Novel methodology for the geophysical interpretation of magnetic anomalies due to simple geometrical bodies using social spider optimization (SSO) algorithm.

The inefficiencies and uncertainties surrounding solutions from existing inversion methods have necessitated investigation for more efficient techniques for the inversion of ill-posed magnetic problems. In this study, the Social Spider Optimization (SSO) algorithm has been modified, adopted and successfully used in modelling physical characteristics of magnetic anomalies originating from simple-shaped geologic structures. The study, aimed at testing the capacity and efficiency of the SSO algorithm to model magnetic data of varying complexity, was successfully conducted on both synthetic data with varying levels of noise and real field data obtained from mining fields in Senegal and Egypt. To assess the mathematical nature of the inverse problem considered, error energy maps were produced for each model parameter pairs in the synthetic examples. These maps enabled the pre-assessment of the resolvability model parameter for the ill-posed problem. In addition, uncertainty analysis aimed at providing insight to the reliability of the obtained solutions was carried out using the Metropolis-Hastings (M-H) sampling algorithm. Results show that the procedure converges fast and generates accurate results even when confronted with constrained multi-parameter non-linear inversion problems. Its outstanding converging speed and accuracy of the results reveal it as an excellent procedure for overcoming agelong problems of local optimal solutions associated with pre-existing algorithms. The consistency of the results with actual values affirms the efficacy of the new procedure which is pioneering in geophysical literature. It is therefore a stable and efficient tool for performing geophysical data inversion and is therefore recommended for use in inverting geophysical data with higher complexities like seismic reflection and gravity data, that require many corrections to be performed before reliable geological interpretations can be made.

Appraisal of protectivity and corrosivity of surficial hydrogeological units via geo-sounding measurements.

The surface resistivity method was used to appraise the protectivity of hydrogeological units and corrosivity of the top soil in Obot Akara County, southern Nigeria. A total of 28 vertical electrical sounding (VES) was undertaken in the area using the Schlumberger electrode configuration. The results of the VES data interpretation reveal 3 to 4 geoelectric layers in the study area. The resistivity of the first layer interpreted as the Motley top soil ranges from 34.7 to 929.7 Ωm with a mean value of 381.1 Ωm. The third layer, with a resistivity range of 99.4 to 2716.7 Ωm, constitutes the aquifer unit in most communities in the area, with an average thickness of 58.3 m, while the fourth layer penetrated in most communities has a resistivity range of 216.1 to 1475.7 Ωm with a mean value of 657.5 Ωm. The longitudinal conductance and resistivity reflection coefficient of the aquifer protective layers vary from 0.04 to 0.76 mhos and - 0.74 to 0.93, respectively. Analysis of these results shows that 89.3% of the hydrogeological units in the area is weakly/poorly protected, 10.7% has moderate to good protection, while 85.7% of the top soil at the sounding stations is noncorrosive and 14.3% is slightly to moderately corrosive. The implication of these results is that most of the hydrogeological units in the area are likely prone to contamination in particular by some ferrugenized materials from the overlying layers. Also, underground metal storage tanks and galvanized and steel pipes can be buried in the topmost layer in most communities in the area without any risk of failure. Although these findings are very promising especially in groundwater management and exploitation in the area, hydrogeochemical and microbiological analyses of groundwater samples from available boreholes are recommended to corroborate the results.

Exploratory assessment of groundwater vulnerability to pollution in Abi, southeastern Nigeria, using geophysical and geological techniques.

The geophysical-based integrated electrical conductivity (IEC) and the groundwater hydraulic confinement-overlying strata-depth to water table (GOD) techniques were used to assess vulnerability levels of aquifers and the extent of aquifer protection in Abi, Nigeria. The IEC indices was generated from constrained one dimensional (1D) inversion of vertical electrical sounding (VES) and two dimensional (2D) electrical resistivity tomography (ERT) data, acquired randomly in the area. The GOD indices were sourced from existing geologic data within the area. Results showed that IEC values vary from <0.1 S in the weakly protected areas to >2.0 S in the strongly protected areas. The GOD indices vary from <0.3 in the lowly vulnerable areas to 0.6 in the highly vulnerable areas. Thus, the groundwater resources in the area need to be properly managed for sustainability and such management practices have been suggested.