Integrated Geophysical Mapping of Near-surface Structures and Utilities in Ile-Ife, Nigeria

Abstract

Magnetic, self-potential (SP) and Ground-penetrating radar (GPR) methods were employed to investigate the near-surface conditions beneath a road in Obafemi Awolowo University Campus, Ile-Ife with the aim of mapping previously suspected dyke, fractures, buried utilities in the area. Fifty stations were established for the magnetic and SP methods along five and four traverses respectively while GPR data was acquired along three traverses, all parallel to the general strike of the rocks in the area (NW-SE). The magnetic and SP readings obtained were interpreted qualitatively and quantitatively using least-squares to identify the location/trends and determine source parameters of features of interest. Also, GPR sections were analyzed using basic radar processing tools. The magnetic field readings ranged from -310nT to 220nT while the SP readings with range -126mV to 55mV. The anomaly source was delineated on low closures on the iso-magnetic map while the inflexion points on the SP profiles lie above the target source. Results of quantitative interpretation of magnetic and SP anomalies revealed a spherical, sub-vertical source with depth to top values of 1.6m to 3.85m. The radargrams revealed the positions and depths of utilities such as pipes and cables, shallow-dipping fractures, a road culvert and partially weathered bedrock layers overlying fresh basement in the area. The penetration depth of the GPR was 14 m and the depth to top of the basement ranged from 4 m to 6 m. The techniques employed mapped the subsurface structural/cultural features in the study area and determined their positions and depth extents. crust than the other deeper layers. As a result increased understanding of the dynamics of this part of the Earth is paramount (1). In this paper, the primary aim was to map the shallow portions of the earth within a defined area in Ile-Ife; up to a depth of about 15 m, and confirm the presence of previously suspected fractures, fault, dyke and man-made utilities. Three geophysical methods namely Magnetic, Self-potential (SP) and Ground Penetrating Radar (GPR) were employed in the present study. The magnetic method involves the measurement of variations in total magnetic field of the earth, caused by local differences in the magnetization of the subsurface rocks and soils. It has a wide range of applications and has been used in locating buried metalliferous man-made objects such as cables and pipelines, location of boundaries between rocks, faults, and dykes (2). The SP method is a passive geophysical technique that responds to naturally-occurring potential differences generated mainly by electrochemical, electrokinetic, and thermoelectric sources in the subsurface and has many applications ranging from mineral exploration, hydrogeological, environmental, geotechnical and engineering investigations. There is recorded success in the use of the SP method in shallow subsurface mapping of simple geometrical bodies (3) and sheet-like bodies (4). The GPR technique (which is a novel technique in Nigeria) detects electrical discontinuities due to features such as bedding planes, fault planes etc. in the shallow subsurface (usually less than 50m) by the generation, transmission, propagation, reflection and reception of discrete pulses of high-frequency electromagnetic energy with high-resolution imagery (5). GPR is effective for subsurface fracture and fracture zone imaging (6), soil and rock stratigraphy mapping (7) and underground utility mapping (8). Consequently, this integrated study was done to verify the presence of the geologic structures and cultural utilities in the area, and also to demonstrate the efficacy of the GPR technique in shallow subsurface mapping in a basement complex terrain of southwestern Nigeria.