Integrated Geophysical Methods to Determine Subsurface Structure of PSK-1 Mini Hydro Power Plant, Citeurep Creeping Area

Citeurep area of PSK-1 mini hydro power plant is one of the areas passed by waterway to flow water into the power plant turbine and it resides on a foothill with some natural water springs. As time passes, this waterway suffered continuous damage due to continuous slow movement of land mass (creeping). Therefore, integration of geological data, drill data, remote sensing (drone data), Vertical Electrical Sounding (VES), Electrical Resistivity Tomography (ERT), and Very Low Frequency (VLF) were carried out, creating a 3D VES model, 2D ERT model, and 2D VLF model. Geological data, drill data, and the drone data are used for elevation model and utilized as supporting data to create a better electrical and VLF model to determine the subsurface lithology. VES and ERT models show a subsurface structure that formed an appearance of a natural "weir" from breccia rock of Koleberes Formation (140-380 Ωm) and thick low resistivity zones that contain sandy grains up to 50 meters deep (6-11) μm in the Citeurep area. VLF model shows some conductive zones that are interpreted as a flow of underground water. All processed data indicates that Citeurep area is an alluvial that is dominated with breccia as a bedrock. It can be concluded that the causes of creeping are due to: 1) heavy load from the top of the hill coupled with thick sedimentary layers saturated with water which influence the addition of mass and 2) lack of layer cohesion. The absence of sudden landslides is largely caused by hard breccia rocks that form a structure in appearance of a natural weir.