Note: Descriptions are shown in the official language in which they were submitted.
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A Seismic Source, a Marine Seismic Surveying Arrangement, a Method of
operating a Marine Seismic Source, and a Method of De-ghosting Seismic Data
The present invention relates to a seismic source, in particular to a source
for use in
marine seismic surveying. The present invention also relates to a marine
seismic
surveying arrangement including a source, to a method of operating the source
and to a
method of de-ghosting marine seismic data.
The principle of marine seismic surveying is shown schematically in Figure 1.
Seismic
energy emitted in a generally downwards direction from a source of seismic
energy 1 is
reflected by the sea bed 2 and by the earth strata or geological structures
beneath the sea
bed, and is received by an array of seismic receivers 3 such as hydrophones.
Analysis
of the energy received at the receiving array 3 can provide information about
the earth
strata or geological structures beneath the seabed. In the marine seismic
surveying
arrangement shown in Figure 1, the source of seismic energy 1 is suspended
from a
survey vessel 4 and the array of seismic receivers 3 is towed by the survey
vessel 3.
One problem associated with conventional marine seismic surveying is that of
"ghost
reflections". Ghost reflections occur when upwardly travelling seismic energy
is
reflected or scattered downwards at the sea surface. A related problem in
marine
seismic surveying is that of "reverberations". Reverberations occur when
seismic
energy is reflected between the sea surface and the sea-bed. The problems of
ghost
reflections and reverberations are explained in Figures 2(a) to 2(d).
Figure 2(a) shows a "primary reflection". Seismic energy is emitted downwards
by the
source l, is reflected by a geological feature below the sea bed,,and the
reflected signal
is detected at the receiver 3. An analysis of the seismic signal generated by
the primary
reflection provides information about the geological feature responsible for
reflecting
the seismic energy. (In practice, refraction may occur at the sea-bed, but
this has been
omitted from Figures 2(a) to 2(d) for clarity.)
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Figure 2(b) shows a ghost reflection. Seismic energy that has been emitted
upwards by
the source is reflected or scattered downwards by the sea surface. The seismic
energy
that is reflected or scattered downwards may then be incident on the target
geological
feature, undergo reflection, and be reflected to the receiver. Seismic energy
that follows
the path shown in Figure 2(b) will have a different travel time from the
source to the
receiver than will energy that follows the primary path of Figure 2(a). Ghost
reflections
are an undesirable source of contamination of seismic data since they tend to
obscure
the interpretation of data produced by the primary reflection.
Figures 2(c) and 2(d) show reverberations, in which seismic energy undergoes
reflections between the sea-bed and the sea-surface. Reverberations can occur
in the
case of seismic energy emitted in an up-going direction by the source (Figure
2(c)) and
also in the case of seismic energy emitted in a down-going direction by the
source
(Figure 2(d)). As is the case for ghost reflections, reverberations are an
undesirable
source of contamination of seismic data, since they obscure the interpretation
of the
primary reflection from the earth's interior.
Figures 2(b), 2(c) and 2(d) show source-side ghost reflections and
reverberations - that
is, ghost reflections and reverberations that occur before the seismic energy
is reflected
by the target geological structure. (Indeed it will be noted that the path of
seismic
energy shown in Figure 2(d) does not involve a reflection by the target
geological
structure.) Ghost reflections and reverberations can also occur after the
seismic energy
has been reflected from the target geological structure, and these are known
as receiver-
side ghost reflections or reverberations.
A number of'scherties'for minimising the effect of ghost reflections and
reverberations
on seismic data have been proposed. For most survey arrangements, the
attenuation of
ghost reflections and reverberations is equivalent to separating the up-going
and down-
going seismic wave fields.
F. J. Barr and J. J. Saunders have proposed, in a paper presented at the 59th
SEG
Meeting (1989), a method of attenuating ghost reflections and reverberations
by
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recording the reflected seismic signal using two different types of seismic
receivers,
namely using both hydrophones and geophones. The up-going wave field is
recorded
by the hydrophone and the geophone with the same polarity, while the down-
going
wave field is recorded by the hydrophone and the geophone with opposite
polarities.
The difference between the signal recorded by the hydrophone and the signal
recorded
by the geophone allows the up-going wavefield to be separated from the down-
going
wavefield.
