Electrical Resistivity Tomography (ERT) for investigating subsurface structures in the karst area of the Ek Leuntie Cave, Aceh Province

Abstract

Ek Leuntie Cave, in the southern city of Banda Aceh, Aceh Province, is recognized as an educational media based on geopark sites because it holds a 7400-year paleotsunami track record. As a karst area, Ek Leuntie Cave is vulnerable to the impact of geological disasters such as earthquakes, sinkholes and subsidence. As an effort to reduce the negative impact of geological disasters, geophysical studies have been applied in the Ek Leuntie Cave Karst area. The study aims to identify the subsurface geological structure of the Karst area. The identification was carried out using the Electrical Resistivity Tomography (ERT) method of Wenner-Schlumberger configuration applied to two measurement trajectories that have South-North and West-East directions in front of the Ek Leuntie Cave door. ERT data acquisition results were then modeled in inversion using Res2Dinv software to produce a 2D model of the subsurface geological structure of the Ek Leuntie Cave karst area based on variations in rock resistivity values. Interpretation of subsurface geological structure modeling is validated with shallow drilling data (Hand Auger). The inversion modeling results show that the rock resistivity value in the Ek Leuntie Cave area reaches a depth of 30 m which has a resistivity value of 0 - 1000 Ωm. The results of the interpretation of resistivity values and validation of drill data show that the Ek Leuntie Cave area is covered by a layer of sand with fine to coarse texture and the layer below is a layer of limestone at a depth of ± 0.4 meters with a resistivity value of 50 - 250 Ωm. Limestone generally shows porosity with the possibility of caves or pores in the rock formation at Ek Leuntie Cave, making it vulnerable to damage from earthquakes, landslides, and subsidence. The results of the study are expected to provide updated information in efforts to develop the Ek Leuntie Cave geopark area.