The author was the Project Leader of the British Geological Survey (BGS) Research Projects on Basement Aquifers and Collector Wells. This introductory paper constitutes an overview of crystalline basement aquifer occurrence, geological and climatic controls, evolution, geometry, hydraulic parameters, resources and recharge, combined with a discussion on exploration and management. Data from the BGS Research Projects Report Series are incorporated.

The hydrogeology of crystalline basement aquifers in Africa; introduction

In a context of global change and increasing anthropic pressure on the environment, monitoring the Earth system and its evolution has become one of the key missions of geosciences. Geodesy is the geoscience that measures the geometric shape of the Earth, its orientation in space, and gravity field. Time-variable gravity, because of its high accuracy, can be used to build an enhanced picture and understanding of the changing Earth. Ground-based gravimetry can determine the change in gravity related to the Earth rotation fluctuation, to celestial body and Earth attractions, to the mass in the direct vicinity of the instruments, and to vertical displacement of the instrument itself on the ground. In this paper, we review the geophysical questions that can be addressed by ground gravimeters used to monitor time-variable gravity. This is done in relation to the instrumental characteristics, noise sources, and good practices. We also discuss the next challenges to be met by ground gravimetry, the place that terrestrial gravimetry should hold in the Earth observation system, and perspectives and recommendations about the future of ground gravity instrumentation. 
Plain Language Summary: In a context of global change and increased human vulnerability to terrestrial hazard, monitoring the Earth system is one of the key challenges of geoscience. In particular, terrestrial gravimetry, with its precision at the level of one part of a billion, allows the monitoring of many phenomena, from water resource availability to volcanic activity. This paper reviews the technique, its advantages and limitations, how it has been used in the Earth monitoring, and the next challenges to be met by ground gravimetry.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2017RG000566

Geophysics From Terrestrial Time-Variable Gravity Measurements

Isostatic geoid anomalies are usually interpreted using a flat-earth, one-dimensional idealization. Isostatic anomalies on the spherical, self-gravitating earth differ from this idealization because: (1) degree one terms in the spherical harmonic expansion vanish; (2) each term in the spherical harmonic expansion is multiplied by (l + 2)/(l + 0.5) relative to the flat-earth case; (3) mass in cones rather than straight-sided columns is constant; and (4) further deformation of the earth is induced by the gravitational attraction of the deformation caused by the isostatic potential anomaly. When the effect of each of these is quantified, the second, third, and fourth nearly cancel, leaving the degree one, “over the horizon” effect providing the most important difference. Calculations of model isostatic geoid anomalies for the spherical analogues (developed here) of the plate and boundary layer thermal models show that this effect can bias estimates of geoid slopes by over 20%, although the effect is usually less than 5%. The geoid anomalies for these two models are quite different over old ocean basins, but they are unlikely to be distinguishable on the basis of global geoid observations owing to the presence of other larger perturbations in the geoid. Stripping the effects of plate aging and a hypothetical uniform, 35 km thick, isostatically-compensated continental crust from the observed geoid emphasizes that the largest-amplitude geoid anomaly is the geoid low of almost 120 m over West Antarctica. This anomaly is a factor of two greater in amplitude than the low of 60 m over Sri Lanka.

Global isostatic geoid anomalies for plate and boundary layer models of the lithosphere

Tanneries located in an industrial development area of Ranipet (India) manufactured chromate chemicals during 1976–1996. A large quantity of associated hazardous solid wastes has been stacked about 5-m high above ground level, spread over 3.5 ha inside one of the factory premises. The study area receives an average annual rainfall of 1,100 mm. The granitic formation in the northern part of Palar River catchment has high infiltration rates and has resulted in fast migration of the contamination to the water table. Chromium levels in the groundwater were found up to 275 mg/l. The available hydrogeological, geophysical and groundwater quality data bases have been used to construct a groundwater flow and mass transport model for assessing the groundwater contamination and it has been calibrated for the next 30 years. The migration has been found to be very slow, with a groundwater velocity of 10 m/year. This is the first field-scale study of its kind in this industrial area. The findings are of relevance to addressing the groundwater pollution due to indiscriminate disposal practices of hazardous waste in areas located on the phreatic aquifer. Further, it has been reported that the untreated effluent discharge adjacent to the chromium dump site is most influential in the migration of contaminants.

Assessment of groundwater contamination from a hazardous dump site in Ranipet, Tamil Nadu, India

In this study, finite difference method is used to solve the equations that govern groundwater flow to obtain flow rates, flow direction and hydraulic heads through an aquifer. The aim therefore is to discuss the principles of Finite Difference Method and its applications in groundwater modelling. To achieve this, a rectangular grid is overlain an aquifer in order to obtain an exact solution. Initial and boundary conditions are then determined. By discretizing the system into grids and cells that are small compared to the entire aquifer, exact solutions are obtained. A flow chart of the computational algorithm for particle tracking is also developed. Results show that under a steady-state flow with no recharge, pathlines coincide with streamlines. It is also found that the accuracy of the numerical solution by Finite Difference Method is largely dependent on initial particle distribution and number of particles assigned to a cell. It is therefore concluded that Finite Difference Method can be used to predict the future direction of flow and particle location within a simulation domain.

