Datensatz

Zusammenfassung

Combined analysis of seismic $P$ and $S$ velocity information provides a basis for petrologic interpretation of seismic cross-sections. The methodology presented here allows extraction of rock-body composition and shape from well-defined $P$ velocities and Poisson's ratios that are derived from tomographic $P$ and $S$ velocity models. Refraction/reflection seismic data were acquired along a 46-km long line perpendicular to the San Andreas Fault centered on the SAFOD drill site. A fixed array of 912 three-component stations at a 25-50 m spacing recorded 62 explosive shots (25-100 kg) at a 0.5-1 km spacing. Over 45000 $P$-wave and 20000 $S$-wave travel times could be picked. P- and S-velocity models were determined independently by travel-time tomography of direct refracted phases, resulting in high-resolution images of the upper crust down to 5-km depth. The 2-D models contain strong velocity anomalies for both $P$ and $S$ waves. Resolution tests have been carried out to evaluate the quality of the tomographic models. $V_p$/$V_s$ ratios, calculated from the $P$ and $S$ wave velocity are generally high, exceeding values of 2. Errors in forming the $V_p$/$V_s$ ratio from the $P$ and $S$ wave models are low in the shallower parts of the model and become larger at greater depth. Cluster analysis procedures have been applied to the $P$- and $V_p$/$V_s$ models. First, prominent clusters were identified in plots of $V_p$/$V_s$ versus $P$-wave velocity. Secondly, the identified clusters were mapped back into spatial distributions in the seismic depth section. These clusters are interpretable as specific rock types; they constrain rock types better than $P$ or $S$ velocities alone. Prominent features are the sedimentary bodies and the high-velocity granitic rocks of the Salinian terrane.