM 18 without too much oscillation about that
average value. If pole 50 pitches with the vessel, ADCM 18 will
pitch as well. However a gimbaling system (FIGS. 4A and 4B) may be
combined with this system so as to ensure the ADCM beam(s) are
always horizontal, or nearly so. This applies for roll as well.
Gimbaling the roll motion transmits the beams at the same, or
nearly the same water depth.
[0059] FIG. 8 is a schematic block diagram of several control
schemes that may be utilized to control vessel motion using a
vessel-mounted current meter and information supplied by other
sensors, databases, and survey parameters. FIG. 8 represents a few
options in control schemes that are useful in practicing the
invention, and those of ordinary skill in the art will recognize,
after reading this disclosure, that variations abound. As long as
the control scheme uses data gathered by a vessel-mounted ADCM or
H-ADCM to assist in steering one or more spread elements, the
control scheme is considered within the invention. A supervisory
controller 60 may receive data 61 from a vessel-mounted ADCM 18;
data 75 from a database or other source 64 of non-current
environmental data (for example wind data, water salinity data,
water temperature, air temperature, and the like); data 77 from a
survey design database 66; and manual data 83 or other input from a
human operator 70 through a visual display 68 receiving data 79
from supervisory controller 60. Interaction 81 between visual
display 68 and human operator 70 may be visual, audio, touch or any
other transmission means. Supervisory controller 60 may also
receive feedback or status data 73 from one or more spread control
elements 72N (vessel rudder, deflectors, steerable birds, and the
like). Vessel-mounted ADCM 18 may directly control one or more
spread control elements 72N via local controllers 62N as indicated
by command lines 63 and 71. In these instances, spread control
elements 72N would feedback data to local controllers 62N, and may
feedback data 73 to supervisory controller 60. In one option,
supervisory controller 60 sends commands 65 directly to spread
control elements 72N. In another option, supervisory controller 60
may send master commands 69 to one or more local controllers 62N in
cascade control fashion, modifying feedback signals 67 as desired
by operator 70 or programmed by supervisory controller 60. If data
61 on current velocity obtained by ADCM 18 is sent to supervisory
controller 60, the latter may transmit the raw or modified data 85
to an optional motion compensation software application 74, which
may utilize one or more software algorithms, for example
interpolation, extrapolation, and the like, and return modified
data 87 to supervisory controller 60 for use in controlling one or
more spread control elements 72N. Alternatively, optional software
application 74 may be locally stored in one or more local
controllers 62N. Other options include supervisory controller 60
sending data 89 (which may include some or all of the input data
61, 75, 77, 83 and motion compensation data 87) to a software
application 76 for calculating optimum tracks 93 using a software
application 78.
[0060] In use, systems and methods of the invention are
particularly adept for 3D and so-called 4D seismic data acquisition
surveys. More specifically, the systems and methods of the
invention may be integrated into the seismic towing vessel steering
strategy, and may be integrated into positioning strategies for the
other spread elements. In time-lapse or so-called 4D seismic, the
source and receivers may be positioned to within a few meters of a
baseline survey in order to gather a good picture of the evolution
of a reservoir over time. The geophysical requirement for the
accuracy of the repositioning varies with the geological structure
and the expected time-difference signal, but generally a 10 meter
positioning discrepancy is allowed, and often a bigger mismatch is
allowed due to practicalities regarding the historical
repositioning abilities. It is desired to position the source to
within 5 meters, and the streamers to within about 10 meters of
their previous tracks. Knowing, or at least having a good
approximation of, the current ahead of the vessel may be helpful in
order to meet these targets as it allows for corrective actions to
be taken before it is too late. One use of systems and methods of
the invention is to make an approximate positioning by seismic
towing vessel steering and to fine tune by positioning the
individual spread elements behind the seismic towing vessel, i.e.
the source and the streamers, if present. One optional strategy
involves manual control combined with closed loop control of the
individual steering elements. A second optional strategy involves
use of fully integrated, multilayer regulators. In both strategies
at least a horizontal component of the current velocity ahead of
the vessel is determined or closely approximated using a
vessel-mounted ADCM. In the first optional strategy the steering
software suggests inputs to the steering elements (vessel, source
streamer) based on current, wind, and other external forces. Then
it is up to the human operator to judge the sanity of the
information that comes in and approve or correct the steering
commands. In the second optional strategy, the vessel and the other
positionable elements accept the data obtained from the
vessel-mounted ADCM about the current, as well as other non-current
environmental data, survey design data, and the like, and steer
accordingly in order to minimize the re-positioning error.
