Borehole Ground Penetrating Radar Surveys Of Fractured Limestone

Two Borehole Ground Penetrating Radar (GPR) surveys were conducted to evaluate<br>fractured limestone bedrock formations. One survey was conducted at the proposed location for<br>a bridge pylon and one at a limestone quarry. The objective for both surveys was to identify<br>small voids and fractures in the limestone. At the proposed pylon site, the voids and fractures<br>were mapped to evaluate the limestone’s bearing capacity and to design a grouting program. At<br>the quarry site, the voids and fractures were mapped to help identifying permeable layers.<br>Borehole GPR is the only geophysical technique capable of imaging individual small voids and<br>fractures that do not intersect a borehole. A MALA Geoscience borehole GPR tool configured<br>with 250 and 100 MHz antennas was employed for these investigations.<br>Nine boreholes were drilled into the limestone formation at the footprint of the proposed<br>pylon. All nine boreholes were surveyed in reflection mode using 100 MHz and 250 MHz<br>borehole GPR antennas. Two cross-hole tomographic surveys were also conducted at the site.<br>The reflection surveys identified multiple layers that indicated voids and fractures of varying intensity.<br>The tomography data provided information regarding the bulk characteristics of geological units<br>and identified smaller areas with anomalous high porosity that could indicate high density of voids or<br>fractures.