Use of Vertical Electrical Sounding in Mapping Lateral and Vertical Changes in Subsurface Lithologies: A Case Study of Olbanita, Menengai Area, Nakuru, Kenya

The response of the earth to the flow of electrical current is the basis of electrical resistivity, under which Vertical Electrical Sounding (VES) is the commonest resistivity field application for conventional examination of the subsurface. Much study has been done in the study area linking Vertical Electrical Sounding (VES) interpreted results to lithologies in the subsurface though only tend to indicate the vertical changes with the aim of mapping the occurrence of groundwater aquifers. Several boreholes have been drilled in the study area, though not much has been done to compare the vertical and lateral lithologic changes in the study area. This study has purposed to evaluate the lithological changes both laterally and vertically using VES as the main method of investigation. This research is based on VES modelled geoelectric layers compared from point to point and using borehole logs as control data to establish inferences of certain lithology in the subsurface. The inversion of each VES curve was obtained using an AGI Earth Imager ID inversion automated computer program and resistivities and thicknesses of a geoelectric model were estimated. The analyzed VES data interpretation achieved using the curve matching technique resulted in mapping the subsurface of the area as portraying H-type;p1> p2 <p3 , K-type;p1 <p2 >p3 , A-type; p1 < p2 <p3 , Q-type; p1 > p2 > p3, representing a 3-Layer subsurface and subsequently a combination of HK, HA and KHK types of curves representing 4-Layer and 5-Layer in the subsurface. The analysis further deployed the use of the surfer software capabilities which combined the VES data to generate profiles running in the west-east and the north-south direction. A closer analysis of the curve types indicates that there exists a sequence showing a shifting of the order of arrangement between the west and the east fragments which incidentally coincides with VES points 8, 9 and 10 in the West-East profiles. The lateral change is noted from the types of curves established and each curve indicates a vertical change in the subsurface. Control log data of lithologies from four boreholes BH1, BH2, BH3 and BH5 to show a qualification that different resistivity values portent different lithologies. Indeed, an analysis at borehole BH3 lithologies are dominated by either compacted rocks or soils, insinuating a scenario of compression experienced in this part of the subsurface which confirmed compression of subsurface formations. A correlation of the VES curve types and their change from one point to another in the study area are evident. This change supported by the surfer-generated profiles from the modeled VES data shows that there exists and inferred fault line running in the north-south in the area. The inferred fault line by VES mapping, is magnificently outlined by the geological map. There is exuded evidence from this study that the application of VES is able to help map the lateral and the vertical changes in the subsurface of any area but the evidence of the specific lithologies has to be supported by the availability of borehole log control data. The VES data was able to enumerate vertical layering of lithologies, lateral changes and even mapping vertical fault lines in the study area. Further analysis using surfer profiling confirmed the departure/transition area which was established to be a fault line through the further analysis of the borehole logs and the area geological map.