Potential Impacts of Climate Change and Variability on Groundwater Resources in Nigeria

Climate change observed over the past decades has been consistently associated with modifications of components of the hydrological systems such as precipitation patterns, sea surface temperature, accelerated melting of glacier and ice caps, soil temperature and moisture, surface runoff and stream flow. Such changes are known to influence subsurface hydrological systems, which could lead to changes in groundwater recharge, discharge and storage of many aquifers. Although, there are uncertainties in the characterisation of climate change induced groundwater impacts due to largely multi-scale local and regional heterogeneity, there is need to evaluate groundwater resources, quality and vulnerability to climate change and variability. This paper attempts to assess the potential impacts of climate change and variability on groundwater resources availability and sustainability in Nigeria

Anomaly Effects of Arrays for 3d Geoelectrical Resistivity Imaging using Orthogonal or Parallel 2d Profiles

The effectiveness of using a net of orthogonal or parallel sets of two-dimensional (2D) profiles for threedimensional (3D) geoelectrical resistivity imaging has been evaluated. A series of 2D apparent resistivity data were generated over two synthetic models which represent geological or environmental conditions for a typical weathered profile and waste dump site, respectively, commonly associated with geophysical applications for hydrogeological, environmental and engineering investigations. Several minimum electrode separations and inter-line spacing were used to generate the apparent resistivity data for each electrode array with a view to determining the optimum inter-line spacing relative to the minimum electrode separation. The 2D apparent resistivity data for each array were collated to 3D data sets. The effectiveness and efficiency of the arrays in 3D geoelectrical resistivity imaging were evaluated by determining the mean absolute anomaly effects of the electrodes arrays on the synthetic models. The anomaly effects observed in dipole-dipole (DDP), pole-dipole (PDP) and Wenner- Schlumberger (WSC) arrays were generally larger than that observed in other arrays considered. The least anomaly effect on the synthetic models was observed in pole-pole (PP) array. This indicates that DDP, PDP and WSC arrays are more sensitive to 3D features. In all the arrays, the anomaly effects observed in the data set generated using the conventional square grids were slightly larger than those from parallel or orthogonal 2D profiles. This slight increase is attributed to the increased data density and is insignificant when compared with those of parallel and orthogonal 2D profiles. Hence, the use of parallel or orthogonal 2D profiles for 3D geoelectrical resistivity survey is effective

Effectiveness of 3D Geoelectrical Resistivity Imaging using Parallel 2D Profiles

Acquisition geometry for 3D geoelectrical resistivity imaging in which apparent resistivity data of a set of parallel 2D profiles are collated to 3D dataset was evaluated. A set of parallel 2D apparent resistivity data was generated over two model structures. The models, horst and trough, simulate the geological environment of a weathered profile and refuse dump site in a crystalline basement complex respectively. The apparent resistivity data were generated for Wenner–alpha, Wenner–beta, Wenner–Schlumberger, dipole–dipole, pole–dipole and pole–pole arrays with minimum electrode separation, a (a = 2, 4, 5 and 10 m) and inter-line spacing, L (L = a, 2a, 2.5a, 4a, 5a and 10a). The 2D apparent resistivity data for each of the arrays were collated to 3D dataset and inverted using a full 3D inversion code. The 3D imaging capability and resolution of the arrays for the set of parallel 2D profiles are presented. Grid orientation effects are observed in the inversion images produced. Inter-line spacing of not greater than four times the minimum electrode separation gives reasonable inverse models. The resolution of the inverse models can be greatly improved if the 3D dataset is built by collating sets of orthogonal 2D profile

2D and 3D Geoelectrical Resistivity Imaging: Theory and Field Design

The development of resistivity surveying techniques has been very rapid in the last three decades. The advent of automated data acquisition systems, inversion codes, and easy access to powerful and fast computers has tremendously increased the practical applicability of the geophysical method. Geoelectrical resistivity imaging is increasingly being used in environmental, engineering and hydrological investigations as well as geothermal and mineral prospecting, where detailed knowledge of the subsurface is sought. In this paper, the historical development and basic principles of geoelectrical resistivity surveying techniques are presented. Past researches and on-going developments in the survey designs and field procedures in two-dimensional (2D) and threedimensional (3D) geoelectrical resistivity surveys are discussed. Current development in the acquisition geometry for 3D geoelectrical resistivity imaging data is emphasize

Geoelectrical Resistivity Imaging in Environmental Studies

The presence of contaminants in the environment requires a precise characterization of the nature and extent of contamination for effective remediation. Conventional environmental monitoring has focused largely on point sampling, which involves intrusive processes such as grid drilling. This approach is expensive and provides information only on effects at the sample sites, and hence may not be a true representation of the complex and subtle subsurface geology associated with environmental investigations. Alternative methods that have been used in environmental studies are geophysical methods such as geoelectrical resistivity techniques. Geoelectrical resistivity imaging is used in estimating the resistivity distributions of the subsurface based on several measurements of discrete voltage and current. This paper evaluates the effectiveness of geoelectrical resistivity imaging in environmental applications

Application of Vertical Electrical Soundings to Characterize Aquifer Potential in Ota, Southwestern Nigeria

