Surface-Wave Group-Velocity Tomography For Shallow Structures

In order to identify shallow structures using fundamental-mode Rayleigh waves,<br>we use the multiple filter technique and measure group velocity across a study area.<br>First, we identify the mean velocity by stacking the power densities along constant<br>velocities for the complete set of filtered traces. Second, we identify peaks in the power<br>density consistent with the fundamental mode and eliminate those associated with higher<br>modes. Third, we perform a constrained inversion for optimal arrival times using<br>minimal deviations in travel time between stations corrected for differences in distance.<br>The resulting travel times are used in a tomographic inversion for group velocities in the<br>study area. The tomographic inversion is repeated for many frequencies giving a<br>dispersion curve for each point in the study area. Then, the vertical velocity structure at<br>any point in the study area can be interpreted from its dispersion curve. Velocity<br>anomalies of 10 percent are easily mapped by this technique. Rayleigh waves in the<br>frequency range of 2 to 50 Hz are sensitive to soil velocities at depths of 1 to 10 meters.<br>This depth range contains the near-surface structures that control dispersion of<br>underground contamination or which were introduced by the burial of wastes. The use of<br>tomography eliminates the need to make measurements directly above contamination.