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81.
Martin Nekola René Hudec Martin Jelínek Matůš Kocka Petr Kubánek Filip Münz Cyril Polášek Vojtěch Šimon Jan Štrobl 《Experimental Astronomy》2010,28(1):79-85
We report on two small aperture robotic telescopes called BART and D50 operated in Ondřejov. Both telescopes are capable of
automatic observation of gamma ray burst (GRB) optical afterglows. Coordinates of GRBs are taken from alerts distributed via
Internet. Telescopes observe other interesting high energy sources when there is not any alert. The smaller telescope BART
has aperture D = 254 mm. The bigger telescope D50 has a primary mirror of diameter D = 500 mm. Both telescopes are controlled
by free software package RTS2 and are accessible through Internet. We describe the two telescopes and related software and
show some results such as our first observed optical counterpart of GRB. 相似文献
82.
On the origin of rock fragment mulches on Vertisols: A case study from the Ethiopian highlands 总被引:1,自引:2,他引:1
Jan Moeyersons Jan Nyssen Jean Poesen Jozef Deckers Mitiku Haile 《Geomorphology》2006,76(3-4):411-429
Many Vertisols in Tigray, Ethiopia, typically carry a discontinuous rock fragment (RF, size 0.5–> 40 · 10− 2 m) cover with 10 to 100 RFs m− 2. Such RF mulches are of agricultural and environmental significance because they influence the water balance in the underlying soils and the crop yield. Natural RF concentrations are mostly considered as eolian or hydraulic lag deposits, or as the result of lateral transport over the soil surface from a rock outcrop, upslope. In cultivated areas RF mulches can develop by tillage.This paper presents the case of a natural RF mulch whose lithology indicates that the RFs are up-squeezed by the local Vertisol. The study site is located in the pass of Enda Maryam, Tigray, Northern Ethiopia (39°8′ E and 13°36′ N). A circular area of 10 m diameter, about 200 m away from the water divide in the valley has been cleared annually between 01/1999 and 05/2003. During this period, 625 RFs, 17 being > 7.5 · 10− 2 m in size, totalling a mass of nearly 62 kg, have been collected. After correction for measurement procedures, the rate of RF up-warping by the Vertisol at Enda Maryam is assessed at 5 RFs m− 2 in 3 years. At this rate of appearance, the formation of current RF concentrations on top of active valley Vertisols is only a matter of 101–2 years, provided the availability of RFs below the soil surface.Although important underground displacements were measured in the Vertisol between 01/1999 and 05/2002, the supposed link between up-squeezing of RFs and plastic deformations of ‘chimney’, ‘diapir’ or ‘intrusion’-like type in the Vertisol could not be evidenced. Instead, RFs are clearly concentrated on the soil surface as well as in depth, along the existing vertical desiccation cracks, often > 1 m deep which display polygonal configurations at the soil surface. Further, bundles of slickensides containing some RFs, have been mapped at the base of the Vertisol. The slickenside configuration suggests that the RF-bearing substrate is being scraped off.While the underground displacement of RFs along active slickensides seems normal, the process of RFs ascending in ‘upright’ position in the edge of desiccation cracks needs explanation. The closure of a desiccation crack is a peristaltic-like movement, following ascent or descent of the capillary fringe. It is hypothesized that this movement gradually pushes the RF to the surface or to another place or level in the soil profile where the crack closes in last instance.The apparent young age of the valley Vertisol mulches in Ethiopia might indicate the very recent formation of yearly recurrent desiccation cracks of Vertisols in the area. Available information confirms that most valleys in the study area used to be perennially marshy. Under these conditions no movements of RFs in the soil profile are expected to occur. Gullying, leading to pronounced seasonal desiccation of the Vertisols, started in several cases not more than 50 years ago. 相似文献
83.
84.
N. Tvis Knudsen P. Nørnberg Jacob C. Yde Bent Hasholt Jan Heinemeier 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2013,113(1):137-142
Geografisk Tidsskrift, Danish Journal of Geography 108(1):137–142, 2008 During field observations in August 2005 antler remains of a reindeer were found at a recently deglaciated site at about 500 m asl., and bones from a polar bear were found at about 300 m asl. along the margin of Mittivakkat Glacier, Southeast Greenland. Radio carbon dating determined the age of the samples to 720 14C years and 350 14C years, respectively. In August 2006 old surface vegetation covering peaty material became exposed due to ice recession close to the site where the antler was found. The radio carbon age of small roots from the material was determined to 1530 14C years, and is in agreement with dating of woody remains of Salix glauca found close by, at the top of a nearby nunatak in 1999. The antler indicates that reindeer lived in the area when the glacier began to advance from a position where it was close to or smaller than today. The vegetation surface and peaty material indicate that the climate was warmer before the onset of the Little Ice Age in Southeast Greenland than today. 相似文献
85.
86.
Jan Kryński 《Journal of Geodesy》1979,53(1):19-36
Summary The concept of satellite-to-satellite tracking measuring the relative velocity of two orbiting satellites spaced some hundreds
kilometers on a close orbit, provides now possibilities for the investigation of the Earth’s gravity field. In the paper only
medium and short wave length effects affecting the measured relative velocity have been considered. Collocation is used in
such an analysis of local geoid improvement, because this method allows to combine heterogeneous data in a consistent way.
Covariance functions relevant for the particular case of a circular equatorial orbit are given. Two kinds of observation equations
have been formulated. The choice of observation equation with regard to satellites configuration is discussed.
