Determining Subsurface Fracture Characteristics from the Azimuthal Square Array Resistivity Survey at Igarra, Nigeria

Abstract

Fractures are deformations in rocks with discontinuity. They are important in a number of ways. Their presence significantly influences the strength and engineering properties as well as the hydraulic characteristics of rocks. Fractures may extend to the surface where they are observed and studied at outcrops. On the other hand, they may terminate in the subsurface or may be covered by overburden which makes them impossible to be studied and characterized at the ground surface. There has been an increasing interest in the location and characterization of fractures by earth scientists, engineers and other scientists, both at the sur face and the subsurface. However, the unavailability or inaccessibility of good outcrops makes it imperative to develop methods and tools for studying fractures in the subsurface. Geo physical methods such as the resistivity methods have been very useful in this regard. The Azimuthal Square Array Resistivity Survey was used in this project to locate and characterize subsurface fractures in the crystalline rocks at Igarra. Results from the analysis and interpretation of the field data showed that the dominant fracture strike orientation is in the NNW–SSE direction. This compares well with the results of surface geologic mapping data which gave the general fracture strike orientation as N–S; however, the major large and extensive fractures are striking NNW–SSE. This information is very useful in modeling groundwater flow and contaminant transport; planning proper waste management pro grams as well as the Environmental Impact Assessment analysis for the study area. This study once more illustrates the satisfactory use of non-invasive geophysical methods in characterizing fractures in the subsurface especially where quality outcrops are not available or inaccessible.