Near-surface Characterization via Seismic Surface-wave Inversion
Roy, Soumya 1981-
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Characterization of the near-surface is important in identifying shallow properties and structures. In this dissertation, special emphasis is placed on estimating near-surface shear (S)-wave velocities (V_S) which can be used for exploration seismology as well as geotechnical purposes; and even for planetary studies. A frequency-based surface-wave (Rayleigh-wave or ground-roll) inversion method (MASW: Multichannel Analysis of Surface Waves) has been used to estimate 1D and 2D S-wave velocities. The method has been applied on varied seismic datasets related to numerical modeling, physical modeling, and field surveys. The field seismic datasets are from different geological settings and geographical locations: 1) La Marque, Texas, 2) Barringer (Meteor) Crater, Arizona, 3) YBRA field camp, Montana, 4) Hockley fault survey, Texas, and 5) Bradford, Pennsylvania. Estimated S-wave velocities range from as low as 100-300 m/s (La Marque, Hockley) to as high as 3400-3500 m/s (physical model: blank glass block). For the Meteor Crater survey, an unconsolidated near-surface structure (ejecta-blanket) and its thinning thickness trend (thickness decreasing from 20 m to 5 m) has been successfully identified using 2D V_S structure (400-1200 m/s). The depths of investigation for S-wave velocities vary from only 10 m (Hockley survey) to 180 m (Bradford survey) depending on acquisition geometries and source types. Apart from the identification of geological structures; S-wave velocities have been used to calculate S-wave statics and predict densities. The long-wavelength S-wave statics have been calculated for Bradford and Meteor Crater surveys. The densities have been successfully predicted from V_S for modeling experiments and field data (Bradford and YBRA surveys). All predicted densities are consistent with known values with a maximum error of 6%. The effect of lateral heterogeneity on MASW has also been evaluated using different numerical and physical models (dipping layers varying from 10º to 90º). MASW works well for gentle heterogeneity but provides smeared velocity structures for sharp heterogeneities (physical model experiment and Hockley fault survey). A basic full-waveform inversion scheme has been applied on a numerical model with a vertical interface (i.e. 90º dip) showing its potential to handle lateral heterogeneity problems.