[0061] Systems and methods of the invention may employ any number
of spread control elements, which may include one or more
orientation members, a device capable of movements that may result
in any one or multiple straight line or curved path movements of a
spread element in 3-dimensions, such as lateral, vertical up,
vertical down, horizontal, and combinations thereof. The terms and
phrases "bird", "cable controller", "streamer control device", and
like terms and phrases are used interchangeably herein and refer to
orientation members having one or more control surfaces attached
thereto or a part thereof. A "steerable front-end deflector" (or
simply "deflector") such as typically positioned at the front end
of selected streamers, and other deflecting members, such as those
that may be employed at the front end of seismic sources or source
arrays, may function as orientation members in some embodiments,
although they are primarily used to pull streamers and steer
sources laterally with respect to direction of movement of a tow
vessel. Horizontal separation between individual streamers may
range from 10 to about 200 meters. In the embodiment of FIG. 1 the
horizontal streamer separation may be consistent between one
streamer 6 and its nearest neighboring streamers 6. Horizontal
and/or vertical control of streamers 6 may be provided by
orientation members (not illustrated) which may be of any type as
explained herein, such as small hydrofoils or steerable birds that
can provide forces in the vertical and/or horizontal planes. One
suitable orientation member is the device known under the trade
designation Q-FIN.TM., available from WesternGeco LLC, Houston,
Tex., and described in U.S. Pat. No. 6,671,223, describing a
steerable bird that is designed to be electrically and mechanically
connected in series with a streamer; another suitable device is
that known under the trade designation DigiBIRD.TM., available from
Input/Output, Inc., Stafford, Tex. Other streamer positioning
devices, such as the devices described in U.S. Pat. Nos. 3,774,570;
3,560,912; 5,443,027; 3,605,674; 4,404,664; 6,525,992 and EP patent
publication no. EP 0613025, may be employed.
[0062] Current meters in general, including the
vessel-mountedcurrent meters useful in the present invention,
measure water velocity referring to the vessel fixed coordinate
system. In order to extract the current related to the earth fixed
coordinate system, supervisory controller 60 in FIG. 8 may have the
capability to relate the current velocity measurements obtained in
the vessel fixed coordinate system to the earth fixed coordinate
system by measuring the current speed over ground by using any kind
of positioning system relating to an earth fixed coordinate system.
These calculations are well-known and require little further
explanation here.
[0063] Systems of the invention may communicate with the outside
world, which may be the vessel to which it is attached, or another
vessel or vehicle, a satellite, a hand-held device, a land-based
device, and the like. The way this may be accomplished varies in
accordance with the amount of energy the system requires and the
amount of energy the system is able to store locally in terms of
batteries, fuel cells, and the like. If housing 18 is large enough,
batteries, fuels cells, and the like may be housed therein, and
wireless communication may be sufficient. Alternatively, or in
addition, there may be a hard-wire power connection and a hard wire
communications connection to another device, this other device able
to communicate via wireless transmission.
[0064] In use the systems and methods of the invention may work in
feed-forwarded fashion with existing control apparatus and methods
to position not only the seismic tow vessel, but seismic sources
and streamers. Sources and streamers may be actively controlled by
using GPS data or other position detector sensing the position of
the streamer (e.g. underwater acoustic network), or other means may
sense the orientation of one or more individual streamers (e.g.
compass) and feed this data to navigation and control systems.
Gross positioning and local movement of spread components may be
controlled on board a tow vessel, on some other vessel, locally, or
indeed a remote location. By using a communication system, either
hardwire or wireless, information regarding current velocity ahead
of the vessel may be sent to one or more local controllers, as
described herein. The local controllers in turn are operatively
connected to spread control elements comprising motors or other
motive power means, and actuators and couplers connected to the
orientation members (flaps), and, if present, steerable birds,
which function to move the spread components as desired. This in
turn adjusts the position of the spread element, causing it to move
as desired. Feedback control may be achieved using local sensors
positioned as appropriate depending on the specific embodiment
used, which may inform the local and remote controllers of the
position of one or more orientation members, distance between
streamers, a position of an actuator, the status of a motor or
hydraulic cylinder, the status of a steerable bird, and the like. A
computer or human operator can thus access information and control
the entire positioning effort, and thus obtain much better control
over the seismic data acquisition process.
[0065] Although only a few exemplary embodiments of this invention
have been described in detail above, those skilled in the art will
readily appreciate that many modifications are possible in the
exemplary embodiments without materially departing from the novel
teachings and advantages of this invention. Accordingly, all such
modifications are intended to be included within the scope of this
invention as defined in the following claims. In the claims, no
clauses are intended to be in the means-plus-function format
allowed by 35 U.S.C. .sctn. 112, paragraph 6 unless "means for" is
explicitly recited together with an associated function. "Means
for" clauses are intended to cover the structures described herein
as performing the recited function and not only structural
equivalents, but also equivalent structures. Thus, although
electronic and hydraulic motion platforms may not be structural
equivalents in that an electronic motion platform employs one type
of actuator, whereas a hydraulic motion platform employs a
different type of actuator, in the environment of motion platforms
for motion compensation, electronic and hydraulic motion platforms
may be equivalent structures.
* * * * *