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61.
The Henry formulation, which couples subsurface flow and salt transport via a variable-density flow formulation, can be used to evaluate the extent of sea water intrusion into coastal aquifers. The coupling gives rise to nontrivial flow patterns that are very different from those observed in inland aquifers. We investigate the influence of these flow patterns on the transport of conservative contaminants in a coastal aquifer. The flow is characterized by two dimensionless parameters: the Péclet number, which compares the relative effects of advective and dispersive transport mechanisms, and a coupling parameter, which describes the importance of the salt water boundary on the flow. We focus our attention on two regimes – low and intermediate Péclet number flows. Two transport scenarios are solved analytically by means of a perturbation analysis. The first, a natural attenuation scenario, describes the flushing of a contaminant from a coastal aquifer by clean fresh water, while the second, a contaminant spill scenario, considers an isolated point source. 相似文献
62.
Qualitative and quantitative aspects of the evolution of the planktonic populations in Lake Lugano 总被引:2,自引:0,他引:2
This study is a summary of all available information on the long-term and on the more recent development of the planktonic populations in Lago di Lugano with regard to its trophic evolution. The first effects of the lake eutrophication became manifest beginning in the 1950's and brought to important changes in the planktonic community as the appearance and the rapid increase of the filamentous algaeOscillatoria rubescens andStephanodiscus hantzschii, and the disappearance of the zooplanktonic Diaptomidae. Since 1980 the phosphorus concentration began to decrease in lake's epilimnion and the planktonic community showed a new composition, evidencing the presence of others predominant species:Oscillatoria redekei, Lyngbya limnetica, Stephanodiscus sp. (little form). Since 1989 algal standing crop reduced its value below 2 g m–2 (dry weight); furthermore a strong decrease of the Cyanophceae and the appearance of new predominant species (Tabellaria fenestrata, div. Ulotrichales) occurred. At the same time an increase of the herbivorous zooplankton (Daphnia hyalina) and the come-back of the Diaptomidae was observed. Only in the northern basin a decrease tendency of the primary production was observed, changing from 480 to about 300 g C m–2 yr–1 during the last 10 years. 相似文献
63.
Marco Franzini 《Contributions to Mineralogy and Petrology》1969,21(3):203-224
A discussion of the transition from the ideal hexagonal mica structure to the ideal ditrigonal one, leads to the conclusion that the single mica layer may have two different structures (labelled A and B). The recent literature data show that both the A and B structures have been detected in some triocahedral layer lattice silicates found in nature. An examination of the structural stability of the A and B structures suggests that the last one may not be realized by dioctahedral layer lattice silicates. The concept of two structurally different mica layers, which however have the same lattice constants, greatly improves the understanding of polymorphism and twin laws in layer lattice silicates.The structural features of the tetrahedral sheet, octahedral sheet and interlayer region are carefully examined. Thus we can reach the following conclusions: the tetrahedal sheet is not entirely free to reduce its lateral dimensions by the mechanism of tetrahedal rotation owing to the repulsion among Obas atoms; the octahedral sheet in layer lattice silicates, may increase or reduce its lateral dimensions as compared to the lateral dimensions it has in the hydroxide minerals; the interlayer region is characterized by a regular octahedral coordination of the Obas around the interlayer cation. On the ground of these conclusions, new structural models for some selected layer lattice silicates are proposed.Notations Obas
basal oxygen atoms of the (Al, Si)O4 tetrahedra
- Oap
apical oxygen atoms of the (Al, Si)O4 tetrahedra
-
b
tetr
b dimension which the tetrahedral sheet would assume if unconstrained
-
b
oct
b dimension which the octahedral sheet has in the hydroxide minerals
-
b
obs
observed value of b
- c
oct
*
thickness of the octahedral sheet
-
d
o
distance between an octahedral cation and an Oap atom
-
d
int
distance between an interlayer cation and an Obas atom
-
average tetrahedral rotation from ideal hexagonal symmetry 相似文献
64.
Mario Boccaletti Roberto Mazzuoli Marco Bonini Teresa Trua Bekele Abebe 《Journal of African Earth Sciences》1999,29(4):315
Deformation and magmatism within the 90 km wide northern Ethiopian Rift system is concentrated along a narrow zone - the Wonji Fault Belt. Two key areas (the Nazret-Dera and Asela-Ziway areas), located along the eastern margin of the north-northeast to northeast trending Main Ethiopian Rift, have been investigated in order to reconstruct the recent tectonomagmatic evolution of the northern branch of the Main Ethiopian Rift. In these areas, Early Pleistocene volcanic products (Wonji Group) overlie Pliocene volcanic rocks (Eastern Margin Unit). Detailed stratigraphical reconstructions have revealed the presence of several tectonomagmatic units which can be correlated between the two study areas. The stratigraphical and petrological study of these units outlined (1) the bimodal composition (basalts-pantellerites) of the oldest and youngest units and the unimodal character (pantellerites) of the products erupted during the intervening period; (2) the mainly fissural origin of the ignimbrites and oldest basalts; and (3) a mafic/felsic volumetric ratio of 1:5.The geological data suggest that, around the Pliocene-Quaternary boundary, a change in the stress field occurred in this Main Ethiopian Rift sector, passing from a direction of extension roughly orthogonal to the rift shoulders, to oblique rifting related to an east-west trending extension. In this framework the change in the style of volcanism observed in the Nazret-Dera and Asela-Ziway areas can be related to the change of the stress field. A new geodynamic model is presented for the Late Pliocene to Recent evolution of this sector of the Main Ethiopian Rift. According to this model, a large volume of rhyolitic products was erupted during an oblique rifting phase, following a previous period of pure extension. The change in the tectonic regime favoured partial melting of the underplated basalts as a decrease in the pressure and an elevation of isotherms occurred. 相似文献
65.
