Remote sensing and geophysical mapping of lineaments and evaluation of the hydrogeological significance: a case study for the Ipogun Drainage Basin, Southwestern Nigeria
摘要
Public water supply to the Ipogun Drainage Basin, southwestern Nigeria, is grossly inadequate thereby limiting the inhabitants to the alternatives of surface and groundwater. The surface water in streams and rivers are polluted by runoffs and anthropogenic activities (requiring expensive treatment) thereby making groundwater development the only alternative source of potable water. However, recent audit of wells and boreholes drilled within the basin has established a generally low groundwater yield. But lineaments in basement complex terrain, as the referenced basin, enhance groundwater accumulation, transmission and yield. Remotely sensed and aeromagnetic data were therefore, used to map lineaments within the basin while existing borehole yield data and Vertical Electrical Sounding (VES) derived geoelectrical parameters were applied to assess the hydrogeological significance of the delineated lineaments. Remotely sensed Landsat 8 Operational Land Imager/Thermal Infrared Sensor (Band 6 and 7), Digital Elevation Model imageries and aeromagnetic data were processed and interpreted to map surface/near-surface and deep-seated lineaments. The VES data were quantitatively interpreted for subsurface layer sequence and geoelectrical characterization and aquifer identification. The remotely sensed data mapped few (39) shallow lineaments in the upper 30 m while magnetic derived attributes of Total Horizontal Derivative, Second Vertical Derivative, Tilt Derivative and 3D Euler Deconvolution (ED) mapped 155 near-surface to deep-seated lineaments (in the depth range of 0–300 m). The remote sensing/magnetic generated composite lineaments trend ENE–WSW, N–S, NE–SW, NNE–SSW and W–E. Lineament density and intersection ranged between 0.0 and 10.44 km/Km2 and 0.0 and 1.51 P/Km2 respectively. The subsurface sequence includes the topsoil/laterite, weathered layer, partly weathered/fractured basement and fresh basement. Audited records of One hundred (100) boreholes drilled to shallow depths (generally < 40 m) within the basin penetrated majorly the weathered layer aquifer and are characterised by generally low groundwater yields (< 1.00 l/s). The low groundwater potential is attributable to the generally thin (< 20 m) and clayey (resistivity of < 150 Ω m) weathered layer with low permeability, and low near-surface lineament density. Statistical analysis showed that the influence of geologic structures (lineaments) on groundwater yield seem insignificant at shallow depth. However, the mapping of many deep-seated magnetic lineaments (up to 300 m depth), presents the prospect of an enhanced groundwater potential, if validated with further study.