Overview of geothermal surface exploration methods

Abstract

Geothermal surface exploration invariably entails a multi-geoscientific process, which are holistically aimed at defining the geometry and characteristics of the geothermal system prior to drilling. The scientific disciplines commonly involved are geology, geochemistry, and geophysics. Geological approach generally aims at understanding the various lithologies, volcanological evolution, structural controls, and hydrological regimes of the system. Geophysical exploration helps in determining the geometry (shape, size and depth) of the heat sources, reservoir and cap rock. It also aims at imaging structures that are responsible for the geothermal system, and delineating the areal extent of the geothermal resource. The most commonly used geophysical methods are electromagnetic/electric, gravity, magnetics and seismics. These methods ultimately depend on the various intrinsic properties of rocks such as resistivity/electrical conductivity, density, magnetic susceptibility, elastic moduli/velocity respectively. Geochemical exploration relies mostly on sampling and analysis of water, steam and gas from the thermal manifestation in order to characterize the fluids, estimate equilibrium reservoir temperature, determine the origin, evaluate mixing scenarios, determine the suitability of the fluids for the intended use and locate recharge areas and direction of fluid flow. Additional geochemical studies entails soil diffuse degassing measurements aimed at identifying gas leakages that usually mimic active faults and structures.