Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Patent
1997-09-12
1999-11-30
Oda, Christine
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
702 14, G06F 1900
Patent
active
059959053
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of determining a signature of a source of energy and to a method of reducing the effects of multiple reflected energy. Such a method may be used in medical imaging and non-destructive evaluation, where a sample is irradiated with energy, for instance in the form of x-rays or ultrasound, in order to determine the internal structure of the sample non-invasively. Such a method may also be used with marine seismic reflection data obtained by means of seismic sources and receivers located in water and towed behind a seismic exploration vessel. These methods may be used during actual surveying and/or subsequently with recorded data from such surveys.
During seismic surveying, a seismic source is repeatedly actuated and seismic receivers, such as hydrophones in marine seismic surveying, receive energy direct from the sources and reflected from various boundaries or interfaces. In the case of marine seismic surveying, energy propagates into the earth and is reflected back to the hydrophones from subterranean boundaries or interfaces, for instance between strata of different types. The recorded seismic data can be processed to reveal information about the structure of the earth in the area being surveyed. However, such reflections are contaminated by other reflection paths. For instance, energy from the sources is reflected at the sea surface directly to the hydrophones. Also, energy can be reflected more than once between the sources and the receivers. Such multiple reflections can take place within the earth. Also, energy initially travelling downwards from the sources can be reflected upwardly and then downwardly again from the surface of the sea before arriving at the hydrophones. Reflections of this type are referred to as "free-surface multiple reflections". Free-surface multiple reflections can be classified according to their order, which is equal to the number of reflections from the free-surface. Thus, first order free-surface reflections comprise energy initially travelling downwardly from the sources (as opposed to "ghosting" where energy travels upwardly and is reflected from the free-surface), is reflected upwardly from the sea bed or a boundary below the sea bed, and is then reflected downwardly from the free-surface to the hydrophones. Second order free-surface multiple reflections undergo two downward reflections from the sea-surface before being detected by the hydrophones, and so on.
In order to remove or reduce the effects of multiple reflections including free-surface multiple reflections in seismic data, a good knowledge of the seismic source signature is required. The seismic source signature is the wave shape e.g. the pressure waveform with respect to time, of the seismic energy emitted by the seismic sources. Removal or reduction of the effects of multiple reflections in seismic data is performed before "stacking", the known process of summing seismic data related to each sub-region of the region being explored so as to improve the effective signal-to-noise ratio of the data. Good knowledge of the seismic source signature also enhances other data processing techniques such as deconvolution, migration and inversion for elastic parameters. Thus, good knowledge of the seismic source signature can significantly enhance the final data processing products.
There are known techniques for determining the signature of a seismic source. One such known technique relies on a "convolution model" for seismic data, in which the model is defined as the convolution of the source signature with the impulse response of the earth (including the effects of reflections, refractions, multiples, and diffractions). This technique requires known information about some part of the sub-surface, such as the sea bottom, so as to allow estimation of the source signature to be treated as a linear inverse problem. However, such sub- surface information is often not available and the technique must then rely on statistical methods which require more severe assumptions, for instance
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Ikelle Luc Thomas
Roberts Graham
Batzer William B.
Oda Christine
Schlumberger Technology Corporation
Smith Keith G. W.
Taylor Victor J.
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