Subsurface Geological and Hydrogeological Conditions of the Matale District, Sri Lanka: Inferred from Vertical Electrical Sounding Curves

Abstract

The electrical resistivity method which has important applications in mineral resources and groundwater exploration is used to map variations in the electrical properties of the subsurface. The study area, the District of Matale forms a part of the Central Province of Sri Lanka. The major rock types of the area consist of crystalline limestone, garnet-sillimanite gneiss, charnockitic gneiss and quartzite. Fifteen vertical electrical soundings (VES) were carried out in the Matale District using the Schlumberger configuration. Initially, fifteen apparent resistivity field curves were analyzed manually. Subsequently IPI2WIN open source software was used for computational interpretation. The VES points were located mainly in three lithologies; quartzite, crystalline limestone and gneiss. Electrical conductivity of groundwater in the region was found to be uniform and had no significant salinity-based impact on the resistivity variations. The majority of resistivity layers obtained from the VES correlated with the drill logs of same points indicating a considerable degree of relevance for their application as a tool to investigate subsurface geological and hydrogeological conditions.Introduction The Direct Current (DC) resistivity method is used to measure the electrical resistivity of a geological section which includes soil, rock and groundwater. Accordingly, the method may be used to assess the lateral changes in the physical properties of the vertical cross sections of the natural geological settings. DC resistivity techniques measure the Earth's resistivity by applying a DC signal into the ground through a pair of surface electrodes. The resulting potential differences (voltages) are measured at the surface between a second pair of electrodes. The subsurface resistivity can be calculated by knowing the electrode separation, geometry of the electrode configuration, applied current and measured voltage. Thus from obtained data, the electrical properties of the Earth (the geo-electrical section) can be derived. Resistivity methods are most commonly used for the identification of the water table (Lowrie, 1997). The method is also extensively used to