An improved de-ghosting method and system that utilizes multi-component marine seismic data recorded in a fluid medium. The method makes use of two types of data: pressure data that represents the pressure in the fluid medium, such as sea water, at a number of locations; and vertical particle motion data that represents the vertical particle motion of the acoustic energy propagating in the fluid medium at a number of locations within the same spatial area as the pressure data. The vertical particle motion data can be in various forms, for example, velocity, pressure gradient, displacement, or acceleration. A spatial filter is designed so as to be effective at separating up and down propagating acoustic energy over substantially the entire range of non-horizontal incidence angles in the fluid medium. The spatial filter is applied to either the vertical particle motion data or to the pressure data, and then combined with the other data to generate pressure data that has its up and down propagating components separated.
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1. A method of reducing the effects in seismic data of downward propagating reflected and scattered acoustic energy travelling in a fluid medium comprising the steps of:
receiving pressure data representing at least the pressure in the fluid medium at a first location and a second location, the first location being in close proximity to the second location ;
receiving vertical particle motion data representing at least the vertical particle motion of acoustic energy propagating in the fluid medium at a third second location and a fourth third location, the third second location being in close proximity to the fourth third location, and the first, second, and third and fourth locations being within a spatial area;
calculating a plurality of spatial filter coefficients based in part on the velocity of sound in the fluid medium, the density of the fluid medium and a plurality of acquisition parameters, thereby creating a spatial filter which is designed so as to be effective at separating up and down propagating acoustic energy over a range of non-vertical incidence angles in the fluid medium;
applying the spatial filter to the vertical particle motion data to generate filtered particle motion data;
combining the filtered particle motion data with the pressure data to generate separated pressure data, the separated pressure data having up and down propagating components separated; and
analysing at least part of the up or down propagating component of the separated pressure data,
and wherein said vertical particle motion data is measured using one or more multi-component streamers, or over/under twin streamers, or vertical cables having receivers located substantially above the sea floor.
0. 30. A data processor, comprising:
a central processing unit;
a memory;
a program residing on the memory that, when executed by the central processing unit, performs a method including:
receiving pressure data representing at least the pressure in the fluid medium at a first location;
receiving vertical particle motion data representing at least the vertical particle motion of acoustic energy propagating in the fluid medium at a second location and a third location, the second location being in close proximity to the third location location, and the first, second, and third locations being within a spatial area;
calculating a plurality of spatial filter coefficients based in part on the velocity of sound in the fluid medium, the density of the fluid medium and a plurality of acquisition parameters, thereby creating a spatial filter which is designed so as to be effective at separating up and down propagating acoustic energy over a range of non-vertical incidence angles in the fluid medium; applying the spatial filter to the vertical particle motion data to generate filtered particle motion data;
combining the filtered particle motion data with the pressure data to generate separated pressure data, the separated pressure data having up and down propagating components separated; and analysing at least part of the up or down propagating component of the separated pressure data,
and wherein said vertical particle motion data is measured using one or more multi-component streamers, or over/under twin streamers, or vertical cables having receivers located substantially above the sea floor.
18. A method of reducing the effects in seismic data of downward propagating reflected and scattered acoustic energy travelling in a fluid medium comprising the steps of:
receiving pressure data representing at least variations in pressure in the fluid medium at a first location, the variations caused in part by a first source event and a second source event, the first source event and the second source event being within a spatial area;
receiving vertical particle motion data representing at least the vertical particle motion of acoustic energy propagating in the fluid medium at a second location, the acoustic energy being caused in part by the first source event and the second source event;
calculating a plurality of spatial filter coefficients based in part on the velocity of sound in the fluid medium and the density of the fluid medium, thereby creating a spatial filter which is designed so as to be effective at separating up and down propagating acoustic energy from the first source event and second source event over a range of non-horizontal incidence angles in the fluid medium;
applying the spatial filter to the vertical particle motion data to generate filtered particle motion data;
combining the filtered particle motion data with the pressure data to generate separated pressure data, the separated pressure data having up and down propagating components separated; and
analysing at least part of the up or down propagating component of the separated pressure data, and
wherein said vertical particle motion data is measured using one or more multi-component streamers or vertical cables having receivers located substantially above the sea floor.
