|Publication number||USRE41656 E1|
|Application number||US 11/501,195|
|Publication date||Sep 7, 2010|
|Filing date||Mar 21, 2000|
|Priority date||Mar 22, 1999|
|Also published as||US6529445, US6775618, USRE43188, WO2000057206A1, WO2000057207A1|
|Publication number||11501195, 501195, PCT/2000/1074, PCT/GB/0/001074, PCT/GB/0/01074, PCT/GB/2000/001074, PCT/GB/2000/01074, PCT/GB0/001074, PCT/GB0/01074, PCT/GB0001074, PCT/GB001074, PCT/GB2000/001074, PCT/GB2000/01074, PCT/GB2000001074, PCT/GB200001074, US RE41656 E1, US RE41656E1, US-E1-RE41656, USRE41656 E1, USRE41656E1|
|Inventors||Johan Robertsson, Julian Edward Kragh, James Edward Martin|
|Original Assignee||Westerngeco L. L. C.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (29), Non-Patent Citations (17), Referenced by (3), Classifications (10), Legal Events (1)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application is a Reissue of U.S. patent application Ser. No. 09/936,863, filed 18 Sep. 2001, now U.S. Pat. No. 6,775,618, which is a U.S. National Phase of International Application PCT/GB00/01074, filed 21 Mar. 2000, which designated the U.S. and claims priority to G.B. Application No. 9906456, filed 22 Mar. 1999. More than one Reissue Application has been filed for the reissue of U.S. Pat. No. 6,775,618. The Reissue Applications are 11/501,195 and 12/264,784, which is a Divisional Reissue Application of Reissue Application 11/501,195.
The present invention relates to the field of reducing the effects of sea-surface ghost reflections in seismic data. In particular, the invention relates an improved de-ghosting method that utilises measurements or estimates of multi-component marine seismic data recorded in a fluid medium.
Removing the ghost reflections from seismic data is for many experimental configurations equivalent to up/down wavefield separation of the recorded data. In such configurations the down-going part of the wavefield represents the ghost and the up-going wavefield represents the desired signal. Exact filters for up/down separation of multi-component wavefield measurements in Ocean Bottom Cable (OBC) configurations have been derived by Amundsen and Ikelle, and are described in U.K. Patent Application Number 9800741.2. An example of such a filter corresponding to de-ghosting of pressure data at a frequency of 50 Hz for a seafloor with P-velocity of 2000 m/s, S-velocity of 500 m/s and density of 1800 kg/m3 is shown in FIG. 2. At this frequency, the maximum horizontal wavenumber for P-waves right below the seafloor is k=0.157 m−1, whereas it is k=0.628 m−1 for S-waves. Notice the pole and the kink due to a zero in the filter at these two wavenumbers, making approximations necessary for robust filter implementations.
The OBC de-ghosting filters have been shown to work very well on synthetic data. However, apart from the difficulty with poles and zeros at critical wave numbers, they also require knowledge about the principles of the immediate sub-bottom locations as well as hydrophone/geophone calibration and coupling compensation.
A normal incidence approximation to the de-ghosting filters for data acquired at the sea floor was described by Barr, F. J. in U.S. Pat. No. 4,979,150,issued 1990, entitled ‘System for attenuating water-column reflections’, (hereinafter “Barr (1990)”). For all practical purposes, this was previously described by White, J. E., in a 1965 article entitled ‘Seismic waves: radiation, transmission and attenuation’, McGraw-Hill (hereinafter “White (1965)”). However, this technique is not effective when the angle of incidence is away from vertical. Also, this technique does not completely correct for wide-angle scattering and the complex reflections from rough sea surfaces. Additionally, its is believed that the OBC techniques described have not been used successfully in a fluid medium, such as with data gathered with towed streamers.
Thus, it is an object of the present invention to provide a method of de-ghosting which improves attenuation of noise from substantially all non-horizontal angles of incidence.
