Near-surface velocity reconstruction using surface wave inversion

This thesis examines the feasibility of inverting surface waves in common shot point (CSP) scismic data for S- and P-wave velocities. Several approaches to surface wave inversion are examined: 1) separate Love and Rayleigh wave inversions for S- and P-wave velocities; 2) Love wave inversion for S-wave velocities that is then followed by Rayleigh wave inversion for S- and P-wave velocities; and 3) Love wave inversion for S-wave velocities that are then used in Rayleigh wave inversion for P-wave velocities. Inversion of synthetic data suggests that a combination of Love and Rayleigh wave inversion will provide the best results, especially if the Love waves are first used to reconstruct the S-wave velocities and the Rayleigh waves are then used to recover the P-wave velocities. Results also suggest that S-wave velocities inverted from Love waves may be more reliable than those from Rayleigh wave inversion, and that combining Love and Rayleigh wave inversion will provide the most accurate S-wave velocity reconstruction from field data. Density determination appears impractical using surface wave inversion. To verify the practicality of surface wave inversion, a nine-component surface wave experiment was performed in northeast Texas (courtesy of Arco Research). Tau-p and Fourier transforms are applied to the YY (cross-line component source recorded on cross-line component gcophones) and ZZ (z-component source recorded on z-component geophones) CSP gathers to recover Love and Rayleigh wave dispersion curves respectively. These curves show clearly the fundamental and higher harmonic modes from about 2 Hz to 14 Hz. Inversion of the fundamental mode data suggests shear velocities of 180 m/s near the surface with an abrupt change to 400 m/s within the first 10 meters followed by a gradual increase to 650 m/s at a depth of 65 meters. This is in good agreement with the shear velocities measured from vertical seismic profile (VSP) data. Results suggest that either Love or Rayleigh wave inversion can be used to provide shear wave statics and near-surface layer velocities, but that a combination of the two can improve the reliability of the velocity reconstruction.