Effect of Geodynamic Activities on an Existing Dam: A Case Study of Ojirami Dam, Southern Nigeria

Abstract

Geodynamics is the study of the forces and their effects on motion and physics of the processes and the phenomena attending the steady evolution of the earth and the changes that are still going on. This study revealled the significance of electrical resistivity method on effect of geodynamic activities on an existing Dam monitoring at Ojirami Dam, Edo State, Nigeria. The electrical resistivity method involved three techniques: 2D Electrical Resistivity Tomography (ERT), Vertical Electrical Sounding (VES) and Horizontal Profiling (HP). 2D ERT using Dipole-Dipole electrode array with inter-station separation of 5 m and an expansion factor that varied from 1 to 5 with Nine (9) VES were carried out using Schlumberger array with current electrode spacing varying from 1.0 to 65.0 m and HP using Wenner array with an electrode spacing of 20 m and electrode movement at 5 meters. The 2-D imaging (Dipole-Dipole) gave information on the subsurface characteristic which section delineated five major geologic layers comprising of the topsoil, weathered basement, fractured zone, partly fractured basement and the fresh basement. The geoelectric sections identified three to four geoelectric/geologic subsurface layers along the traverse. The HP revealled the pattern of resistivity variations within the subsurface. The entire results correlate well with one another showing that all the techniques used were complemented. The combination of these techniques has proved effective and useful in geodynamic activities of the existing dam. Ojirami Dam is at a critical point of yielding to activities of geodynamic processes that may occur from the main axis, of major weak zones as observed between 75 to 95.00 m and 115 to 145.00 m. Hence, there is need to call government attention for further confirmatory test using other geophysical methods and subsequently putting in place remedial measures to prevent its future occurrence.