Engineering Geophysical Investigation Around Ungwan Doka, Shika Area within the Basement Complex of North-Western Nigeria

Abstract

Geophysical investigation for engineering or environmental studies was carried out around Ungwan Doka of Shika area which falls within the Basement Complex of North-Western Nigeria. The study is aimed at evaluating the competence of the near surface formation as foundation materials, and to unravel the subsurface profile which in turn determines if there would be any subsurface lithological variation(s) that might lead to structural failure at the site and evaluating the groundwater potential of the site and determining the level of safety of the hydrogeologic system. Vertical Electrical Sounding (VES), using Schlumberger configuration was adopted. A total of 18 VES was conducted. The data obtained were subjected to 1-D inversion algorithm to determine the layer parameters. The geoelectric section revealed two to four lithologic units defined by the topsoil, which comprises clayey-sandy and sandy lateritic hard pan; the weathered basement; partly weathered/fractured basement and the fresh basement. The resistivity values range from 26?m - 373?m; 77?m - 391?m; 473?m - 708?m; and 1161?m - 3600?m in the topsoil, weathered, fractured basement and fresh basement respectively. Layer thicknesses vary from 0.38m – 6.58m in the topsoil, 1.1m – 33.04m in the weathered layer, 5.86m – 34.1m in the fractured basement. Depth from the surface to bedrock/fresh basement generally varied between 2.65m and 37.75m. Based on the resistivity values, it is concluded that the subsurface material up to the depth of 25m is competent and has high load-bearing capacity. However, resistivity values less than 100?m at depths of 10m-15m indicate high porosity, high clayey sand content and high degree of saturation which are indications of soil conditions requiring serious consideration in the design of massive engineering structures. The hydrogeologic system at the site is vulnerable to contamination. Hence, the result reasonably provide a basis for which groundwater potential zones are appraised for safety in case  potential sources of groundwater contamination sites such as septic tanks and sewage channels are planned for the area under study.

Key words: VES, Top soil, Weathered Basement, Partly weathered or Fractured Basement, Fresh ,Basement