Geoelectric/electromagnetic VLF survey for groundwater development in a basement terrain – a case study
Keywords: electromagnetic, electrical resistivity, lithological log, groundwater development
AbstractA 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
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