The protective capacity of aquifers is a prerequisite for groundwater quality in areas prone to contamination from the ground surface. Aquifers without protective layers are more susceptible to contaminations from point and non-point sources. The protective capacity of an aquifer significantly correlates with the thickness of clay and organic materials that mantled it. To assess aquifer protective capacity, electrical conductivity property of saturated clay was determined from filtered imaginary and real components of Very Low-Frequency Electromagnetic (VLF-EM). The filtered real components against distance and current density pseudo-section produced from real components were concomitantly used for interpretation. Clays capping aquifers were delineated from those without capping from low and high values of filtered imaginary and real components and current density pseudo-section. The distribution of clay laterally across the area indicates that aquifers are poorly protected and susceptible to contaminations from a point and non-point sources. The apparent agreement between inferred geology from VLF-EM interpretation, borehole data and resistivity data underscores the efficacy of VLF-EM as an important tool that can be used or combined with other geophysical methods and borehole information/data for assessment of the protective capacity of the aquifer.

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An integrated geophysical survey involving Very Low Frequency Electromagnetic (VLF-EM) and electrical resistivity sounding was conducted at AuchiSouthwestern Nigeria, in order to delineate the fissured zones and associated groundwaterbearing media for borehole drilling in the study area. The area is underlain by the Precambrian Basement Complex rocks and in some places sedimentary rocks. The VLF-EM traverses were established along two traverses using ABEM-WADI equipment, with a station interval of 10 m and with lengths ranging from 200 to 240 m. Linear fissures presumed to be geologic fissures inferred from the filtered real and filtered imaginary assisted in selecting seven VES points that were further probed using ABEM SAS 1000 Resistivity meter. The spreading were conducted using the convectional Schlumberger electrode configuration with half-current electrode separation (AB/2) that ranged from 1 to 225 m,was used for the sounding. The VES data were presented as depth sounding curves and were suitably iterated using Schlumberger O'Neil software. The VLF filtered real profile showed a high peak trend signifying a fracture signature. The delineated weathered and basement column constituted the aquifer units for borehole drilling. Six lithologic formations were delineated which included the top soil, clay/shale, fine grained sand, coarse grained sand, fractured bedrock and fresh bedrock. Four Model field curves were delineated which included HAKQ, HKQQ, KHAA and KHKH. There are two main targets for groundwater exploration in the study area. These included the weathered zone (fine and medium grained sand) and the fractured zone. The thickness of these zones varied between 15.2 to 50.1 m. Based on the geophysical parameters such as thin overburden thickness, clayey weathered layer and low fractured basement characterized by the study area, it is therefore inferred that the groundwater potential of the area is low. Nevertheless, the study has justified the use of integrated geophysical mapping as a better tool in evaluating the groundwater potential in crystalline terrains. ©JASEM https://dx.doi.org/10.4314/jasem.v21i4.9

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Geoelectric investigation for groundwater prospects in Ejeme-Aniogor and environs, Aniocha-South Local Government Area of Delta State, Nigeria was carried out with a view to determining the near subsurface lithologies and depths; as well as thickness of the various aquiferous zones/units in the study area. Fourteen vertical electrical soundings (VES) technique, employing the Schlumberger configuration were used in investigating the groundwater prospects, with maximum current electrode spacing of 500m. The acquired field data were interpreted quantitatively by partial curve matching and computer iteration. The result showed six to eight geoelectric layers with resistivities ranging from 120 to 1700 Ω for the topmost layer and 276 to 163000 Ω for the bottommost layer. The result also revealed that the depths to the various aquiferous zones ranged from 28.5 to 162 m while the thickness of the aquiferous zones ranged from 20.3 to 58.7 m. The aquiferous units in the study area were mostly confined and the results obtained from the study were found to be consistent with the lithologic logs obtained from borehole drilled in the study area. A generalized depth to the water table (acquiferous zones) cannot be deduced for the study area because it varied from one locality to another. However, the result showed that the area has good prospects for groundwater development. ©JASEM