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Summary. A residual map of the total magnetic field (above 25 000 nT base) is presented for a portion of the central crystalline shield area of Nigeria and overlapping small portions of the Chad basin and the Benue rift (8°30'−12° 00'lat, and 7°−10°30' long). The map (based on a dataset digitized from recently released aeromagnetic sheets of Nigeria) leads to four results. (1) A magnetic boundary, evident on the map, separates the Younger Granite complexes into two groups. The groups are petrologically different, and the boundary may be a fault line with uplift to the south. (2) South of the boundary the map is dominated by a system of sub-parallel anomalies striking NE–SW, possibly representing major tectonic trends, and a set of fractures through which the Younger Granite complexes were intruded. The trend of the system parallels the Benue rift and lineaments in the oceanic crust off West Africa. (3) Negative magnetic anomalies lie over most of the known ring complexes, and over some suspected buried ring complexes and other intrusions. (4) 2½-and 3-D modelling shows that the larger complexes extend to 12 km depth, and the smaller ones to 6 km. They have nearly vertical sides, and magnetization contrasts range from 0.3 to 0.5 A m−1 . 相似文献
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In an attempt to discover planetary eclipses, we observed the open cluster NGC 7086. On one night we observed the cluster and standard stars through the B and V filters, enabling us to make a colour-magnitude diagram. Plots of the differential magnitudes were inspected for variability, but no planetary eclipses were found. New variable stars were discovered and their light curves show some of them to be eclipsing binary stars. The location on the colour-magnitude diagram of some of the variable stars is consistent with cluster membership. 相似文献
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Joanne Muller Malin Kylander Antonio Martinez-Cortizas Dominik Weiss Kevin Blake Raquel Garcia-Sanchez 《Geochimica et cosmochimica acta》2008,72(2):449-463
The Lynch’s Crater peat deposit in NE-Australia is a sensitive environmental archive located in the tropical Southern Hemisphere. This unique deposit illustrates that local and regional changes had a profound effect on the local Australian ecosystem over the past 55 kyr. To obtain a proxy of past climate changes, trace and major element geochemistry analyses were applied to a 13 m peat core from the crater. Principle component analysis (PCA) was used to identify the main factors that control elemental distribution in the peat and to add interpretative strength to the geochemical behavior of selected major and trace elements. For example, Sc, Al, Cu, and Pb were found to be related to increased erosion of the basin soils, and from this, several periods of significant flux from atmospheric input and/or terrigenous run-off were identified. Geochemically mobile elements during rock weathering and pedogenesis, such as Mg, Ca, and Sr helped to identify the peat ombrotrophic-minerotrophic boundary at ∼1.5 m depth and offered important information about fluxes of these nutrients to the mire and their dynamics within the deposit. Arsenic and V comparisons between the peat record (high concentrations in some peat sections) and in local basin rocks (very low concentrations), suggested the presence of a long range, atmospheric dust source early in the formation of the mire. The Lynch’s Crater peat record presents a continuous record of environmental change in tropical Australia and contributes new understanding to geochemical processes in peatlands. 相似文献
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Laboratory spectra of SiO particles of 1 m radius show a broad structureless extinction peak at 9.6m. The wavelength dependence of extinction from SiO, an amorphous silicon oxide, provides a good match to that of interstellar dust. 相似文献
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John R. Greenwood Joanne E. Norris Jo Wint 《Geotechnical and Geological Engineering》2006,24(3):467-481
The procedure for geotechnical site investigation is well established but little attention is currently given to investigating
the potential of vegetation to assist with ground stability. This paper describes how routine investigation procedures may
be adapted to consider the effects of the vegetation. It is recommended that the major part of the vegetation investigation
is carried out, at relatively low cost, during the preliminary (desk) study phase of the investigation when there is maximum
flexibility to take account of findings in the proposed design and construction. The techniques available for investigation
of the effects of vegetation are reviewed and references provided for further consideration. As for general geotechnical investigation
work, it is important that a balance of effort is maintained in the vegetation investigation between (a) site characterisation
(defining and identifying the existing and proposed vegetation to suit the site and ground conditions), (b) testing (in-situ and laboratory testing of the vegetation and root systems to provide design parameters) and (c) modelling (to analyse the
vegetation effects). 相似文献