IMPROVING CONCEPTUAL MODEL OF CRYSTALLINE BASEMENT AQUIFERS USING ELECTRICAL RESISTIVITY AND SEISMIC REFRACTION TOMOGRAPHY

Crystalline basement aquifers are characterized by high hydraulic heterogeneities with flow controlled by complex secondary permeability fields in the fractured rocks. A detailed conceptual model capturing the variation in weathered overburden thickness, fracture distribution and connectivity is needed to implement a groundwater flow and transport model for these complex reservoirs. In this study, we combine both electrical resistivity and seismic refraction tomography to improve a conceptual understanding of the heterogeneities within these aquifers. We first simulated the variation in resistivity distribution as a function of varying weathered overburden thickness as well as fracture size and orientation. Results of these numerical studies guided field experimental design and interpretation of model results. Field investigations were conducted at the Ibadan Hydrogeophysics Research Site (IHRS), a 50 m × 50 m-controlled volume field-laboratory at the University of Ibadan, Nigeria for investigating fractured aquifers in a predominantly augen- and banded-gneiss terrain. We acquired 12, 126 m long electrical resistivity tomography (ERT) and 2, 72 m long seismic refraction profiles in an East-West and North-South grid. ERT profiles were acquired with a Campus Tigre resistivity meter using a Wenner electrode array with 2 m unit electrode spacing. Both forward and inverse resistivity modeling were done using a MATLAB script and the AGI EarthImager resistivity inversion code. The seismic refraction profiles were acquired using an ABEM, 48 channel Terraloc Pro Seismograph with 4.5 Hz geophones, a geophone spacing of 0.5 m and shorts at every meter using a piezoelectric hammer. A 3-step roll along was used to cover a length of 72 m at each profile. We processed the seismic data with SeisImager using the Pickwin and Plotrefa modules for picking the first arrivals and modeling the velocities. The resulting electrical resistivity and P-Wave velocity tomograms compare relatively well. In addition to differentiating subsurface layers, the geophysical models reveal regions with higher fracture concentration which could be used to improve a flow model calibration for these aquifers but these models are limited in identifying each fracture plane.