An alternative method for attenuating ghost reflections and reverberations is
to use two
receivers located at different depths. This method is based on the principle
that waves
travelling in different directions will have spatial derivatives of different
signs, so that
comparing the signal obtained at one receiver with the signal obtained by the
other
receiver will allow the up-going wavefield to be separated from the down-going
wavefield.
These prior art methods separate the up-going and down-going wave fields at
the
receiver location. That is, they attempt to remove the ghost reflections and
reverberations that arise after the seismic energy has been reflected by the
target
geological structure. This is known as receiver-side deghosting. These prior
art
methods do not, however, address the problem of the ghost reflections and
reverberations that occur before the seismic energy is reflected by the target
geological
structure.
A first aspect of the present invention provides a marine seismic source
comprising: a
first array of N emitters of seismic energy, where N is an integer greater
than l; and a
second array~of N.emitters of seismic energy; wherein, in use, the first array
is disposed
at a first depth and the second array is disposed at a second depth greater
than the first
depth, and the j'h emitter of the first array (j = l, 2...N) is displaced by
anon-zero
horizontal distance dH from the jih emitter of the second array along a first
direction; and
the jth emitter of the first array and the jth emitter of the second array
both lie in a
vertical plane parallel to the first direction.
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The use of two axrays of emitters of seismic energy at different depths allows
the up-
going and down-going seismic wavefields to be separated from one another, as
will be
described below. The effect of source-side ghost reflections and
reverberations on the
seismic data can be reduced or eliminated.
A second aspect of the present invention provides a marine seismic surveying
arrangement comprising a marine seismic receiver; and a seismic source as
defined
above; means for moving the seismic source; and one or more seismic receivers.
A third aspect of the present invention provides a method of operating a
marine seismic
source as defined above, the method comprising the steps of: moving the
seismic source
at a speed v along the first direction; firing one of the first and second
arrays of emitters
of seismic energy; and firing the other of the first and second arrays of
emitters of
seismic energy after a time dr1/v. The time delay of dH/v between the firings
of the two
arrays of seismic sources ensures that each emitter of one array is fired at
the same point
in the x- and y-directions as the corresponding emitter of the other array,
but at different
depths. This allows the seismic data generated by one of the arrays to be used
to de-
ghost the seismic data generated by the other of the arrays.
A fourth aspect of the present invention provides a method of processing
marine seismic
data comprising the steps of firing a first emitter of seismic energy at a
point in a fluid
medium having components (x1, yi, z~), and detecting the resultant first
seismic data at a
receiver array; firing a second emitter of seismic energy at a point in the
fluid medium
having components (x~, y~, z2), where z~ ~ z2, and detecting the resultant
second seismic
data at the receiver array; and using the second'seismic data to reduce the
effects of
source-side reflections and/or scattering at.the sea surface on the first
seismic data.
Preferred features of the invention are set out in the dependent claims.
Preferred embodiments of the present invention will now be described by way of
illustrative examples with reference to the accompanying figures, in which:
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Figure 1 is a schematic view of a typical marine seismic surveying
arrangement;
Figures 2(a) to 2(d) are schematic illustrations of the problems of ghost
reflections and
reverberations;
Figure 3 is a schematic view of a vertical source array illustrating the
principles of the
de-ghosting method of the present invention;
Figure 4 is a schematic illustration of a vertical source array according to
an
embodiment of the present invention;
Figure 5 shows a typical seismic signal recorded by a receiver in a marine
seismic
surveying arrangement that contains a seismic source according to an
embodiment of
the present invention;
Figure 6 shows the signal of Figure 5 after processing to attenuate source-
side ghost
reflections and reverberations;
Figure 7 illustrates the average amplitude spectrum of the signal of Figure 5;
and
Figure 8 illustrates the average amplitude of the signal of Figure 6.
Figure 3 illustrates the general principle of the de-ghosting method of the
present
invention. Figure 3 shows'a vertical source array that consists of two
emitters of
seismic energy S 1 and S2 that have identical emission characteristics to one
another.
The emitters are disposed in the water at two different depths. The upper
emitter S 1 is
disposed substantially vertically above the lower emitter S2.
The source array generates a seismic wavefield that has both up-going and down-
going
components. The wavefield travelling upwards generates source-side ghost
reflections
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and up-going reverberations in the water layer. The wavefield travelling
downwards
from the source array generates the primary reflection and also generates down-
going
reverberations.
