Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2000-12-07
2002-09-03
McElheny, Jr., Donald E. (Department: 2862)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
C367S045000
Reexamination Certificate
active
06446008
ABSTRACT:
This invention relates to seismic data acquisition and to methods of processing seismic data. It relates to a process for filtering coherent noise and interference from seismic data by an adaptive beamforming method. In another aspect, it relates to adaptively filtering coherent noise and interference from seismic data while preserving seismic signals with arbitrary spectral content in the frequency-wavenumber domain. In yet another aspect, it relates to adaptively filtering coherent noise and interference that is temporally and spatially nonstationary. In a further aspect, it relates to adaptively filtering coherent noise and interference that has been recorded by a sensor array in the presence of perturbations.
BACKGROUND OF THE INVENTION
In seismic surveys, a seismic source induces seismic waves at or near the surface of the earth. Explosives, vibrating devices and airguns are examples of seismic sources. These waves propagate through the earth and are reflected, refracted, and diffracted by formations within the earth, and can be detected by a plurality of sensors (typically geophones or hydrophones) at the earth's surface. Each such receiver monitors the seismic wavefield, which is then recorded. The data received by a receiver and then recorded are collectively called a trace. The collection of traces are stored for further processing to gain information about the earth's subsurface. Such information is commonly interpreted to detect the possible presence of hydrocarbons, or to monitor changes in hydrocarbon bearing rocks.
Seismic data in general contains coherent noise signals, along with seismic reflection signals. These noise signals, hereafter referred to as the noise, interfere with the interpretation of the seismic signals, and degrade the quality of the subsurface images that can be obtained by further processing. It is therefore desirable to suppress the noise that is present in the recorded data before processing it for imaging.
In land seismics, source-generated noise like ground-roll and air-waves are the dominant noise types, and can lead to severe degradation in data quality. In marine seismics, energy propagating as waves trapped in the water column and near-surface layers is a significant source, as well as swell noise and bulge-wave noise which result from waves propagating along the streamers of receiver devices. Other sources of coherent noise in marine seismics include passing vessels, other vessels acquiring seismic data in the vicinity, or any drilling activity close to the survey area.
An important feature of the so-called coherent noise present in seismic data is the distance over which the noise appears coherent. In many circumstances, the noise is coherent over only a few meters. In other cases, although the noise is mostly coherent, there exists spatially impulse noise. in such cases, filtering methods which have large spatial extent, like the known frequency-wavenumber filtering generate undesirable artifacts, which are mistakenly identified as seismic events after further processing and imaging.
Another feature of the noise present in seismic data is that it is often non-stationary as a function of time; i.e. its characteristics change as a function of time.
During recent years there have been suggested a variety of methods employing the central concept of applying adaptive signal processing ideas to the problem of suppressing coherent noise in seismic data. Booker and Ong, in: “Multiple-constraint adaptive filtering,” Geophysics, Vol.36, pp. 498-509, 1971, have derived an algorithm for multi-channel time-series data processing, which maintains specified initial multiple filter constraints for known signal or noise sources while simultaneously adapting the filter to minimize the effect of the unknown source field. The constraints are of the multiple look-direction constraints type, where look-directions must be precisely specified.
In the International Patent Application WO97/25632, Ozbek has described a class of adaptive signal processing techniques for attenuation of dispersive, nonstationary and aliased coherent noise in seismic data, in the presence of phase and amplitude perturbations. The methods developed can be classified as multi-channel adaptive interference cancellers. Since a signal-free noise reference is not readily available in seismic data acquisition, various preprocessing techniques are introduced to generate the coherent noise reference channels. In the single-component version of the method, moveout (apparent velocity) and spatio-temporal coherence are used as the criteria for differentiating between the signal and the noise. In the multi-component version, polarization is used as an additional attribute for differentiation. Once single or multiple noise reference channels are established, coherent noise in the primary channel is canceled using data-adaptive least-squares multi-channel filter banks.
U.S. Pat. No. 4,556,962 attempts to attenuate the ground roll from a surface seismic source by placing a sensor close to the source to detect the interfering noise. The interfering noise is scaled, delayed and summed with signals from a more distant geophone array and then cross-correlated with the original vibrational source. This patent also suggests that an adaptive filter be used so as to modify the delayed signal to correspond more closely to that detected by the more distant geophone array. However, ground roll is in general of an inhomogeneous nature; due to dispersion and scattering from near surface anomalies the ground roll measured at one point increasingly deviates in character from that measured at another with increasing distance. Hence, the ground roll measured close to the source may be substantially different from that received by the geophone array, and the adaptive filter may not be able to deal with this. It is also difficult to measure seismic signals (ground roll) close to the source. Often the nearest offset is 100 meters. For close measurements, more robust sensors may be needed and detector ‘character’ matching should be an important preliminary step.
In U.S. Pat. No. 4,890,264 a method is proposed for suppressing non-uniformly distributed noise generated by surface wave propagation, wind, and machinery. A number of horizontally sensitive geophones are distributed amongst the conventional vertically oriented geophones. The outputs of the surface wave detectors are utilized in conjunction with an adaptive filter to cancel the effects of the surface wave interference. This method for the suppression of ground roll is inherently a multi-component method, and cannot be used in conjunction with single component acquisition. In addition, it neglects the fact that some seismic body wave energy also is detected by horizontally sensitive geophones, and this may cause signal cancellation.
In UK Patent Application GB-A-2273358 linearly constrained adaptive beamforming and adaptive interference canceling beamforming is used for ground roll suppression. In linearly constrained adaptive beamforming, signals measured by an array of geophones are filtered and summed so as to preserve signals incident from a preferred direction while suppressing interferences incident from other directions. In applying adaptive interference canceling, the moveout differential between the seismic reflections and the ground roll is used to form primary and reference channels. The filtering is performed using a continuously adaptive method such as the LMS (least-mean-square) algorithm. The suggested application is in seismic while drilling, where the horizontal offset range is very small, so that the seismic reflections have an almost vertical angle of incidence and there is effectively a lot of data available from each source-receiver position since the roller cone drill bit used as the seismic source moves very slowly. The statistics of the noise then change very slowly, allowing stochastic gradient type of algorithms like the LMS to converge. However, in surface seismic experiments the ground roll present is often non-stationa
Batzer William B.
McElheny Jr. Donald E.
Ryberg John J.
Schlumberger Technology Corporation
Wang William L.
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