24. A computer-readable medium which can be used for directing an apparatus to reduce the effects in seismic data of downward propagating reflected and scattered acoustic energy travelling in a fluid medium comprising:
means for retrieving pressure data representing at least the pressure in the fluid medium at a first location and a second location, the first location being in close proximity to the second location;
means for retrieving vertical particle motion data representing at least the vertical particle motion of acoustic energy propagating in the fluid medium at a third location and a fourth location, the third location being in close proximity to the fourth location, and the first, second, third and fourth locations being within a spatial area;
means for calculating a plurality of spatial filter coefficients based in part on the velocity of sound in the fluid medium, the density of the fluid medium and a plurality of acquisition parameters, thereby creating a spatial filter which is designed so as to be effective at separating up and down propagating acoustic energy over a range of non-vertical incidence angles in the fluid medium;
means for applying the spatial filter to the vertical particle motion data to generate filtered particle motion data;
means for combining the filtered particle motion data with the pressure data to generate separated pressure data, the separated pressure data having up and down propagating components separated; and
means for analysing at least part of the up or down propagating component of the separating pressure data, and
wherein said vertical particle motion data is measured using one or more multi-component streamers or vertical cables having receivers located substantially above the sea floor.
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0. 12. A method of reducing the effects in seismic data of downward propagating reflected and scattered acoustic energy travelling in a fluid medium comprising the steps of:
receiving pressure data representing at least the pressure in the fluid medium at a first location and a second location, the first location being in close proximity to the second location;
receiving vertical particle motion data representing at least the vertical particle motion of acoustic energy propagating in the fluid medium at a third location and a fourth location, the third location being in close proximity to the fourth location, and the first, second, third and fourth locations being within a spatial area;
calculating a plurality of spatial filter coefficients based in part on the velocity of sound in the fluid medium and the density of the fluid medium, thereby creating a spatial filter which is designed so as to be effective at separating up and down propagating acoustic energy over a range of non-horizontal incidence angles in the fluid medium;
applying the spatial filter to the pressure data to generate filtered pressure data;
combining the filtered pressure data with the vertical particle motion data to generate separated pressure data, the separated pressure data having up and down propagating components separated; and
analysing at least part of the up or down propagating component of the separated pressure data, and
wherein said vertical particle motion data is measuring using one or more multi-component streamers or vertical cables having receivers located substantially above the sea floor.
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By comparison to equation (1), we see that this is a normal incidence approximation, which occurs when kx is zero. This is implemented as a simple scaling of the vertical velocity trace followed by its addition to the pressure trace.
Equation (1) can also be formulated in terms of the vertical pressure gradient (dp(x)/dz). The vertical pressure gradient is proportional to the vertical acceleration:
dp(x)/dz=ρdνz(x)/dt (3)
Integrating in the frequency domain through division of iω, and substituting in equation (1) gives:
Adequate spatial sampling of the wavefield is highly preferred for the successful application of the de-ghosting filters. For typical towed streamer marine data, a spatial sampling interval of 12 m is adequate for all incidence angles. However, to accurately spatially sample all frequencies up to 125 Hz (for all incidence angles), a spatial sampling interval of 6.25 meters is preferred. These spacings are determined according to the Nyquist spatial sampling criterion. Note that if all incidence angles are not required, a coarser spacing than described above can be used. The filters can be applied equally to both group formed or point receiver data.
The processing described herein is preferably performed on a data processor configured to process large amounts of data. For example,
The filters described herein are applicable to, for example, measurements of both pressure and vertical velocity along the streamer. Currently, however, only pressure measurements are commercially available. Therefore, engineering of streamer sections that are capable of commercially measuring vertical velocity is preferred in order to implement the filters.
In an alternative formulation, the filters make use of vertical pressure gradient measurements. An estimate of vertical pressure can be obtained from over/under twin streamers (such as shown in
An important advantage of multiple streamer configurations such as shown in
The filters described here are applied in 2D (along the streamer) to data modelled in 2D. The application to towed streamer configurations naturally lends itself to this implementations, the cross-line (streamer) sampling to the wavefield being usually insufficient for a full 3D implementation. Application of these filters to real data (with ghost reflections from 3D sea surfaces) will give rise to residual errors caused by scattering of the wavefield from the cross-line direction. This error increases with frequency though is less than 0.5 dB in amplitude and 3.6° in phase for frequencies up to 150 Hz, for a 4 m SWH sea. These small residual noise levels are acceptable when time-lapse seismic surveys are to be conducted.
Invoking the principle of reciprocity, the filters can be applied in common receiver domain to remove the downward travelling source ghost. Reciprocity simply means that the locations of source and receiver pairs can be interchanged, (the ray path remaining the same) without altering the seismic response.
While preferred embodiments of the invention have been described, the descriptions and figures are merely illustrative and are not intended to limit the present invention.
Martin, James Edward, Robertsson, Johan, Kragh, Julian Edward
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