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1.
Anjali Bahuguna Shailesh Nayak Dam Roy 《International Journal of Applied Earth Observation and Geoinformation》2008,10(2):229-1
Tsunami waves struck the Indian coast on 26th December 2004 affecting the Andaman and Nicobar group of islands. A quick assessment of the status of the vital coastal ecosystems has been made using pre- and post-tsunami Advance Wide Field Sensor (AWiFS) data of Indian satellite RESOURCESAT with an accuracy of 87–90% and the Kappa ranging from 0.8696 to 0.9053. Among the coastal ecosystems the coral reefs have suffered the maximum with the Nicobar reefs (69% eroded and 29% degraded) bearing the brunt more than the Andaman reefs (54% eroded and 22% degraded). Significant improvement to the condition of the reef damaged due to backwash has been noted. About 41% of the Sentinel reef area has undergone significant improvement. The continuance of the erosion of the southwestern Andaman reefs is due to the impact of recurring earthquakes. The impact on mangroves of both the groups of islands has been due to uprooting as well as inundation of seawater and resulting stagnation. Changes are expected in community structure of mangroves as a result of tsunami. 相似文献
2.
3.
Dam Quang-Minh Manfred Frechen Tran Nghi Jan Harff 《International Journal of Earth Sciences》2010,99(8):1731-1740
In Vietnam, the coastal sand barriers and dunes located in front of the steep slopes of the high rising Truong Son Mountains
are sensitive to climate and environment change and give evidence for Holocene sea-level rise. The outer barrier sands were
deposited shortly before or contemporaneous with the local sea-level high stand along the Van Phong Bay postdating the last
glacial maximum (LGM). Optically stimulated luminescence (OSL) dating yielded deposition ages ranging from 8.3 ± 0.6 to 6.2 ± 0.3 ka
for the stratigraphically oldest exposed barrier sands. Further periods of sand accumulation took place between 2.7 and 2.5 ka
and between 0.7 and 0.5 ka. The youngest period of sand mobilisation was dated to 0.2 ± 0.01 ka and is most likely related
to reworked sand from mining activities. At the Suoi Tien section in southern central Vietnam, the deposition of the inner
barrier sands very likely correlate with an earlier sea-level high stand prior to the last glaciation. OSL age estimates range
from 276 ± 17 to 139 ± 15 ka. OSL dating significantly improves our knowledge about the sedimentary dynamics along the coast
of Vietnam during the Holocene. 相似文献
4.
Picard SODISM, a Space Telescope to Study the Sun from the Middle Ultraviolet to the Near Infrared 总被引:1,自引:0,他引:1
M. Meftah J.-F. Hochedez A. Irbah A. Hauchecorne P. Boumier T. Corbard S. Turck-Chièze S. Abbaki P. Assus E. Bertran P. Bourget F. Buisson M. Chaigneau L. Damé D. Djafer C. Dufour P. Etcheto P. Ferrero M. Hersé J.-P. Marcovici M. Meissonnier F. Morand G. Poiet J.-Y. Prado C. Renaud N. Rouanet M. Rouzé D. Salabert A.-J. Vieau 《Solar physics》2014,289(3):1043-1076
5.
Strategies to mitigate aliasing of loading signals while estimating GPS frame parameters 总被引:7,自引:5,他引:2
Xavier?CollilieuxEmail author Tonie?van?Dam Jim?Ray David?Coulot Laurent?Métivier Zuheir?Altamimi 《Journal of Geodesy》2012,86(1):1-14
Although GNSS techniques are theoretically sensitive to the Earth center of mass, it is often preferable to remove intrinsic
origin and scale information from the estimated station positions since they are known to be affected by systematic errors.
This is usually done by estimating the parameters of a linearized similarity transformation which relates the quasi-instantaneous
frames to a long-term frame such as the International Terrestrial Reference Frame (ITRF). It is well known that non-linear
station motions can partially alias into these parameters. We discuss in this paper some procedures that may allow reducing
these aliasing effects in the case of the GPS techniques. The options include the use of well-distributed sub-networks for
the frame transformation estimation, the use of site loading corrections, a modification of the stochastic model by downweighting
heights, or the joint estimation of the low degrees of the deformation field. We confirm that the standard approach consisting
of estimating the transformation over the whole network is particularly harmful for the loading signals if the network is
not well distributed. Downweighting the height component, using a uniform sub-network, or estimating the deformation field
perform similarly in drastically reducing the amplitude of the aliasing effect. The application of these methods to reprocessed
GPS terrestrial frames permits an assessment of the level of agreement between GPS and our loading model, which is found to
be about 1.5 mm WRMS in height and 0.8 mm WRMS in the horizontal at the annual frequency. Aliased loading signals are not
the main source of discrepancies between loading displacement models and GPS position time series. 相似文献
6.
