Use of GPR and ERT Methods to Image Shallow Structures around a Prospective Paleotsunami Geopark Cave in Aceh Besar, Indonesia

Abstract The Ek Leuntie Cave is a prospective paleotsunami geopark that preserved 12 tsunami layers dating back 7500 years in Aceh. Landslides pose many challenges to the development facilities in karst areas. Before development, the area was assessed using high-resolution Ground Penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT). The 700 and 250 MHz of GPR frequencies were used to image shallow subsurface structures, while the ERT method can penetrate deeper. Data measurements were conducted on a long 130m profile perpendicular to the cave entrance. The GPR radargrams clearly show subsurface structure up to 1.5 m depth. The uppermost thin and flat layer is superimposed on a concave layer. The lowest structure begins to fade due to the limitation of penetration depth. The ERT model images up to 30 m depth with resistivity vary 0 to 1000 Ωm. The uppermost and the second layers have low to medium resistivity values superimposed by a very high resistivity layer. The top layer filled with anthropogenic soil covers the sandy layer at a depth of 0.3 to 0,4 m. While the third layer is interpreted as limestone. Shallow borehole data drilled along the profile agree with the interpretation. The 700 MHz of GPR radargram shows a very clear interface between the sandy and the top soil layers as well as the limestone beneath it, while the ERT model penetrates deeper but the interface between layers is blurry due to smoothing effects. Both methods complement each other and can be used for geotechnical assessment of the area.