APPLICATION OF THE PASSIVE SEISMIC GEOPHYSICAL METHOD TO VERMONT’S VARIED LANDSCAPE

In order to build out capabilities during field investigations, the Vermont Geological Survey (VGS) recently adopted the horizontal-to-vertical spectral ratio (HVSR) technique as a non-invasive approach to subsurface exploration. HVSR is a passive geophysical method that uses ambient seismic noise from earthquakes, ocean activity, wind, and even anthropogenic activities to interrogate the subsurface rather than active sources. Important lithological and structural changes, including the location and geometry of the bedrock-overburden interface, can be identified where there are significant changes in seismic energy propagation.

This talk presents three applications in which the HVSR method has enhanced our approach to field data acquisition and the questions that we are able to ask. First, we will speak to challenges in municipal infrastructure design, including source (water supply) to sink (wastewater) issues, that benefitted from higher-resolution subsurface information. Second, we will present a high-level summary of a PFAS investigation at the Vermont Fire Academy in Pittsford, VT where variably thick deltaic sediments overlie a largely undefined, high-relief fractured bedrock surface. Finally, we will show how HVSR has benefited traditional geologic mapping, where the lack of depth-to-bedrock information had previously stymied efforts to “map” features such as ancestral (pre-glacial) channels in the subsurface.

In addition to highlighting these case studies, a brief introduction of the HVSR method will be provided, including details on the data collection process, the site parameters that are required for reliable interpretation, and the all-important step of data calibration. In two years of applying this relatively inexpensive technique, the VGS has closed significant data gaps and improved the science we delivered to Vermonters.