A knowledge of hydrogeophysical parameters of aquifers is essential for groundwater resource assessment, development and management. Traditionally, these parameters are estimated using pumping test carried out in boreholes or wells; but this is often costly and time consuming. Surface geophysical measurements can provide a cost effective and efficient estimates of these parameters. In the present work, geoelectrical resistivity data has been used to characterize and evaluate the aquifer potential at Covenant University, Ota, southwestern Nigeria. Some thirty-five vertical electrical soundings (VESs) were conducted using Schlumberger array with a maximun half-current electrode spacing (AB/2) of 240 m. The geoelectrical parameters obtained were used to estimate longitudinal conductance and transverse resistance of the delineated aquifer. Both the longitudinal conductance and transverse resistance, which qualitatively reflects the hydraulic properties of the aquifer, indicate that the aquifer unit is characterized with high values of hydraulic parameters; consequently a good groundwater potential. Thus, groundwater resource development and management in the area can be effectively planned based on these parameters

Field Evaluation of 3D Geo-electrical Resistivity Imaging for Environmental and Engineering Studies Using Parallel 2D Profiles

Eight parallel two-dimensional (2D) geo-electrical resistivity profiles were generated in hard-rock (Pulivendla) area of Andhra Pradesh, India using a Lund imaging multi-electrode system adopting Wen-ner array. The aim of the survey was to experiment-tally evaluate the effectiveness of using parallel 2D profiles for three-dimensional (3D) geo-electrical resistivity imaging for better understanding of aquifer geometry and its characteristics. The observed 2D apparent resistivity data were independently inverted, and then collated to 3D data set. The inversion of the resulting 3D data set was carried out using a full 3D inversion code. The 3D inverse model of resistivity images obtained are presented as horizontal depth slices. The 2D images extracted from 3D inverse models showed no distortions that are observed in 2D models obtained by 2D inversion. The 3D inverse model resis-tivity appears to be more realistic, considering the hydrogeology of the area. The unusually high resistiv-ity values observed in the 2D inverse models were not observed in the 3D inverse models. The very low near-surface inverse model resistivity observed is thought to be structurally influenced. The results, which are consistent with numerical evaluation, show that high resolution 3D geoelectrical resistivity imaging can be successfully conducted using parallel 2D profiles if appropriate survey parameters are carefully chosen

Anomaly Effects of Orthogonal Paired-arrays for 3D Geoelectrical Resistivity Imaging

A series of 2D apparent resistivity data were generated over two synthetic models representing different geological or environmental conditions commonly associated with geophysical applications for hydrogeological, environmental and engineering investigations. The apparent resistivity data were generated for the following arrays: Wenner-alpha (WA), Wenner-beta (WB), Wenner– Schlumberger (WSC), dipole–dipole (DDP), pole–dipole (PDP) and pole–pole (PP) arrays, which were paired such that apparent resistivity data for 2D profiles in a parallel direction are obtained with a particular array type and those in a perpendicular direction are observed with a different array type. The 2D apparent resistivity data for the orthogonal paired-arrays were then collated to 3D data sets. The effectiveness and efficiency of the orthogonal pairedarrays in 3D geoelectrical resistivity imaging were evaluated by determining the mean absolute anomaly effects of the electrode configurations on the synthetic models. The results show that DDP–PDP, DDP–PP, DDP–WSC, PDP– PP, DDP–WB, PDP–WB and WB–WSC orthogonal paired-arrays produced higher anomaly effects on the synthetic models. This indicates that DDP–PDP, DDP–PP, DDP–WSC, PDP–PP, DDP–WB, PDP–WB and WB–WSC orthogonal paired-arrays are more sensitive to 3D features of the geologic models than the other orthogonal pairedarrays investigate

Application of Artificial Neural Network for the Inversion of Electrical Resistivity Data

The inversion of most geophysical data sets is complex due to the inherent non-linearity of the inverse problem. This usually leads to non-uniqueness of solutions to the inverse problem. Artificial neural network (ANN) has been used effectively to address several non-linear and non-stationary inverse problems. This study is essentially an assessment of the effectiveness of estimating subsurface resistivity model parameters from apparent resistivity measurements using ANN. Multi-layered earth models for different geologic environments were used to generate synthetic apparent resistivity data. The synthetic apparent resistivity data were generated using linear filter method embedded in the RES1D program. Neural network toolbox on MATLAB was used to design, train and test a developed neural network that was employed in the inversion of the apparent resistivity sounding data sets. Resilient feed-forward back propagation algorithm was used to train the network. The network was trained with 50% of the synthetic apparent resistivity data sets and their corresponding multi-layered earth models. 25% of the data set was used to test the network and the network was validated with another 25% of the data set. The network was then used to invert field data obtained from Iyanna- Iyesi, southwestern Nigeria. The results obtained from ANN responses were compared with that of a conventional geoelectrical resistivity inversion program (WINRESIST); the results indicate that ANN is effective in the inversion of geoelectrical resistivity sounding data for multi-layered earth models

Seismic driven reservoir characterization, offshore Niger Delta, Nigeria

This paper presents the subsurface characterization by applying integrated three dimensional seismic attributes analysis on a 3D seismic dataset from OPO field, within the western Niger Delta basin. The volume attributes aimed at extracting features associated with hydrocarbon presence detection, net pay evaluation and porosity estimation for optima reservoir characterization. Neural network (NN) derived chimney properties prediction attribute was used to evaluate the integrity of the delineated structural traps. Common contour binning was employed for hydrocarbon prospect evaluation, while the Seismic coloured inversion was also applied for net pay evaluation. Amplitude anomalies were used to delineate bright spots and flat spots; good reservoirs in term of their porosity models, and fluid content and contacts (GOC & OWC) were identified in the area through common contour binning, seismic colour inversion and supervised NN classification