Studies of basement‐bounded canyons in West Greenland show that these were long‐lasting features that extended inland for several hundreds of kilometres, acting as prominent sediment conduits sourcing the Albian–Palaeocene Nuussuaq Basin during several phases of basin evolution. The Ilulissat Icefjord canyon was the major conduit for sediment into the basin and provenance data indicate that it had a huge catchment area that extended to East Greenland. The Uparuaqqusuitsut canyon was also an important conduit for sediment in the northern part of the basin. It is suggested that the initial canyon formation occurred during uplift events in the Late Triassic and Late Jurassic when the deeply weathered basement surface formed during Early–Middle Triassic time was eroded. The recognition of these canyons as long‐lasting sediment conduits have huge implications for understanding the sediment distribution, source‐to‐sink studies and the palaeogeography of the North Atlantic basins. 相似文献
7.
Xavier Collilieux Laurent Métivier Zuheir Altamimi Tonie van Dam Jim Ray 《GPS Solutions》2011,15(3):219-231
The International GNSS Service (IGS) contributes to the construction of the International Terrestrial Reference Frame (ITRF)
by submitting time series of station positions and Earth Rotation Parameters (ERP). For the first time, its submission to
the ITRF2008 construction is based on a combination of entirely reprocessed GPS solutions delivered by 11 Analysis Centers
(ACs). We analyze the IGS submission and four of the individual AC contributions in terms of the GNSS frame origin and scale,
station position repeatability and time series seasonal variations. We show here that the GPS Terrestrial Reference Frame
(TRF) origin is consistent with Satellite laser Ranging (SLR) at the centimeter level with a drift lower than 1 mm/year. Although
the scale drift compared to Very Long baseline Interferometry (VLBI) and SLR mean scale is smaller than 0.4 mm/year, we think
that it would be premature to use that information in the ITRF scale definition due to its strong dependence on the GPS satellite
and ground antenna phase center variations. The new position time series also show a better repeatability compared to past
IGS combined products and their annual variations are shown to be more consistent with loading models. The comparison of GPS
station positions and velocities to those of VLBI via local ties in co-located sites demonstrates that the IGS reprocessed
solution submitted to the ITRF2008 is more reliable and precise than any of the past submissions. However, we show that some
of the remaining inconsistencies between GPS and VLBI positioning may be caused by uncalibrated GNSS radomes. 相似文献
8.
Ruud P. Bartholomeus Jan-Philip M. Witte Peter M. van Bodegom Jos C. van Dam Rien Aerts 《Journal of Hydrology》2008,360(1-4):147-165
Effects of insufficient soil aeration on the functioning of plants form an important field of research. A well-known and frequently used utility to express oxygen stress experienced by plants is the Feddes-function. This function reduces root water uptake linearly between two constant pressure heads, representing threshold values for minimum and maximum oxygen deficiency. However, the correctness of this expression has never been evaluated and constant critical values for oxygen stress are likely to be inappropriate. On theoretical grounds it is expected that oxygen stress depends on various abiotic and biotic factors. In this paper, we propose a fundamentally different approach to assess oxygen stress: we built a plant physiological and soil physical process-based model to calculate the minimum gas filled porosity of the soil (gas_min) at which oxygen stress occurs.First, we calculated the minimum oxygen concentration in the gas phase of the soil needed to sustain the roots through (micro-scale) diffusion with just enough oxygen to respire. Subsequently, gas_min that corresponds to this minimum oxygen concentration was calculated from diffusion from the atmosphere through the soil (macro-scale).We analyzed the validity of constant critical values to represent oxygen stress in terms of gas_min, based on model simulations in which we distinguished different soil types and in which we varied temperature, organic matter content, soil depth and plant characteristics. Furthermore, in order to compare our model results with the Feddes-function, we linked root oxygen stress to root water uptake (through the sink term variable F, which is the ratio of actual and potential uptake).The simulations showed that gas_min is especially sensitive to soil temperature, plant characteristics (root dry weight and maintenance respiration coefficient) and soil depth but hardly to soil organic matter content. Moreover, gas_min varied considerably between soil types and was larger in sandy soils than in clayey soils. We demonstrated that F of the Feddes-function indeed decreases approximately linearly, but that actual oxygen stress already starts at drier conditions than according to the Feddes-function. How much drier is depended on the factors indicated above. Thus, the Feddes-function might cause large errors in the prediction of transpiration reduction and growth reduction through oxygen stress.We made our method easily accessible to others by implementing it in SWAP, a user-friendly soil water model that is coupled to plant growth. Since constant values for gas_min in plant and hydrological modeling appeared to be inappropriate, an integrated approach, including both physiological and physical processes, should be used instead. Therefore, we advocate using our method in all situations where oxygen stress could occur. 相似文献
9.
