Laboratory Measurement of the Electrical Resistivity of some Nigerian Crystalline Basement Complex Rocks

Abstract

The electrical resistivity of fresh Pre-Cambrian to Upper Cambrian crystalline basement rocks in southwestern Nigeria, hitherto inferred from sounding interpretation, has been determined from laboratory measurements. The rock types consist of granite gneiss, banded gneiss, augen gneiss, biotite granite, charnockite, granodiorite, amphibolite and quartzite. The samples were cut into uniformly thick slices and four non-polarisable silver-silver chloride electrodes set up at the boundary of each specimen. The geometry of the electrode array could take any arbitrary configurations. Electrical contact with the sample is made by injecting the filter tip of the electrode with 0.1 molar KCl solution. The potentials and currents corresponding to the four cyclic permutations on the rock were measured for the five boundary arrays. The specific resistivity of each sample was determined using the Van der Pauw equation. One hundred and two rock samples were considered in the experiment. The resistivities are generally very high, ranging from 1010 to 2.8 * 106 ohm-m. The granite gneisses have the lowest resistivities (1.01 * 103 to 7.76 * 103 ohm-m); they are followed by banded gneiss (2.43 * 103 to 6.29 * 103 ohm-m); augen gneiss (4.32 * 103 to 1.43 * 104 ohm-m); biotite granite (8.0 * 103 to 6.2 * 104 ohm-m); charnockite (2.1 * 104 to 1.75 * 105 ohm-m); granodiorite (4.0 * 104 to 1.1 * 105 ohm-m); amphibolite (9.1 * 104 to 3.73 * 105 ohm-m); and quartzite (5.06 * 104 to 2.82 * 106 ohm-m). There is a considerable overlap in the resistivity of the various rock types. Moreover, even for different samples from the same rock exposure there may be a wide range of resistivities; these may be attributed to differences in the mineralogical composition. In general, there is a tendency for rocks which are not readily susceptible to chemical weathering (e.g. quartzite) to have high resistivities; conversely, gneisses which are more easily weathered, are characterized by relatively low electrical resistivities.

(Af. J. of Science and Technology: 2002 3(1): 93-97)