USE OF MULTIPLE SEISMIC METHODS TO ESTIMATE DEPTH TO BEDROCK AT SELECTED SITES WITHIN THE COVENTRY QUADRANGLE, COVENTRY, CONNECTICUT

Information on the depth to bedrock is important for hydrogeologic investigations. In this study, three seismic methods were applied at selected sites within the Coventry Quadrangle, Connecticut, to determine the depth to bedrock. This effort was part of a larger investigation to evaluate methods for mapping relatively shallow (less than 30 meters) surficial materials at a quadrangle scale.

Seismic refraction is an active-source method traditionally used for determining the thickness of surficial materials overlaying bedrock. This well established method was used to evaluate results from two relatively new seismic methods: the passive horizontal-to-vertical spectral ratio (HVSR) method and the multichannel analysis of surface waves (MASW) method.

The HVSR method records the ambient seismic wave field using a single three-component seismometer. The ambient seismic data were processed to obtain the resonance frequency that was converted to bedrock depth using site-specific regression equations, which were calibrated to known depth to rock at boreholes. HVSR data can be collected at a single location in under 1 hour, however for this study, a large HVSR data set consisting of many measurements, was required to establish the site-specific regression equation.

MASW is an active seismic method for determining depth to bedrock to about 30 meters below land surface. Extraction of the fundamental mode dispersion curve results in a shear-wave velocity structure of the shallow subsurface. The shear-wave velocity distribution was interpreted to determine the depth to bedrock.

In this study, for sites where the depth to bedrock was greater than about 3 meters, results from the three methods were in good agreement. For depths less than 3 meters however, the MASW dispersion curves were poorly developed, and the HVSR frequency peaks were broad and difficult to interpret. Results of this investigation support the integrated use of seismic refraction, HVSR, and MASW techniques to provide reliable estimates of bedrock depth.