Analysis of Ambient Seismic Noise to Estimate Unconsolidated Sediment Thickness: Field Examples of the Horizontal-to-Vertical Spectral Ratio (HVSR) Method

Ambient seismic noise can be analyzed to estimate unconsolidated sediment thickness and delineate bedrock surface topography using the horizontal-to-vertical spectral ratio (HVSR) method. At sites with strong acoustic impedance contrasts between bedrock and overlying sediments, naturally occurring seismic noise induces resonance in the sediments at frequencies in the range of about 10^-1 to 10² Hertz (Hz). The ratio of the average horizontal- and vertical-component amplitude spectrums produces a spectral ratio curve with peaks at fundamental and higher-order resonant frequencies.  Resonant frequencies are a function of sediment layer thickness and average layer shear-wave velocity. Therefore, observed HVSR resonant frequencies can be used to estimate sediment thicknesses through (1) regression equations developed from seismic observations adjacent to boreholes known to penetrate bedrock or (2) direct or indirect measurement of sediment shear-wave velocity.

 Here we present results of HVSR studies conducted in different geologic settings in New England and compare the HVSR-derived estimates of bedrock depth to the results of other seismic and electrical geophysical methods and ground-truth from drilling.