Electrical resistivity survey involving 2D Dipole-Dipole subsurface imaging and 1D Vertical Electrical Sounding (VES) and an integrated magnetic and electrical resistivity survey involving magnetic profiling and 1D VES/2D electrical imaging techniques were carried out at two sites underlain by grey gneiss and pegmatised schist, respectively, within the Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria. The surveys were carried out with a view to delineating the subsurface layers and determining the geoelectrical characteristics; identify geological structures such as faults, fractured, jointed and sheared basement zones that are favourable to groundwater accumulation and transmission and estimate hydrogeologically significant borehole drill depth in an area with previous experience of failed borehole drilling attempts. At the two sites (A&B) investigated, the 2D resistivity images identified suspected fractured basement zones as low resistivity vertical discontinuities within high resistivity fresh basement host rock. The magnetic profiles at site B displayed typical magnetic anomaly of a thick dyke (two magnetic lows with central high in low magnetic latitude) over each vertical resistivity discontinuity. The follow-up VES delineated maximum of five subsurface layers which include the topsoil/laterite, weathered layer, partly weathered/fresh basement, fractured basement and the fresh basement. The weathered/fractured basement constitutes the main aquifer unit with thicknesses varying from 17.7-125 m. Two boreholes, one per site, were drilled to 80 m at site A and 70 m at site B. Both boreholes encountered the predicted fractured basement zones and the borehole logs correlated well with the interpretation models of the drilled VES locations. The boreholes were very productive (yields of about 1.5 l/s and 2.2 l/s at sites A & B respectively). The study demonstrated the effectiveness of integrated geophysical methods and techniques in providing information on the subsurface sequence and the structural disposition required for a successful groundwater development through borehole drilling, in a borehole-failure-prone basement complex terrain.
A detailed ground magnetic survey was carried out in a basement complex underlain Modomo community in southwestern Nigeria with a view to delineating the subsurface structures, estimate the overburden thicknesses and assess the relevance of the geophysical method in groundwater investigation in the locality. The total field component of the earth’s magnetic field was measured at station intervals of 10–100 m along access routes within the study area. The data were corrected for diurnal variation and offset and subsequently reduced to the magnetic equator (RTE). Data enhancement techniques including the second vertical derivative, total horizontal derivative and Euler deconvolution were applied to the RTE data to map edges and estimate depths to the structures. Overburden thicknesses were estimated from 2D magnetic subsurface modeling along eight profiles. The magnetic survey derived lineaments (structures) and overburden thicknesses were validated with resistivity survey derived 2D structures and overburden thicknesses and borehole log data from the study area. Twenty-four lineaments with lengths ranging from 150 to 777 m were identified from the magnetic map. The lineament orientations were E–W, ENE–WSW, WNW–ESE, NNW–SSE, NW–SE and NE–SW directions. Many of the identified lineaments correlated significantly with structures derived from the 2D resistivity images. Comparable thicknesses were observed between correlated magnetic derived overburden thicknesses (4.41–29.4 m) and depths from wells and boreholes (5.48–27.1 m). The study concluded that the magnetic method could be reliably used for overburden thickness estimation and structure mapping required in groundwater potential assessment in a typical basement complex terrain.
Integrated geotechnical and geophysical investigation of a wetland dumpsite in Epe, Lagos State, Nigeria, was carried out with a view to determining the hydraulic properties of the underlying formation for possible leachate transmission and subsequent subsoil pollution. Eleven boreholes were drilled, lithologically logged, sampled and analysed for hydraulic/hydrogeological parameters. Geophysical investigation involved 1-D Vertical Electrical Soundings (VES) (8 Nos.) and 2-D Dipole-Dipole profiling along five traverses. The survey area is underlain by fine-medium grained sands with coefficient of uniformity values of < 6. The fresh water saturated sandy formation is characterized by relatively high layer resistivity values (5681-9889 Ωm). The mean bulk-3-4-4 density is 1.89±0.162 g/cm ; porosity: 37.84±8.42%; and permeability: 3.0 x 10 ± 1.1 x 10 cm/sec. The 2D images identify leachate impacted zone beneath the waste dump site and its periphery characterized by relatively low resistivity values (4 to 165 Ωm). The study concluded that the sandy nature of the subsoil and its moderate permeability aided subsoil and groundwater pollution by leachate to depths in excess of 15 m.
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