Geotechnical characterisation of Obudu damsite, Obudu, south-eastern Nigeria

The Obudu dam is being built across Abeb river in Obudu area of the Cross River State (Nigeria). The earthfill dam will be approximately 18 m high with a crest length of 385 m. The dam site is located within part of the Obudu crystalline basement plateau which is a region of low seismicity. The terrain is smoothly undulating and low lying and was known to be composed of unclassified basement and decomposed bedrock (overburden). The present study was carried out to assess the suitability of the chosen dam axis based on the determination of the nature and geotechnical characteristics of the overburden and bedrock. The investigation included geophysical surveys, bearing capacity tests (cone penetrometer and standard penetration), classification and grain size distribution as well as tests for compaction, consolidation and compressive strength. The results show that the bedrock is heterogeneous, including gneisses sillimanite, biotite and granite types), dolerite, quartzite and pegmatite with an overall moderate strength (about 76.04 MN m⁻² average) and fair rock mass rating (RMR). The bedrock along the dam axis is apparently lacking in major fractures which could lead to short circuiting of the future impoundment. The overburden (soil) comprises silt (MH, ML), silty clay (CL) and silty sand (SM) with a combined thickness of about 2–20 m, increasing away from the valley floor towards the shoulders. Generally the overburden is of suitable compressive strength (150–300 KN m⁻²), low to medium plasticity and swelling potential and low permeability (up to 1.41 × 10⁻⁷ ms⁻¹) which would ensure a tight reservoir. The material settlement is expected to be small and slow. Accordingly, excavation of up to 5 m, decreasing towards the valley shoulders has been suggested to expose the recommended bearing medium: fresh/competent bedrock and overburden in the valley floor and flanks, respectively. While a detailed investigation of burrow areas was not part of the present study, a few potential sites in the dam axis and reservoir areas have been suggested based on the evaluation of material properties.     

Assessment of hydrogeological conditions in basement aquifers of the Precambrian Oban massif, southeastern Nigeria

A combined analysis of lineament length density from radar imagery and surface resistivity data is used to assess the hydrogeological conditions in the Oban massif, Nigeria. The results show that the data guided the qualitative and quantitative estimation of some aquifer parameters. These include resistivity of the water bearing formations (280–740 ω m), thickness (5–140 m), limited hydraulic conductivity (8.53-13.18 m/day) and transmissivity (410.65–725.88 m²/day) data. In addition, the lineament length density for the area ranged between less than 0.2 to slightly more than 0.4. Site evaluation for the location of productive boreholes/wells using a groundwater potential index (GWPI) indicates that areas with a GWPI of greater than 35 are consistent with relatively high yield.     

Hydrogeological and hydrochemical character of the regolith aquifer, northern Obudu Plateau, southern Nigeria

Photogeological, geoelectrical, and drill-hole data were used to determine the hydrogeological conditions of the regolith layer mantling the Precambrian rocks in the northern Obudu plateau of southern Nigeria. The regolith thickness is 3.8–89.5 m (mean 38 m). Water for domestic purposes is extracted by dug wells <15 m deep and shallow boreholes <50 m. The mean value for permeability of the saturated regolith is 7.5×10^–1 m/d and for transmissivity, 3.8×10^–1 m²/d. The regolith wells have a high specific capacity, 40–270 m³/d/m and a high yield, 700–4,050 m³/d. Using statistical analysis, well yields and specific capacities can be estimated if the saturated thickness and apparent resistivity of regolith layers are known from drill-hole data and vertical electrical sounding. Chemical data show that the major groundwater types are calcium-bicarbonate (Ca-HCO₃) and sodium+potassium bicarbonate [(Na+K)-HCO₃] and the water quality meets national and international standards for drinking, domestic and agricultural purposes. However, for future proper management of water resources in the area, care should be taken, that waste-disposal sites for both solid and human waste should not be located near waste sources.

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Resumen Se utilizaron datos fotogeológicos, geoeléctricas y de barrenación para determinar las condiciones hidrogeológicas de la capa de regolito que cubre rocas precámbricas en la meseta obudu septentrional del sur de Nigeria. El regolito tiene un espesor de 3.8–89.5 m (promedio38m). El agua para uso doméstico se extrae medio de pozos excavados hasta profundidades de 15 metros y por barrenos no muy profundos de 50 metros. La permeabilidad promedio del regolito saturado es 7.5×10^–1 m/d, con transmisibilidad de 3.8×10^–1 m²/d. Los pozos dentro del regolito tienen alta capacidad específica de 40–270 mm²/d/m y un alto rendimiento de 700–4,050 mm³/d. Se puede estimar el rendimiento de los pozos y capacidades específicas usando análisis estadístico si el espesor saturado y la resistividad aparente de las capas de regolito se conocen a partir de los datos de barrenación y sondeo geoeléctrico. Los datos químicos muestran que los tipos de aguas subterráneas son principalmente de bicarbonato de calcio (Ca-HCO₃) y bicarbonato de sodio y potasio [(Na+K)-HCO₃] y el agua cumple las estándares de calidad nacional y internacional para agua potable y para uso doméstico y agrícola. Sin embargo para el futuro manejo apropriado del recurso de agua en el área es importante que se tenga cuidado de que los sitios de desechos de basura solida y humana no sean ubicados cerca de los fuentes de agua.

