Self-Potential Method in Geothermal Investigations at Saline Hot Spring, Krabi

Abstract

The shallow subsurface is subject to various human activities, and the place of occurrence of geohazards, e.g. shallow active faults. The identification of the shallow subsurface structure such as sedimentary layers and faults can be vital for infrastructure development. Resistivity survey is one type of geophysical method, which is widely used for shallow subsurface investigations. General objectives of resistivity surveys are to study the distribution of resistivity values in the subsurface to identify and understand subsurface structures. There are many types of resistivity surveys classified by survey dimensions: ID vertical electrical sounding (VES) is mainly for studying changes of resistivity in vertical direction; 2D electrical resistivity tomography (ERT) is for studying the distribution of resistivity values in horizontal and vertical directions; whereas 3D resistivity survey is for studying the distribution of resistivity values in horizontal sections and along vertical directions. The aim of this study was to develop a low-cost 3D resistivity survey system, with reasonable survey time for shallow subsurface investigations. The study area is in Songkhla Province, Thailand, located in an old quarry where faults could be identified in outcrops. The study area consists of Area 1, Area 2 and Area 3. All three areas were designed to cover the expected faults. For each area, 100 electrodes arranged in a 10×10 square grid with an electrode spacing of 3 meters along x- and y-axis. Each electrode in turn was used as a current and potential electrode using a dipole-dipole array. Field data have been processed and interpreted using Res3DINV. Results, presented in horizontal depth slices and vertical xz- and yz-cross sections, revealed through differences in resistivity down to 8 m depths a complex structural setting with four shallow faults and dipping sedimentary rock layers. In conclusion, this study has shown that a 3D resistivity survey can image complex tectonic structures, thus providing a far more insight into the shallow subsurface.