Differential Surface Wave Detection of Changes In Fluid Saturation In Soils

Differential surface-wave analysis is a new approach to the interpretation of temporal variations<br>in surface wave dispersion. The analysis technique utilizes the difference between traces recorded<br>before and after a change in shear-wave velocity. A multiple filter technique is used in order to measure<br>perturbations in dispersion as a function of frequency for each point along a refraction line. The traces<br>are normalized at the time of arrival of the phase of interest. The amplitude of the difference between<br>the normalized traces is a direct indicator of the phase difference and, hence, travel-time difference. The<br>differential surface wave technique allows determination of perturbations in shear-wave velocities with<br>much greater precision than is possible for the direct determination of structure. The perturbed structure<br>can then be computed relative to a reference structure that need only approximate the actual structure.<br>We tested this technique by recording traces along a refraction line near a shallow injection well. We<br>detected a 2% change in velocity over a distance of 3 meters for water injected at a depth of 0.5 meters.