Configuration Of Near-Surface Shear-Wave Velocity By Inverting Surface Wave

The shear (S)-wave velocity of near-surface materials (such as soil, rocks, and<br>pavement) and its effect on seismic wave propagation are of fundamental interest in many<br>groundwater, engineering, and environmental studies. Ground roll is a Rayleigh-type<br>surface wave that travels along or near the surface of the ground. Rayleigh wave phase<br>velocity of a layered earth model is a function of frequency and four earth parameters: Swave<br>velocity, P-wave velocity, density, and thickness of layers. Analysis of the Jacobian<br>matrix in a high frequency range (5-30 Hz) provides a measure of sensitivity of dispersion<br>curves to earth model parameters. S-wave velocities are the dominant influence of<br>the four earth model parameters. With the lack of sensitivity of the Rayleigh wave to Pwave<br>velocities and densities, estimations of near-surface S-wave velocities can be made<br>from high frequency Rayleigh wave for a layered earth model. An iterative technique<br>applied to a weighted equation proved very effective when using the Levenberg-<br>Marquardt method and singular value decomposition techniques. The convergence of the<br>weighted damping solution is guaranteed through selection of the damping factor of the<br>Levenberg-Marquardt method. Three real world examples are presented in this paper. The<br>first and second examples demonstrate the sensitivity of inverted S-wave velocities to<br>their initial values, the stability of the inversion procedure, and/or accuracy of the inverted<br>results. The third example illustrates the combination of a standard CDP (common<br>depth point) roll-along acquisition format with inverting surface waves one shot gather by<br>one shot gather to generate a cross section of S-wave velocity. The inverted S-wave<br>velocities are confirmed by borehole data.