249 8. Seismic anisotropy applied to geothermal prospection Figure 8.5 Correlation (left) and energy (right) map computed on the full stack As a base for the VVAZ approach, isotropic interval velocities from RMS velocities have been deduced through Dix formula (Figure 8.6). Trials and errors have been used to set the interval parameter to 40 ms; beyond, the obtained velocities are less accurate, below, the obtained velocities contain gridding artifacts, as observable, attesting of the too high sampling compared to the original RMS picking. Figure 8.6 interval velocity using Dix formula for 40 ms (left) and for 10 ms (right). While aligning the stacks, detecting shifts according to the reference, tests (errors and trials) have been undertaken to establish the final parameters. As displayed in Figure 8.7, the shifts obtained are subtle, mainly between ±5 ms. They are directly linked to the average velocities, while their vertical gradients are linked to the interval velocities. Therefore, constant shifts (vertically) indicate no anomaly, while abrupt (vertical) changes indicate a presence of VVAZ anomaly.
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