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81.
Richard E. Matheson David K. Camp Susan M. Sogard Kimberly A. Bjorgo 《Estuaries and Coasts》1999,22(2):534-551
The fauna of seagrass-covered mud banks in Florida Bay, documented in the mid 1980s prior to recent seagrass die-off, phytoplankton blooms, and other ecosystem changes, was reexamined in the mid 1990s for faunal changes that might be associated with environmental perturbations. During both decades, decapod crustaceans and fishes were collected with 1-m2 throw traps from seagrass beds at six sites that differ in the amount of freshwater and/or marine influence and in seagrass community metrics. The most common faunal changes were declines in seagrass-canopydwelling forms and increases in benthic forms. At three sites with relatively lush seagrass meadows, above-ground seagrass standing crop declined and abundance of the benthic predatory fishOpsanus beta increased. The degree of faunal change among these sites appeared to be related either to salinity variability or to the degree of exposure to the ecosystem changes that have taken place in Florida Bay. At two sites with poorly developed seagrass meadows, seagrass standing crop and canopy height did not change significantly between decades, but there was an increase in shoot density and total leaf area. The animal communities at these sites were characterized by significant increases in the abundance of benthic crustaceans. At the site on the edge of Rankin Lake, the basin where seagrass die-off was first observed in Florida Bay during 1987, seagrass standing crop, canopy height, shoot density, and leaf area declined significantly between decades, but species richness of both crustaceans and fishes increased. The abundance of canopy-dwelling crustaceans and fishes declined markedly at this site, whereas the abundance of benthic forms less dependent on seagrass cover generally increased. In retrospect, we believe the fauma at this site during the 1980s, characterized by high productivity but few species, was already showing signs of the stresses that led to the seagrass die-off that began in 1987. 相似文献
82.
Lack of filtration and rapid transport of groundwater and particulate matter make karst aquifers susceptible to bacterial contamination. This study utilized quantitative polymerase chain reaction (qPCR) to examine the transport and attenuation of two nonvirulent isolates of Escherichia coli (E. coli) in relation to traditional groundwater tracers (rhodamine WT dye and 1-µm diameter latex microspheres) in a karst-conduit aquifer in central Kentucky. Bacterial isolates were labeled with stable isotopes (15N and 13C). All tracers were detected more than 6 km downstream from the injection site and demonstrated overlapping breakthrough curves, with differential transport observed between the two bacterial strains. The E. coli isolate containing the kps gene (low attachment) arrived at sampling sites 1.25 to 36 h prior to the bacterial isolate containing the iha gene (high attachment) and was detected in samples collected following storm events in which the iha isolate was not detected. The storage potential of contaminants within karst systems was demonstrated by the remobilization of all tracers during storm events more than 1 month after injection. Bacteria-sized microspheres were more easily remobilized during periods of increased discharge compared to other tracers. The study demonstrated that molecular biology techniques such as qPCR can be utilized as a sensitive analysis of bacterial tracers in karst aquifers and may prove to be a more sensitive analytical technique than stable isotope analysis for field-scale traces. 相似文献
83.
Tanya V. Kizovski Kimberly T. Tait Veronica E. Di Cecco Lee F. White Desmond E. Moser 《Meteoritics & planetary science》2019,54(4):768-784
Northwest Africa (NWA) 6342 is an intermediate, poikilitic shergottite, found in Algeria in 2010. It is comprised of two distinct petrographic areas; poikilitic domains with rounded Mg‐rich olivine chadacrysts enclosed by large low‐Ca pyroxene oikocrysts, and a nonpoikilitic domain mainly comprised of subhedral olivine and vesicular recrystallized plagioclase. Oxygen fugacity conditions become more oxidizing during crystallization from the poikilitic to the nonpoikilitic domain (QFM?3.0 to QFM?2.2). As such, it is likely that NWA 6342 experienced a two‐stage (polybaric) crystallization history similar to that of the enriched poikilitic shergottites. NWA 6342 also experienced relatively high levels of shock metamorphism in comparison to most other poikilitic shergottites as evidenced by the fine‐grained recrystallization texture in olivine, as well as melting and subsequent crystallization of plagioclase. The recrystallization of plagioclase requires an extended period of postshock thermal metamorphism for NWA 6342 and similarly shocked intermediate poikilitic shergottites NWA 4797 and Grove Mountains 99027 most likely due to launch from Mars. The similarities in petrology, chemistry, and shock features between these three meteorites indicate that they have similar crystallization and shock histories; possibly originating from the same source area on Mars. 相似文献
84.
