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931.
Jürg Schweizer Martin Schneebeli Charles Fierz Paul M. B. Föhn 《Surveys in Geophysics》1995,16(5-6):621-633
Knowledge about snow mechanics and snow avalanche formation forms the basis of any hazard mitigation measures. The crucial point is the snow stability. The most relevant mechanical properties - the compressive, tensile and shear strength of the individual snow layers within the snow cover - vary substantially in space and time. Among other things the strength of the snow layers depends strongly on the state of stress and the strain rate. The evaluation of the stability of the snow cover is hence a difficult task involving many extrapolations.To gain insight in the release mechanism of slab avalanches triggered by skiers, the skier's impact is measured with a load cell at different depths within the snow cover and for different snow conditions. The study focused on the effects of the dynamic loading and of the damping by snow compaction. In accordance with earlier finite-element (FE) calculations the results show the importance of the depth of the weak layer or interface and the snow conditions, especially the sublayering.In order to directly measure the impact force and to study the snow properties in more detail, a new instrument, called rammrutsch was developed. It combines the properties of the rutschblock with the defined impact properties of the rammsonde. The mechanical properties are determined using (i) the impact energy of the rammrutsch and (ii) the deformations of the snow cover measured with accelerometers and digital image processing of video sequences. The new method is well suited to detect and to measure the mechanical processes and properties of the fracturing layers. The duration of one test is around 10 minutes and the method seems appropriate for determining the spatial variability of the snow cover. A series of experiments in a forest opening showed a clear difference in the snow stability between sites below trees and ones in the free field of the opening. 相似文献
932.
933.
934.
Small blue crabs (Callinectes sapidus Rathbun, 43–70 mm carapace width) can influence the size-distribution of the gastropod,Bittium varium, strongly reducing the contribution of snails >3 mm shell length in field enclosures. We test the hypothesis that these size-dependent effects are due to size-selective predation rather than size-dependent emigration from the field enclosures. In laboratory feeding trials, blue crabs showed negative selectivity for snails <2.5 mm and positive selectivity for snails >3.3 mm. When feeding, blue crabs crushBittium shells, but the opercula are deposited undamaged in the sediment. Sediment from a field enclosure experiment contained 6.5× moreBittium opercula from enclosures with blue crabs than from enclosures without blue crabs. We reconstructed the size distribution ofBittium killed by blue crabs from the opercula recovered from sediment. This distribution qualitatively matched those “missing” from the size frequency distribution of surviving snails. Estimates of selectivity from laboratory feeding trials predicted the pattern of size-selection fromBittium killed in the enclosures. We also estimated Strauss's linear index of selectivity and Chesson's α based on the size distribution of snails available during the field enclosure experiment. These indices predicted both the pattern of selectivity and the size distribution ofBittium killed in enclosures with blue crabs. We conclude that size-selective predation by blue crabs can explain the observed shifts inBittium size distributions. 相似文献
935.
Melt inclusions in high-An plagioclase from the Gorda Ridge: an example of the local diversity of MORB parent magmas 总被引:2,自引:0,他引:2
R. L. Nielsen Jennifer Crum Rene Bourgeois Kaylea Hascall Lance M. Forsythe Martin R. Fisk David M. Christie 《Contributions to Mineralogy and Petrology》1995,122(1-2):34-50
The use of ocean floor basalt chemistry as a tool to investigate mantle composition and processes requires that we work with
basalts that have been modified little since leaving the mantle. One source of such basalts is melt inclusions trapped in
primitive crystals. However, obtaining information from these melt inclusions is complicated by the fact that melt inclusions
in natural basalts are essentially always altered by post-entrapment crystallization. This requires that we develop techniques
for reconstructing the original trapped liquid compositions. We conducted a series of experiments to reverse the effects of
post-entrapment crystallization by re-heating the host crystals to their crystallization temperature. For these experiments
we used plagioclase crystals separated from a single Gorda Ridge lava. The crystallization temperature for these crystals
was determined by a set of incremental re-heating experiments to be ∼1240–1260° C. The inclusions are primitive, high Ca-Al
basaltic melts, saturated with plagioclase, olivine and Al-rich chromite at low pressure. The inclusion analyses can be linked
to the host lava composition by low pressure fractionation. The major element composition of the re-homogenized melt inclusions
within each crystal is relatively constant. However, the incompatible element analyses have extremely wide ranges. The range
of La/Sm and Ti/Zr from inclusions analyzed from a single sample from the Gorda Ridge exceeds the range reported for lavas
sampled from the entire ridge. The pyroxene compositions predicted to be in equilibrium with the melt inclusion trace element
signature cover much of the range represented by pyroxenes from abyssal peridotites. The volumetric proportions of the magmas
entering the base of the crust can be evaluated using frequency distribution of melt inclusion compositions. This distribution
suggests that the array of magmas was skewed towards the more depleted compositions, with little evidence for an enriched
component in this system. This pattern is more consistent with a dynamic flow model of the mantle or of a passive flow model
where the melts produced in the peripheral areas of the melting regime were not focused to the ridge.
Received: 5 January 1995 / Accepted: 13 June 1995 相似文献
936.
