VELOCITY STRUCTURES IN FOUR DIMENSIONS

LONG, L.T., and TOTEVA,T., School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332-0340 tim.long@eas.gatech.edu.

The monitoring of time variations in velocity and/or scattering intensity promises to be an important tool in seismology. While already an important tool in monitoring depletion of oil reservoirs, applications to seismology are just starting. Successful applications will depend on the ability to identify or create consistency in the source function and seismometer placement. We have been investigating two applications of 4-D seismic analysis, temporal variations in scattering coda and near-surface velocity structure using surface waves. In both cases the analysis techniques must be modified to maximize sensitivity to changes in velocity or scattering intensity. For surface-wave analysis we have developed a time-domain differential technique that has proven to be highly sensitive to velocity perturbations. The analysis technique utilizes the difference between traces recorded before and after a change in shear-wave velocity. A multiple filter technique is used in order to measure perturbations in dispersion as a function of frequency for each point along a refraction line. The traces are normalized at the time of arrival of the phase of interest. The amplitude of the difference between the normalized traces is a direct indicator of the phase difference and, hence, travel-time difference. The differential surface wave technique allows determination of perturbations in shear-wave velocities with much greater precision than is possible for the direct determination of structure. The perturbed structure can then be computed relative to a reference structure that need only approximate the actual structure. We tested this technique by recording traces along a refraction line near a shallow injection well. We detected a 2% change in velocity over a distance of 3 meters for water injected at a depth of 0.5 meters.