Application of resistivity soundings for borehole siting in crystalline basement areas of Southwestern Nigeria

Abstract

This paper establishes the reasons why the resistivity sounding technique is the preferred geophysical tool in borehole siting in crystalline basement areas of southwestern Nigeria. It also identifies ways in which its use could be optimized. Factors for the widespread use of the technique include its relative cheapness, accuracy of depth determination, suitability in differentiating between the overburden and the fresh bedrock, speed of survey, popularity (and acceptability), and availability oi equipment. Due to the Superposition of lateral variations in resistivity on sounding data, some 62% of the sounding curves from the study area are characterized by a terminal branch rising at an angle exceeding 45°. In all the sounding data presented the terminal branch of the curve Starts rising at Wenner electrode spacing of less than or equal to 32 m. It is recommended that the maximum electrode spacing (AB/2 or a) should not be more than 150 m. Moreover, due to the irregular nature of the bedrock topography the Separation between adjacent sounding centres should not exceed 100 m, thus permitting an adequate sampling of the subsurface. This approach might prove helpful in other basement areas.