Integrated geophysical methods for determination of lithology and unstable slope, case study: Cipogor area, PSK-1 mini hydro power plant

Cipogor is an area in PSK-1 Power Plant located at West Java. This area has a damaged flushing canal due to landslides at 2018. Therefore, integration of two-dimensional resistivity (ERT), one-dimensional resistivity (VES), and Digital Elevation Model is conducted to determine lithology and unstable slope that characterized by water saturated weak zone. Resistivity models from ERT and VES method are integrated with elevation data from DEM. Lithologies from interpreted VES model are as follows: Alluvial (6 – 70.06 Ωm), Tuff sediments (12 – 28.44 Ωm), sandstone sediments (1.96 – 9 Ωm) and Breccia (42.07 – 88.6 Ωm) from Koleberes Formations. There is an anomaly (12 – 28.98 Ωm) within sandstone layer. From ERT model, there is low resistivity zone (2-5 Ωm) interpreted as weak zone and moderate resistivity zone (16-24 Ωm) around 9-17 m in depth, interpreted as sediments transported during previous landslide. From the interpretation, it can be concluded that there is weak zone in depth 5-6.91 m and 17-20 m which is a weathered sandstone of Koleberes Formation and a sediment layer that transported from previous landslide in depth 9-17 m. Heaving is occurred in this layer and preventing any large landslide to occur but cannot prevent soil creeping in the area.