The study tries to utilize vertical electrical sounding (VES) and 2D resistivity tomography to evaluate the region of influence of the leachate plume on the groundwater around a dumpsite at Onitsha expressway, southeastern Nigeria. The borehole log data were acquired and their respective geoposition logged with Garmin GPS device. In addition, four 1D (VES) soundings and 2D electrical profile data were acquired in the field utilizing the Schlumberger and Wenner profiles respectively. Petrozenith PZ-03 Resistivity meter was used to acquire the electrical data, while RES2DINV and WinResist software were used to interpret the 2D and 1D data respectively. The resulting geoelectic layers were correlated with the borehole logs and were interpreted according to their resistivity distribution. Results of the 2D inversion at profiles 1 and 3 showed low resistivity zones indicating influence from the leachate plume. Profiles 2 and 4 gave low resistivity zone within 14.6 and 44.3 Ωm from surface to between 0.375 and 3.60 m depths indicating influence from leachate plume. Likewise, profiles 1 and 3, which penetrated groundwater, also showed very low resistivity with resistivity ranging from 3.12 to 8.7 Ωm, from surface to few meters depths. This indicates that it has been polluted by the leachate. In contrast, Profiles 2 and 4, from the 2D inversion, has no leachate influence on the groundwater. The VES result showed that the depth to the water table at location 1, 2, 3 and 4 are 21.7 m, 17.9 m, 15.9 m and 12.2 m respectively, with the leachate plume flowing in the southeast direction in line with the groundwater flow direction.
Pre-stack seismic inversion, well log analysis approach and empirical relations were adopted in this study to better estimate geomechanical properties of Ruby field with minimum error. The use of conventional well log empirical method alone to evaluate geomechanical properties in oil/gas fields sometimes becomes problematic. Geomechanical properties were divided into: elastic moduli [Young’s modulus, shear modulus, bulk modulus and Poisson ratio (PR)] and rock mechanical strength properties (closure stress ratio (CSR), brittleness (BRI) and compressibility). Four geomechanical earth models (CSR, BRI, Young’s modulus and PR) were generated from the inversion analysis to understand the distribution of rock strength properties across the field. The results deciphered high Young’s, shear and bulk modulus in the reservoir zone compared to the cap/seal rocks and a decrease in PR. This implies that, the cap/seal are more ductile and less compressible than the reservoir rocks, indicating that the reservoirs are highly brittle. CSR result reveals high in cap/seal indicating that the cap/seal rock are harder to fracture and has a greater chance to withstand higher compressive stress before failing as opposed to reservoir rocks. The inverted earth model shows that, Young’s modulus and brittleness increase toward the northeastern part of the field, while CSR and PR increase toward the southwestern part of the field. These results suggest that harder, stiffer, highly compressible and easily fractured rocks are found in the northern and eastern part of the field as opposed to the southern to western part of the field that is ductile.
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