MASW DETERMINATION OF SURFACE LAYER THICKNESS AND VS RECONCILED WITH MICROTREMOR RESONANCE ANALYSIS – GREENE COUNTY, OHIO

Multi-channel analysis of surface waves (MASW) was used to define the thickness and shear-wave velocity (Vs) structure of glacial drift at a site in Greene County, Ohio, where glacial drift varies from 10 m to 30 m thick and overlies limestone bedrock. The results were compared to that of microtremor resonance analysis of three-component passive seismic data collected with broadband seismometers at the same site. Both MASW and microtremor analyses resolved the Vs structure and apparent depth to bedrock to depths of ~20m. However, modeling experiments by J. Ivanov (SEG, 2011) suggest that settings such as this, with a low velocity surface layer over high velocity bedrock, require MASW data with larger source offsets than is conventional. It was suggested that longer source offsets are required to distinguish the dispersion of the fundamental and higher modes at longer periods. This phenomenon does not appear to pose a problem for these data. However, additional data are being collected with longer source offsets which will be analyzed to address this question.