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