Shallow S-Wave Structure Can Be Interpreted From Surfacewave Group-Velocity Tomography

Surface-wave group-velocity tomography provides an efficient way to measure<br>the shallow S-wave structure at waste sites and other areas where measurements are not<br>advisable directly above the structure. In this technique, measurements are made only on<br>the boundary of a test area. The wavelength and area dimensions determine the depth of<br>resolution. For example, structures in the depth range of 1 to 8 m may be interpreted<br>from 16 to 48 Hz surface waves traversing a 30 m square. Given sufficiently dense<br>record traces, the horizontal resolution is limited principally by wavelength. A 40 Hz<br>wave will resolve structures less than 2 m in diameter near the surface. Unlike phase<br>velocity measurements (e.g. SASW) corrections for energy delays introduced by the<br>instrument response and geophone ground coupling are required for accurate velocity<br>determination. The multiple filter technique is used to determine group arrival times and<br>their uncertainties at individual frequencies. The SIRT algorithm is used to obtain a<br>tomographic image of the group velocity at each frequency. The images for individual<br>frequencies can be used to interpret geologic structure, or they can be combined to<br>generate group-velocity dispersion curves and their associated S-wave structure for any<br>position in the test area.