Modelling the effect of geo-matrix conduction on the bulk and pore water resistivity in hydrogeological sedimentary beddings

Abstract

Generally, the electrical method of geophysics is potentially valuable in typifying the subsurface conductivity and its surrounding medium. Sixteen datasets of 1-D electrical resistivity survey was integrated with 8 core samples, which were taken in the neighbourhood of eight of the sixteen VES datasets. The two sets of data were intertwined with geological and hydrogeological datasets to enhance the realization of unique results used in assessing the aquifer geo-matrix and pore water geo-resistivity in four counties in the coastal province of Akwa Ibom State, southern Nigeria. The current electrode separations of 1-D resistivity data were extended up to 300 m to ensure that the prolifically exploited water-bearing units were assessed. The VES data were manually and electronically modeled and each of them showed characteristic four geo-electric layers with KH, HK, KQ, HA and A group of curves. The core samples, which cut across the local Government Areas under survey, included sandy clay (12.5%), fine-grained sand (50.0%), medium-grained sand (12.5%) and coarse sand (25.0%). Bulk resistivities of water-bearing units and other primary geo-electrical indices were measured in all sixteen locations. However, in the eight aquifer core sample locations, water resistivities were measured in situ. The core samples were taken to the laboratory for measurements of fractional porosity and volumetric water content. The measured fractional porosity ranged from 0.189 to 0.267 with an average of 0.232, whilst volumetric water content varied from 0.23 to 0.31 with an average value of 0.268. The transmission coefficients were computed from volumetric water content and other parameters in the work. The values ranged from 0.4135 to 0.5612 with an average value of 0.4945. Fractional porosity was also modeled from field parameters and the results showed a good match with laboratory measurement as the root—mean square error was just 1.3%. Intrinsic specific resistance and conductivity of soil aquifer matrix were found to be $3.333\times {10}^3\mathrm{\Omega }\text{m}$ and $3.000\times {10}^{-4}{\text{Sm}}^{-1}$ respectively. The study has provided useful information about the geo-electrical and geo-pore properties of the prolifically exploited aquifers in the study area. The aquifer parameter contour maps have been delineated and these, together with other parameter relations generated in this study could help in predicting the volumetric conductivity between pore water and the matrix texture of hydrogeological units in the study area and other areas with similar geology.