Determining subsurface fracture characteristics from azimuthal resistivity surveys: A case study at Nsawam, Ghana

Abstract

We conducted azimuthal resistivity surveys (ARS) using the square-array configuration to characterize the subsurface fractured rock mass at selected farmland sites in Nsawam District, Ghana. This study is the first of its kind in Ghana, and it provides useful information for future hydrological studies of the area, where groundwater is suspected to be contaminated as a result of the indiscriminate use of pesticides and fertilizers by local farmers. We estimate the fracture orientation, fracture porosity, and coefficient of anisotropy of the fractured rock mass at the selected sites from the azimuthal resistivity measurements; the specific surface area is estimated from field geological mapping of outcrops. High correlations exist between the specific surface area and the real and imaginary parts of the measured resistivity of the fractured rock mass. Fractures at localities with relatively high values of coefficient of anisotropy possess relatively high fracture porosity and relatively low specific surface area and are thus more likely to be intensely fractured and permeable. Results from this integrated geological and geophysical study indicate two dominant fracture directions in the study area, with other minor orientations that may influence groundwater and contaminant transport. The dominant orientations of the fracture systems at Kitase are northwest-southeast in the northern part and northeast-southwest in the southern part. At Amanfrom, the fractures are oriented northwest-southeast, and at Nsakye they are northeast-southwest. These sources of information from a noninvasive geophysical method are useful in assessing the transport properties of the fractured rock mass in the study area.