Communications – electrical: acoustic wave systems and devices – Signal transducers – Underwater type
Reexamination Certificate
2002-02-15
2003-03-04
Lefkowitz, Edward (Department: 2862)
Communications, electrical: acoustic wave systems and devices
Signal transducers
Underwater type
C702S014000
Reexamination Certificate
active
06529445
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of reducing the effects of surface ghost reflections in seismic data obtained in a fluid medium. In particular, the invention relates to a method of correcting for the effects of a rough sea surface on marine seismic data.
BACKGROUND OF THE INVENTION
FIG. 1
is a schematic diagram showing reflections between a sea surface (S), sea floor (W) and a target reflector (T). Various events that will be recorded in the seismogram are shown and are labelled according to the series of interfaces they are reflected at. The stars indicate the seismic source and the arrowheads indicate the direction of propagation at the receiver. Events ending with ‘S’ were last reflected at the rough sea surface and are called receiver ghost events. Down-going sea-surface ghost reflections are an undesirable source of contamination, obscuring the interpretation of the desired up-going reflections from the earth's sub-surface.
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.
Ghost reflections from the sea surface will occur in all sea conditions. Rough seas are a further 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 numbers.
FIG. 2
shows an example of rough sea impulses along a 400 m 2D line (e.g. streamer) computed under a 2 m SWH 3D rough sea surface. The streamer shape affects the details of the impulses, and in this example the streamer is straight and horizontal.
FIG. 2
shows, from top to bottom: The ghost wavelet (white trough) arrival time, the ghost wavelet maximum amplitude, a section through the rough sea realisation above the streamer, and the computed rough sea impulses. The black peak is the upward travelling wave, which is unperturbed; the white trough is the sea ghost reflected from the rough sea surface. The latter part of the wavelet at each receiver is the scattering coda from increasingly more distant parts of the surface. Notice that the amplitude and arrival time ghost perturbations change fairly slowly with offset. The arrival time perturbations are governed by the dominant wavelengths in the sea surface, which are 100-200 m for 2-4 m SWH seas, and the amplitude perturbations are governed by the curvature of the sea surface which has an RMS radius of about 80 m and is fairly independent of sea state. The diffraction coda appear as quasi-random noise following the ghost pulse.
The rough sea perturbations cause a partial fill and a shift of the ghost notch in the frequency domain. (The “ghost notch” is a minimum in the spectrum caused by destructive interference between the direct signal and the ghost signal). 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.
FIG. 3
shows an example of how such an error can be significant for time-lapse surveys. The panel on the top left shows a post-stack time-migrated synthetic finite difference seismic section. The top middle panel shows the same data but after simulating production in the oil reservoir by shifting the oil water contact by 6 m and introducing a 6 m partial depletion zone above this. The small difference is just noticeable on the black leg of the reflection to the right of the fault just below 2 s two-way travel-time. The panel on the right (top) shows the difference between these two sections multiplied by a factor of 10. This is the ideal seismic response from the time-lapse anomaly.
The left and middle bottom panels show the same seismic sections, but rough sea perturbations for 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.
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. 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, these prior art techniques require measurements of both velocity and pressure. Moreover, they do not completely correct the complex reflections from rough sea surfaces, and a satisfactory method for eliminating or reducing the effects of a rough sea surface on seismic data is required.
SUMMARY OF THE INVENTION
The present invention provides a method of reducing the effects in seismic data of downwardly propagating reflected and/or scattered seismic energy travelling in a fluid medium, the method comprising the steps of:
a) obtaining seismic data using a seismic source and a seismic receiver disposed within the fluid medium;
b) determining the height of at least one portion of the surface of the fluid medium as a function of time; and
c) processing the seismic data using the results of the determination of the height of the surface of the fluid medium to correct for variations in the height of the fluid medium. Thus, the present invention provides a method of correcting for the effects of a rough sea surface on marine seismic data. Moreover, in contrast to the prior art techniques mentioned above, the invention requires only a measurement of the height of the sea surface.
The portion of the sea surface of which the height is measured may be located over th
Batzer William B.
Ryberg John J.
Schlumberger Technology Corporation
Taylor Victor J.
Wang William L.
LandOfFree
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