Analysis of principles of equivalence and suppression in resistivity sounding technique

The principles of equivalence and suppression in resistivity sounding technique are phenomena where different resistivity models may produce resistivity curves that are essentially the same. The understanding of these phenomena could be of great importance in using resistivity sounding technique for citing boreholes in places underlain by Basement Complex rocks. In such areas, it would be difficult to identify the fractured/weathered bedrock from the sounding interpretation. This is because the resistivity of the weathered layer is usually intermediate between those of the adjacent layers (i.e. residual overburden) on top and the fresh bedrock below. Therefore, it is important to analyse the problems of suppression and equivalence in resistivity sounding data for estimating thickness and resistivity of the subsurface layers. Several resistivity models were generated ranging from two-layer to five-layer models to investigate these phenomena. The apparent resistivity curves were plotted for these models. The results show that the problems of suppression and equivalence exist in resistivity sounding data. Hence, geophysicists should not make any a priori quantitative inferences using the shape of the resistivity sounding curve

Understanding the interrelationship between electrical anisotropy and groundwater yield in a typical Basement Complex area

The rocks in the Basement Complexof southwestern Nigeria exhibit electrical anisotropy as a result of their heterogeneous nature caused by weathering, near-surface effects or presence of fractures. It is known that groundwater yield (Y) in a typical Basement Complex aquifers is related to its coefficient of anisotropy (). Therefore, it is essential to study the correlation between Y and in the basement complex. Ten (10) vertical electrical sounding (VES) points using Schlumberger array were studied in areas underlain by migmatite gneiss and quartzite rocks. The current electrode spacing (AB/2) varies from 1 m to a maximum spread length of 100 m. The quantitative interpretation of VES curves was done by using partial curve matching and computer assisted program called WinResist version 1.0 software. Dar Zarrouk parameters were estimated from the interpreted VES curves and thus coefficient of anisotropy () was calculated from these parameters. Data on the groundwater yield at the VES points were also obtained. The results show that as increases, Y also increases with coefficient of correlation (R2) of 0.86 and 0.79 for migmatite gneiss and quartzite rocks respectively. To further investigate the relationship between Y and, a regression analysis was performed. The regression analysis that was performed on the dataset shows that contributes significantly to the regression models of the two rocks. The relationship between Y and reveals that groundwater yield could be predicted from the values of in a given locality