A GROUND-PENETRATING RADAR SURVEY OVER FLINT RIDGE CAVE, AUSTIN, TEXAS FOR KARST AQUIFER RECHARGE STUDIES

The objective of this research was to explore the uses and limitations of ground penetrating radar (GPR) in helping to understand karst aquifer recharge and connectivity. Flint Ridge Cave in south Austin, Texas is a recharge cave for the Edwards aquifer and is of great interest to the Travis County Watershed Protection and Development Review Department. Flint Ridge Cave possesses a good control environment for this geophysical research. The cave's rooms and passageways have recently been mapped using traditional means and a radio survey of the cave allows for positions within the cave to be known on the surface. A precisely measured 14 x 19 meter grid was laid out over one of the main chambers of the cave. A Sensors and Software PulseEkko 100 GPR was used to acquire a dense grid of data at 25, 50 and 100 MHz. This range of frequencies was needed in order to accurately map the roof of the cave (~ 15 m depth) as well as the smaller recharge features. The data were collected in parallel lines with grid line spacing of 1 meter for the 25 MHz and 50 MHz antennas and ½ meter spacing for the 100 MHz antennas. The step size for each frequency was the maximum step size allowed without violating the Nyquist, in order to avoid aliased data. The data were then cleaned and processed by removing bad points, dewowing, applying a gain and then interpolating for any missing data. The data will then be processed using seismic data analysis software including migration to more accurately locate features in the subsurface.