Integrated electrical resistivity methods for evaluation of fracture terrain groundwater potentials, case study of indurated shale of Lower Benue trough, Southeastern Nigeria

Abstract

The incessant borehole failure in the Abakaliki area suggests that the application of vertical electrical sounding (VES) in prospecting for groundwater within the fractured terrain is relatively inefficient. Therefore, this research aimed at using integrated resistivity methods (azimuthal resistivity survey (ARS) and VES) to defined the structures and porosity parameters in the area to overcome the lapses experienced in applying VES method alone. The data acquisition was done along four azimuthal angles (0°,45°, 90°, and 135°), adopting the Wenner electrode array expanded symmetrically about the center point in increments of 5, 7, 10, 14, 20, 28, 40, and 50 m, whereas VES data was acquired using routine Schlumberger array and the resistivity of the subsurface was recorded using resistivity meter. The data was processed with IPI2WIN™, INTERPEX™, GRAPHER™, and SURFER™, and interpretated with respect to the geology of the area. The data delineated the fractures based on mean resistivity contrast. The fractures strike within the area are NW-SE, NE-SW and N–S, E-W (37.07%, 33.33%, 18.52%, and 11.11% respectively). The interconnectivity of the fractures was portrayed in some locations and they define the permeability of the area, coefficient of anisotropy, transmissivity, and longitudinal conductance defines the groundwater possible yield. The coefficient of anisotropy ranges from 1.05 to 9.54, and fracture porosity extends from 0.0006 to 1.95, with an average of 0.34. The transmissivity of groundwater in the area support withdrawal of marginal regional importance. Therefore, groundwater yield and recharge in the area is contingent on the variability and interconnectivity of the subsurface fractures, the number of existing boreholes being recharged by this fracture, and the baseflow dynamics and dipping angle of the major fracture at the borehole site.