oa Using GPR and resistivity methods to detect subsurface karst cavities

Ground penetrating radar (GPR) and resistivity methods were used to detect the subsurface karst cavities in Jubaila Formation, west of Riyadh, Saudi Arabia, These features may cause geological or environmental hazards to buildings and constructions. The aims of this study are to understand the physical properties of subsurface cavities, the response of the formation structure to these geophysical techniques and to evaluate them in geotechnical and engineering studies. By 250 MHz frequency, GPR was found to be clearly effective in detecting variations of the electrical properties of shallow subsurface structure up to a depth of 6 m, and to 8 m with lower resolution. The GPR images detected strong and continuous reflections from horizontal layers with similar<br>positions and depths (at a depth 4.75 m), which have an electrical resistivity value between 180–300 ohm-meter. The GPR sections did not detect any caves in the layers; i.e. the caves are not continuous nor extensive, but rather showed a homogenous structure in the lower section. The GPR results were improved-upon by the electrical resistivity method. This method detected the horizontal layers, which in parts showed zoning of the resistivity. The zones show a distributed field with a semi-circular gradient and a sharp value in the center. Based on drilling, the central point was found to be a limestone layer with facies change, hardness or indurations. The resistivity of the central point reached 280 ohm-meter, while the surrounding media’s resistivity is 120 ohm-meter. The results of GPR and resistivity methods are supported with geotechnical drilling, which were in agreement with the interpretation.