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Interferometry examines the general interference phenomena between pairs of signals in order to gain useful information about the subsurface. Seismic interferometry (SI) utilizes the crosscorrelation of signal pairs to reconstruct the impulse response of a given media. Jon Claerbout's initial (1968) conjecture provided the framework upon which modern theory is based. A signal at a location A can be crosscorrelated with a signal at a location B to reproduce a virtual source-receiver pair using seismic interferometry. Crosscorrelation is often considered the key mathematical operation in this approach, but it is also possible to use convolution to come up with a similar result. The crosscorrelation of passive noise measured at a free surface reproduces the subsurface impulse response. As such, it is possible to obtain information about the subsurface with no need for an active seismic source.〔 This method, however, is not limited to passive sources, and can be extended for use with active sources and computer–generated waveforms.〔 As of 2006 the field of seismic interferometry was beginning to change the way geophysicists view noise. Seismic interferometry uses this previously–ignored noise in models of the shallow subsurface. Potential applications include both research and industry.〔 ==History and Development== Claerbout (1968) first considered seismic interferometry for investigating the shallow subsurface. He later predicted that seismic interferometry could be applied to real world media, which was later proven.〔 The long term average of random ultrasound waves can reconstruct the impulse response between two points on an aluminum block. However, they had assumed random diffuse noise, limiting interferometry in real world conditions. In a similar case, it was shown that the expressions for uncorrelated noise sources reduce to a single crosscorrelation of observations at two receivers. The interferometric impulse response of the subsurface can be reconstructed using only an extended record of background noise, initially only for the surface and direct wave arrivals. Crosscorrelations of seismic signals from both active and passive sources at the surface or in the subsurface can be used to reconstruct a valid model of the subsurface. Seismic interferometry can produce a result similar to traditional methods without limitations on the diffusivity of the wavefield or ambient sources. In a drilling application, it is possible to utilize a virtual source to image the subsurface adjacent to a downhole location. This application is increasingly utilized particularly for exploration in subsalt settings. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Seismic interferometry」の詳細全文を読む スポンサード リンク
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