Geophysical imaging of the subsurface seismic structure of the Chestnut Hill Reservoir earth embankment dam (Massachusetts)

Abstract Degradation within a manmade earth embankment structure is often unobservable from the surface. In order to evaluate the structural integrity of earth embankment levees and dams and identify subsurface zones of weakness that may result in the future failures of such structures, various geophysical methods have been proposed as effective subsurface imaging tools. This study presents the results of using the horizontal‐to‐vertical spectral ratio (HVSR) of seismic noise, seismic refraction analysis and multi‐channel analysis of seismic surface waves (MASW) to estimate subsurface seismic structures for the Chestnut Hill Reservoir earth embankment dam in Chestnut Hill, MA. The HVSR method is used to estimate site fundamental frequency from ambient seismic noise recordings. The fundamental frequency () at a site can be used to estimate depth to bedrock with a known/estimated surface shear‐wave velocity. The MASW and seismic refraction analysis methods are used to estimate seismic velocity structures from seismic refraction lines with active sources. The depth‐to‐bedrock estimates from the seismic refraction analysis and MASW performed in this study confirm that the HVSR method is able to effectively estimate depth to bedrock at sites atop an earth embankment. The MASW was found to resolve a low‐velocity zone in the subsurface seismic structure at the Chestnut Hill Reservoir embankment that the seismic refraction method was unable to image, and this low‐velocity zone is required to best fit a theoretical HVSR to an observed spectrum. Furthermore, the variation and uncertainty in fundamental frequency estimation were quantified by making repeated HVSR measurements at the reservoir embankment.