It is an object of the present invention to provide a method of de-ghosting of seismic measurements made in a fluid medium which improves attenuation of the ghost as well as downward propagating noise from substantially all non-horizontal angles of incidence.
Also, it is an object of the present invention to provide a method of de-ghosting which is not critically dependent on knowledge about the properties of the surrounding fluid medium as well as hydrophone/geophone calibration and coupling compensation.
Also, it is an object of the present invention to provide a method of de-ghosting whose exact implementation is robust and can be implemented efficiently.
According to the invention, a method is described for sea surface ghost correction through the application of spatial filters to the case of marine seismic data acquired in a fluid medium. Using, for example, either typical towed streamer or vertical cable geometries. Preferably, both pressure and vertical velocity measurements are acquired along the streamer. The invention takes advantage of non-conventional velocity measurements taken along a marine towed streamer, for example. New streamer designs are currently under development and are expected to become commercially available in the near future. For example, the Defence Evaluation and Research Agency (DERA), based in Dorset, U.K., claim to have successfully built such a streamer for high frequency sonar applications.
According to an alternative embodiment, the invention is also applicable to seismic data obtained with configurations of multiple conventional streamers. Here, the filters make use of vertical pressure gradient measurements, as opposed to velocity measurements. According to the invention, an estimate of the vertical pressure gradient can be obtained from over/under twin streamer data, or more generally from streamer data acquired by a plurality of streamers where the streamers are spatially deployed in a manner analogous to that described in U.K Patent Application Number 9820049.6, by Robertsson, entitled “Seismic detection apparatus and related method” filed in 1998 (hereinafter “Robertsson (1998)”). For example, three streamers can be used, forming a triangular shape cross-section along their length. Vertical pressure gradient data can also be obtained from pressure gradient measuring devices.
According to the invention, the filters fully account for the rough sea perturbed ghost, showing improvement over other techniques based on normal incidence approximations (see e.g., White (1965)), which have been applied to data recorded at the sea floor.
Advantageously, according to preferred embodiments of the invention, the results are not sensitive to streamer depth, allowing the streamer(s) to be towed at depths below swell noise contamination, hence opening up the acquisition weather window where shallow towed streamer data would be unusable. Local streamer accelerations will be minimised in the deep water flow regime, improving resolution of the pressure, multi-component velocity and pressure gradient measurements.
Advantageously, according to preferred embodiments of the invention, there are no filter poles in the data window, except for seismic energy propagating horizontally at the compressional wave speed in water.
Advantageously, according to preferred embodiments of the invention, the filter is not critically dependent on detailed knowledge of the physical properties of the surrounding fluid medium.
The filters can be simple spatial convolutions, and with the regular geometry of typical towed streamer acquisition the filters are efficient to apply in the frequency-wavenumber wavenumber (FK) domain. The filters can also be formulated for application in other domains, such as time-space and intercept time-slowness (τ-p)
According to the invention, a method of reducing the effects is seismic data of downward propagating reflected and scattered acoustic energy travelling in a fluid medium is provided. The method advantageously 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 distance between the locations that are represented by the pressure data and the vertical particle motion data in each case is preferably less than the Nyquist spatial sampling criterion. The vertical particle motion data can be in various forms, for example, velocity, pressure gradient, displacement, or acceleration.
The spatial filter is created by calculating a number of coefficients that are based on the velocity of sound in the fluid medium and the density of the fluid medium. The 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. The separated data are then processed further and analysed. Ordinarily the down-going data would be analysed, but the up going data could be used instead if the sea surface was sufficiently calm.
According to an alternative embodiment, a method of reducing the effects of downward propagating reflected and scattered acoustic energy travelling in a fluid medium is provided wherein the pressure data and vertical particle motion data represent variations caused by a first source event and a second source event. The source events are preferably generated by firing a seismic source at different locations at different times, and the distance between the locations is preferably less than the Nyquist spatial sampling criterion.
