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.     

The geochemistry of uranium occurrences and speciation in groundwater of Chinnar sub basin,South India

An attempt has been made in Chinnar sub basin of Dharmapuri district, South India to isolate the geochemistry of uranium occurrences in groundwater. The geology of the area is mainly of charnockite and granite gneiss. Groundwater samples were collected for two different seasons post and pre monsoon in two different litho units (granite gneiss and charnockite) and analysed for major, minor and uranium concentrations. Higher uranium (18.45 μg L^?1) has been recorded during pre monsoon season in granite gneiss with increasing pH. The saturation index calculation for the groundwater isolated minerals like uaraninite, coffinite, haiweeite and soddyite to be precipitating and uranium oxides like UO_2.25, UO_2.25beta, UO_2.33beta as oversaturated. The Eh-pH diagram attempted represents solubility of uraninite within the pH range of 6.0 to 8.0. The study isolate uranium in groundwater of the study area is controlled by the presence of (U₄O₉) uranium oxide.     

Seasonally and spatially referenced estimates of recreational shore-angling effort,catch composition,catch rates and total catch in the Goukamma Marine Protected Area,South Africa

A roving creel survey of the recreational shore fishery along the 16.4-km coastline in the Goukamma Marine Protected Area on the south coast of South Africa was conducted from 2009 to 2011. Some 838 patrols were stratified equally among months, areas and years, but intentionally biased towards weekends. Angler densities at Buffalo Bay and Groenvlei were 0.59 and 0.28 anglers km?1, respectively. Weekend densities were double to quadruple weekday densities and fishing during winter was more popular than during summer. Area, habitat and distance to access points explained variation in angler densities. Shannon–Wiener diversity in catches declined from 2.18 in an earlier (1993–2002) survey to 1.79. Although the order of species abundance in the catches remained largely unchanged, blacktail Diplodus capensis dominance increased to 57.3% by number, at the expense of galjoen Dichistius capensis. Habitat explained 27% of the variance in catch composition. The catch per unit effort (CPUE) for the top nine species ranged from 0.19 to 6.35 fish 100-h?1. The CPUE of all species, except spotted grunter Pomadasys commersonnii, declined. Blacktail and galjoen CPUE declined by 17% and 77%, respectively. The total catch estimate was 2 986 fish y?1. Transgressions of size limits were common. The results suggest that the fishery is overexploited and that catch rates are declining.     

东天山伊吾二叠纪花岗质杂岩体的锆石定年、地球化学及其构造意义

伊吾花岗质杂岩体主要由二长花岗岩和碱长花岗岩构成。锆石LA-ICP-MS U-Pb定年分析得到二长花岗岩和碱长花岗岩的侵位年龄分别为284.6±1.4Ma和284.0±1.1Ma。结合地质证据,此年龄表明该杂岩体形成于碰撞之后的二叠纪早期挤压-伸展转折阶段。岩石学、地球化学和同位素等方面的对比研究表明这两种岩石为同一岩浆演化的产物。与二长花岗岩相比,碱长花岗岩表现为硅、碱的含量较高,而铝的含量较低;富Th、U、Nb、Ta,贫Sr、P、Ti、Sm;DI和Rb/Sr升高,Nb/Ta和Zr/Hf下降,从二长花岗岩到碱长花岗岩表现出连续分异演化的趋势。计算得到二长花岗岩和碱长花岗岩的模式年龄(t_(DM))分别为693Ma和763Ma,ε_(Nd)(t)分别为+4.53和+4.64,(~(87)Sr/~(86)Sr)_i分别为0.703858和0.703855,表现出高ε_(Nd)(t)低(~(87)Sr/~(86)Sr)_i的特征。这些特征表明,伊吾岩体的岩浆来自新元古代时从亏损地幔分离出来的初生地壳源区。二长花岗岩岩浆是这种初生地壳岩浆演化的产物,而碱长花岗岩形成于二长花岗岩母岩浆经斜长石、磷灰石、钛铁氧化物、榍石、独居石、褐帘石和锆石等矿物分离结晶后的残留岩浆。     

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     

江西铜坑嶂钼矿和红山铜矿含矿斑岩锆石U-Pb定年及其地质意义

江西铜坑嶂钼矿和红山铜矿是在武夷山成矿带中最近发现的两个斑岩型矿床。本文利用LA-ICP-MS锆石U-Pb定年方法对铜坑嶂花岗斑岩和红山含矿花岗斑岩分别进行了年代学研究,获得铜坑嶂岩体中花岗斑岩的锆石年龄为138±1Ma,代表斑岩体侵位的年龄;红山矿区含矿斑岩的锆石年龄,分作两期,分别为99Ma和49Ma,两期不同的年龄可能代表了两期热事件。研究表明,铜坑嶂钼矿的成岩成矿时代均发生在白垩纪,其岩浆作用与成矿作用基本吻合。综合前人资料,认为铜坑嶂钼矿区的斑岩和红山铜矿区的斑岩可能分别形成于大陆弧后伸展带和岩石圈伸展环境。     

The Auroral Station in Adventdalen, Svalbard

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