ABSTRACT
Geoelectrical measurements using the vertical electrical sounding (VES) method were conducted in Obukpa, parts of Alor-Uno, parts of Ibagwa-Aka and environs in order to determine quantitatively the depth to static water table, stable aquifer thickness and the lithology of ground water host rock/sediments within the area. A signal averaging system resistivity instrument (SAS 300) was used in the field to carry out the measurements. Twelve (VES) profiles were run during the survey out of which four traverses were commuted to one traverse during preprocessing. The generated survey data for nine traverses were then subjected to basic processing treatment, curve analysis and finally quantitatively interpreted using a computer interactive program (RESOUND). Analysis of the VES curves was in terms of multiple horizontal layers. Profiles V, VI, VII, VIII and IX show an ascending A curve and their variants. Profiles II III and IV presented 4 layer KA curves while profile I generated a H curve type. The results of the interpretation indicate the static water table to occur at varying depths of about 215m, 220m, 222m, 220m, 215m, 218m, 212, 218m and 155m for the entire profiles (VES I - VES IX) respectively. The average depth to the water table is thus approximately 211m. The existing borehole data show that the deepest borehole in the area penetrated to a vertical depth of 190m which by inference from correlation with the survey result has not penetrated well enough a stable aquifer in these areas hence the reason for the failure of these boreholes to yield sufficiently all year round. A maximum traverse length AB of one kilometer was reached during the survey but the result of the interpretation shows that this was not enough to establish the thickness of the stable aquiferous horizon; however, VES I, VES II, VES III VES IV and VES VIII have regions with potential perched aquifer existence. In profile I, the thickness is about 55m and about 102m thickness is suggested in VES II. In profile II, the perched aquifer exists at a depth of about 135m from the surface. Correlation and comparative analysis among established resistivity values (ranges) for sediment, resistivity values of the different geoelectric layers in this work and of lithological log of borehole show mainly sandstone units. Evidently clear from the interpretation result is that a thick sandstone layer of thickness within the range of 85m to 192m of dry bed overlies the zone of permanent water saturation (water table).