The present invention is also embodied in a computer-readable medium which can be used for directing an apparatus, preferably a computer, to reduce the effects in seismic data of downward propagating reflected and scattered acoustic energy travelling in a fluid medium as otherwise described herein.
Rough seas are a source of noise in seismic data. Aside from the often-observed swell noise, further errors are introduced into the reflection events by ghost reflection and scattering from the rough sea surface. The rough sea perturbed ghost events introduce errors that are significant for time-lapse seismic surveying and the reliable acquisition of repeatable data for stratigraphic inversion.
The effect of the rough sea is to perturb the amplitude and arrival time of the sea surface reflection ghost and add a scattering coda, or tail, to the ghost impulse. The impulse response can be calculated by finite difference or Kirchhoff methods (for example) from a scattering surface which represents statistically typical rough sea surfaces. For example, a directional form of the Pierson-Moskowitz spectrum described by Pierson, W. J. and Moskowitz, L., 1964 ‘A proposed Spectral Form for Fully Developed Wind Seas Based on the Similarity Theory of S. A. Kitaigorodskii’ J. Geo. Res., 69, 24, 5181-5190, (hereinafter “Pierson and Moskowitz (1964)”), and Hasselmann, D. E., Dunckel, M. and Ewing, J. A., 1980 ‘Directional Wave Spectra Observed During JONSWAP 1973’, J. Phys. Oceanography, v10, 1264-1280, (hereinafter “Hasselmann et al, (1980)”). Both the wind's speed and direction define the spectra. The Significant Wave Height (“SWH”) is the subjective peak to trough wave amplitude, and is about equal to 4 times the RMS wave height. Different realisations are obtained by multiplying the 2D surface spectrum by Gaussian random complex numbers.
The rough sea perturbations cause a partial fill and a shift of the ghost notch in the frequency domain. They also add a small ripple to the spectrum, which amounts to 1-2 dB of error for typical sea states. In the post stack domain this translates to an error in the signal that is about −20 dB for a 2 m SWH sea.
The left and middle bottom panels show the same seismic sections, but rough sea perturbations of a 2 m SWH (as described above) have been added to the raw data before processing. Note that different rough sea effects are added to each model to represent the different seas at the time of acquisition. The difference obtained between the two sections is shown on the bottom right panel (again multiplied by a factor of 10). The errors in the reflector amplitude and phase (caused by the rough sea perturbations) introduce noise of similar amplitude to the true seismic time-lapse response. To a great extent, the true response is masked by these rough sea perturbations. A method for correcting these types of error is clearly important in such a case, and with the increasing requirement for higher quality, low noise-floor data, correction for the rough sea ghost becomes necessary even in modest sea states.
Equation (1) gives the frequency domain expression for a preferred filter relating the up-going pressure field, pu (x), to the total pressure, p(x), and vertical particle velocity, vz(x).
where kz is the vertical wavenumber for compressional waves in the water, ρ is the density of water and * denotes spatial convolution.
The vertical wavenumber is calculated from kz 2=k2−k x 2 for two-dimensional survey geometries, or k2 2=k2−kx 2−ky 2 for three-dimensional survey geometries, with k2=ω2/c2, where c is the compressional wave speed in the water and kx is the horizontal wavenumber for compressional waves in the water. The regular sampling of typical towed streamer data allows kz to be calculated efficiently in the FK domain.
The traditional filter (White (1965), Barr, (1990)) is equation (2):
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:
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.
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|US9625600 *||Dec 4, 2012||Apr 18, 2017||Pgs Geophysical As||Systems and methods for removal of swell noise in marine electromagnetic surveys|
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|U.S. Classification||702/14, 367/24|
|International Classification||G01V1/36, G01V1/00, G01V1/38|
|Cooperative Classification||G01V1/364, G01V1/3808, G01V2210/56|
|European Classification||G01V1/38B, G01V1/36C|