Geoelectric/electromagnetic VLF survey for groundwater development in a basement terrain – a case study

Abstract

A geophysical investigation for groundwater development involving the electrical resistivity and electromagnetic VLF methods was carried out in the premises of the Conference Centre, Obafemi Awolowo University, Ile-Ife, southwestern Nigeria. The investigation involved three reconnaissance VLF-EM profilings and six vertical electrical soundings (VES). The VLF normal and filtered real component anomalies identify five major geological interfaces suspected to be faults/fractured zones. One of the interfaces coincided with a river channel which is suspected to be structurally controlled. The geoelectric section prepared from VES interpretation results delineate four subsurface layers which include the topsoil, weathered layer, partly weathered/ fractured basement and the fresh basement. The weathered layer and the partly weathered/fractured basement constitute the aquifer units. The weathered layer is relatively thin (1.8m - 4.7m) while the partly weathered/fractured basement is significantly thick (15.7m - 67.3m) and extensive with tendency for significant groundwater discharge capacity. The lithological log from a test borehole at one of the VES stations corroborated geophysically predicted subsurface sequence and its structural disposition. Fractured basement columns were identified at depth ranges of 2.5 - 7m and 12 - 45m within a column of fresh basement rock. The borehole whose aquifer is primarily fractured basement discharges about 2.0 L/s. Key Words: electromagnetic, electrical resistivity, lithological log, groundwater development. Ife Journal of Science Vol.6(1) 2004: 74-78