APPLICATION OF GEOPHYSICAL AND HYDROLOGICAL TECHNIQUES TO MAP FRACTURES IN A KARST REGION OF WESTERN OKLAHOMA

An understanding of the orientation and intensity of fractures is crucial for ground water flow modeling of an aquifer. The site selected for our study is a former gypsum quarry in the karstic region of western Oklahoma. In addition to being an ideal place for the investigation of geophysical and hydrological techniques to map fractures in gypsum, the site is also of interest to the Oklahoma Department of Transportation because the karst topography in the area is a potential hazard to the integrity of the roads. The geophysical techniques used in our study included the Ground Penetrating Radar (GPR) and a Global Positioning System (GPS).

Major fractures visible to the naked eye were mapped with GPS and a zone of high fracture intensity was identified. Several GPR reflection profiles were acquired in this highly fractured zone. In addition, twenty-four common-mid point profiles were acquired with GPR to map the velocity structure of the shallow subsurface beneath the quarry floor. A hydrological study conducted at the site included infiltration tests by sealing a PVC pipe to the quarry floor. A recording pressure transducer was placed in the PVC standpipe and the pipe was filled with water. The infiltration through the fractures in the quarry floor was calculated as a parallel plate uniform fracture with a width equal to the diameter of the pipe. Several fractures had infiltration rates that were too high to be determined with this technique because the water drained as fast as it could be added to the pipe. This technique provided good results on fractures with sufficiently low infiltration rates.

Our preliminary conclusions from this work are that 1) GPR can be used to map high angle fractures when the spatial resolution of the data is high 2) the velocity field in the survey area is laterally heterogeneous and may be used to predict underground cavities and 3) the relationship between hydraulic conductivity and the attributes of the GPR data need to be further explored.