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321.
Legchenko A Descloitres M Bost A Ruiz L Reddy M Girard JF Sekhar M Mohan Kumar MS Braun JJ 《Ground water》2006,44(4):547-554
The performance of the Magnetic Resonance Sounding (MRS) method applied to the investigation of heterogeneous hard-rock aquifers was studied. It was shown using both numerical modeling and field measurements that MRS could be applied to the investigation of the weathered part of hard-rock aquifers when the product of the free water content multiplied by the thickness of the aquifer is >0.2 (for example, 10-m-thick layer with a 2% water content). Using a currently available one-dimensional MRS system, the method allows the characterization of two-dimensional subsurface structures with acceptable accuracy when the size of the subsurface anomaly is equal to or greater than the MRS loop. However, the fractured part of hard-rock aquifers characterized by low effective porosity (<0.5%) cannot be resolved using currently available MRS equipment. It was found that shallow water in the weathered part of the aquifer may screen MRS signals from deeper water-saturated layers, thus further reducing the possibility of investigating deeper fractured aquifers. A field study using the NUMIS(plus) MRS system developed by IRIS Instruments was carried out on an experimental watershed in southern India. A heterogeneous unconfined aquifer in a gneissic formation was successfully localized, and MRS results were confirmed by drilling shortly after the geophysical study. The top of the aquifer revealed by MRS was found to be in a good agreement with observed static water level measurements in boreholes. 相似文献
322.
Antonio D. Montero-Dorta Darren J. Croton Renbin Yan Michael C. Cooper Jeffery A. Newman Antonis Georgakakis Francisco Prada Marc Davis Kirpal Nandra Alison Coil 《Monthly notices of the Royal Astronomical Society》2009,392(1):125-134
We measure the dependence of the active galactic nuclei (AGN) fraction on local environment at z ∼ 1 , using spectroscopic data taken from the DEEP2 Galaxy Redshift Survey, and Chandra X-ray data from the All-Wavelength Extended Groth Strip International Survey (AEGIS). To provide a clean sample of AGN, we restrict our analysis to the red sequence population; this also reduces additional colour–environment correlations. We find evidence that high-redshift LINERs in DEEP2 tend to favour higher density environments relative to the red population from which they are drawn. In contrast, Seyferts and X-ray selected AGN at z ∼ 1 show little (or no) environmental dependencies within the same underlying population. We compare these results with a sample of local AGN drawn from the Sloan Digital Sky Survey (SDSS). Contrary to the high-redshift behaviour, we find that both LINERs and Seyferts in the SDSS show a slowly declining red sequence AGN fraction towards high-density environments. Interestingly, at z ∼ 1 red sequence Seyferts and LINERs are approximately equally abundant. By z ∼ 0 , however, the red Seyfert population has declined relative to the LINER population by over a factor of ∼4.5. We speculate on possible interpretations of our results. 相似文献
323.
Allen P. Nutman Clark R.L. Friend Vickie C. Bennett David Wright Marc D. Norman 《Precambrian Research》2010
Chemical (meta)sedimentary rocks in the amphibolite facies ≥3700 Ma Isua supracrustal belt (W. Greenland) are mostly strongly deformed, so there is only a small chance of the survival of features such as stromatolites or microfossils that would be direct proof of a ≥3700 Ma biosphere. Therefore the search for evidence of ≥3700 Ma life in Isua rocks has focused on chemical signatures, particularly C-isotopes. The new approach presented here is based on whole rock chemistry rather than isotopic signatures. Isua chemical sedimentary rocks have Ca–Mg–Fe bulk compositions that coincide with ferroan dolomite – siderite/Fe-oxide mixtures. Most have low Al2O3, TiO2 contents (<0.5 and <0.05 wt% respectively) showing minimal contamination from terriginous materials. Identical seawater-like REE + Y shale-normalised trace element signatures with La, Ce, Eu and Y positive anomalies are found in magnetite-rich banded iron formation (BIF – such as the geochemical standard IF-G), dolomite-rich rocks and quartz–carbonate–calc-silicate rocks. Additionally from a rare, small area of low deformation in Isua, there are ∼3700 Ma pillow lava interstices consisting of quartz + tremolite + calcite derived from pre-metamorphic dolomite + silica. Thus the dolomite in the chemical sediments and the pillow interstice was part of the pre-metamorphic assemblage, and was deposited from seawater and/or low-temperature groundwater (as shown by the REE + Y chemistry). Therefore, at least some Isua carbonate rocks are sedimentary or diagenetic in origin rather than being formed by metasomatism at 600–500 °C as proposed by Rose et al. (1996. American Journal of Science 296, 1004–1044). 相似文献
324.
