Borehole GPR Direct and Refracted Arrivals for Determination of Subsurface Void Dimensions

Ground-penetrating radar (GPR) borehole data were acquired in order to determine the size of shallow coal mining voids in Pittsburg Kansas, USA. A dense grid of drillholes would have been required to accurately map the lateral extent of subsurface mining voids. Borehole GPR was evaluated as a method to image the voids and potentially reduce the number of drillholes necessary to determine void dimensions. Zero-offset transmission profiles showed two distinct arrival modes in the region of the subsurface voids. Velocities estimated for each of the arrival modes indicate that the early (first) arrival corresponds to refracted energy along a carbonate bed over the voids. Slower (second) arrivals correspond to energy that propagated through water saturated void and adjacent coal pillar. The observed radar transmission arrival times were used along with the known separation of the GPR boreholes to determine the proportion of radar wave travelpath through pillar and void present between the boreholes. Using this method, void and pillar dimensions were determined between three boreholes.