Elastic velocity analysis and time-lapse full-waveform inversion

Until recently, a majority of seismic-processing and seismic-imaging techniques has to a large extent ignored amplitude effects and has been designed to use the information contained in the kinematic part of seismic waves. With the introduction of inversion techniques designed to use the amplitude properties of seismic data also, such as full-waveform inversion, one can no longer use the simple acoustic approximation to describe seismic waves but has to employ the full elastic formulation, possibly also including anisotropy. Reverse time migration can be modified easily to take elastic waves into account. This also leads directly to elastic migration velocity analysis, in which the effect of anisotropy can be taken into account in a straightforward way. Full-waveform inversion using the full elastic wave equation gives more accurate results compared with acoustic full-waveform inversion and is capable of exploiting information contained in reflected waves. Time-lapse elastic full-waveform inversion is capable of recovering compressional- and shear-wave velocity changes caused by fluid effects on synthetic data. Real data results are more uncertain but seem to indicate that the elastic approach leads to better results than the conventional acoustic method.

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