Geophysical Investigations in Groundwater and Local Site Effect for Geotechnic Application

Environmental and development potential risks occur during the construction process of Geothermal Power Plants (GPP) site, with the discovery of several groundwater manifestations that arise on the surface due to excavation and development activities. Electrical Resistivity Tomography, Vertical Electrical Sounding, seismic refraction and microtremor surveys have been done to investigate these risks. It was intended to identify the aquifer, to provide information about rock structure and dynamic characteristics of local geological conditions in favour of the necessary of geotechnical engineering. 3D modeling results based on ERT data, geological interpretation and seismic refraction tomography concluded that the aquifers are in the brecciated lava zone as water reservoir while the impermeable layer are lava and clay. Aquifer forms two unrelated channels on the surface on higher elevations and merges into one channel on lower elevation. From the results of microtremor analysis concluded that GPP site area has very low values of seismic vulnerability index and ground shear strain. This low value due to the its geological conditions which is composed by andesitic breccia material on the surface. The location of GPP site is relatively safe from earthquake shocks except at the location of LB-12, LB-11 and LB-05 until geotechnical engineering is done.