Quasi-2D Resistivity Model from Inversion of Vertical Electrical Sounding (VES) Data for Preliminary Geothermal Prospecting

Abstract

Vertical Electrical Sounding (VES) using Schlumberger electrode configuration is practical in the field with rough topography. With moderate investigation depth, VES technique is still commonly performed in preliminary surveys of a geothermal prospect. This paper presents the guided random search algorithm for VES data inversion leading to a quasi-2D resistivity model. At every VES station, resistivities of a layered earth model with fixed thicknesses are selected from "a priori" resistivity values favoring lower misfit. In addition, a higher weight is associated to a resistivity value minimizing the variation of resistivities: (i) between layers at one station (vertical smoothness), and (ii) of the same layer at adjacent sounding sites (horizontal smoothness). Simultaneous inversions of original and interpolated VES data along a profile results in a quasi-2D smooth resistivity model of the subsurface. The algorithm was applied to invert synthetic data and was able to recover the synthetic model satisfactorily. The inversion result of field data from a geothermal field is in good agreement with known local geology of the survey area. The top of conductive cap rocks are well resolved at shallow to moderate depths (approximately 600 m) which are typical accessible target for a geothermal exploration at the preliminary stage using the geo-electrical method.