DETERMINING BEDROCK DEPTH USING THE HORIZONTAL-TO-VERTICAL SPECTRAL RATIO (HVSR) PASSIVE SEISMIC METHOD: EXAMPLES FROM THE NORTHEASTERN UNITED STATES

Ambient seismic noise can be analyzed using the horizontal-to-vertical spectral ratio (HVSR) method to estimate sediment thickness and delineate depth to bedrock. The resonance frequency at a location is a function of sediment thickness (bedrock depth) and the average shear-wave velocity of the sediment materials. Using local or regional regression curves, observed site resonance frequencies can be converted to estimates of bedrock depth. The passive seismic HVSR technique is particularly suited for areas of the Northeastern United States where there is a high acoustic impedance contrast at the boundary between the glacial sediment and the bedrock.

HVSR data can be analyzed independently or integrated with results from other geophysical methods to improve site characterization. For instance, HVSR data can be collected concurrently with electrical resistivity tomography (ERT) data, and the HVSR results can be used as a constraint in the ERT inversions and interpretation. HVSR data collection is relatively easy and inexpensive when compared with traditional seismic surveys. Examples of local- to regional-scale HVSR surveys from locations around the Northeastern United States demonstrate the potential of the method for routine inclusion in studies with a bedrock-surface mapping component.