Investigation of the subsurface becomes a compulsion before waste disposal site is developed. The content of this paper presents integrated geophysical methods in a basement complex terrain, southwestern Nigeria. The dumpsite is located at Oke Saje, Abeokuta, in Ogun state. This was done to investigate the leachate curtailment capacity (LCC) of the existing dumpsite by characterizing the subsurface. A clay overburden thickness of about 15 m which acts as leachate filter is a standard for determining the LCC of a dumpsite. Prior to this work, there is no documented record that such investigation had been done to reveal the LCC before establishment of the dumpsite. Vertical electrical sounding (VES), 2D electrical resistivity tomography (2D-ERT) and multi-channel analysis of surface waves techniques were adopted for the investigation on the existing dumpsite. Eighteen VES was carried out using Schlumberger configuration at 5-10-m intervals to obtain resistivity values and the respective lithological units of the subsurface and three electrical resistivity imaging using dipole-dipole array. MASW techniques were later carried out to ascertain the underlain materials by determining seismic wave arrival times which were processed to obtain the shear wave velocities (SWVs) of the subsurface. The 2D-ERT revealed clearer features of the subsurface as topsoil, weathered, fractured and fresh basement. The result of the VES revealed topsoil with resistivity values ranging from 18 to 1381 Ωm, weathered layer (clay) with resistivity values 15-84 Ωm, fresh basement and fractured layer with resistivity values 192/366-5585 Ωm. The thickness of the clay deposition ranges from 1.8 to 18.5 m. The MASW result using SWVs revealed topsoil, sandy clay, clayey sand and clay with rigidity modulus ranging between 42.4 and 160 N/m 2 at the depth of 30 m. The combined methods carried out at the dumpsites revealed the same lithological units. The waste disposal site is said to have a good LCC due to the presence of clay overburden thickness but not up to 15 m in some parts of the dumpsite.
The geological map of the old western region of Nigeria indicates the presence of iron ore deposit at Iboro village Ogun state (7.9983o - 7.99933o N, 3.5790o - 3.5890o E). Hence a ground magnetic survey was carried out at a location at Iboro village so as to delineate the subsurface magnetic anomalies and to know whether the anomalies favour accumulation of magnetic minerals. The survey was carried out using high resolution proton precession magnetometer model G-856X. Eight traverses were run at 5m separations and earth magnetic intensity values were measured at 10m intervals along each traverse; the acquired data were corrected for drift. The residual anomalies obtained by removal of regional gradient from observed data using trend analysis were presented as profiles and maps. The treated data were qualitatively and quantitatively interpreted and the results gave values for the total ground magnetic anomalies that varied between a minimum and maximum peak values of about -33.0 and 30.6nT respectively. Depth to the basement rock was estimated using Peter’s half slope method which gave a maximum depth of about 13m. The contour maps and the total relative graphs present the subsurface picture of the geological structure that is assumed to harbour the metallic minerals through the action of the field towards the concentration of anomalies. It was suspected that the overburden was relatively thin in the study area and the minerals were at a shallow depth.
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