Borehole, Surface and Water-Borne Geophysical Surveys at the Callahan Mine Superfund Site in Brooksville, Maine: October 2016 to July 2018

From October 2016 to July 2018, the U.S. Geological Survey, in cooperation with the U.S. Army Corps of Engineers and Maine Department of Transportation, collected surface, marine and borehole geophysical surveys to characterize the subsurface materials on land and under the water at a former mine facility in Brooksville, Maine. Three water-based geophysical methods were used to evaluate the geometry and composition of subsurface materials. Continuous seismic profiling (CSP) methods provide the depth to water bottom, and, when sufficient signal penetration can be achieved, delineate the depth to bedrock and subbottom materials. Continuous resistivity profiling (CRP) and frequency domain electromagnetics (FDEM) methods were used to define the electrical properties of the shallow subbottom. All data points were located using global positioning systems (GPS), and the GPS data were used for real-time navigation. The stage of Goose pond was monitored with pressure transducers during the water-borne geophysical surveys. On land, electrical resistivity tomography (ERT), FDEM, shear-wave velocity (Vs) seismic refraction and horizontal-to-vertical spectral ratio (HVSR) seismic methods were used to characterize the subbottom materials and to evaluate the surveys collected on the water. Borehole geophysical logs were collected in five boreholes to identify fluid and electrical properties as well as natural gamma emissions.