Identifying causes of ground-penetrating radar reflections using time-domain reflectometry and sedimentological analyses 总被引:5,自引:0,他引:5
Ground-penetrating radar (GPR) is a geophysical technique widely used to study the shallow subsurface and identify various sediment features that reflect electromagnetic waves. However, little is known about the exact cause of GPR reflections because few studies have coupled wave theory to petrophysical data. In this study, a 100- and 200-MHz GPR survey was conducted on aeolian deposits in a quarry. Time-domain reflectometry (TDR) was used to obtain detailed information on the product of relative permittivity (ɛr ) and relative magnetic permeability (μr ), which mainly controls the GPR contrast parameter in the subsurface. Combining TDR data and lacquer peels from the quarry wall allowed the identification of various relationships between sediment characteristics and ɛr μr . Synthetic radar traces, constructed using the TDR logs and sedimentological data from the lacquer peels, were compared with the actual GPR sections. Numerous peaks in ɛr μr , which are superimposed on a baseline value of 4 for dry sand, are caused by potential GPR reflectors. These increases in ɛr μr coincide with the presence of either organic material, having a higher water content and relative permittivity than the surrounding sediment, or iron oxide bands, enhancing relative magnetic permeability and causing water to stagnate on top of them. Sedimentary structures, as reflected in textural change, only result in possible GPR reflections when the volumetric water content exceeds 0·055. The synthetic radar traces provide an improved insight into the behaviour of radar waves and show that GPR results may be ambiguous because of multiples and interference. 相似文献
10.
As the 7 December 2007 equinox of Uranus approached, collaboration between ring and atmosphere observers in the summer and fall of 2007 produced a substantial collection of ground-based observations using the 10-m Keck telescope with adaptive optics and space-based observations with the Hubble Space Telescope. Both near-infrared and visible-wavelength imaging and spatially resolved near-infrared spectroscopic observations were obtained. We used observations spanning the period from 7 June 2007 through 9 September 2007 to identify and track cloud features, determine atmospheric motions, characterize cloud morphology and dynamics, and define changes in atmospheric band structure. Atmospheric motions were obtained over a wider range of latitudes than previously was possible, extending to 73°N, and for 28 cloud features we obtained extremely high wind-speed accuracy through extended tracking times. We confirmed the existence of the suspected northern hemisphere prograde jet, locating its peak near 58°N. The new results confirm a small N-S asymmetry in the zonal wind profile, and the lack of any change in the southern hemisphere between 1986 (near solstice) and 2007 (near equinox) suggests that the asymmetry may be permanent rather than seasonally reversing. In the 2007 images, we found two prominent groups of discrete cloud features with very long lifetimes. The one near 30°S has departed from its previous oscillatory motion and started a significant northward drift, accompanied by substantial morphological changes. The complex of features near 30°N remained at a nearly fixed latitude, while exhibiting some characteristics of a dark spot accompanied by bright companion features. Smaller and less stable features were used to track cloud motions at other latitudes, some of which lasted over many planet rotations, though many could not be tracked beyond a single transit. A bright band has developed near 45°N, while the bright band near 45°S has begun to decline, both events in agreement with the idea that the asymmetric band structure of Uranus is a delayed response to solar forcing, but with a surprisingly short delay of only a few years. 相似文献