排序方式: 共有66条查询结果,搜索用时 15 毫秒
51.
M. S. Hirt 《大气与海洋》2013,51(2):14-24
The Automatic Picture Transmission (or APT) System developed for weather satellites enables satellite data laboratories, such as the one at the Toronto International Airport, to prepare photographic mosaics covering almost the entire continent of North America, The mosaic can be completed within four hours after the satellite comes within range of approximately 2500 miles of the ground receiving station. One such mosaic is presented here. The cloud features are analyzed .with the aid of the meteorological charts available i n the forecast office. Conclusions drawn show that cloud patterns on the satellite photograph are readily associated 16th the features on the charts available in a forecast office. Conversely, the satellite photograph can be used as an aid in the analysis of these charts. 相似文献
52.
53.
Prediction of vertical deflections from high-degree spherical harmonic synthesis and residual terrain model data 总被引:6,自引:4,他引:2
Christian Hirt 《Journal of Geodesy》2010,84(3):179-190
This study demonstrates that in mountainous areas the use of residual terrain model (RTM) data significantly improves the
accuracy of vertical deflections obtained from high-degree spherical harmonic synthesis. The new Earth gravitational model
EGM2008 is used to compute vertical deflections up to a spherical harmonic degree of 2,160. RTM data can be constructed as
difference between high-resolution Shuttle Radar Topography Mission (SRTM) elevation data and the terrain model DTM2006.0
(a spherical harmonic terrain model that complements EGM2008) providing the long-wavelength reference surface. Because these
RTM elevations imply most of the gravity field signal beyond spherical harmonic degree of 2,160, they can be used to augment
EGM2008 vertical deflection predictions in the very high spherical harmonic degrees. In two mountainous test areas—the German
and the Swiss Alps—the combined use of EGM2008 and RTM data was successfully tested at 223 stations with high-precision astrogeodetic
vertical deflections from recent zenith camera observations (accuracy of about 0.1 arc seconds) available. The comparison
of EGM2008 vertical deflections with the ground-truth astrogeodetic observations shows root mean square (RMS) values (from
differences) of 3.5 arc seconds for ξ and 3.2 arc seconds for η, respectively. Using a combination of EGM2008 and RTM data for the prediction of vertical deflections considerably reduces
the RMS values to the level of 0.8 arc seconds for both vertical deflection components, which is a significant improvement
of about 75%. Density anomalies of the real topography with respect to the residual model topography are one factor limiting
the accuracy of the approach. The proposed technique for vertical deflection predictions is based on three publicly available
data sets: (1) EGM2008, (2) DTM2006.0 and (3) SRTM elevation data. This allows replication of the approach for improving the
accuracy of EGM2008 vertical deflection predictions in regions with a rough topography or for improved validation of EGM2008
and future high-degree spherical harmonic models by means of independent ground truth data. 相似文献
54.
T Sancelme J Goetze S Jaquemet MG Meekan A Flam AM Watts 《African Journal of Marine Science》2020,42(1):95-107
Coastal populations of maritime countries in eastern Africa rely on fish as a primary source of protein, but baseline information on the abundance of fish communities on these coastlines is often lacking. We used baited remote underwater video stations to compare the abundance and diversity of reef fishes targeted by fishing at two sites in southern Mozambique, one at Lighthouse Reef within the Bazaruto Archipelago National Park and the other to the south at San Sebastian Reef on the San Sebastian Peninsula. Fish that are known targets of fisheries (mostly small-scale and artisanal) had an abundance that was almost three-times greater at San Sebastian Reef (80.22 ind. h–1 [SE 18.00]) than at Lighthouse Reef (29.70 ind. h–1 [SE 8.91]). Similarly, there was greater mean species richness at San Sebastian Reef (38.74 species h–1 [SE 2.79]) than at Lighthouse Reef (25.37 species h–1 [SE 3.66]). The main drivers of targeted fish abundance were habitat and depth, with shallow (<15 m) and mixed reef areas having the greatest abundance and richness. More sampling was done over sand habitat at Lighthouse Reef, which likely led to the lower abundance and species richness observed at this site; however, that finding could also be attributable to the fact that protection is provided to only a section of available coral reef habitat in a small area. Nevertheless, fish community structure was comparable between the sites, with similar proportions of carnivores (78–81%), herbivores (12–14%) and omnivores (7–8%). Our findings highlight the variation in species abundance and assemblages of coral-reef fish targeted by fishing in Mozambique and emphasise the importance of localised environmental variables as a driver of these patterns. To ensure maximum protection of Lighthouse Reef fish communities, we recommend an extension of the no-take zone to include the entire reef complex. 相似文献
55.
