3D Resistivity Surveys for Structural Investigations of the Shallow Subsurface at Sadao, Songkhla

Abstract

Geothermal system formed deep inside the earth, but often show surface manifestations in form of hot springs, which are often tourist attractions and their water can be used for recreational or medical spa. Energy from these geothermal systems can be used for electricity generation or direct uses. The main medium which transfers the energy in the system is the geothermal fluid, which carries the heat from deeper reservoirs to the surface through open fractures and faults, which act as higher permeability pathways. Thus, the pattern of fluid flow is a key parameter to obtain details of the deeper parts of the reservoirs. Geophysical surveys were often performed to study and identify these fluid flow pathways and the water circulation in the subsurface in order to estimate the reservoirs characteristic and potential and also provide evidence of geothermal systems that have no surface expressions. This study focusses on self-potential (SP) surveys used to infer permeable pathway at depth as the self-potential response is directly linked to subsurface fluid movements. Moreover, electrical resistivity surveys, ground surface temperature measurements, and geological surveys are integrated in order to provide better interpretation and based on that drawing a hydrogeological model of the study area. The exploration was conducted at Khlongthom Saline Hot Spring, located in Krabi, Southern Thailand. The combination of the quantitative results from all survey methods revealed a pattern of fluid flow in the relative shallow subsurface of the area. Additionally, self-potential data showing that negative SP anomalies correlated well with hot saline water flowing up, and positive SP anomalies can be associated with water flowing down in an overall higher saline environment. The SP response is a result from the sum of streaming potential and diffusion potential. It can be concluded that the saline hot spring water in the area is often very shallow and separated by shallow fractures, and additional hidden hot springs could be identified; locations where the saline water flow upwards but not reaching the surface. Understanding of this unique hot spring system is vital for further development in this area as the results from this study should be considered when designing buildings or other infrastructure projects in this area.