USING GROUND-PENETRATING RADAR TO LOCATE NEAR-SURFACE FAULTS IN THE VIRGINIA PIEDMONT: PRELIMINARY RESULTS FROM THE EVERONA FAULT

The Everona fault is a NE-striking, NW-dipping reverse fault within the NE-striking, SE-dipping Mountain Run fault zone in Orange County, Virginia. It displaced bedrock saprolite over colluvial and fluvial gravels deposited by Mountain Run. The undated gravels are inferred to be Pliocene or early Pleistocene deposits. The Everona fault is not known to be historically seismically active, and whether it may pose seismic risk to nearby cities such as Washington, D.C., and Richmond, Virginia, is undetermined. The Everona fault is no longer exposed at the original excavation, and locating faults in the field has proven to be challenging. We are currently testing the effectiveness of ground penetrating radar for locating buried faults and displaced surficial deposits or bedrock-saprolite transitions in the Virginia Piedmont.

Ground penetrating radar systems with antenna frequencies of 250 and 25 MHz were used to collect profiles across the Mountain Run Valley. The 250 MHz antenna works well at locating gravel deposits, but radar penetration is typically less than 4 m and the system is highly sensitive to interference from overhead power lines and buried cables and pipelines. The resolution of the 250 MHz system is limited to targets and/or sedimentary layers larger than ~0.3 to 1.0 m. The low frequency 25 MHz system penetrates up to 30 m into the subsurface and locally shows reflections that may represent the top of competent bedrock. Its resolution is limited to layers and targets larger than ~0.6 to 2.0 m, but cannot image the upper 3-5 m. The 25 MHz system is less susceptible to interference from power lines and buried cables than the 250 MHz system. Both systems successfully show dipping and displaced reflectors, but some of these may represent bedrock folds and other Paleozoic features, so additional field testing and ground truthing is needed.

Most recently we tested a 100 MHz antenna which shows the most promising results. The 100 MHz system shows both shallow and deep reflectors, is less prone to interference from power lines and other nearby objects, and so far shows the best contrast between subsurface reflectors. All three GPR systems show potential for locating near-surface faults in the Virginia Piedmont.