Marc Blanchard Guillaume Morin Michele Lazzeri Etienne Balan 《Geochimica et cosmochimica acta》2010,74(14):3948-14056
A series of hematite structures containing various amounts of aluminum and hydroxyl groups was modeled using first-principles methods based on the density functional theory. Evolution of the lattice parameters was quantified as a function of Al and H concentrations. The a and c lattice parameters decrease with the aluminum content and increase with the water content. This allows explaining experimental data reported for synthetic hematites, in particular the observed deviation from the Vegard’s rule. Investigation of several hydroxyl configurations associated with cationic vacancies suggests that the speciation of water also significantly affects the hematite structure. The 57Fe and 18O reduced partition functions (β-factor) were determined. Results show a linear dependence of the iron and oxygen β-factors on the aluminum content. An incorporation of 18 mole % Al2O3 in hematite would increase the iron β-factor of about 0.6‰ and the oxygen β-factor of about 5.5‰ at 0 °C. This effect is sufficiently large to be measurable and to affect the interpretation of natural isotopic compositions. On the other hand, the effect of water is found to be negligible for the hydroxyl configuration investigated. 相似文献
325.
Joyce Schmatz Marc Holland Steffen Giese Wouter van der Zee Janos. L. Urai 《Journal of the Geological Society of India》2010,75(1):74-88
The aim of this study is to improve our knowledge of the processes that lead to clay smear during faulting of a layered sand-clay
sequence in an analogue sandbox model. We carefully characterized mechanical properties of the materials used by a series
of geotechnical tests. Displacement field was quantified using PIV (Particle Image Velocimetry). The model is water-saturated
to allow the deformation of wet clay and sand in one experiment comprising a sand package with a horizontal layer of clay
above a predefined rigid basement fault. The thickness and rigidity of the clay layer are the parameters varied in this study.
The model shows a range of structures that are related to competence contrast between sand and different clay types. Results
show ductile shearing of soft clay with a transition to brittle fracturing of stiff clay accompanied by the formation of rotating
clay blocks in the fault zone. Localized deformation is observed through time showing (i) the propagation of one active fault
migrating laterally through the sediment package, and (ii) the formation of a stable prism between two or more active faults
that gets progressively smaller with minor rotation of the hanging wall fault. Continuous clay smear is observed resulting
from the lateral injection of clay as well as from a reworked mixture of sand and clay. 相似文献
326.
Terrigenous organic matter (TOM) transfer from a watershed to a lake plays a key role in contaminants fate and greenhouse gazes emission in these aquatic ecosystems. In this study, we linked physiographic and vegetation characteristics of a watershed with TOM nature deposited in lake sediments. TOM was characterized using lignin biomarkers as indicators of TOM sources and state of degradation. Geographical information system (GIS) also allowed us to integrate and describe the landscape morpho-edaphic characteristics of a defined drainage basin. Combining these tools we found a significant and positive relationship (R2 = 0.65, p < 0.002) between mean slope of the watershed and the terrigenous fraction estimated by Λ8 in recent sediments. The mean slope also correlated with the composition of TOM in recent sediments as P/(V + S) and 3,5Bd/V ratios significantly decreased with the steepness of the watersheds (R2 = 0.57, p < 0.021 and R2 = 0.71, p < 0.004, respectively). More precisely, areas with slopes comprised between 4° and 10° have a major influence on TOM inputs to lakes. The vegetation composition of each watershed influenced the composition of recent sediments of the sampled lakes. The increasing presence of angiosperm trees in the watershed influenced the export of TOM to the lake as Λ8 increased significantly with the presence of this type of vegetation (R2 = 0.44, p < 0.019). A similar relationship was also observed with S/V ratios, an indicator of angiosperm sources for TOM. The type of vegetation also greatly influenced the degradation state of OM. In this study, we were able to determine that low-sloped areas (0-2°) act as buffer zones for lignin inputs and by extension for TOM loading to sediments. The relative contribution of TOM from the soil organic horizons also increased in steeper watersheds. This study has significant implications in our understanding of the fate of TOM in lacustrine ecosystems. 相似文献
327.