56.
A series of large diameter calcite–muscovite aggregates has been prepared from calcite and muscovite powders, in order to gain a better understanding of how texture develops in impure carbonate rocks. The development of the microstructure and the crystallographic preferred orientation (CPO, texture) during the preparation process is described. The synthetic rocks have been fabricated from powders of calcite and muscovite by uniaxial cold-pressing at loads up to 400 MPa and subsequent hot isostatic pressing (HIPping) at pressures of 150 to 170 MPa and a temperature of 670 °C. The resulting textures and microstructures are homogeneous throughout the samples. The calcite CPO is generated by rigid body rotation and twinning during cold-pressing and is not significantly altered by recrystallization during HIPping. Grain growth during HIPping is observed in pure calcite samples, but is inhibited through high porosity and the presence of muscovite in the mixed aggregates. The preferred orientation of the calcite c-axes is found to increase with increasing uniaxial cold pressure, and to be independent of the muscovite content. The magnetic bulk susceptibility of the starting material has been changed by the formation of ferromagnetic impurities during fabrication. Comparison of the samples to natural calcite fabrics from fault zones show the potential of the experiments and fabric analyses presented to analyze and to better understand the deformation mechanisms of fault zones. 相似文献
57.
Triassic stretching directions in Iberia and North Africa inferred from magnetic fabrics 总被引:1,自引:0,他引:1
Ruth Soto Antonio M. Casas‐Sainz Beln Oliva‐Urcia Cristina García‐Lasanta Esther Izquierdo‐Llavall Bennacer Moussaid Jos Carlos Kullberg Teresa Romn‐Berdiel Yolanda Snchez‐Moya Alfonso Sopea Sara Torres‐Lpez Juan Jos Villalaín Hamidou El‐Ouardi Inmaculada Gil‐Pea Ann M. Hirt 《地学学报》2019,31(5):465-478
During the Triassic, Iberia and western North Africa displayed a unique situation in relation with the Central and North Atlantic opening and westward expansion of the Tethys. Unravelling the stretching direction in Triassic deposits of the studied area can help in our understanding of this scenario. The tectonic setting is characterized by localized basins with strong thickness variations greatly influenced by previous post‐Variscan mechanical discontinuities. In this work, we revise and compile magnetic fabric data from eight Triassic depocentres in terms of defining the stretching direction (i.e. magnetic lineation), resulting from extensional deformation of this period. Data show the importance of the opening of the Atlantic rift as the leading process during the Triassic. Dextral transtension can explain the deflection of the extensional direction observed in most studied depocentres that is caused by the activity of previous major oblique faults. 相似文献
58.
Alla Shogenova Anne Kleesment Ann Hirt Enn Pirrus Toivo Kallaste Kazbulat Shogenov Rein Vaher 《Studia Geophysica et Geodaetica》2009,53(1):111-131
The formation of hematite and goethite concretions in different sedimentary rocks including sandstones is an important diagenetic
process in the geologic history of the Earth. Its interpretation can also contribute to understanding the diagenetic history
of Martian iron hydroxide concretions.
A case study of iron-rich concretions from Estonian Middle Devonian sandstones exposed in ancient river valleys in southeastern
Estonia was carried out based on the results of mineralogical, petrographical, geochemical, petrophysical and magnetic analyses.
It was found that the high Fe2O3(total) content (25.0–39.5%), high magnetic susceptibility, bulk and grain density, very low porosity, corrosion and fracturing
of the quartz grains of the platy iron concretions are in contrast with properties of the Devonian host sandstones. However
the ferrous iron content (measured as FeO) of iron-rich concretions was as low as in the other Devonian rocks, suggesting
an oxidizing environment and arid climate during the cementation by iron-hydroxides. The fracturing of quartz grains cemented
by iron hydroxides could take place at near-surface conditions including vadose and phreatic zones in arid climate with high
evaporation rates. Such climatic conditions have been reported for the Baltic region during Devonian, Upper Permian and Triassic
times.