Consider a hypothetical emitter of seismic energy S having identical emission
characteristics to the emitters S 1 and S2, placed at the mid-point between
the upper
emitter S 1 and the lower emitter S2. This emitter S would generate up-going
and down-
going source wavefields at a reference time t. The total wavefield S(t)
emitted by the
hypothetical emitter S is the sum of the up-going and down-going source
wavefields,
that is:
S(t) = t~(t) + d(t) (I)
In this equation, u(t) is the up-going source wavefield and d(t) is the down-
going source
wavefield emitted by the hypothetical emitter S.
The emitters Sl and S2 generate up-going and down-going wavefields. These
wavefields can be described; relative to time t, by the following equations:
Silt) = u(t-dt) + d(t+dt) (2)
S2(t) = u(t+dt) + d(t-dt) (3)
In these equations, S~ is the wavefield emitted by the upper emitter S l and
SZ is the
wavefield emitted by the lower emitter S2. The time dt is the time that
seismic energy
would take to travel from the upper or lower emitter S 1 or SZ to tile
position of the
hypothetical emitter S. Since the hypothetical emitter S is at the mid-point
between the
upper emitter S 1 and the Iower emitter 5~., the time dt is equal to half the
time taken for
seismic energy to travel between the upper emitter S~ and the lower emitter SZ
or vice
versa.
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On the assumption that dt is small, the terms in equations (2) and (3) can be
expanded
using a first-order Taylor expansion, as follows:
S, (t) = ac(t) - u'(t)dt + d(t) + d'(t)dt (4)
S2 (t) = u(t) + u'(t)dt + d(t) - d'(t)dt (s)
In equations (4) and (s), u'(t) and d'(t) are the time derivatives of u(t) and
d(t)I,
respectively.
The sum of the two source wavefields S~ and Sz and the difference between the
two
source wavefields S, and SZ can be derived from equations (~) and (s) as
follows:
Suns = SI(t) + Sa(t) = 2u(t) + 2d(t) (6)
Dif = Salt) - Silt) = 2u'(t)dt - 2d (t)dt (7)
Integrating both sides of equation (7) with respect to time leads to the
following result:
Intdif = 2u(t)dt - 2d(t)dt (8)
Equations (6) and (8) may now be combined, to eliminate u(t). This leads to
the
following expression for the down-going source wavefield d(t):
d(t) _ (Sum - Ir~tdif l dt)l4 (9)
Thus, by using a vertical source array that consists of two emitters of
seismic energy
that have identical emission characteristics, with one emitter disposed above
the other, it
is possible to derive the down-going source wavefield d(t) using equation (9)
above.
This allows the effect of the up-going wavefield u(t) to be eliminated when
seismic data
acquired using the source is processed.
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The principle of reciprocity is a fundamental principle of wave propagation,
and states
that a signal is unaffected by interchanging the location and character of the
sources and
receivers. For example, if a surveying arrangement with an array of seismic
sources at
point A and a receiver at point B gives a certain signal at the receiver, then
using a
receiver array at point A and a single source at point B would lead to the
same signal,
provided that the source array corresponds to the receiver array. (By
"corresponds") it
is meant that the source array contains the same number of sources as the
receiver array
has receivers, and that the sources in the source array are arranged in the
same locations
relative to one another as the receivers in the receiver array.)
One consequence of the principle of reciprocity is that the theory described
above with
relation to equations (1) to (9) above could be used for wave field separation
using two
vertically separated receivers. This would provide a method of receiver - side
de-
ghosting, which would enable the up-going wave field at the receiver, which
contains
the primary reflection, to be separated from the down-going wavefield caused
by
reflection or scattering at the sea surface.
The above discussion relates to a vertical source array that contains just two
emitters,
with one emitter being disposed above the other. However, the same principle
can be
applied to a source that comprises a frst array of two or more emitters of
seismic energy
disposed above a second array of two or more emitters of seismic energy. It
is,
however, necessary for the first and second arrays of emitters to have
substantially
identical emission characteristics to one another - that is, each emitter
array must
contain the same number of emitters, and each emitter in one array must have
identical
emission characteristics to the corresponding emitter in the other array.
Furthermore,
the relative arrangement and separation of the emitters in one array must be
the same as
the relative arrangement and separation of the emitters in the other array.
