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861.
Pascal Bertran 《地球表面变化过程与地形》1993,18(7):645-660
The soil deformation produced by mass movements gives rise to specific microscopic features. The study of some markers of deformation, such as the orientation of rigid clasts and the morphology of fine-grained intercalations, indicates that, in most cases, mass movements can be related to simple shear. The development of these features varies according mainly to the type of mass movement (solifluction, debris flow or earth slide). Different factors play a significant role in their formation: intrinsic soil characteristics such as the grain size of the matrix, water content and matrix/element viscosity ratios, and external factors such as the intensity and homogeneity of deformation. A careful study of the microscopic features leads to a better understanding of the dynamics of mass deposits. 相似文献
862.
863.
864.
865.
Derivation of vegetative variables from a landsat tm image for modelling soil erosion 总被引:1,自引:0,他引:1
Steven M. De Jong 《地球表面变化过程与地形》1994,19(2):165-178
A study was carried out to assess the potential use of satellite thematic mapper (TM) images to produce maps of vegetation-related variables for erosion modelling. In a Mediterranean study area in southern France the (semi-)natural vegetation was described at 33 field plots using four quantitative methods: the Fosberg structural classification system, the cover and management factor of the Universal Soil Loss Equation, the leaf area index and the total percentage cover. After radiometric correction of the image, the spectral TM bands were processed following three different methods. Each method aimed at combining the data of the six spectral TM bands into a single band in such a way that the resulting image displayed optimal information on green vegetation cover. The algorithms used comprise the normalized difference vegetation index, the conventional ‘tasselled cap’ transformation and a locally tuned tasselled cap transformation. Only slight differences were found between the different methods to calculate spectral vegetation indices for this particular case. Furthermore, the correlations between the field variables and image-derived spectral indices are generally small. The largest correlations were found for the normalized vegetation index and the leaf area index (r + 0·71) and for the normalized vegetation index and Fosberg's structural vegetation classes (r + 0·76). However, Fosberg's method results in very general classes, which are of little use for soil erosion models. Furthermore, the spectral indices appeared to be sensitive for the vitality of the vegetation. Consequently, an area covered by a sensed, senescent vegetation will not yield a large value for the spectral index, but its soil is protected against splash erosion. This might lead to a misinterpretation of the soil protective cover when satellite images are used. A final conclusion is that a balance has to be found between the more accurate, but time-consuming field surveys to gather information on erosion-controlling factors and a certain loss of accuracy associated with the use of quick and easy remote sensing methods. 相似文献
866.
In the Vélez Blanco region (province of Almeia), filament limestones occur associated with pellet limestones, crinoidal limestones, radiolarian limestones, Saccocoma limestones and tintinnid limestones. These predominantly Late Jurassic rocks are underlain by Middle Jurassic oolitic limestones and overlain by Cretaceous pelagic limestones and marls.A distinction can be made between long and short filaments. The former are interpreted as undamaged valves, the latter as shell fragments of the pelagic pelecypod Bositra buchi. Bositra valves and fragments have been sorted by weak current (and/or wave) action.In the Saccocoma limestones, evidence for similar sorting of Saccocoma debris has been found.The radiolarian limestones represent a low-energy basin environment, but are not comparable with recent deep-sea radiolarian oozes.Nodular limestone intercalations and hiatuses represent a current- (and/or wave-) swept environment.The occurrence of filament-rich and Radiolaria-bearing, but benthos-poor, intervals within the oolitic limestone indicates that the deposition of such rocks can take place at moderate sea depths. 相似文献
867.
Robert W Risebrough Brock W De Lappe Timothy T Schmidt 《Marine pollution bulletin》1976,7(12):225-228
The bioaccumulation of PCB and DDT compounds by mussels, Mytilus spp., from ambient seawater was determined by measuring concentrations of these chlorinated hydrocarbons in both mussels and seawater at four sites on the Mediterranean coast of France and two sites in California. Bioaccumulation factors were found to vary over an order of magnitude. Ratios of the concentrations of PCB in mussels, consisting predominantly of pentachloro-and hexachlorobiphenyls, to concentrations in ambient seawater ranged between 69 000 and 690 000; the corresponding ratios of concentrations of p,p′-DDE or p,p′-DDT ranged from 40 000 to 690 000. Although mussels appear to be appropriate and convenient indicator organisms of local chlorinated hydrocarbon contamination levels, the factors contributing to this variance must be determined before mussels are widely used in global monitoring programmes. 相似文献
868.
Comparison of the analytical data on the alkenes extracted from both dried and wet Messel shale sediment points to the formation of artifacts, presumably as a result of clay-catalyzed dehydration reactions. 相似文献
869.
Solid source mass spectrometry has been used to determine the relative cumulative fission yields of five elements in three samples of uranium ore from reactor zones in the Oklo mine site. Eighteen fission chains covering the mass range from105 ≤ A ≤ 130 have been measured for Pd, Ag, Cd, Sn and Te. These measurements have enabled a number of nuclear parameters to be calculated including the relative proportions of235U,238U and239Pu involved in the fission process.The concentration of the five elements in the Oklo samples have also been measured using the stable isotope dilution technique. These values have then been compared to the estimates of the amount of these elements produced by fission under the conditions that are appropriate to the three samples. This procedure enables the retentivity of the elements in the reactor zones to be evaluated. Our work confirms the fact that Pd and Te are retained almost in their entirety in the samples, whereas the other three elements have been partially lost from the reactor site. Almost all the Cd fission products have been lost, and more than 50% of the Ag and Sn fission-produced material has been removed. 相似文献
870.
Several independant determinations of the difference in Gibbs free energy of formation (from the elements at 25°, 1 bar) between NaCl0 and KCl0 in aqueous solutions (molality > 0.5) are derived from equilibrium data between alkali feldspars, feldspathoids (nepheline-kalsilite), micas (muscoviteparagonite) and hydrothermal (Na, K)Cl-H2O solutions. These results along with other data from the literature are discussed. The relation: . is proposed from 400 to 800°C and 1 to 2 Kbar. 相似文献