We have found that goethite is prevalent in the cement, replacing clay and carbonate minerals. We assume that this iron-rich
cement is originated from the mobilization of iron in host sandstones by groundwater, associated with tectonic activity at
the end of the Middle Devonian, evidenced by fracturing in Devonian outcrops and caves. Although this mobilization could occur
under reducing conditions, precipitation of goethite and hematite for the cementation could take place in oxidizing environment
along bedding planes close to the surface during short sedimentation breaks. Another possible time for the formation of iron
concretions could be Permian, under the condition of both arid climate and tectonic activity. 相似文献
59.
Mesoscalic estimation of nitrogen discharge via drainage systems 总被引:2,自引:0,他引:2
A complex approach has been developed for estimating mesoscalic nitrogen discharges via drainage systems using spatial information about land use, drainage areas, nitrogen balances and soil and site characteristics. Determining the total drainage area involves certain difficulties for larger areas, as on the one hand, the available databases are incomplete, and on the other hand the localisation and digitalisation of large subsurface drainage areas is a very time-consuming process. Knowledge of the history and causes of drainage systems in landscapes is required. To solve this problem a method has been developed to calculate the drainage areas for large catchments. In order to obtain a complete data set of subsurface drainage areas, representative areas were selected to enable the proportion of subsurface drainage area to be determined for various soil and site characteristics. These proportions were extrapolated to the entire area and the approach tested in the Mulde River Catchment Area in Germany.The rate of drained arable land is about 25.2% of the total area, which can be broken down into grassland (19.0%) and arable land (27.4%). The results differ for sandy soils with up to 8% drained areas and 57.8% for stagnant soils. This shows that the proportion of drained land is highly dependent on the nature of the soil in the catchment area, which has profound implications for approaches to nitrogen modelling.Average nitrogen discharge for the whole catchment area via drainage water was 33 kg ha−1 yr−1 in the 1980s and 10 kg ha−1 yr−1 in the 1990s. The nitrogen discharge varies from one soil type to another: in regions with sandy substrate (11,900 ha) discharge was 34 kg ha−1 yr−1 in the 1980s (14 kg ha−1 yr−1 in the 1990s), while in areas with loess lessivé soils (89,200 ha) it was about 26 kg ha−1 yr−1 in the 1980s (9 kg ha−1 yr−1 in the 1990s). The reduction can be explained by the complete change in farming strategy since the demise of the former German Democratic Republic (GDR).The approach shown is well suited to future model approaches on a regional scale. By creating and integrating new data sets derived from modern GIS operations the approach reduces the uncertainty of water and nitrogen modelling. This gives us a better understanding of nitrogen discharges into surface and groundwater and temporal discharge dynamics. The discharge data are highly valuable to predict environmental protection measurements for streams, lakes, coastal waters and groundwaters. 相似文献
60.
A. Kontny R. Engelmann J. C. Grimmer R. O. Greiling A. Hirt 《International Journal of Earth Sciences》2012,101(3):671-692
Magnetite-bearing mylonitic garnet–micaschists close to the major suture between the Baltica and Iapetus terranes (Seve Nappe
Complex, Scandinavian Caledonides) show very high anisotropy of magnetic susceptibility (AMS) with corrected degree of anisotropy
(P′) up to 4.8. Three different magnetic fabric types can be distinguished. They correspond to protomylonite (type I, P′ < 2), mylonite (type II, 2 < P′ < 3), and ultramylonite (type III, P′ > 3), respectively. The orientation of the ellipsoid axes from all applied magnetic fabric methods in this study is similar
with shallow dips of the metamorphic foliation toward WSW and subhorizontal, mostly NW–SE trending mineral lineation. Differences
between subfabrics were minimized under high shear strain as all markers tend to align parallel with the shear plane. The
very high anisotropies and mostly oblate ellipsoid shapes of type III correlate with high magnetic susceptibility (k
mean up to 55 × 10−3 SI units) and are related to the concentration of magnetite aggregates with shape-preferred orientation. They show a distinct
field dependence of magnetic susceptibility of up to 10% in the k
max-direction. We attribute this field dependence to a “memory” of high strains in the domain walls of the crystals acquired
during synkinematic magnetite growth during shear zone fabric development at temperatures of 550–570°C. 相似文献