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Résumé Des données photogéologiques, géoélectriques et de forage ont été utilisées afin de déterminer les conditions hydrogéologiques de laquifère régolithique qui recouvre les roches précambriennes dans le plateau nord dObudu au sud du Nigeria. Lépaisseur de régolithe varie entre 3.8 et 89.5 m avec une moyenne de 38 m. Leau destinée aux usages domestiques est extraite de puits de surface (< 15 m de profondeur) et de puits artésiens (< 50 m). La valeur moyenne de perméabilité du régolithe saturé est de 7.5×10^–1 m/j et la transmissivité moyenne de 3.8×10^–1 m²/j. Les puits qui exploitent laquifère de régolithe ont une capacité spécifique variant entre 40 et 270 m²/j/m et un rendement élevé de 700 à 4 500 m³/j. Avec laide dune analyse statistique, le rendement et la capacité spécifique des puits peuvent être estimés si lépaisseur saturée et la résistivité apparente des couches de régolithe sont déterminées avec des forages et des relevés électriques verticaux. Des données sur la composition chimique de leau montrent que les types deau majeurs sont bicarbonaté-calcique et bicarbonaté sodique + potassique et que la qualité de leau rencontre les standards nationaux et internationaux pour leau potable, lutilisation domestique et à fins dagriculture. Malgré tout, dans loptique dune gestion appropriée des ressources en eau dans la région, une attention particulière doit être portée aux sites denfouissement des déchets solides et humains qui ne devraient pas être localisés près des sources deau.

     

Low velocity layer characterization in the Niger Delta: Implications for seismic reflection data quality

The problems of poor data quality and statics in seismic surveys have been attributed to lack of proper understanding of the low velocity layer characteristics of the area from which such data was acquired. Downhole seismic refraction survey was therefore conducted at twenty (20) borehole locations within parts of Niger Delta, Nigeria to determine the low velocity layer characteristics of the area using the Geometrics Stratavisor NZ11 instrument. The data was processed using Udysys software with spatial representation of the results presented. Static corrections were carried out on reflection seismic data acquired from the study area using Geoscribe II software to determine the effects of the low velocity layer on reflection seismic data in the area. Results of the study revealed that the velocity of the low velocity layer ranged between 144 and 996m/s with a regional average of 407m/s. The thickness of the low velocity layer varied between 3.0 and 9.6m with a mean value of 5.0m. Similarly, the velocity of the consolidated layer ranged between 1449 and 1812m/s with a mean value of 1738m/s. Results of the static correction carried out on the seismic reflection data revealed a substantial improvement in the resolution of the data after static correction. Based on these findings, it is therefore, recommended that shots for reflection seismic survey should be located at a minimum depth of 9.6m in the area to eliminate the effects associated with the low velocity layer.     

Application of remote-sensing data to groundwater exploration: A case study of the Cross River State, southeastern Nigeria

 The Cross River State, Nigeria, is underlain by the Precambrian-age crystalline basement complex and by rocks of Cretaceous to Tertiary age. The exploration for groundwater in this area requires a systematic technique in order to obtain optimum results, but the non-availability of funds and facilities has made it extremely difficult to carry out site investigations prior to the drilling of water wells. Therefore, the failure rate is as high as 80%. In order to delineate areas that are expected to be suitable for future groundwater development, black and white radar imagery and aerial photographs were used to define some hydrological and hydrogeological features in parts of the study area. Lineament and drainage patterns were analysed using length density and frequency. Lineament-length density ranges from 0.04–1.52; lineament frequency is 0.11–5.09; drainage-length density is 0.17–0.94, and the drainage frequency is 0.16–1.53. These range of values reflect the differences in the probability of groundwater potentials. Results were then used to delineate areas of high, medium, and low groundwater potential. Study results also indicate that correlations exist between lineament and drainage patterns, lithology, water temperature, water conductivity, well yield, transmissivity, longitudinal conductance, and the occurrence of groundwater. Received, August 1994 · Revised, March 1996, June 1996 · Accepted, October 1996     

A regional study of saltwater intrusion in southeastern Nigeria based on the analysis of geoelectrical and hydrochemical data.

