FACULTY OF SCIENCE
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Item Geophysical siting of boreholes in crystalline basement areas of Africa(Pergamon Press Ltd, 1992) Olayinka, A.I.This paper assesses the effectiveness of surface geophysical methods namely electrical resistivity, electromagnetic, seismic refraction, magnetic, gravity and induced polarization for groundwater exploration in crystalline basement complex areas. Most of these geophysical techniques can provide quantitative information on the characteristics of the weathered zone which relate to the occurrence of an economic aquifer. The critical factors in the choice of a particular method include the local geological setting, the initial and maintenance costs of the equipment, the speed of surveying, the manpower required as field crew, the degree of sophistication entailed in data processing to enable a geologically meaningful interpretation, and anomaly resolution. The particular advantages and limitations of each technique are highlighted. Several case histories from Nigeria and the rest of Africa indicate that electrical resistivity (both vertical sounding and horizontal profiling) is the most widely used, followed by electromagnetic traversing. These are often employed in combination to improve upon the percentage of successful boreholes. Due to the high cost of equipment, large scale of the field operations and difficulties in data interpretation, seismic refraction is not widely adopted in commercial- type surveys. Similarly, magnetic, gravity and induced polarization are used only sparinglyItem Error in depth determination from resistivity soundings due to non-identification of suppressed layers(Elsevier Science, 1997) Olayinka, A.I.The magnitude of errors in the determination of depth to bedrock from Wenner and Schlumberger resistivity sounding curves, caused by the nonidentification of a suppressed layer, has been investigated. The principal objective is to evaluate how the layer thicknesses and resistivities affect the accuracy of depth estimates. In the computations, the intermediate layer in a 3-layer model, in which the resistivity increases with depth, is removed and the 2-layer sounding curve that is electrically equivalent to the 3-layer curve is generated. The results indicate that there is a possibility for large depth underestimations when the resistivity contrast between layers 1 and 2 is very large. This is manifested in a steeply rising terminal branch on the sounding curve. There is a slight decrease in the depth underestimation as the resistivity contrast between layers 2 and 3 increases. Conversely, if the intermediate layer is fairly thick and the resistivity contrasts are not too large, the best-fit 2-layer curve shows large deviations from the 3-layer curve, in such cases, the intermediate layer can be identified, resulting in reliable depth estimates. A field example from Nigeria is presented in which the sounding data has been interpreted so as to account for a prebasement layer of intermediate resistivity, indicative of a fractured granite.Item Electromagnetic profiling for groundwater in precambrian basement complex areas of Nigeria(1990) Olayinka, A.I.Ground electromagnetic profiling, using a Geonics EM34-3 instrument, has been employed to identify areas of high conductivity in a Precambrian basement complex terrain of Nigeria. Field examples, conducted as part of a rural water supply programme, are presented. They indicate that the apparent conductivities are generally lower than about 60 mmho m-1. Subsequent borehole drilling suggests a good correlation between high EM34 anomalies, deep weathering and high well yield (> 1 1 s-1). On the other hand, boreholes sited on conductivity lows penetrated a thinner regolith with relatively lower yields.Item Electromagnetic profiling and resistivity soundings in groundwater investigation near Egbeda- kabba, Kwara state(1990) Olayinka, A.I.As part of a borehole siting programme for rural water supply in a Precambrian crystalline basement terrain, 36 Offset - Wenner electrical soundings and about 4.5 line-kilometres of ground conductivity profiling with a Gconics EM34-3 equipment were made near Egbeda-Kabba. Kwara Slate, Nigeria. The EM34-3 measurements provided a rapid reconnaissance tool in identifying high conductivity anomalies thought to be due to deep weathering and/or bedrock fissuring. The apparent conductivities arc generally less than 50 mmho m'1. A quantitative interpretation of the vertical sounding data indicated that the resistivity of the weathered zone varies over a wide range, from about 10 to 200 ohm.m, and that the overburden is generally less than 40m thick. Three abstraction wells, each having a yield of about 11s"1, were drilled at deeply weathered sites (depth to bedrock > 20m) identified from the surface geophysical measurements.