Application of Ground Penetrating Radar to detect orientation of cavities and caverns developed due to tectonic implication in Baratang Island, Middle Andaman, India

Ground Penetrating Radar (GPR) is an effective tool to detect small (<; 10m diameter) caves and fissures in karst terrain. 170 caves have been identified within 1 km² area in uplifted carbonate rocks in Baratang Island of Middle Andaman. Due to oblique subduction of Indian plate with Burmese plate in NW part of Andaman induced NE- SW and NW- SE trending sutures. The Study aims to validate the orientation of the fault patterns in carbonate rocks through Ground Penetrating Radar. To conduct GPR surveys two cave sites 400 m apart are selected. GPR - SIR-3000 instrument with dipole antenna in bistatic arrangement in reflection mode with stationary point mode is employed. Four depth (time) slices computed from the transect data for 40 MHz antenna with 500 ns range reflected across the cave No. 1 to a length of 19 m reveal bright linear features and intense spots positioned at the depths of 3 m and below 11 m to 18 m and vertical shaft connecting discontinuities are traced to 25 m depth in all plots. Five depth slices at an interval of 4.0 m to a total distance of 28 m with 40 MHz antenna with 400 ns have reflected bright regions at the depths from 8.5 m to 12.5 m and scanning up to depth of 20 m in cave No.2. Seven depth slices by using the 80 MHz antenna with time window of 250 ns across the cave No.2 have reflected to 12.5 m depth, show bright regions with high resolutions at the depth range of 8.5 to 12.5 m. Increase of antenna frequencies with lowering of time range has resolved high resolution reflections of cavities, cracks and reduction of depth in the cave No. 2 than the in cave No. 1. Induction of cracks and fissures in carbonate rocks due to erstwhile tectonic repercussion were subsequently increased their sizes through rain water percolation and dissolution. Depth (time) slices of GPR in all transects across the cave orientations in two sites proved that continuation of caves are aligned in NW-SE and NE-SW directions.