Panoramic azimuthal Schlumberger Vertical Electrical Sounding for fracture orientation and anisotropy quantification

Abstract

Vertical electrical sounding (VES) data acquired with the Schlumberger configuration is popularly used to image the electrical resistivity variation with depth at a single azimuth. Apart from the random subjective choice of the single azimuthal direction by the field geophysicists, important hydrological information such as fracture orientation and anisotropic coefficients needed for understanding resultant groundwater flow direction are by design lost in the process. Panoramic (0°–360°) azimuthal VES data were acquired at two data points at the Federal University of Technology, Akure (FUTA) at angular step of 15°, making a total of 24 data sets per data point. Each azimuthal VES data was inverted using equal number of layers in order to confirm the presence of anisotropy, quantify the anisotropic coefficients and image the orientation of fracture at a particular depth. Little to large apparent resistivity data and model suggested the presence of anisotropy which otherwise would have been lost in a single azimuthal survey. Elliptical fit of each layer azimuthal inverted resistivity was used to quantify the fracture orientation and coefficient of anisotropy with depth. From the results, it is established that anisotropy is present only at the near-surface: and the anisotropic coefficient increases from the surface to 7m. The result also showed the presence of an isotropic unit from 8m to the fresh basement. In agreement with existing published results on the geology of the area, the majority of the fractures trend North West and North East at stations 1 and 2 respectively. We hope that the methodology will foster detailed 3D panoramic imaging of the fracture network within and outside the study location, which will help in designing better groundwater management scheme and understanding resultant groundwater flow direction for contaminant and pollutant prevention and for flood control.