If the upper and lower emitters S I and S2 were fired simultaneously, a
receiver would
record the combination of the wavefield generated by the upper emitter S1 and
the
wavefield generated by the lower emitter S2. It would therefore not be
possible to
apply the de-ghosting method outlined above, since the difference between the
two
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wavefields would not be known. To apply the method using the seismic source
shown
in Figure 3, it would be necessary to maintain the source stationary in the
water, and fire
the two emitters one after the other. This would generate two distinct
wavefields S~, S'2
that could be recorded separately and processed according to equations (I) to
(9).
However, it would be inconvenient in practice to have to hold the source
stationary in
the water.
In principle, the two separate wavefields required for the de-ghosting method
could also
be obtained by using firing a single emitter at one depth, altering the depth
of the
emitter, and firing the emitter again. However, this method would also be
inconvenient
to carry out.
In a preferred embodiment of the present invention, therefore, a staggered
vertical
source is used consisting of two emitters or of two emitter arrays, with, in
use, one
emitter or emitter array being disposed at one depth and the other being
disposed at a
different depth. The two emitters, or two emitter arrays, are displaced
horizontally with
respect to one another. In use, the source is moved through the water in the
direction
along which the emitters, or emitter arrays, are displaced. There is a time
delay between
the firing of one of the emitters or emitter arrays and the firing of the
other emitter or
emitter array. The time delay between the firings and the speed of movement of
the
source are chosen such that, in the case of a source having just two emitters,
the point at
which the upper emitter is fired has the same x- and y-co-ordinates as the
point at which
the lower emitter is fired. In the ease of a source having two arrays of
emitters, the time
delay between firing one array and firing the other array is chosen so that
the point at
which an emitter in one array is fired has the same x- and y-co-ordinates as
the point at
which the corresponding emitter in the other~array is~ired, for all emitters
in the array.
Thus, the invention makes it straightforward to generate identical seismic
wavefields at
different depths but at the same x- and y-co-ordinates. The seismic data
generated by
one wavefield can then be used to de-ghost the seismic data generated by the
other
wavefield, using equation (9) above.
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Figure 4 shows an embodiment of the invention in which the source includes two
arrays
10, 1 I each having two emitters of seismic energy S I 1, S 12; 521, S22. The
four
emitters S 11, S 12; 521, S22 have substantially identical emission
characteristics to one
another. The separation between the two emitters S 1 I, S 12 of the first
array I O is
substantially equal to the separation between the two emitters S21, S22 of the
second
array. In this embodiment, one array I O is disposed at a depth of four
metres, whereas
the other array 11 is disposed at a depth of 10 metres. The axis of each
emitter array is
preferably horizontal, so that each emitter SI I, SI2 of the first array 10 is
at a depth of
4 metres and each emitter S2I, S22 of the second array 11 is at a depth of
10m. The
source is intended to be moved through the water at a speed v, and this is
most
conveniently done by towing the source from a survey vessel, as shown in
Figure 1.
In addition to being separated in the vertical direction (z-direction), the
two arrays are
also displaced in a horizontal direction. The direction of displacement of the
two arrays
is the direction in which the source is towed in use. The arrays are displaced
by a
horizontal distance dH. In Figure 4, the direction in which the arrays are
displaced, and
in which the source is moved in use, is chosen to be the x-direction for
convenience of
description.
The two arrays are not displaced in the direction perpendicular to the
direction of
movement of the source (in Figure 4 this is the y-direction and extends out of
the plane
of the paper). An emitter of one array and the corresponding emitter of the
other array
are both disposed in a common vertical plane, that is parallel to the
direction of
movement of the source.
The difference in depth between the first and second emitter arrays should be
chosen
such that 1/dt < fmax, where fmax is the maximum frequency in the seismic
data. The
time dt is determined by the depth difference between the two emitter arrays
and by the
velocity of seismic energy in water, which is a known quantity. The embodiment
of
Figure 4 is intended for use with a maximum frequency fmaX <_ 90Hz, and a
depth
difference of 6m has been found to be acceptable in this case.
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As noted above, the two emitter arrays of the source shown in Figure 4 have a
horizontal displacement, dH. The horizontal displacement is measured between
an
emitter of the array nearer the towing vessel and the corresponding emitter of
the array
further from the towing vessel.
The marine seismic source shown in Figure 4 can be used in a marine seismic
surveying
arrangement. In addition to the source, the arrangement would also comprise
one or
more seismic receivers, and means, such as a towing vessel, for moving the
source
through the water. The marine seismic surveying arrangement would also
comprises
control means for firing the emitters, and recording means for recording
seismic data
acquired by the receiver(s).
