Integrated Resistivity Sounding and Anisotropy to Hydrogeologic Investigation of Alakuta-Apete Area of Ibadan Southwestern Nigeria

Abstract

Fast growing population of Alakuta - Apete area of Ibadan has warranted the need to develop the water availability in the area. The area lies within the basement complex terrain of southwestern Nigeria typified by banded gneiss and minor intrusion of pegmatite and quartz vein. This study was carried out to characterize the aquifer units and anisotropic properties of fractures for evaluation of groundwater development in the area. Hydrogeologic investigation was carried out on forty-five wells to study the groundwater system and dynamics in shallow aquifers. This study was integrated with twenty-two Schlumberger vertical electrical soundings to further investigate the different subsurface geomaterials with the aim of delineating the thickness and continuity of the aquiferous zone. Six radial soundings were conducted to study the directional properties of the anisotropic rock and thus indicate the orientation fractures and extent of the fracturing. Measured Static water level and head measured vary from 0.8 to 9.9m and 182m to 209m respectively indicating that groundwater generally flow towards the eastern and southwestern part from two main discharge sites in the northwestern part of the study area. Interpretation of the twenty-two Schlumberger vertical electrical soundings generally shows three layered earth structure notably top soil, saturated/sandy/lateritic clay and weathered/fractured/fresh basement which are mostly of the “H” curve type. Overburden isopach map revealed that the depth to the bedrock varies from 3m to 23m. The main aquifer (weathered basement) is relatively shallow and most wells in the study area terminate in the second layer. The main water – bearing unit in the area of study is the weathered basement and the fractured basement which are within the second and third geoelectric layers respectively. The weathered/fractured basement resistivity values vary from 62 Ohm-m and 9807 Ohm-m with thickness values ranging from 2.2m to 36m. Polygons produced from six radial sounding showed the predominant structural trends of fractures on the banded gneiss.  The radial resistivity survey results show that there is significant anisotropy between 0 – 50 m depth generally striking NE-SW, NW-SE and N-S showing the major structural trend of basement fractures. Coefficient of Resistivity Anisotropy ranges between 1.03 and 1.38. Variation of apparent resistivity is strongest at the eastern and southern parts of the study area with coefficients of 1.33, 1.38, and 1.30. This high coefficient of anisotropy implies higher- permeability anisotropy. The directionality of the trends could be responsible for the depressions in the weathered/fractured basement. The regions with thick weathered/fractured basement/depressed zones are likely to be most promising sites for borehole drilling.

Keywords: coefficient of resistivity anisotropy