U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12, 1999--Volume 3 of 3--Subsurface Contamination From Point Sources, Water-Resources Investigations Report 99-4018C

Integration of Surface Geophysical Methods for Fracture Detection in Bedrock at Mirror Lake, New Hampshire

By C.J. Powers, Kamini Singha, and F. Peter Haeni

ABSTRACT

Five surface geophysical methods were used to determine the locations of fracture zones in crystalline bedrock for predicting fluid flow and chemical migration at the U.S. Geological Survey Fractured Rock Research Site at Mirror Lake, Grafton County, New Hampshire. Two methods of direct-current (dc) resistivity (two-dimensional (2D) and crossed square-array profiling), two methods of inductive terrain conductivity, and very-low-frequency electromagnetics (VLF) were used over survey lines extending about 200 meters. The results of the five methods were correlated to locate fracture zones; anomalies that were detected in one or two of the results were eliminated, increasing the confidence in the interpretation of anomalies detected in all of the results.

Two low resistivity anomalies were detected with all the geophysical methods in the southeast part of the study area. Based on the geophysical, outcrop, and photolinear data, the anomalous areas were interpreted as steeply dipping fracture zones approximately 10-meters wide. One interpreted fracture zone strikes approximately north 45 degrees east and the other strikes approximately north 17 degrees east.

Results of dc-resistivity surveys were analyzed to estimate the secondary porosity of the two interpreted fracture zones. Crossed square-array dc-resistivity profiling data indicates the secondary porosity is between 0.65 to 0.75 percent, whereas the 2D dc-resistivity profiling results indicate the secondary porosity to be 1.6 to 1.9 percent. Estimates from the 2D dc-resistivity profiling could indicate the effects of alteration and/or iron precipitate observed in outcrops near the survey lines.