Modeling Hydraulically Significant Subsurface Features Utilizing MASW for Environmental Applications

Abstract

The non‐invasive seismic method of multi‐channel analysis of surface waves (MASW) was utilized to model hydraulically significant features in the subsurface geology for an environmental project located in the central area of Georgia's Piedmont geophysical region. This method was used to supplement the information derived from a preliminary environmental exploration program which included soil borings and monitoring wells. The findings from the MASW survey provided a 2‐dimensional plan view for the site which modeled contours for the erratic bedrock encountered in the preliminary borings and monitoring wells. MASW data was acquired across various ground surfaces including asphalt pavement, soil, grass, and gravel. The data acquisition involved the use of two deployment systems; a twenty‐four (24) channel land streamer deployment of pressure coupled geophones, and a forty‐eight (48) channel deployment of spike coupled geophones embedded in the ground in lines traversing the study area. At paved areas of the site, the land streamer deployment was utilized, and data was gathered as the line was pulled along the transect at 5 feet intervals. For soil, grass, and gravel areas, the spike coupled deployment was used. Data was gathered by electronically rolling through the spread at 5 ft shot intervals, collecting twenty‐four soundings, before “leap‐frogging” the deployment along the transect to cover the study area. The MASW data successfully modeled low and high velocity zones in the site's underlying stratum. A shear wave velocity (Vs) was established for the underlying granitic gneiss bedrock based on our past experience with MASW Surveys in the local Piedmont geology, a review of preliminary borings log data, and a review of literature from the International Building Code. The site bedrock contour map modeled from the MASW data was utilized by the project environmental scientists to identify hydraulically significant features in order to more accurately locate and optimize additional cost‐effective monitoring wells. This provides a strong basis for continuing the application of non‐invasive geophysical techniques for the purposes of site specific subsurface modeling.