Integrated Geophysical Approach to Aquifer Delineation in Crystalline Basement Environment

Abstract

Aquifer delineation and characterization could be somewhat challenging in Crystalline Basement Complex environment, where they are associated with either saturated weathered Basement or fractured Basement. Increasing demand for water occasioned by domestic and industrial purposes resulted in delineation and characterization of aquifer properties at a site within the crystalline basement environment, where two boreholes have been drilled (BH1 and BH2). Very Low Frequency Electromagnetic (VLF-EM) and Electrical resistivity methods were used. Twelve (12) VLF-EM profiles of length 120 – 160 m, with 10 m station separation oriented in east-west direction and twenty-five (25) VES stations were occupied. Three peak positive raw/filtered real VLF-EM anomalies identified are presumed to be water-filled fractures or faults; thus implying possible locations suitable for groundwater development. Four geoelectric layers were delineated namely: top soil, clayey sand/sandy clay, weathered layer and basement rock. The weathered layer is characterized by resistivity values which range from 28 to 1309 Ωm, and thickness values of 2.9 to 15.1 m. Longitudinal unit conductance (S), Transverse unit resistance (T), and coefficient of Anisotropy (λ) have values ranging from 0.03 – 0.16 mhos, 382 – 8517 Ωm2, and 1.05 – 2.24 respectively. These results are indicative of the aquifer protective capacity, hydraulic conductivity, aquifer productivity and hydrogeologic significance. BH1 failed because it was drilled into the weathered basement at a location characterized by fairly low transverse resistance, while BH2 was drilled into a localized fractured basement with fairly high recharge attributed to extent of fracture network, high transverse resistance and coefficient of anisotropy. Thus, fracture network and its depth extent are of great importance in selecting fractured basement aquifer as groundwater development site. Integration of electromagnetic profiling with electrical resistivity methods has shown to be complimentary tools in determination of groundwater potential and reduces uncertainties in predicting drillable positions. Robust assessment of the weathered basement and fractured basement aquifer parameters has assisted in locating productive borehole position in basement complex environment. This creates opportunities for efficient resources management, and reduces the risk of sinking unproductive boreholes.