Surface-wave group-velocity tomography for shallow structures

In order to identify shallow structures using fundamental-mode Rayleigh waves, we use the multiple filter technique and measure group velocity across a study area. First, we identify the mean velocity by stacking the power densities along constant velocities for the complete set of filtered traces. Second, we identify peaks in the power density consistent with the fundamental mode and eliminate those associated with higher modes. Third, we perform a constrained inversion for optimal arrival times using minimal, deviations in travel time between stations corrected for differences in distance. The resulting travel times are used in a tomographic inversion for group velocities in the study area. The tomographic inversion is repeated for many frequencies giving a dispersion curve for each point in the study area. Then, the vertical velocity structure at any point in the study area can be interpreted from its dispersion curve. Velocity anomalies of 10 percent are easily mapped by this technique. Rayleigh waves in the frequency range of 2 to 50 Hz are sensitive to soil velocities at depths of I to 10 meters. This depth range contains the near-surface structures that control dispersion of underground contamination or which were introduced by the burial of wastes. The use of tomography eliminates the need to make measurements directly above contamination.