Shear Wave Velocity Inversion from the Group Velocities of Seismic Surface Waves

Abstract

This project's main focus is on a method for estimating the shear wave velocity profile of the upper seafloor from the dispersive characteristics of the seismic surface waves. The surface waves travel along the boundary between water and the seafloor. The propagational parameters of surface waves (dispersive phase and group velocities) are directly linked to shear strength of the near surface sediments. This allows a straight forward inversion for the shear velocities from measured surface waves' group and phase velocities. An inversion code written by Andrea Caiti was used to carry out the shear wave velocity profile estimation. In order to obtain reliable results the code was tested for instability and possible limitations. The inversion was performed on the synthetic and real field data examples. The surface waves used in this project are measured during the seismo-acoustical survey in the area between the Pir and Munkholmen in the Trondheimsfjord, and from a synthetic data set generated in a program called OSIRIS. The dispersion of the surface waves are detected by using Morlet wavelet analysis, Adaptive wavelet analysis and Moving window analysis. The inversion was performed with one group mode only, and not for several modes. Several limitations in the inversion code were found and analyzed. These limitations concern the initial assumptions of the shear wave velocities for the geological model: the value of the regularization parameter and the frequency content in the dispersion input file.