Combined use of Very Low Frequency Electromagnetic (VLF-EM) and electrical resistivity survey for evaluation of groundwater potential of Modomo/Eleweran area, south western Nigeria

Very Low Frequency Electromagnetic (VLF-EM) and electrical resistivity surveys were conducted at Modomo/Eleweran, along Ede-road, south western Nigeria, with a view to delineate the hydrogeophysical characterization of the study area. The area is underlain by the Precambrian Basement Complex rocks. The VLF-EM traverses were established along 6 traverses with a station interval of 10 m with lengths ranging from 130 to 360 m. Linear features presumed to be geologic fissures inferred from the filtered real pseudo-sections helped in selecting twenty-nine VES points that were further probed using ABEM SAS 300 C Resistivity Meter. The spreading were carried out using the convectional Schlumberger electrode  configuration with half-current electrode separation (AB/2) varying from 1 to 100 m was used for the sounding. The VES data were presented as depth sounding curves and were appropriately iterated using RESIST version (1.0) software. The VLF  filtered real profile displayed a low peak trend depicting poor or no fracture signature. Four lithological formations were delineated which included the topsoil, weathered layer, partly weathered/fractured basement and fresh bedrock. The delineated weathered and fractured basement columns constituted the aquifer units.Additionally, from the geophysical parameters viz a viz thin overburden thickness, clayey weathered layer and low fractured frequency characterized by the study area, it is inferred that the groundwater potential of the area varies between poor and low. However, the study justified the use of a combined geophysical investigation as a better tool in evaluating the groundwater potential in the basement complex.Key words: Weathered layer, geological fissures, aquifer, electrical resistivity, geoelectric section, electromagnetic

Investigation of basement fault propagation in Chad Basin of Nigeria using high resolution aeromagnetic data

This study was conducted to investigate the basement fault propagation into the overlying sedimentary cover in parts of the Nigerian sector of Chad Basin. The Total Magnetic Intensity (TMI) map was compiled from the digital aeromagnetic data and was reduced to the equator to produce the Reduced-to-Equator (RTE) map. Residual Magnetic Intensity (RMI) map of the study area was obtained after the removal of regional trend from the RTE data. Regionalresidual separation of the RMI map was carried out using upward continuation filtering technique adopting the depths obtained from spectral analysis to produce magnetic anomaly maps associated with the basement and intra-sedimentary magnetic sources. The maxima of the Horizontal Gradient Magnitude (HGM) of the basement and intra-sedimentary magnetic anomaly maps were computed and used to delineate faults that produced the structural maps of the basement and the intra-sedimentary column, respectively. Upward continuation of the RMI map at various altitudes and the maxima of their HGM were used to highlight faults from shallow to deep depths, as well as their strikes and dips. Both major and minor faults dominated the study area. The faults strike in the directions NE-SW (Pan-African trend), ENE-WSW, NW-SE, and E-W. Two profiles were drawn on the basement and intrasedimentary maps, respectively, to model the subsurface structures. The results of this study revealed that the sedimentary section was affected by the tectonics of the underlying basement, with faults propagating from the basement upwards into the sedimentary cover. These faults constitute potential structural traps for oil accumulation or conduit for oil migration