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

Characterizing Fractures in a Bedrock Outcrop Using Ground-Penetrating Radar at Mirror Lake, Grafton County, New Hampshire

ABSTRACT

A study incorporating numerical modeling, physical modeling, and field surveys at the U.S. Geological Survey Fractured Rock Research Site at Mirror Lake, Grafton County, New Hampshire, was conducted to test the use of ground-penetrating radar (GPR) surface reflection methods to delineate fractures in heterogeneous bedrock. Results of one- and 2.5-dimensional numerical modeling correlate with results of laboratory-scale physical modeling and establish different GPR reflection characteristics for saturated and unsaturated (dry) fractures. Saturated fractures generate higher amplitude reflections than unsaturated fractures and have an opposite phase. GPR reflection data collected over a highway bedrock outcrop near Mirror Lake were processed to reduce noise and clutter, to correct geometric and topographic distortions, and to enhance weak reflections from structures more than 15 meters (m) deep.

Guided by the results of numerical modeling, 18 reflectors with lengths ranging from 4 to 32 m and dips ranging from 5 to 40 degrees were interpreted as fractures in the processed GPR field records. All of the interpreted fractures that project above land surface correlated with fractures recorded by detailed outcrop mapping. Interpretation of the processed field data was limited to reflectors dipping less than 45 degrees, although more steeply dipping fractures exist at the field site. Spatial aliasing effects and other limitations constrain the range of dip-angles that GPR reflection methods can usefully image from the surface. The results of the field study together with the results of the numerical and physical modeling indicate that surface GPR reflection methods can be used to help characterize fractures in heterogeneous bedrock.