2-D RESISTIVITY IMAGING AND DETERMINATION OF HYDRAULIC PARAMETERS OF THE VADOSE ZONE IN A CULTIVATED LAND IN SOUTHWESTERN NIGERIA. Abel I. Olayinka, University of Ibadan, Ibadan, Nigeria. Michael A. Oladunjoye, University of Ibadan, Ibadan, Nigeria. Abstract A combined 2-D resistivity imaging survey and determination of hydraulic parameters of the vadose zone in a seasonally-flooded part of southwestern Nigeria that has been cultivated for the past 18 years is presented. The survey area is about 22 000m 2 (2.2 ha) with the bedrock geology comprising Pre-Cambrian granite gneiss. Imaging data were measured along 11 lines, with a minimum inter-electrode spacing of 1 m; control data were obtained along two lines nearby. Soil samples were collected from 30 shallow pits at depths of 0.5, 1.0 and 1.5 m, respectively. The resistivity of the near-surface sandy-clay lithology is low at less than 50 Ωm while that at the control site is much higher. The natural moisture content ranges between 3.3 and 27.4% with a mean of 12.2%; porosity ranges between 9.9 and 36.8%, with a mean of 14.4%. The coefficient of permeability ranges between 9.61x10 -7 and 1.01x10 -5 cm sec -1 . The study has shown that changes in electrical properties of the soil could be due to variations in moisture content and that there is a close relationship between the sandy clay lithology and the recharge. Moreover, the soil could constitute an environmental problem at the peak of the rainy season. Introduction Valley bottom soils otherwise known, as hydromorphic soil/fadama or inland valley swamps are soils formed by the influence of pedogenic processes. They are classified by having a special environment, in which water gathers in sufficient volume and time to produce the effect of graying or reducing regime giving the profile a particular characteristic, which are different from those of well-drained soils. They are widely distributed throughout all the agro-ecological zones of Africa and many parts of the world (Andriesse, 1986). This soil is particularly common in Nigeria and covers about 5% of the cultivable land. The potentials of the valley bottom soils for high and sustainable agricultural production hinge mainly on their inherent characteristics of shallow water, deposition and accumulation of organic matter and residual available moisture for farming. However, the inland valley swamps have been little studied and hence under-utilized especially in southwestern Nigeria. With the increasing population and high demand for fixed land for agricultural and non-agricultural uses, there is no doubt therefore that the valley bottom soils, if properly utilized would enhance agricultural productivity especially during the dry season (Ekemezie, 2002). 2-D surveys measure resistivity changes in the vertical as well as in the horizontal direction along the survey line thus providing better resolution of the subsurface. In many 245 Downloaded 02/13/13 to 128.59.246.78. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/