Michele Bossi Gianantonio Guerrero Luciano Mantegazza Marco Scardia 《Astrophysics and Space Science》1982,85(1-2):25-29
Photoelectric observations of the Delta Scuti star KW 207 (HD 73576) are given. In spite of the low variations, the analysis of the light curves shows at least two sighificant periodicities, which seem to be related to non-radial pulsations.A third frequency, corresponding to the radial fundamental mode, is probably bame out by our data. 相似文献
66.
Stefan Becker Marco Gemmer Tong Jiang 《Stochastic Environmental Research and Risk Assessment (SERRA)》2006,20(6):435-444
Precipitation trends in the Yangtze River catchment (PR China) have been analyzed for the past 50 years by applying the Mann-Kendall trend test and geospatial analyses. Monthly precipitation trends of 36 stations have been calculated. Significant positive trends at many stations can be observed for the summer months, which naturally show precipitation maxima. They were preceded and/or followed by negative trends. This observation points towards a concentration of summer precipitation within a shorter period of time. The analysis of a second data set on a gridded basis with 0.5° resolution reveals trends with distinct spatial patterns. The combination of classic trend tests and spatially interpolated precipitation data sets allows the spatiotemporal visualization of detected trends. Months with positive trends emphasize the aggravation of severe situation in a region, which is particularly prone to flood disasters during summer. Reasons for the observed trends were found in variations in the meridional wind pattern at the 850 hPa level, which account for an increased transport of warm moist air to the Yangtze River catchment during the summer months. 相似文献
67.
68.
A. Marco G. Bernabeau J.M. Torrejón J. Fabregat 《Astrophysics and Space Science》1998,263(1-4):299-302
We are developing a long term programme to determine abundances of peculiar stars, as a function of age, in open clusters
with ages ≤ 5 × 107 yr. A fundamental part will be to deduce the ages and physical properties of these clusters with higher accuracy than in
previous studies. As part of this effort we present here CCD uvbyβ photometry of two open clusters, NGC 884 and NGC 869.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
69.
Microlevelling procedures applied to regional aeromagnetic data: an example from the Transantarctic Mountains (Antarctica) 总被引:1,自引:0,他引:1
The extensive application of digital enhancement and filtering as a powerful tool for aeromagnetic interpretation, not only of high resolution but also of regional data, requires an improved levelling. Two microlevelling techniques were thus compared in order to find an effective but relatively simple procedure to remove, or at least to reduce, residual magnetic errors remaining after standard levelling processes. This study was carried out on regional aeromagnetic data recently acquired at high magnetic latitudes along the Transantarctic Mountains in Antarctica, where it is particularly critical to remove time-dependent magnetic variations. Two-dimensional FFT filters applied to the gridded data, namely the Butterworth and a directional cosine filter, proved to be more effective than previously proposed one-dimensional space-domain filters in the reduction of the 'residual corrugation' not removed by statistical levelling. Tectonic interpretation of trends detected in the total field magnetic anomaly map and in the 3D analytic signal improved after application of frequency-domain microlevelling. However, we also show that when interpreting microlevelled data, two factors must be considered: (i) the possible presence of real geological trends aligned along the flight lines; (ii) modifications in the results yielded by depth estimates of magnetic sources due to the FFT filters applied during the microlevelling procedure. Such changes were seen both in the well-established 2D FFT method, based on the slope of the energy spectrum, and in the more recent 3D Euler deconvolution technique. Overall our results indicate that microlevelling could profitably be applied to older gridded aeromagnetic data sets in Antarctica, thus improving the accuracy and geological significance of future regional magnetic compilations, as already seen in other continents. 相似文献
70.
High-resolution white-light pictures are analyzed to study the differences between the granular size near sunspot penumbrae and in light-bridges presenting granular structure and that of the quiet photosphere. No difference is found between the mean granular diameter in light-bridges and the quiet photosphere. The dispersion found in the results corresponding to different zones around the sunspots indicates that the size of the granulation may vary from place to place near the sunspots, its mean value not differing significantly from that of the quiet photosphere. A possible systematic bias in the selection of the granules by Macris (1979) is found. 相似文献