Elastic Wave Equation Traveltime and Waveform Inversion Of Crosswell Data
Changxi Zhou and Gerard T. Schuster
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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