Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
1999-09-15
2001-07-17
McElheny, Jr., Donald E. (Department: 2862)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
Reexamination Certificate
active
06263285
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to seismic data waveform processing and more particularly to the estimation of amplitude spectra using up-going and down-going wavefields.
2. Related Prior Art
Much of seismic exploration today is being done offshore, that is, in the coastal waters io within several hundred miles of land. In some cases, offshore means slightly off the coast, such as wells off the coast of Louisiana or California. In other cases, offshore can be two hundred miles off the coast, such as the Outer Banks oil fields off the coast of Newfoundland. However, both cases have a common problem, seismic data acquisition is complicated by the presence of both upgoing and downgoing acoustic waves.
In situations where both upgoing and downgoing are present, pressure and particle velocity detectors may be used to separate upward traveling waves, (U), from downward traveling waves, (D). Pressure and particle velocity detectors on the water bottom can be used to separate upward traveling wavefields, (U), from polarity reversed downward traveling wavefields, (D), as demonstrated in U.S. Pat. No. 5,754,492 issued to Starr.
Many ways of determining the up going and down going vector wavefields may be used in separate upward traveling wavefields, from polarity reversed downward traveling wavefields. One method includes locating pairs of seismic receivers at the surface of a water column. This surface or reflecting interface may be the air water interface at the top surface or the water bottom when water column reverberations in marine seismic data is considered. This surface may also be the interface between geologic layers in subsurface formations. Further, in some cases, seismic data may be collected with pressure and particle velocity response receivers located at the same location while in other cases, seismic data may be collected with vertically spaced receivers. In the one case, up going and down going wavefields are directly detected by the two receivers in the one location. In the other case, up going and down going wavefields can be separated by comparing the sequential outputs of the receivers.
For the specific case where the pressure and particle velocity detectors are located on the water bottom, the impulse response of the earth recorded on the U data is the same as that which is recorded on the D data except for a linear phase shift associated with the two-way travel time through the water column, (Z). An autocorrelation is the time domain equivalent of the amplitude spectra in the frequency domain. In cases where a single measurement is made of a signal, such as single phone recordings, the amplitude spectra is estimated by autocorrelating the signal. Autocorrelation of a signal causes problems for a signal containing noise. The noise correlates with itself giving an error in the spectral estimation.
SUMMARY OF THE INVENTION
The present invention provides an improved method of amplitude spectra estimation using up-going and down-going wavefields. Pressure and particle velocity detectors are used to separate upward traveling wave, (U), from downward traveling waves, (D), in a seismic experiment. For the specific case where the pressure and particle velocity detectors are located on the water bottom, the impulse response of the earth recorded on the U data is the same as that which is recorded on the D data except for a linear phase shift associated with the two-way travel time through a water column, Z (see FIG.
1
). An autocorrelation of seismic data is the time domain equivalent of the amplitude spectra in the frequency domain. In cases where a single measurement is made of a signal (single phone recordings), the amplitude spectra is estimated by autocorrelating the signal. This causes problems for a signal containing coherent noise or any noise other than random white noise. In an autocorrelation, coherent noise correlates with itself giving an error in the spectral estimation. The U and D signals are independent measurements of the same reflection sequence separated by a linear phase shift. Hence, the U and D signals can be crosscorrelated to improve the estimation of the amplitude spectra. This improvement comes from the fact that the noise contaminating the two signals are different and will not correlate as they would in an autocorrelation of the signal. For U, upgoing signals, and D, downgoing signals, that contain noise, an improved estimation of the amplitude spectra can be generated by using the amplitude component of the crosscorrelation of the upgoing and downgoing waves rather than that of the autocorrelation of the upgoing wave or the autocorrelation of the downgoing wave.
The present invention provides a method of amplitude spectra estimation for seismic data using up-going and down-going wavefields. The method of the present invention includes gathering up-going and down-going wavefields, separating the up-going and down-going wavefields from each other and crosscorrelating the separated wavefields. From the result of the crosscorrelation of the up-going and the down-going wavefields an estimate of the amplitude spectra of the seismic data can be obtained. In this method for amplitude spectra estimation, the gathering may include providing pressure and particle velocity detectors to distinguish between the up-going and the down-going wavefields. Also, in this method for amplitude spectra estimation, the gathering may include measuring the down-going waves and the separating may include removing the down-going waves from the seismic data. In the alternative, the gathering may include measuring the up-going waves and the separating may include removing the up-going waves from the seismic data.
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Loewenthal, et al., “Source Signature Estimation using Fictitious Source and Reflector, ” Geophysics, vol. 54, No. 7, Jul. 1989, pp. 916-920.
Lowenthal, et al., “Deterministic Estimation of a Wavelet Using Impedance Type Technique, ” Geophysical Prospecting 33, 1985, pp. 956-969.
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Arnold & Associates
McElheny Jr. Donald E.
PGS Tensor Inc.
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