Surface-wave group-velocity tomography for shallow structures at a waste site

We use the multiple-filter technique to measure group velocities along multiple independent ray paths across a test area and then we use tomographic inversion methods to define the distribution of group velocity inside the test area. Group velocities of surface waves are more sensitive to shear-wave velocity structure than phase velocities because they vary over a wider range of velocities, but they are more difficult to measure precisely. In order to minimize the scatter in the arrival times of wave groups, we constrain the travel times for each frequency to fall within a small time increment of arrivals at adjacent traces. The vertical shear-wave velocity structure for each element in the test area can be interpreted from its dispersion curve. We have applied surface-wave group-velocity tomography to areas ranging in diameter from 20 to 300 meters. Velocity anomalies of 10 percent are easily mapped. For these test sites, the shear-wave velocity structure in a depth range of 1 to 10 meters was obtained from surface waves with frequencies from 10 to 40 Hz. The advantages of surface-wave group-velocity tomography include the ability to measure structure without placing instruments directly above the structure, ease in measurement because the surface waves are usually the largest observed phase, and sensitivity to a depth range critical for evaluation in many waste dumps.