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

Abstract

The shear (S)-wave velocity of near-surface materials (such as soil, rocks, and pavement) and its effect on seismic wave propagation are of fundamental interest in many groundwater, engineering, and environmental studies. Ground roll is a Rayleigh-type surface wave that travels along or near the surface of the ground. Rayleigh wave phase velocity of a layered earth model is a function of frequency and four groups of earth parameters: S-wave velocity, P-wave velocity, density, and thickness of layers. Analysis of Jacobian matrix in a high frequency range (530 Hz) provides a measure of sensitivity of dispersion curves to earth model parameters. S-wave velocities are the dominant influence of the four earth model parameters. With the lack of sensitivity of the Rayleigh wave to P-wave velocities and densities, estimations of near-surface S-wave velocities can be made from high frequency Rayleigh wave for a layered earth model. An iterative technique applied to a weighted equation proved very effective when using the LevenbergMarquardt method and singular value decomposition techniques. The convergence of the weighted damping solution is guaranteed through selection of the damping factor of the Levenberg-Marquardt method. Three real world examples are presented in this paper. The first and second examples demonstrate the sensitivity of inverted S-wave velocities to their initial values, the stability of the inversion procedure, and/or accuracy of the inverted results. The third example illustrates the combination of a standard CDP (common depth point) roll-along acquisition format with inverting surface wave one shot gather by one shot gather to generate a cross section of S-wave velocity. The inverted S-wave velocities are confirmed by borehole data.