Small-Scale Mineral Cavity Exploration Utilizing High-Resolution Ground-Penetrating Radar

In order to test the exploration potential of ground-penetrating radar (GPR) for mineral-lined cavity<br>detection, a GPR survey was conducted over a relatively smooth, -5 m2 bedding plane surface located within<br>an open-pit limestone mine, east-central New York State. Mineral cavities within the siliceous limestone are<br>observed on near-vertical surfaces and range between 0.2 to 0.8 m below the exposed bedding surface, upon<br>which the survey was performed. For this investigation, a hand-held, shielded 1000 megahertz (MHz) transceiver<br>antenna was utilized, and subsurface reflections were sampled over a 20 nanosecond (ns) time window which<br>provided a maximum penetration depth of -1 m. A 1 x 1 ft (0.3 x 0.3 m) grid was marked on the exposed<br>bedding surface, and GPR profile data were then collected by sliding the 1000 MHz antenna at a relatively<br>constant rate over the established grid lines in perpendicular, intersecting directions.<br>All GPR data were collected as analog chart records which allowed real-time, in-field interpretations.<br>Radar facies observed during this survey include laterally continuous, high-amplitude reflections that have a<br>subparallel to parallel geometry. These reflections are prominent between 4 and 14 ns two-way travel time<br>(TWT), and are interpreted as either internal bedding plane interfaces, fractures, or solution (stylolite) surfaces.<br>Several high-amplitude reflectors between -4 and 10 ns TWT were observed and characterized by an arching,<br>hyperbolic-shaped geometry. Steeply inclined diffractions that cross-cut prominent lateral, parallel reflections<br>are also associated with the hyperbolic reflections. This facies relationship is attributed to the contact between<br>bedrock and air-filed voids within bedrock. A total of 10 air-filed voids were predicted from the GPR survey.<br>Physical sampling of these GPR anomalies uncovered a total of six, near-circular to ovoid, dome-shaped, minerallined<br>cavities. Based on the survey fmdmgs, the high-resolution 1000 MHz GPR system can be used to accurately<br>map internal bedrock structures, in particular, shallow (c 1 m subsurface) air-filled vugs.