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Delineation of Active Fractures in a Gully Erosion Area Using Geophysical Methods : Case study of the Okigwe-Umuahia Erosion Belt , Southeastern Nigeria

The Okigwe-Umuahia Erosion Belt, Southeastern Nigeria is characterized by severe gullies and occasional landslides. The present study was then designed to delineate the linear structures associated with the area and to evaluate the structural and tectonic trends in the study area using electrical anisotropy and drainage lineament analysis. The study area was divided into grid cells measuring 10 x 10km and within each grid cell, azimuthal resistivity soundings and drainage lineament analysis were carried out. Similarly, vertical electrical soundings were carried out using Schlumberger array to a maximum current electrode spacing of 1000m. The azimuthfrequency diagrams and the anisotropy figures revealed that, generally NE-SW and NW-SE trends are preponderant over other trends in the study area. Results revealed that the axes of maximum electrical anisotropy correlated well with the measured strikes of the geologic formations and the azimuth-frequency diagrams obtained from drainage lineament analysis. Similarly, there exists a relationship between the drainage density, depth to the water table and the coefficient of anisotropy. Higher coefficients of anisotropy (1.31-1.79) were obtained at locations where drainage density and depths to water table are high (0.27 – 0.47 and 160 – 194m respectively). The correlation of high coefficient of electrical anisotropy and drainage density values with deep gullies suggest that the network of fractures in the area serves mainly as channels for surface runoff, rather than zones of groundwater infiltration. This may also account for accelerated removal of surface materials leading to gully initiation.