OBS Data Analysis
Traveltime analysis. We will obtain an initial Vs function by raytrace modeling of P-S conversion traveltimes. The depth of converting boundaries will be determined by analysis of near-offset arrival times and moveouts. Simple 1-D modeling of a converted S-wave recorded in the 1995 OBS data demonstrates the feasibility of this approach. Placing the conversion depth at 3.07 km (270 mbsf), which is the shallowest converting boundary on the walkaway VSP S-arrivals, yields a Vs of 265 m/s. For this conversion depth, however, predicted S-wave arrivals arrive too early at far offsets. By raising the conversion depth to 2.91 km (110 mbsf) and lowering Vs to 100 m/s, we obtain a good fit to all picked arrivals. The average Vs through the gas zone can be calculated by correlating the highly reflective S-wave zone beneath the BSR to the free gas zone indicated by high reflectivity in MCS sections, the depth of which is fairly well constrained.
Waveform analysis. A modification of the full waveform inversion of Kormendi and Dietrich will be applied to converted S-wave reflections to study their fine-scale velocity structure. The models of interest produce different waveforms, as demonstrated by reflectivity modeling of four combinations of high- and low-Vs in thin layers or beneath velocity steps. P-velocity was obtained by holding bulk modulus (K) constant and computing changes in Vp caused by varying shear modulus (G). Amplitudes for the step discontinuities were computed using the Zoeppritz equations. Changes in shear modulus create strong P- to-S conversions without generating strong P-reflections, whereas a large change in Vp without a corresponding change in Vs and density does not generate converted shear waves. Wave signatures differ considerably in all four cases. Velocity structure will be derived using waveform inversion, which has been successfully applied to BSR reflections in reflection data and OBH data. Singh et al. have recently modified this code to invert for converted S-waves. The option of accounting for P-wave anisotropy is currently being added by Singh and co- workers at the Univ. of Cambridge (U.K.). Finally, the waveform inversion code will be used to investigate the fine- scale Vp structure of the "stringer" reflection. The different Vp models of interest can be clearly distinguished by waveforms. S-wave arrivals from the stringer will be analyzed as described for the shear-wave experiment above.
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