Prof. Henno Martin 《International Journal of Earth Sciences》1964,54(1):115-127
The Late Precambrian Numees Formation contains besides very thick deposits of tillite-like habit also laminated siltstones resembling glacial varved rocks. These enclose coarser sand grains, pebbles and boulders of all sizes which have dropped from above into the soft sediment. As only floating ice can have transported these large components, the conclusion is inescapable that the laminated siltstones are true glacial varves and that the tillite-like rocks are true tillites.This evidence for Late Precambrien glacial conditions makes it highly probable that other formations of a similar age, which have been described as tillites, are also of glacial origin. The Buschmannsklippe Formation begins locally with a tillite which seems to be more or less contemporaneous with the Numees Formation. A high percentage of facetted and deeply striated pebbles and boulders makes it probable that this tillite originated as a basal moraine.The Nama tillite in the Klein Karas Mountains is stratigraphically not very far removed and in time probably not very much younger than the Numees tillite. Striated and grooved floors are associated with this tillite.The Chuos tillite and the Otavi tillite of the Damara System are contemporaneous deposits of Late Precambrian age. Their age relative to the Numees tillite is not known. The Chuos- and the Otavi tillite are probably glaciomarine drifts. For these a glacial origin cannot be proved with the same degree of conclusiveness as for the above described deposits. However, the existence of glacial conditions in the Late Praecambrian having been proved for an adjoining area, a glacial origin may reasonably be assumed for the tillite-like rocks of the Damara System too.Both the Numees tillite and the Otavi tillite are intimately associated with sedimentary iron ore deposits. Oxygen deficiency in stagnating bottom waters, caused by an ice cover, is thought to be responsible for this peculiar combination of sediments.The excellent evidence for the existence of glacial conditions during parts of the Late Precambrian, found in South West Africa, strongly supports the assumption that the many tillite-like deposits of a similar age, which have been described from Central and West Africa are also of glacial origin.
Published under Government Printer's Copyright Authority No. 3320 of 30/4/64. 相似文献
Published under Government Printer's Copyright Authority No. 3320 of 30/4/64. 相似文献
937.
938.
Thomas Esch Hubert Asamer Felix Bachofer Jakub Balhar Martin Boettcher Enguerran Boissier 《International Journal of Digital Earth》2020,13(1):136-157
ABSTRACTThe digital transformation taking place in all areas of life has led to a massive increase in digital data – in particular, related to the places where and the ways how we live. To facilitate an exploration of the new opportunities arising from this development the Urban Thematic Exploitation Platform (U-TEP) has been set-up. This enabling instrument represents a virtual environment that combines open access to multi-source data repositories with dedicated data processing, analysis and visualisation functionalities. Moreover, it includes mechanisms for the development and sharing of technology and knowledge. After an introduction of the underlying methodical concept, this paper introduces four selected use cases that were carried out on the basis of U-TEP: two technology-driven applications implemented by users from the remote sensing and software engineering community (generation of cloud-free mosaics, processing of drone data) and two examples related to concrete use scenarios defined by planners and decision makers (data analytics related to global urbanization, monitoring of regional land-use dynamics). The experiences from U-TEP’s pre-operations phase show that the system can effectively support the derivation of new data, facts and empirical evidence that helps scientists and decision-makers to implement improved strategies for sustainable urban development. 相似文献
939.
Martin Sudmanns Dirk Tiede Hannah Augustin Stefan Lang 《International Journal of Digital Earth》2020,13(7):768-784
ABSTRACT Sentinel-2 scenes are increasingly being used in operational Earth observation (EO) applications at regional, continental and global scales, in near-real time applications, and with multi-temporal approaches. On a broader scale, they are therefore one of the most important facilitators of the Digital Earth. However, the data quality and availability are not spatially and temporally homogeneous due to effects related to cloudiness, the position on the Earth or the acquisition plan. The spatio-temporal inhomogeneity of the underlying data may therefore affect any big remote sensing analysis and is important to consider. This study presents an assessment of the metadata for all accessible Sentinel-2 Level-1C scenes acquired in 2017, enabling the spatio-temporal coverage and availability to be quantified, including scene availability and cloudiness. Spatial exploratory analysis of the global, multi-temporal metadata also reveals that higher acquisition frequencies do not necessarily yield more cloud-free scenes and exposes metadata quality issues, e.g. systematically incorrect cloud cover estimation in high, non-vegetated altitudes. The continuously updated datasets and analysis results are accessible as a Web application called EO-Compass. It contributes to a better understanding and selection of Sentinel-2 scenes, and improves the planning and interpretation of remote sensing analyses. 相似文献
940.
Efstratios Stylianidis Daniela Poli Martin Hofer Armin Gruen Victor Sanchez Martin 《International Journal of Digital Earth》2020,13(8):854-885
ABSTRACT A 3D forest monitoring system, called FORSAT (a satellite very high resolution image processing platform for forest assessment), was developed for the extraction of 3D geometric forest information from very high resolution (VHR) satellite imagery and the automatic 3D change detection. FORSAT is composed of two complementary tasks: (1) the geometric and radiometric processing of satellite optical imagery and digital surface model (DSM) reconstruction by using a precise and robust image matching approach specially designed for VHR satellite imagery, (2) 3D surface comparison for change detection. It allows the users to import DSMs, align them using an advanced 3D surface matching approach and calculate the 3D differences and volume changes (together with precision values) between epochs. FORSAT is a single source and flexible forest information solution, allowing expert and non-expert remote sensing users to monitor forests in three and four (time) dimensions. The geometric resolution and thematic content of VHR optical imagery are sufficient for many forest information needs such as deforestation, clear-cut and fire severity mapping. The capacity and benefits of FORSAT, as a forest information system contributing to the sustainable forest management, have been tested and validated in case studies located in Austria, Switzerland and Spain. 相似文献