HYDROGEOLOGICAL SIGNIFICANCE OF GEOELECTRIC SOUNDING AT IJEBU-ODE AREA, SOUTHWEST NIGERIA

Abstract

Twenty eight (28) Vertical Electrical Resistivity Soundings have been carried out in three selected areas of Ijebu-ode (Ijebu-ode, Igbeba and Ikangba) with maximum current electrode separation (AB) of 500m.

The geoelectric deductions show that Ijebu-ode is underlain by the sedimentary rock of Abeokuta formation and consist of one major groundwater aquifer unit suitable for sustainable industrial development. This unit is composed of coarse sand in Ijebu-ode area with resistivity ranging from 733 to 1394Wm and thickness of 45 to 81m while at Igbeba the aquifer unit is composed of fine-medium sand and sandy clay layer with resistivity ranging from 59 to 446Wm and thickness of 20 to 104m. Depth to bedrock in these areas vary from 70 to 163m. The existence of thick highly compacted and highly resistive clay formation at general depth of about 3 – 56 m delineated may be responsible for the failure of hand dug wells and some boreholes in these areas.

At Ikangba, a shallow and relatively thin aquifer unit composed of sandy clay and fine-medium sand with resistivity ranging from 57 to 523Wm and thickness of 1 to 15m was delineated while the depth to bedrock vary from 2 to 30m. This area is suspected to be a transition between the basement complex rock and the sedimentary rock of Abeokuta formation. Thus for Ijebu-ode and its environs, a thorough geophysical survey is required before citing a borehole to guide against failure or dry hole.