Mitchell Craig

GEOPHONE ARRAY FORMATION AND SEMBLANCE EVALUATION

The performance of a variety of areal geophone arrays was evaluated using seismic data recorded on a dense receiver grid in a walkaway survey conducted in the Permian Basin of west Texas. Surface waves, trapped waves, and scattered energy have long been recognized as a significant noise problem in this area. Arrays were formed by extracting sets of traces from the main data set and stacking them to produce individual traces of a receiver gather. We calculated semblance of each receiver gather to evaluate array performance. High values of semblance indicate that an array effectively removes surface waves while preserving reflections. Differences in data quality associated with variations in geophone-array design are often subtle and difficult to discern through simple inspection of field records. By calculating frequency-dependent semblance, we were able to detect and quantify differences in array performance.

AFTERSHOCK OBSERVATION AND S-WAVE VELOCITY SURVEYS AFTER THE 2014 SOUTH NAPA EARTHQUAKE

A M6.0 earthquake occurred in South Napa County on August 24, 2014. We recorded aftershocks and conducted S-wave velocity (VS) surveys using active and passive surface wave methods at four sites in Napa. Portable accelerometers were deployed at three locations for aftershock observation on the day after the mainshock. One of the three sites is located directly on the line of the surface rupture. The accelerometers recorded continuous data for two weeks. At least 550 aftershocks with magnitudes between -0.6 and 3.9 were recorded during a one-week period starting the day after the mainshock. Surface wave surveys were conducted at each of the three aftershock observation sites and also at Napa Valley College. Dispersion curves from active and passive methods were combined and phase velocities were obtained to a minimum frequency of 0.3 Hz. VS profiles were determined to a depth of 100 to 2000 m by joint inversion of dispersion curves and horizontal to vertical spectral ratio (HVSR) curves. Depth to bedrock with VS higher than 3000 m/s appears to be at least 2000 m at three aftershock observation sites. The VS profiles obtained by the surface wave surveys are generally consistent with site-specific differences in amplification and S-P time observed in aftershock observations.