Elastic Wave Equation Traveltime and Waveform Inversion Of Crosswell Data

Changxi Zhou and Gerard T. Schuster

ABSTRACT

A method is presented for reconstructing P- and S-velocity distributions from elastic traveltimes and waveforms; the input data consist of crosswell hydrophone records. Borehole effects are partially accounted for by using a low-frequency Green's function to simulate the pressure generated in the fluid filled receiver well. The tube waves in the borehole are ignored based on the assumption that they can be removed from the field data by median filtering. In addition, the source radiation pattern is partially accounted for by inverting for the equivalent stress components acting on the earth at the source location. The elastic wave equation traveltime and waveform (WTW) inversion method is applied to both synthetic crosswell data and the McElroy field crosswell data. As predicted by theory, results show that elastic WTW tomograms provide a sharper interface image than delineated in the traveltime tomograms. The spatial resolution of the McElroy traveltime tomogram is about 20 m compared to about 3 m and 1.5 m, respectively, for the associated P- and S-velocity WTW tomograms. The correlation between the P- and S-velocity tomograms is quite reasonable and the velocity profiles extracted from the WTW tomograms show, in many places, a very good agreement with the sonic logs. From these tomograms detailed porosity maps of the interwell geology are constructed.

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