It is found that it is sufficient to have a limited number of satellite-to-satellite observations in a 7o×7o area around the estimation point with distances between profiles of about 1o.5 and between the two satellites forming the pair of 200+350 km; the altitude of satellite-to-satellite observations should
be as low as possible. The accuracy of the geoid determination strongly depends on the degree and order of the reference field
used. An accuracy of about ±1 m can be achieved with an assumed reference field of (40,40).
The influence of measuring errors is discussed and it is shown that only satellite-to-satellite observations with accuracy
better then 0.1 mm/sec will give an improvement of the geoid. Finally, some results on the combination of low-low satellite-to-satellite
tracking and terrestrial gravity data are given.
The proposed method seems to be especially interesting for unsurveyed areas. Furthermore, it has the practical advantage that
only a local coverage data is needed. 相似文献
87.
Wu Xiangjie Pang Xiong Shi Hesheng He Min Shen Jan Zhang Xiangtao Hu Dengke 《中国地质大学学报(英文版)》2009,20(1)
To study the deep dynamic mechanism leading to the difference in rifting pattern and basin structure from shelf to oceanic basin in passive continental margin,we constructed long geological sections across the shelf,slope and oceanic basin using new seismic data.Integrated gravity-magnetic inversion and interpretation of these sections were made with the advanced dissection method.Results show that the basement composition changes from intermediate-acid intrusive rocks in the sheff to intermediate-basic rocks in the slope.The Moho surface shoals gradually from 31 km in the sheff to 22.5 km in the uplift and then 19 km in the slope and finally to 13 km in the oceanic basin.The crust thickness also decreases gradually from 30 km in the northern fault belt to 9 km in the oceanic basin.The crustal stretching factor increases from the shelf toward the oceanic basin,with the strongest extension under the sags and the oceanic basin.The intensity of mantle upwelling controlled the style of basin structures from sheff to oceanic basin.In the Zhu 1 depression on the shelf,the crust is nearly normal,the brittle and cold upper crust mainly controlled the fault development;so the combinative grabens with single symmetric graben are characteristic.In the slope,the crust thinned with a large stretching factor,affected by the mantle upwelling.The ductile deformation controlled the faults,so there developed an asymmetric complex graben in the Baiyun (白云) sag. 相似文献
88.
An elasto-plastic constitutive model is introduced for rock joints under cyclic loading, considering the additional shear resistance generated by the asperity damage in the first forward shear cycle and sliding mechanism for further shearing. A series of cyclic loading direct shear tests was conducted on artificial joints with triangular asperities and replicas of a real rock asperity surface under constant normal stiffness (CNS) conditions. The model was calibrated and then validated using selected data sets from the experimental results. Model simulations were found to be in good agreement with the rock joints behaviour under cyclic loading and CNS conditions both in stress prediction and dilation behaviour. In addition, dynamic stability analysis of an underground structure was carried out, using Universal Distinct Element Code and the proposed constitutive model. 相似文献
89.
Kerstin S. Treydte David C. Frank Gerhard Helle Jan Esper 《Geochimica et cosmochimica acta》2009,73(16):4635-7303
We present one millennium-long (1171-year), and three 100 year long annually resolved δ13C tree-ring chronologies from ecologically varying Juniperus stands in the Karakorum Mountains (northern Pakistan), and evaluate their response to climatic and atmospheric CO2 changes. All δ13C records show a gradual decrease since the beginning of the 19th century, which is commonly associated with a depletion of atmospheric δ13C due to fossil fuel burning. Climate calibration of high-frequency δ13C variations indicates a pronounced summer temperature signal for all sites. The low-frequency component of the same records, however, deviates from long-term temperature trends, even after correction for changes in anthropogenic CO2. We hypothesize that these high-elevation trees show a response to both climate and elevated atmospheric CO2 concentration and the latter might explain the offset with target temperature data. We applied several corrections to tree-ring δ13C records, considering a range of potential CO2 discrimination changes over the past 150 years and calculated the goodness of fit with the target via calibration/verification tests (R2, residual trend, and Durbin-Watson statistics). These tests revealed that at our sites, carbon isotope fixation on longer timescales is affected by increasing atmospheric CO2 concentrations at a discrimination rate of about 0.012‰/ppmv. Although this statistically derived value may be site related, our findings have implications for the interpretation of any long-term trends in climate reconstructions using tree-ring δ13C, as we demonstrate with our millennium-long δ13C Karakorum record. While we find indications for warmth during the Medieval Warm Period (higher than today’s mean summer temperature), we also show that the low-frequency temperature pattern critically depends on the correction applied. Patterns of long-term climate variation, including the Medieval Warm Period, the Little Ice Age, and 20th century warmth are most similar to existing evidence when a strong influence of increased atmospheric CO2 on plant physiology is assumed. 相似文献
90.
Six samples of non-mineralized black shale from a Kupferschiefer section in the northern part of the Polish Zechstein Basin
(Zdrada IG-8 drill hole: 1026.16–1026.90 m) were studied for Re–Os isotopes and selected major and trace elements. These black
shales, averaging 5.82 wt.% Corg and 1.69 wt.% Stot, display very low base metal values (up to 106 ppb Cu, 792 ppb Pb, and 144 ppb Zn) and have abundances of 64–1376 ppb Re
and 0.37–1.25 ppb Os, with a 187Os/188Os ratio of 6.95–22.5. The regression of all Re–Os data yields an age of 247 ± 20 Ma, which is within the range of uncertainties
of previous Rb–Sr and K–Ar geochronological studies. The scatter in the Re–Os data can be explained in terms of fluctuations
in sedimentary conditions, i.e., restricted basin with terrestrial influence. 相似文献