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Finite Difference Method of Modelling Groundwater Flow

Groundwater flow modelling of the Kwa Ibo River watershed in Abia State of Nigeria is presented in this paper with the aim of assessing the degree of interaction between the Kwa Ibo River and the groundwater regime of the thick sandy aquifer. The local geology of the area comprises the Quaternary to recent Benin Formation. Potential aquifer zones that were delineated earlier using geoelectrical resistivity soundings and borehole data for the area formed the basis for groundwater flow modelling. The watershed has been modelled with a grid of 65 rows by 43 columns and with two layers. Lateral inflow from the north has been simulated with constant heads at the Government College, Umuahia, and outflow at Usaka Elegu in the south. The Kwa Ibo River traverses the middle of the watershed from north to south. The river-stage data at Umudike, Amawom, Ntalakwu and Usaka Elegu have been used for assigning surface water levels and riverbed elevations in the model. Permeability distribution was found to vary from 3 to 14·5 m day−1. Natural recharge due to rainfall formed the main input to the aquifer system, and abstraction from wells was the main output. A steady-state groundwater flow simulation was carried out and calibrated against the May 1980 water levels using 26 observation wells. The model computations have converged after 123 iterations. Under the transient-state calibration, the highest rainfall (and hence groundwater recharge) over the 10-year study period was recorded in 1996, whereas the lowest was recorded in 1991. The computed groundwater balance of 55 274 m3 day−1 was comparable to that estimated from field investigations. Results from the modelling show that abstraction is much less than groundwater recharge. Hence there is the possibility for additional groundwater exploitation in the watershed through drilling of boreholes. Copyright © 2008 John Wiley & Sons, Ltd.

Groundwater flow modelling of Kwa Ibo River watershed, southeastern Nigeria

Groundwater flow modeling of Kwa Ibo River Watershed in Abia State of Nigeria is presented in this paper with the aim of assessing the degree of interaction between the Kwa Ibo River and the groundwater regime of the thick sandy aquifer. The local geology of the area, called Benin Formation, is of Quaternary to Recent age. Potential aquifer zones earlier delineated using the geoelectrical resistivity soundings and well inventory in the area formed the basis for groundwater flow modeling. The watershed has been modeled with a grid of 65 rows x 43 columns and with two layers Lateral inflow from the north has been simulated with constant heads at the Government College Umuahia and outflow at Usaka Elegu in the south. The Kwa Ibo River traverses the middle of the watershed from north to south. The river stage data at Umudike, Amawom, Ntalakwu and Usaka Elegu have been used for assigning surface water levels and riverbed elevations in the model. Permeability distribution was found varying from 3 to 14.5 m/day. Natural recharge due to rainfall formed the main input to the aquifer system and abstraction from wells, the output. A steady-state groundwater flow simulation was carried out and calibrated against the May 1980 water levels using 26 observation wells. The model computations have converged after 123 iterations. Under the transient-state calibration, the highest rainfall (and hence groundwater recharge) over the ten-year study period was recorded in 1996, while the lowest was recorded in 1991. The computed groundwater balance of 55274m 3 /day was comparable to that estimated from field investigations. Results from the modeling show that abstraction is very less compared to groundwater recharge. Hence there is the possibility for additional groundwater exploitation in the watershed through drilling of bore holes. Keywords : groundwater, modeling, permeability, recharge, Kwa Ibo River Global Journal of Geological Sciences Vol. 3(2) 2005: 169-177

/pdf/groundwater-flow-modeling-of-kwa-ibo-river-watershed-1dzfbyz99d.pdf

Groundwater flow modeling of Kwa Ibo river watershed, Southeastern Nigeria

Coupled geophysical characterization of shallow fluvio-clastic sediments in Agwagune, southeastern Nigeria

Seismic refraction using the longitudinal or P wave method of survey was carried out at some highway locations with the object of evaluating the compaction quality of subgrade and subbase at the locations. These highways traverse different geologic terrain. The survey delineated two layers in most of the locations with various values of velocities that generally increase with depth. Soil samples were also obtained for the different subgrades. Compaction and classification tests were carried out on the soil samples. Seismic refraction theory was reviewed and was used to obtain a profile of the layers. The profile pattern of the layers was found to be indicative of the quality of compaction of the subgrade.

Evaluating Compaction Quality Using Elastic Seismic P Wave

We here present a method for improved lithology and gas sand detection in the sandy Formations of the Niger Delta, Nigeria. The absence of Full-waveform sonic log data necessitated a theoretically generated S-wave log using the Greenberg-Castagna equation ) / ( 2844 the density log data yield the Poisson's Dampening-Factor (PDF) log, PDF= D/ρ. Here, PDF is demonstrated to have higher values in gas saturated sands (0.3-0.98ms/m*cc/g) than in brine sand and shaly (0.02-0.10 ms/m*cc/g) regions of each of the two wells where available data permit such estimate and estimate of other useful Petrophysical parameters. Lack of useful data at certain prospective regions of these wells limit the success chance of the method at detecting more gas reservoirs. One out of the two wells sampled at the depth of 614-652m show favourable result representing a 12% success rate of the sample wells. At this well, Poisson's Dampening-Factor (PDF) value is highest, 0.98 ms/m*cc/g. Extensive characterisation of lithology in well 02 is done through Cross plots to highlight sand quality. While well 01 have typically low values of Poisson's Dampening -Factor (PDF), 0.02-0.10 ms/m*cc/g which are typical values for shale and brine sands. However, shale have the lowest values of Poisson's Dampening-Factor (PDF) promising the basis for shale and brine sand differentiation.

/pdf/detection-of-gas-sands-in-the-niger-delta-by-estimation-of-2m9jpso81z.pdf

E. E. Okwueze

Papers

Groundwater flow modelling of Kwa Ibo River watershed, southeastern Nigeria

Groundwater flow modeling of Kwa Ibo river watershed, Southeastern Nigeria

Coupled geophysical characterization of shallow fluvio-clastic sediments in Agwagune, southeastern Nigeria

Evaluating Compaction Quality Using Elastic Seismic P Wave

Detection of Gas Sands in the Niger Delta by Estimation of Poisson's Dampening-Factor (PDF) Using Wireline Log Data