Geology
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Item Smooth and sharp-boundary inversion of two-dimensional pseudosection data in presence of a decrease in resistivity with depth(Geophysical Press, 2002) Olayinka, A.I.; Yaramanci, U.The smooth and sharp-boundary inversion of two-dimensional (2D) apparent resistivity pseudosection data in cases where the half-space has a lower resistivity than the overburden is investigated. The study entailed calculation (by forward modelling) of the synthetic data over simple 2D geologic models and inversion of the data. The 2D structures modelled include vertical fault, graben and horst. The Wenner array was used. The results show that there is generally an improvement in the model misfit with iteration number in smooth inversion; the algorithm can then be expected to iterate to the best solution at a high iteration number where the model resistivity of the substratum approaches the true bedrock resistivity. Inversion of the data using sharp boundaries indicates that the range of 2D equivalence, for which a reasonable interpretation could be attained, is relatively narrow. For the equivalent solutions, the data misfit between the observed and the calculated data is very small while model interpretations that are wrong can be readily identified on account of very large data misfits. A field example is given from Nauen, northern Germany, where partly-saturated sand of high resistivity is underlain in succession by less resistive saturated sand and glacial till; the smooth and sharp-boundary inversion results are in good agreement with the geo-radar and surface magnetic nuclear resonance (SNMR) and borehole informationItem Smooth and sharp-boundary inversion of two- dimensional pseudosection data in presence of a decrease in resistivity with depth(ELSEVIER, 2002) Olayinka, A.I.; Yaramanci, U.The smooth and sharp-boundary inversion of two-dimensional (2D) apparent resistivity pseudosection data in cases where the half-space has a lower resistivity than the overburden is investigated. The study entailed calculation (by forward modelling) of the synthetic data over simple 2D geologic models and inversion of the data. The 2D structures modelled include vertical fault, graben and horst. The Wenner array was used. The results show that there is generally an improvement in the model misfit with iteration number in smooth inversion; the algorithm can then be expected to iterate to the best solution at a high iteration number where the model resistivity of the substratum approaches the true bedrock resistivity. Inversion of the data using sharp boundaries indicates that the range of 2D equivalence, for which a reasonable interpretation could be attained, is relatively narrow. For the equivalent solutions, the data misfit between the observed and the calculated data is very small while model interpretations that are wrong can be readily identified on account of very large data misfits. A field example is given from Nauen, northern Germany, where partly-saturated sand of high resistivity is underlain in succession by less resistive saturated sand and glacial till; the smooth and sharp-boundary inversion results are in good agreement with the geo-radar and surface magnetic nuclear resonance (SNMR) and borehole informationItem Integrated use of geoelectrical imaging and hydrochemical methods in delineating limits of polluted surface and ground water at a landfill site in Ibadan area, Southwestern Nigeria(Nigerian Mining and Geosciences Society, 2001) Olayinka, A.I.; Olayiwola, M.A.The Impact of solid waste disposal at the Ring Road district of Ibadan metropolis, southwestern Nigeria, on the adjoining surface and ground-water sources, has been Investigated. The basement rocks include Precambrian quartzites and quartz-schists that have been extensively weathered. In this study, the geophysical survey Involved 5 electrical imaging lines, while the hydrochemical database comprised 4 samples of surface water and 6 samples of groundwater from dug wells. The samples were analysed for turbidity, pH, conductivity, total hardness, total dissolved solids (TDS), alkalinity, Cl, No3, SO42, PO43, Na, K+, Mg2 and Fe2. The results show that water bodies located in a down gradient direction and in close vicinity of the landfill, has received significant inputs of inorganic pollutants. Statistical analyses indicate positive correlation (p<0.05) between the following pairs of parameters: Ca2 and Mg2. (R=0.83; Na+ and K+ (R=0.95); (Na2+Mg2) and Cl (R = 0.92); (Na2 + K2) and Ca2 (R=0.96); pH and Cr (R = 0.87). Moreover. the electrical conductivity shows a positive correlation (p<0.05) with the concentrations of K+ (R = 0.97); Na+ (R = 0.96); Ca2+ (R = 0.93); Mg2+ (R = 0.82); Fe2+ (R = 0.85); Cl (R = 0.91) and TDS (R = 0.98). On the other hand, the conductivity shows a negative correlation with SO42 (R = 0.78). Corroborating evidence was provided from the geoelectrical imaging data which Indicate that the thickness of the waste dump ranges between about 2m to 17m, while its resistivity is low at between 4 and 8ohm-m. The low resistivity is due to the presence of leachate emanating from the site and this has also polluted the surface and ground-waters in the Immediate delineation of the limits of the probable polluted zone.Item Aspects of equivalence in two-dimensional resistivity imaging and modelling(Nigerian Mining and Geosciences Society, 2000) Olayinka, A.I.The possibility for geoelectrical equivalence in two-dimensional (2-D) inversion of apparent resistivity data has been investigated. This involved the calculation of synthetic pscudosection data for simple geological structures using a finite difference approach. With the aid of statistical F-test, it is shown that identical or near-identical pseudosections can be generated from more than one 2-D model. In particular, the apparent resistivity pscudosection measured over2-D structures like basement fault, trough and horst resemble those arising from lateral variations In the overburden resistivity.