A. I. Olayinka
University of Ibadan
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Featured researches published by A. I. Olayinka.
First Break | 1990
D.H. Griffiths; J. Turnbull; A. I. Olayinka
The MRT system is effectively a resistivity imaging system, having been designed to provide information from which the distribution of subsurface resistivity in crosssection along a profile can be determined. It is of particular value in are as where the strata show both lateral and vertical variation in electrical properties. The field technique involves making repeated constant-separation apparent resistivity traverses along the chosen profile, the spacing being incremented at each pass.
Journal of Applied Geophysics | 2000
A. I. Olayinka; Ugur Yaramanci
Abstract The ability of a block inversion scheme, in which polygons are employed to define layers and/or bodies of equal resistivity, in determining the geometry and true resistivity of subsurface structures has been investigated and a simple strategy for deriving the starting model is proposed. A comparison has also been made between block inversion and smooth inversion, the latter being a cell-based scheme. The study entailed the calculation (by forward modelling) of the synthetic data over 2-D geologic models and inversion of the data. The 2-D structures modelled include vertical fault, graben and horst. The Wenner array was used. The results show that the images obtained from smooth inversion are very useful in determining the geometry; however, they can only provide guides to the true resistivity because of the smearing effects. It is shown that the starting model for block inversion can be based on a plane layer earth model. In the presence of sharp, rather than gradational, resistivity discontinuities, the model from block inversion more adequately represents the true subsurface geology, in terms of both the geometry and the formation resistivity. Field examples from a crystalline basement area of Nigeria are presented to demonstrate the versatility of the two resistivity inversion schemes.
Journal of Applied Geophysics | 1997
A. I. Olayinka; A. Weller
Abstract A methodology is presented for the inversion of two-dimensional (2-D) geoelectrical data for solving hydrogeological problems in crystalline basement areas. The initial step entails compiling an earth model using all available geological, borehole and geophysical information. This model then served as the input to a 2-D inversion algorithm based on the Simultaneous Iterative Reconstruction Technique (SIRT). The algorithm tries to find a model that is as close as possible to the starting model. To demonstrate the usefulness of this procedure, two field examples from Nigeria, conducted as part of a borehole siting programme, are described. In the first example, borehole information regarding the thickness of the weathered zone overlying a gneissic bedrock was used to constrain the 1-D inversion of sounding data and the model thus compiled was used as the starting model for 2-D inversion. In the second example, only sounding information was used to determine the starting model. If the starting model has incorporated all the available information as constraints, it is generally possible to compute a model that not only fits the measured data but is also a good approximation of the subsurface geology, more so when several 2-D models can fit the same set of field measurements on account of the limitations posed by equivalence.
Seg Technical Program Expanded Abstracts | 2009
A. P. Aizebeokhai; A. I. Olayinka; V. S. Singh
Summary Field design for 3D data acquisition in geoelectrical resistivity imaging using a net of orthogonal sets of 2D profiles was numerically investigated. A series of 2D apparent resistivity pseudosections were generated over a synthetic horst structure representing the geological environment of a crystalline basement in low latitude areas using RES2DMOD code. Different minimum electrode separations and inter-line spacing were used with a view of determining the optimum inter-line spacing relative to the minimum electrode separation. The 2D apparent resistivity data were collated to 3D data set and then inverted using RES3DINV, a full 3D inversion code. The relative effectiveness and imaging capabilities of Wenner-alpha (WA), Wenner-beta (WB), Wenner-Schlumberger (WSC), dipole-dipole (DDP), pole-dipole (PDP), and pole-pole (PP) arrays to image the structure using a net of orthogonal set of 2D profiles are presented. The normalized average sensitivity of the inversion results show that WSC, DDP, and PDP arrays are more sensitive to the 3D structure investigated. Interline spacing of not greater than four times the minimum electrode separation gives reasonable resolution.
Journal of African Earth Sciences | 1992
A. I. Olayinka
Abstract 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 sparingly.
60th EAGE Conference and Exhibition | 1998
A. I. Olayinka; Ugur Yaramanci
The objective in the two-dimensional (2-D) inversion of de apparent resistivity data is to determine the true subsurface resistivity distribution over any arbitrarily shaped 2-D structure. To do this successfuliy, it is essential that a good inversion scheme be able to simultaneously minimize the data rms misfit and the model rms misfit. With field data the true model is rarely, if ever, known precisely. Through model studies, such as is presented here, where the true solutions are known, realistic and assessment of the robustness of the inversion can be accomplished. The models investigated, for the Wenner array, include horizontal layering, fault, low and high resistivity fills, localised inhomogeneities, as well as the influence of noise contamination.
4th EEGS Meeting | 1998
A. I. Olayinka; Ugur Yaramanci
Modelling and inversion of synthethic data over 2D geologie models have shown that, at high resistivity contrasts, there could be an increase in the model misfit between the inverted model and the true model for successive iterations. Hence, the optimum model is attained at a low iteration number. A case history, from data measured over a waste dump site, is given in which the optimum model is the structure at the second iteration; the inverted resistivities are very low at less than 30 Om down to a depth of about 10 m. These are consistent with the analyses of surface - and ground - water which indicate the presence of a low resistivity contaminant.
Journal of African Earth Sciences | 1997
A. I. Olayinka
Abstract The magnitude of errors in the determination of depth to bedrock from Wenner and Schlumberger resistivity sounding curves, caused by the non-identification 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.
Journal of African Earth Sciences | 1990
A. I. Olayinka; R. D. Barker
Abstract A simple and rapid procedure has been devised for an approximate interpretation of Wenner resistivity pseudosection profile data collected using a Microprocessor- controlled Resistivity Traversing System. The method is based on the estimation of the total longitudinal conductances of the weathered mantle from vertical sets of pseudosection data. Field results, made as part of hydrogeological investigations in basement areas of Nigeria, show that the method is very useful for determining the overburden resistivity and the depth to bedrock. The interpretation provides an initial model which can then be refined using finite difference modelling techniques.
Geophysical Prospecting | 2000
A. I. Olayinka; Ugur Yaramanci