Integrated geophysical investigation for the characterisation of valley bottom soil at Ilora, Southwestern Nigeria

Abstract

ABSTRACT Hydromorphic soils have potential for sustainable agricultural production due to shallow water deposition, accumulation of organic matter and residual available moisture for farming during dry season. Integrated geophysical investigations comprising Ground Penetrating Radar (GPR) and Vertical Electrical Sounding (VES) were conducted on a valley bottom soil at Ilora, Southwestern Nigeria to determine subsurface geometry, lithology and water table in relation to its agricultural significance. Seventeen GPR profiles were established while twenty eight VES were conducted using Schlumberger array. Three test pits were excavated to a depth of 1 m for ground truth. Horizons revealed on the GPR are; sub-parallel reflections assumed to be fine muddy sand rich in organic matter, low amplitude and weak reflections indicating attenuated signal because of high moisture content and mutiple chaotic, non-parallel reflectors. The VES results show system of two to four geo-electric layers with average overburden thickness of about 7 m. The excavated pits show intercalation of clayey and loamy soil and revealed water level to be 0.8 m. The integrated geophysical approach applied has successfully identified the spatial pattern of clayey soil, overburden thickness and water level of the valley bottom soil which help retention of water during dry season.