In a particularly preferred embodiment, the horizontal displacement between
the two
emitter arrays is substantially equal to the shot point interval of the marine
seismic
surveying arrangement. Thus, for a seismic surveying arrangement that
generates a shot
point interval of, for example, 25m, the horizontal displacement of the
emitter arrays of
the seismic source is preferably approximately 25m.
In this embodiment, the emitter arrays are fired in a "flip-flop" sequence at
equal
intervals of, in this example, 25m. That is to say, the emitters on the array
nearer the
towing vessel are fired initially and they may be fired consecutively, or
simultaneously.
After a time delay that is equal to the time required for the towing vessel to
travel 25m,
the emitters of the array further from the boat are f red. This results in two
shot records
generated at points having the same x-co-ordinate and the same y-co-ordinate,
but at
different depths.
In Figure 4 the array at the shallower depth is shown as the array nearer the
towing
vessel. The invention is not limited to this, however, and the array at the
shallower
depth could be the array further from the towing vessel.
The signals generated at the receiver or receiver array as the result of
firing the first
emitter array and subsequently firing the second emitter array are recorded in
any
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conventional manner. Since, as explained above, the signals were emitted by
the two
emitter arrays at the same x- and y- co-ordinates but at different z-co-
ordinates, the
results can be analysed using the theory outlined above with regard to
equations (1) to
(9). In particular, by calculating the sum of the two signals and the integral
with respect
to time of the difference between the two signals, it is possible to compute
the down-
going source wavefield using equation (9). Thus, the present invention enables
the
effects of the up-going source wavefield to be removed from the processed
seismic data.
The effect of source-side ghost reflections and reverberations is thus
eliminated, or at
least signifcantly reduced.
Results obtained using a seismic source according to the present invention and
the de-
ghosting method of the present invention are illustrated in Figures 5-8. These
figures
relate to a survey carried out using a source having two emitter arrays, each
array
having two marine vibrator arrays as the seismic emitters. The source was
towed with
the arrays at depths of 4m and l Om respectively, with a 25m in line
displacement (by
"in-line displacement" is meant displacement along the towing direction)
between the
two arrays. The average water depth was 52m. An ocean bottom cable (OBC) dual
sensor cable, 1 Okm in length, disposed on the sea bed was used as the
receiver. The two
arrays of marine vibrators were fired in a flip-flop mode as described above.
The parameters of the survey arrangement are as follows:
Number of receiver stations: 204
Receiver interval: 25m
Receiver depth: 52m
Sweep bandwidth: 5-90Hz
Fold: 90.
The data recorded in the OBC sensors as a result of firing the emitter array
at a depth of
l Om is shown in Figure 5. This shows the data after preliminary processing
operations.
The emitter array at a depth of 4m generated another record (not shown) at the
same x, y
location.
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Figure 6 illustrates the data of Figure 5 after processing, using equation (9)
and the data
recorded using the emitter array at a depth of 4m, to remove the up-going
wavefield.
That is, Figure G shows the data of Figure 5 after de-ghosting to remove the
effect of
source-side ghost events and reverberations.
Figures 7 and 8 show the average amplitude spectra for the seismic data of
Figures 5
and 6 respectively. It will be seen that the resolution and the signal-to-
noise ratio have
both been improved by de-ghosting process.
In the preferred embodiment described above, the seismic source consists of
two arrays
each containing two marine vibrator units. The present invention is not,
however,
limited to this precise arrangement. For example, each of the source arrays
could
contain more than two emitters of seismic energy. Moreover the de-ghosting
method of
the present invention could in principle be applied if seismic data acquired
using a
single seismic emitter at one depth and seismic data acquired using an emitter
having
identical emission characteristics at a different depth (but at the same x-
and y-co-
ordinates) is available.
In the embodiment shown in Figure 4, each receiver array is an in-line emitter
array -
that is, the emitters of each array are arranged along the axis of the array.
The axis of
each array is coincident with the towing direction when the source is in use.
The
invention is not, however, limited to use with in-line emitter arrays.
Furthermore, the seismic source of the invention is not limited to a source
that contains
marine vibrator units. The source could also consist of arrays of other
emitters of
seismic energy such as, for example, air guns.
