Preliminary result of electrical resistivity and electromagnetic methods to determine the bedrock

Infrastructure development such as buildings, especially in multi-story buildings, really requires strong foundation support. In the construction of a multi-story building, usually using a pile that is embedded into a zone that has a hard rock arrangement or bedrock to support the building above it. This is a challenge for multistory buildings in areas of volcanic deposits or pyroclastic deposits. The rock layer below the surface in this area is dominated by less dense rocks such as tuff. This rock layer has more challenging when using for building foundations because it tends to be permeable, so a special method is needed to obtain information on hard rock or bedrock for building foundation needs. In other hand, the presence of weak zones, below the surface can cause sudden land subsidence which of course will damage the buildings on it. This study aims to determine the depth of the bedrock zone, in the pyroclastic deposit area using the integration of geophysical methods, resistivity and electromagnetic (EM) method. The resistivity method is more sensitive for areas that have large types of resistance or areas that are resistive, such as rocks that have low porosity and permeability such as igneous rocks. Meanwhile, the EM method is more sensitive in layers that have high porosity and permeability values or in conductive areas such as sediment layers. Nevertheless, the conductor layer is also influenced by the mineral content in the rock, so a good understanding of geology is needed in matching the data and conditions in the field. This is expected to be a reference in designing building structures, especially in pyroclastic deposit.