Comparison of Passive MASW with HVSR Measurements at a Tennessee Dike

Information on the depth and competency of the bedrock surface is essential to hydrogeologic characterization, development of a hydrogeologic framework for contaminant hydrology, groundwater modeling, and geotechnical assessments of dams and levees. Horizontal-to-vertical spectral ratio (HVSR) data were acquired along a dike along Watts Bar Lake in eastern Tennessee as part of a larger effort to understand possible applications of HVSR for dam and levee assessment. The HVSR method is commonly used to estimate depth to bedrock, based on the dominant spectral peak in passive seismic data. Beyond the peak frequencies, the HVSR records have additional features (e.g. secondary spectral peaks and troughs) that can be used to better understand site attributes. Passive multichannel analysis of surface waves (MASW) data were acquired at the site to assess the bedrock depths derived from HVSR measurements, and to assess the secondary peaks and troughs in the HVSR measurements. The two methods are in agreement regarding depth to bedrock and concur with a local boring log. The HVSR bedrock profile is helpful in confirming the selection of peaks in the dispersion curves when MASW data are being processed. This study shows how structural features, represented by an MASW profile, can affect the shape and amplitude of HVSR peaks.