BEDROCK DEPTH DETERMINATION AND MAPPING BY CHARACTERIZING SHEAR WAVE VELOCITY AND FUNDAMENTAL RESONANCE OF THE GLACIAL DRIFT SURFACE LAYER AT A SITE IN NE OHIO

An experiment was conducted at a location of variable thickness glacial drift southwest of Clinton, Ohio, where an extensive earlier dataset of 3-component passive seismic data was available. The goal was to examine the relationship f₀ = Vs/4h between the fundamental resonant frequency (f₀) of the drift surface layer, its shear wave velocity (Vs) and thickness (h), and attempt to produce maps of bedrock depth and bedrock topography for the area. Three sites were examined with MASW and shear wave refraction surveys to independently determine the local drift Vs and depth to bedrock for comparison with local well data and the f₀ determined from 3-component passive seismic data at each site. These surveys indicate that the variation of the average Vs of the glacial drift of the three sites is approximately 10% of the mean despite a bedrock depth variation of over 100% of the mean. A theoretical f₀ calculated at the three survey sites, using the local average drift Vs and depth to bedrock, compared well with the H/V peak spectral frequency (f₀) determined using the associated 3-component seismometer data.  However, the average Vs determined using MASW, rather than the shear wave refraction survey, appeared to better model the bedrock depths using the f₀ determined from the 3-component passive data. As a result of these surveys an average Vs was deemed suitable to be used to solve for h at the several prior sites of 3-component seismometers in the area in order to produce drift thickness and bedrock topography maps. ArcGIS^® was used to produce drift thickness maps using local water and gas well information.  These maps include the depths calculated from the average Vs of drift and the f₀ determined from the prior 3-component passive data. The bedrock depths calculated from the 3-component seismometer array correlate well with the major trends indicated by the surrounding water and gas wells. Final contour maps of bedrock depth and topography incorporated depth to bedrock both observed in the water and gas wells as well as that calculated from geophysical methods. This study demonstrates how studies of surface layer resonance can effectively map variations of bedrock depth and topography in an area of significant bedrock topography.