Electrical Resistivity as a Geophysical Mapping Tool; A Case Study Of The New Art Department, Knust- Ghana

Continuous vertical electrical sounding (CVES) surveys were carried out on KNUST campus to ascertain the electrical properties of the formations in the area in order to determine its suitability for the construction of heavy structures. Eight 2D CVES profiles were conducted at the site in the study area with a wenner array using electrode separations of 1, 2.5 and 4 m. The apparent resistivity data were inverted using the least square inversion technique into subsurface electrical structures. The results on the profiles running N-S indicate a well defined boundary in the electrical resistivity structure between the wet granites at the base and the dry undifferentiated granites on top of it. Keywords; continuous vertical electrical sounding (CVES), KNUST, resistivity, Wenner array, inversion, imaging INTRODUCTION Electrical resistivity surveys are based on the response of the earth to the flow of electrical current. Artificially generated electric currents are introduced into the ground and the resulting potential differences are measured at the surface (Telford et al., 1990; Lowrie 2007). All materials, including soil and rock, have an intrinsic propertyresistivity that governs the relation between the current density and the gradient of the electrical potential. In general, the main principle in any geophysical explorations is to non-intrusively gather data on the area of interest (Scollar et al., 1990). Variations in the resistivity vertically or laterally produces variations in the relation between the applied current and the potential distribution as measured on the surface and thereby reveal something about their composition, extent, and physical properties of the material.Resistivity is therefore, one of the most variable physical properties (Keary et al., 2002)ranging between 1.6×10 -8 Ωm for native silver to about 10 16 Ωm for pure sulphur. The resistivity method has varied applications in mining, groundwater detection, and subsurface geological structure among others. The use of geoelectrical method as an effective tool for gaining knowledge into the subsurface structure, in particular, for identifying anomalies and defining the complexity of the subsurface geology and is fast gaining grounds (Lapenna et al., 2005;Siddiqui and Osman, 2012). This may mainly be due to the fact that on electrical resistivity tomographies, faults lines and other geological formations such as fractures etc, easily stand out due to their low resistivity values compared to the surrounding (Aning et al., 2013). These features are normally identified as anomalies in the electrical resistivity tomographies as they differ from the host material. Field studies by Ozegin et al., (2011) predicted that a geologic structure which was most probably a fracture was established and confirmed to be a potential source of building failure in a site, and this happens when building is constructed across the geologic structure. Garg (2007) found that if a building is constructed at a site, without properly considering the underground strata or its loadbearing capacity, it may settle excessively or differentially, causing development of cracks in the building which may ultimately lead to its failure and collapse. Subsurface geological features such as fractures, voids, and nearness of water table to the surface are among the inconveniences that pose constraint to constraint to building constructions especially to their foundations (Andrews et al., 2013). The electrical resistivity method is very good tool for resolving geological problems ranging from the delineating of hidden underground structures (i.e. fractures, faults water accumulation etc.).It has also made the spatiotemporal evolution of groundwater flow relative to landslide occurrence to studies improve greatly (Aning et al., 2013). In this work, the electrical resistivity distribution of the subsurface of the site would be measured by the application of the CVES method for imaging the subsurface of study area. STUDY AREA The survey site can be located behind the Engineering labs close to the new college of Arts building at the Kwame Nkrumah University of Science and Technology in Kumasi – Ghana. The project site falls within the wet sub-equitorial setting with mean minimum temperature of about 21.5 o C and maximum average temperature of 30.7 o C. The estimated area of the site is (160x100) International Journal of Scientific and Research Publications, Volume 4, Issue 1, January 2014 2 ISSN 2250-3153 www.ijsrp.org m 2 with the land sloping gently in the South-North direction. The area is underlain by Dahomeyan formation, mainly granitoid undifferentiated rocks as shown on fig. 1 and can be located on 6.6849083 °N and 1.5705194 °W. The Dahomeyan formation, consisting of mainly metamorphic rock such as gneiss and schist, occupies the south-southeastern corner of Ghana and occurs as four alternate belts of acid and basic gneisses, trending southsouthwest to northeast direction (Griffis et al, 2002). Fig.1: Geological map of Kumasi Metropolis showing the study area in red (Geological Survey Department,Ghana, 2009) DATA ACQUISITION AND PROCESSING The multi-electrode ABEM Lund Resistivity Imaging System was used to carry out the electrical resistivity measurements on 8 profiles (Fig.2). The system operates automatically once the geometrical parameters (array type, electrode separation and minimum current) are set. The Wenner array with 41 electrodes connected to four 40 m long multi-core cables was used to collect the data.This configuration (Wenner array) is able to give better resolution of the subsurface resistivity distribution(Hamzah et al., 2006).The Continuous Vertical Electrical Sounding (CVES) was used to acquire the data on the field. The equipment used for the survey includes the ABEM terrameter SAS 4000, power supply (car battery), electrodes and multi-conductor cables. For this survey, the electrode separations were as shown in Table 1. International Journal of Scientific and Research Publications, Volume 4, Issue 1, January 2014 3 ISSN 2250-3153 www.ijsrp.org Figure 2 Profile layout Table 1: Table showing electrode spacing and length of profile lines Lines Electrode separation /m Length of line /m