Geophysical assessment of subsurface conditions at proposed building sites: implications for foundation failure and building collapse

Building collapse has been a recurrent environmental hazard in Nigeria in the last two decades. This is a corollary of inadequate foundation investigation prior to construction, poor government policies, and general lack of awareness on the importance of geophysical and geotechnical investigations. In this study, geological mapping and detailed geophysical investigation using Electrical Resistivity Imaging (ERI) and Vertical Electrical Sounding (VES) were carried out to understand the suitability of proposed building sites at the main campus of the Olabisi Onabanjo University (OOU), Ago-Iwoye, Nigeria for construction. Both Wenner array and dipole-dipole were used for profiling and Schlumberger for sounding. Four transverses and VES were used in each of the three areas investigated. Our results show that the subsurface of the study areas is underlain by Precambrian basement rock of Nigeria. Rocks in the study area include banded gneiss, porphyroblastic gneiss, biotite-hornblende granite and quartzite schist. The sounding stations across the three areas and 2D resistivity imaging revealed three principal geoelectric layers, the topsoil, the weathered layer and the fractured/fresh basement with varied resistivity values for each layers. At the VES stations, the three geoelectric layers have resistivity values of 62 to 1182 Ωm, 3.2 to 1360Ωm and 87 to 4680 Ωm. On the 2D resistivity imaging profiles, the resistivity of the three layers varies from 2 to 1182 Ωm, 30to 1360 Ωm, and 40 to 2904 Ωm for the topsoil, the weathered basement, and fractured/fresh bedrock. Our work demonstrates that some of the proposed sites are structurally incompetent for engineering or foundation purposes. Excavation of the topsoil and reinforcement are required to sustain the proposed structures

Structural architecture of the Middle Niger Basin, Nigeria based on aeromagnetic data: An example of a non-volcanic basin

The Middle Niger Basin is located within the west-central half of Nigeria, as a NW-SE trending Campano-Maastrichtian depo-centre which extends from the southern end of the Sokoto Basin west of Kainji reservoir southwestwards to the convergence of the Benue and Niger Rivers at Lokoja. The aeromagnetic anomaly data of the Middle Niger Basin was interpreted to characterize the structural architecture and depth to the magnetic basement of the basin. This is to expand the current knowledge of the region. The analysis of the data has been facilitated by the application of derivative and source parameter imaging techniques. The results from the application of total gradient to the aeromagnetic data provided here signify a rift origin of the basin and NW-SE trending fault systems in the surrounding basement complex terrain. The absence of magnetic highs on the first vertical derivative of the reduced-to-pole aeromagnetic data reveals lack of volcanic rocks within the sedimentary layers of the basin. Additionally, the consistent NW-SE trending source parameter imaging depth solutions within the basin confirm the internal geometry and NW-SE orientation of the basin with sediments not more than 1100 m thick