Vertical electrical sounding (VES) and hydrochemical data have been used to examine the extent of saltwater intrusion into shallow aquifers (depth<300 m) beneath the coastal plains of southeastern Nigeria underlain by siliciclastic sedimentary rocks of the Cenozoic Niger Delta. The VES data indicate that the coastal regions not affected by saltwater are characterised by K-and H-type geoelectrical curves and models in which high-layer resistivities (in excess of about 100 ohm-m) reflect the presence of freshwater aquifers at depths greater than 5 m below the surface. By contrast, Q-type curves and models denote saltwater-infiltrated islands with the upper boundary of the saline zone at depths of about 25 to 30 m. Chemical analysis results of precipitation and ground water show that, compared to coastal and continental sites, island localities have higher concentrations of major ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl^-, HCO₃^-, SO₄^2-, NO₃) due to saltwater intrusion. In addition, these ions, which also characterise seawater, show very good correlations (>0.70). Furthermore, the coastal sites are marked by Ca-Cl and Ca-HCO₃ water types whereas Na-Cl dominated both the islands ( at depths greater than 20 m) and the seawater. This gives an indication of seawater contamination. Seawater contamination is also indicated by the results of five assessment parameters as follows: high chloride concentration (>500 mg/l), high saturation index (halite>10^-6), high ionic strength (>0.05), low formation resistivity (<25 ohm-m), and low formation factor (<0.05). Two of these parameters (chloride concentration, ionic strength) have been used to compose an assessment scheme with five major classes (AA, BB, CC, DD, EE) corresponding to various degrees of salinity. Generally in the study area, saltwater contamination is a feature of island locations at depths greater than about 20 m. Water within the inland coastal regions is fresh.     

A study of some Nigerian carbonate rocks for the building construction industry

A number of geotechnical analyses were carried out on selected carbonate rock samples from ten locations in Nigeria. This was to assess the suitability of these rocks as building construction aggregates. The analyses included uniaxial compressive strength, tensile strength porosity, water absorption and dynamic fragmentation.

Evaluation of these carbonate rocks on the basis of a quality index (QI) scheme show that some lithofacies (i.e., calcareous siltstone, pisolitic limestone, marly limestone, massive limestone, sandy limestone, oncolitic limestone and fossiliferous limestone) which were obtained from Abini, Ashaka, Calabar Ikom road, Ikot Okpora, Mfamosing and Nkalagu are the best for construction purposes. However, it is recommended that the susceptibility of each deposit to solution should be investigated.     

Principles of justice in proposals and policy approaches to avoided deforestation: Towards a post-Kyoto climate agreement

This paper offers a normative analysis of the current negotiations on reducing emissions from deforestation and forest degradation (REDD) under the United Nations Framework Convention on Climate Change (UNFCCC). Drawing on existing theories of distributive justice, we seek to determine which interpretations of equity are embodied in the key proposals and policy approaches to REDD in the run up to a post-Kyoto climate agreement. Our analysis indicates that whilst the various proposals are characterised by different and sometimes contradictory notions of equity, it is the ideas that are more consistent with neoliberal concepts of justice that tend to prevail. The result is that despite abiding contestations and controversies, emerging REDD policy solutions for the post-2012 climate regime looks very likely to reflect a commitment to market-based approaches to forest governance. However, whilst such market-based approaches might serve the preferences of powerful players, their effectiveness in terms of forest preservation, the protection of indigenous peoples and sustainable community development remains extremely dubious. On a broader note, our analysis reinforces the growing realization that the international arena is not beyond the pale of moral arguments but rather that the governance of global environmental change implicates elemental ethical questions regarding which ways of life human beings ought to pursue.     

Policy responses to rapid climate change: An epistemological critique of dominant approaches

This paper reviews existing policy responses to rapid climate change and examines possible assumptions that underpin those responses. The analysis demonstrates that current policy responses to rapid climate change make unwarranted epistemological and ethical assumptions. Specifically, we argue that the assumptions about the possibility of predicting the climate system including tipping points linked to utilitarian ethical assumptions in the form of cost–benefit analysis are open to contestation and should be subject to global public debate. The paper considers alternative normative approaches and briefly proposes complementary policy responses.     

Global environmental sustainability: Intragenerational equity and conceptions of justice in multilateral environmental regimes

With specific focus on two environmental regimes (the Basel Convention on the Transboundary Movements of Hazardous Wastes and the Climate Change Convention), this paper seeks to indicate the prospects and limitations of the aspirations for distributive justice by the political South within the context of sustainability in general, and the institutions for global environmental governance in particular. It is argued that while these aspirations have produced important normative shifts in the rule-structure of global environmental management, they have not proved momentous enough to generate policies outside of what the prevailing neoliberal socio-economic regime might permit. Hence, although the texts of global environmental agreements accommodate concepts that express egalitarian notions of justice, core policies remain firmly rooted in market-based neoliberal interpretations of justice, which mainly serve to sustain the status quo.