INTEGRATED GEOPHYSICAL METHODS IN MAPPING SANDFILLED CHANNEL AT MTU CAMPUS, SOUTHWESTERN NIGERIA

Abstract

Integrated geophysical survey has been used in mapping a sandfilled channel within Mountain Top University. The width of the sandfilled region has been mapped with the aid of historical satellite images but the thickness and depth are unknown. Therefore, Ground Penetrating Radar (GPR), and Electrical Resistivity Tomography (ERT) methods were integrated to delineate the thickness and depth of the sandfilled/river channel, characterise the infill materials and deduce its engineering implications. Nine traverses each about 60 m long were established at inter-traverse spacing of 10 m across the strike of the river channel. 2D ERT data were acquired with ABEM LS and 1m electrode spacing using dipole-dipole array. The resistivity data were inverted using smooth-model inversion with 2D EarthImager. The parallel 2D ERT lines were further combined and inverted with 3D EarthImager to generate an 3D inverted resistivity cube of the subsurface. The GPR data were acquired with Mala ProEx and 160MHz antenna to obtain high resolution and good depth of penetration. Several processing steps including time-zero correction, energy decay, background removal and normalisation were applied on the radargram. The results of 2D ERT have characterised the subsurface into two main geoelectric units within the depth of 13.8m. The first region is represented by varying resistivity (5 to 1576 Ωm) and thickness range of 3 to 8m. At lateral distances of about 16 to 48 m, low resistivity anomalies (5 to 49 Ωm) were observed at a depth range of about 1.5 to 3.8m. The configuration of this low resistivity anomaly is analogous to the configuration of an incised channel against the background/country rock (relatively high resistivity range of 30 to 1576 Ωm). This region is therefore associated with the main course of the sandfill/river channel region. The augmented 3D view of the parallel 2D ERT lines and inverted 3D ERT have shown that the regions proximal to the river channel are infilled with relatively highly conductive materials compared to distal regions. This may indicate varying infill materials (such as peat/clay against sand/clayey sand) or water saturation levels and therefore calls for adequate engineering precautions in foundation designs if the region will be considered for future development. The GPR method has provided a high-resolution image of the subsurface and has been able to identify two main anomalous plane continuous reflectors interpreted as the sandfilled and channel fill boundaries at respective depth range of about 0.5 to 1.3 m and 1.5 to 3.5m.