Bedrock Mapping using Shear Wave Velocity Characterization and H/V Analysis

An experiment was conducted to constrain the HVSR (Horizontal to Vertical Spectral Ratio) or H/V spectral ratio method at a glaciated site in northeast Ohio. Multiple methods were used to determine the shear wave velocity (Vs) and depth (h) to bedrock in relation to the fundamental resonant frequency (fo) determined from 3-component seismic data, as defined by the relationship f0=Vs/4h. The shear wave velocity structure was determined at three sites using MASW (Multi-channel Analysis of Surface Waves) and shear wave refraction methods, and the fundamental resonant frequency was passively observed using 3-component Guralp broadband seismometers. The Vs and bedrock depth results from both refraction and MASW produced comparable calculated theoretical f0 to that observed by the 3-component broadband seismometers. However, the bedrock depth and glacial drift Vs results were consistently lower for refraction than for MASW. Part of the calculations used with the generalized reciprocal method (GRM) method could yield bedrock depths that are underestimated proportionally with the Vs. Notably, the MASW results appear to be improved by combining overtones of multiple source offsets. The average Vs from the MASW and refraction surveys of this study were each used to calculate bedrock depth using the f0 observed for a suite of 73 seismometers previously deployed across the surrounding area as part of another study. Maps of these calculated bedrock depths correlate with the major dipping trends indicated by the water and gas wells in the area. At the site where the closest comparison could be made, the MASW determined Vs yielded a depth to bedrock that was significantly closer to the measured bedrock depth than the refraction determined Vs. This study suggests that an average shear wave velocity for glacial drift determined from a few MASW surveys in a region is sufficient to determine a viable average Vs to convert an array of 3-component f0 observations to produce a map of bedrock topography.