Yves Géraud Michel Rosener Fabrice Surma Joachim Place Édouard Le Garzic Marc Diraison 《Comptes Rendus Geoscience》2010,342(7-8):566-574
In EGS projects, fault zones are considered as the structures controlling deep flow at the reservoir scale. Using a large set of petrophysical properties (porosity, density, permeability, thermal conductivity [TC]) measured on cores collected along the EPS-1 borehole, a model of fault zone is proposed to describe them. A fault zone is a complex structure, showing different parts with different kinds of deformations and/or materials that could explain chemical and physical processes observed during fluid-rock interactions. The different parts composing the fault zone are: (1) the fault core or gauge zone; (2) the damage zone; (3) and the protolith. They are usually heterogeneous and show different physical properties. The damage zone is a potential high permeability channel and could become the main pathway for fluids if secondary minerals seal the fault core. Porosity is the lowest within the protolith, between 0.5 and 1%, but can go up to 15% in the fault zone. Permeability ranges from 10?20 m2 in the fresh granite to, at least, 10?15 m2 in the fault core, and TC ranges from 2.5 W K?1m?1 to 3.7 W K?1m?1. Finally, variations in specific surface are set over two orders of magnitude. If the lowest values usually characterize the fresh granite far from fault zones, physical properties could show variations spread over their whole respective ranges within these fault zones. 相似文献
328.
329.
330.
Asmae Mhamdi Alaoui Mohamed Choura Mohamed Maanan Bendahhou Zourarah Marc Robin Maria Freitas Conceição César Andrade Mehdi Khalid Christian Carruesco 《Environmental Earth Sciences》2010,61(2):275-286
The metal content in surface sediments (0–2 cm, 26 samples), in a sediment core (120, 1 cm slices), taken from Moulay Bousselham
(Morocco) was investigated. Concentrations of Al, Fe, Mn, Pb, Zn, Cu, Ni, Cr, Cd, As, and Hg were evaluated in surface and
cored sediments of Moulay Bousselham lagoon. Significantly high concentrations in μg g−1 dw of Pb (31.7–6.2), Zn (758.9–167), Cu (310.7–22), Ni (96–10.5), Cr (113–18.9), Cd (0.84–0.02), As (1–0.1), and Hg (0.61–0.02)
were found in sediment samples from Moulay Bousselham lagoon. Calculated enrichment factors [EFMe = (Me/Al)sample/(Me/Al)background], using Al as a normalizer, and correlation matrices showed that metal pollution in Merja Zerga of Moulay Bousselham lagoon
was the product of anthropogenic sources, while the metal content in Merja Kehla was of natural origins. The results suggest
that a major change in the sedimentary regime of the lagoon, associated with internal trapping and re-distribution of heavy
metal, has been occurring in the past few decades. The cause would appear to be the construction of a Nador Canal at the lagoon.
Probable effects concentrations (PEC) were often exceeded for heavy metals in the lagoon sediments, especially for Zn, Cu,
Ni, and Cr, and four stations, stations MZ-11, MZ-12, MZ-13, MZ-14, MZ-16, and MZ-17, had multiple metals at presumptively
toxic levels. These comparisons suggest that sediment metal levels in the river are clearly high and probably pose an environmental
risk at some stations. The levels of most of the metals were not greatly enriched, a consideration that is of the utmost importance
when contamination issues are at stake. Metal concentrations found in Moulay Bousselham lagoon were comparable to aquatic
systems classified as contaminated from other regions of the world. 相似文献