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.

Development of azimuthal resistivity survey data processing program with graphical user interface for anisotropic coefficient quantification

“…The anisotropy observed by the ARS method is the horizontal effect, as the measurement is done with electrode lines placed on the earth's surface. In the ARS method, VES measurement is done with various user-defined azimuths, commonly with a constant increment of azimuth [1]. The result is VES data for various azimuths.…”

“…LA is the length of the major axis and LB is the length of the minor axis. By doing ellipse fitting we can also know the degree of declination, which is used to determine the fracture orientation [1]. We can also get the degree of optimization from a circular model.…”

“…ARS data can be used to study the anisotropy of underground material. It is done by calculating the variation of the anisotropy coefficient with depth [1]. The flowchart of this process is given in figure 7.…”

“…The result is apparent resistivity data paired with the electrode spacing value. In the ARS method, the VES measurement is done with various azimuths while maintaining the same middle point [1]. The result is VES data for various electrode line azimuths.…”

“…The ARS can be used to calculate the anisotropic coefficient variation against depth. Furthermore, we can use the ARS data to predict the fracture orientation beneath the earth [1]. This is done by doing inverse modeling for each VES data and ellipse fitting for the ARS data.…”