Geophysical Post-construction Integrity Assessment of the Subsurface Conditions of a Church Auditorium in Ado-Ekiti, Southwestern Nigeria

Abstract

The recent obvious manifestation of rising damp, flaked plaster, blistered paint and cracks which are subtle evidences of structural distress necessitated a post-construction integrity assessment of the immediate vicinities of a church auditorium in Ado-Ekiti southwestern Nigeria. The Spontaneous Potential (SP) and Electrical Resistivity (ER) methods of geophysical prospecting were adopted for the assessment. Using a Total Field array spacing of 2 m and Dipole-dipole array spacing of 5 m respectively, SP and ER measurements were taken along three geophysical traverses. SP data were plotted as profiles of SP against distance and were filtered using the 3 periods moving average algorithm of Microsoft Excel®. Resistivity data were processed with the aid of the Dipro for Windows version 4.0 software which migrated raw field data into 2-D subsurface images. The SP profiles were interpreted semi-quantitatively by visual inspection to identify typical anomalies and their locations while ER data were interpreted quantitatively by identifying and estimating the dimensions of anomalies. Potential troughs which are typical streaming potential anomalies arising from mobile fluids within subsurface capillaries were observed on the SP profiles. Anomalously low (< 10 ohm-m) resistivity zones which are indicative of oversaturation of clay substratum cum subsurface incompetence were identified on the 2-D Electrical Resistivity Images (ERI). Synthesis of the SP profiles and 2-D images showed appreciable semblance in the interpretation of the results of both methods. It was further discovered that the identified anomalous zones delineated by the two methods were coincident on the locations of the two rear corner cracks and the rising damp, flaked plaster and blistered paint on the walls of the building. The depth to competent bedrock was found to be at about 10 m. The evidences of distress on the auditorium were suspected to be attributable to unsatisfactory/unsuitable subsurface conditions.