Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Offshore prospecting
Reexamination Certificate
2002-05-24
2003-09-09
Lobo, Ian J. (Department: 3663)
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Offshore prospecting
Reexamination Certificate
active
06618321
ABSTRACT:
FIELD OF THE INVENTION
The invention relates in general to operations performed at sea for acquiring geophysical data by means of a ship towing one or more cables associated with hydrophones.
More precisely, the invention relates to a method enabling the deformation of the cables towed by a ship to be predicted effectively, and it enables advantage to be taken of such prediction.
BACKGROUND OF THE INVENTION
The purpose of geophysics is to describe the structure of the subsoil. The technique in widest use is reflection seismic surveying. When surveying off-shore, the principle is to emit a high power sound pulse towards the subsoil; the sound wave created in this way is reflected partially at the interfaces between the successive geological layers it encounters, and it returns towards the surface where hydrophones transform the sound signal into electrical signals.
In off-shore surveying, the sound source is generally constituted by air guns, and the hydrophones are grouped together in groups which are integrated in cables known as “streamers” that are towed by the ship.
The number of sound sources and streamers, and the lengths of the streamers can be varied. Depending on required resolution, the distance between two consecutive groups varies over the range 12.5 meters (m) to 25 m. A simple configuration having two streamers S
1
and S
2
, a single sound source Sa, and a plurality of groups T is shown in FIG.
1
.
This figure also shows the so-called “common midpoints” (CMPs) that, for each {source, group} pair correspond to the subsurface point of reflection.
In practice, interaction between sea currents and immersed streamers gives rise to geometrical deformations in the system constituted by said streamers and the set of towed elements, thereby compromising the uniformity of coverage in the zone whose subsoil is to be characterized. These deformations vary in time and give rise to coverage “holes” which need to be filled in by additional passes of the boat, a process known as “infilling”.
This constitutes a major drawback since additional passes increase the time required to perform operations and can give rise to very significant increases in cost (which can be as much as 20%).
In addition, the extra time and cost associated with infilling can vary very greatly from one operation to another, and it is therefore not possible to predict them accurately, thus preventing operators from giving accurate predictions concerning the time and cost of a projected operation; this constitutes an additional drawback for operators.
It will thus be understood that there exists a manifest need to reduce infilling and also to predict the amount of infilling that will be required in a projected data acquisition operation. In order to satisfy these needs, it is necessary to characterize the influence of current on streamer deformation.
In this respect, attempts have been made to model the deformation of a streamer towed by a ship and subjected to current. For example, reference can be made to the article “The shape of a marine streamer in a cross-current” by P. P. Krail and H. Brysk, published in Vol. 54, No. 3 of the journal of the Society of Exploitation Geophysicists.
However, such attempts do not reproduce real current conditions (the article mentioned assumes in particular that the current is steady and the ship follows a uniform rectilinear path) and a result the results thereof are unsuitable for being used directly so the above drawbacks remain.
BRIEF SUMMARY OF THE INVENTION
The object of the invention is to enable those drawbacks to be reduced.
To achieve this object, the invention firstly provides a method of simulating the positioning of a streamer towed by a ship during an operation of acquiring geophysical data at sea, said acquisition operation making use of shots from at least one sound source, the method implementing a hydrodynamic model of the interaction between marine current, the path of the ship, and the streamer, the method being characterized in that it includes determining variations in the current over time and in space.
Other preferred, but non-limiting features of the method of the invention for simulating the positioning of a streamer are as follows:
the method comprises:
receiving primary current values as measured and/or predicted;
defining vector fields or ‘current objects’ of respective types corresponding to different representations of the current and built up from said primary current values; and
selecting a ‘current object’ as a function of the intended application;
‘current object’ selection takes account of proximity in time between the instant for which the prediction is made and the instant at which prediction is performed;
‘current object’ selection takes account of correlation between earlier ‘current object’ predictions and measurements of current performed at the instants for which said earlier predictions were made;
the coordinates of at least some ‘current objects’ comprise values measured on site;
the coordinates of at least some ‘current objects’ comprise extrapolated values predicting current;
some ‘current objects’ are computed by using a predictor filter enabling a current data series to be extrapolated from measurements of current made in the acquisition zone;
the defined types of ‘current object’ comprise the following types:
1) total current as measured by a current meter;
2) tidal current as derived from meteorological bulletins, or as deduced from measurements of current by harmonic analysis;
3) the sum of a tidal current plus a residual current, said tidal current being derived from meteorological bulletins or being deduced from measurements of current by harmonic analysis, and said residual current being taken from meteorological bulletins;
4) an extrapolation from total current as measured by a current meter;
5) the sum of a tidal current and a computed residual current, said tidal current being taken from meteorological bulletins or being deduced from measurements of current by harmonic analysis, and said residual current being obtained by subtracting said tidal current from the current measured in the acquisition zone;
6) a history of past extrapolations of the total current as measured by a current meter; and
7) the sum of a tidal current and a history of past extrapolations of a series of values constituted by the total current as measured by a current meter from which a tidal current has been subtracted, said tidal current being taken from meteorological bulletins or being deduced from measurements of current by harmonic analysis;
while computing ‘current objects’ of types 4, 5, 6, or 7, the processed data series is considered as a second order non-centered steady random process;
while computing values of a ‘current object’ of type 4, 5, 6, or 7, weights are given to the measurements of the data series for extrapolation, which weights are inversely proportional to their age, for the purpose of anticipating sudden changes due to the residual current;
while computing a particular value of a ‘current object’ of type 4, 5, 6, or 7, a variance function of the difference between the predicted value and the exact value of the current or the residual current at the instant for which the prediction was computed is minimized, where said variance function has the form:
G
=
(
1
-
1
-
∑
i
=
3
P
+
1
a
i
a
3
⋯
a
p
+
1
)
Γ
U
(
1
-
1
-
∑
i
=
3
P
+
1
a
i
a
3
⋮
a
P
+
1
)
while computing a particular value of a ‘current object’ of type 4, 5, 6, or 7, an autocorrelation function of the current or residual current data series is computed, and then a linear system of equations is set up and solved;
while computing a particular value of a ‘current object’ of type 4, 5, 6, or 7, the linear system to be solved is conditioned by implementing a descent method, preferably the conjugate gradient method;
the method provides the option of computing extrapolated values on a series of measured current values from which a tidal current has previously been subtracted
CGG Marine
Duane Morris LLP
Lobo Ian J.
LandOfFree
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