Impact of Sandstone Maturity on Groundwater Resistivity in Geophysical Groundwater Exploration in Parts of Edo Central and Agbor, Delta State, Nigeria

Several geoscientists have concluded that some terrains have no water by simply taken very high resistivity layers in the subsurface as a dry zone in areas with sandstone aquifer. However, this present work intends at ascertaining the reasons high resistivity sedimentary areas give high yield aquifer. Eighteen rock samples from aquifers that underlie the study area were obtained. Petrographic study was done on eleven sandstone samples, two igneous, three gneiss and two schist collected from the boreholes. The maturity indexes of the sandstones were computed. Vertical Electrical Sounding was conducted at each point to determine the subsurface resistivity and the wells were drilled to depths at which aquifers were encountered. The aquifer depths were correlated with the VES and petrographic results. The results of the petrographic showed that the sandstones are arenite and wacke sandstone aquifers. The VES result showed that arenite aquifers gave high resistivity, ranged from 5900Ωm to 15000Ωm signature while wacke gave low resistivity signature, ranged from, 37Ωm to 80Ωm and both gave good water yield at these depths. This study has shown that arenites are second cycle sediments that have high percentage of quartz mineral while wackes are rich in feldspar than arenites. Weathered feldspar isgood conductor of electricity in the presence of fresh water, which is the reason for their low resistivity value while quartz has high resistivity that makes arenite aquifers to give high resistance.