Improving the GNSS positioning stochastic model in the presence of ionospheric scintillation 总被引:1,自引:0,他引:1
M. Aquino J. F. G. Monico A. H. Dodson H. Marques G. De Franceschi L. Alfonsi V. Romano M. Andreotti 《Journal of Geodesy》2009,83(10):953-966
Ionospheric scintillations are caused by time- varying electron density irregularities in the ionosphere, occurring more often
at equatorial and high latitudes. This paper focuses exclusively on experiments undertaken in Europe, at geographic latitudes
between ~50°N and ~80°N, where a network of GPS receivers capable of monitoring Total Electron Content and ionospheric scintillation parameters was
deployed. The widely used ionospheric scintillation indices S4 and sj{\sigma_{\varphi}} represent a practical measure of the intensity of amplitude and phase scintillation affecting GNSS receivers. However, they
do not provide sufficient information regarding the actual tracking errors that degrade GNSS receiver performance. Suitable
receiver tracking models, sensitive to ionospheric scintillation, allow the computation of the variance of the output error
of the receiver PLL (Phase Locked Loop) and DLL (Delay Locked Loop), which expresses the quality of the range measurements
used by the receiver to calculate user position. The ability of such models of incorporating phase and amplitude scintillation
effects into the variance of these tracking errors underpins our proposed method of applying relative weights to measurements
from different satellites. That gives the least squares stochastic model used for position computation a more realistic representation,
vis-a-vis the otherwise ‘equal weights’ model. For pseudorange processing, relative weights were com- puted, so that a ‘scintillation-mitigated’
solution could be performed and compared to the (non-mitigated) ‘equal weights’ solution. An improvement between 17 and 38%
in height accuracy was achieved when an epoch by epoch differential solution was computed over baselines ranging from 1 to
750 km. The method was then compared with alternative approaches that can be used to improve the least squares stochastic
model such as weighting according to satellite elevation angle and by the inverse of the square of the standard deviation
of the code/carrier divergence (sigma CCDiv). The influence of multipath effects on the proposed mitigation approach is also
discussed. With the use of high rate scintillation data in addition to the scintillation indices a carrier phase based mitigated
solution was also implemented and compared with the conventional solution. During a period of occurrence of high phase scintillation
it was observed that problems related to ambiguity resolution can be reduced by the use of the proposed mitigated solution. 相似文献
85.
GPS Solutions - Strong ionospheric electron content gradients may lead to fast and unpredictable fluctuations in the phase and amplitude of the signals from Global Navigation Satellite Systems... 相似文献
86.
Julia L. Barringer Pamela A. Reilly Dennis D. Eberl Alex E. Blum Jennifer L. Bonin Robert Rosman Barbara Hirst Marzooq Alebus Kimberly Cenno Miroslawa Gorska 《Applied Geochemistry》2011,26(5):763-776
Glauconite-bearing deposits are found worldwide, but As levels have been determined for relatively few. The As content of glauconites in sediments of the Inner Coastal Plain of New Jersey can exceed 100 mg/kg, and total As concentrations (up to 5.95 μg/L) found historically and recently in streamwaters exceed the State standard. In a major watershed of the Inner Coastal Plain, chemical “fingerprints” were developed for streambed sediments and groundwater to identify contributions of As to the watershed from geologic and anthropogenic sources. The fingerprint for streambed sediments, which included Be, Cr, Fe and V, indicated that As was predominantly of geologic origin. High concentrations of dissolved organic C, nutrients (and Cl−) in shallow groundwater indicated anthropogenic inputs that provided an environment where microbial activity released As from minerals to groundwater discharging to the stream. Particulates in streamwater during high flow constituted most of the As load; the chemical patterns for these particulates resembled the geologic fingerprint of the streambed sediments. The As/Cr ratio of these suspended particles likely indicates they derived not only from runoff, but from groundwater inputs, because As contributed by groundwater is sequestered on streambed sediments. Agricultural inputs of As were not clearly identified, although chemical characteristics of some sediments indicated vehicle-related inputs of metals. Sediment sampling during dry and wet years showed that, under differing hydrologic conditions, local anthropogenic fingerprints could be obscured but the geologic fingerprint, indicating glauconitic sediments as an As source, was robust. 相似文献
87.
88.
Natalie A. Kruse Kimberly Brewster Brian Blair Kaabe Shaw 《Environmental Earth Sciences》2013,68(5):1419-1428
Middleton Run, a severely acid mine drainage (AMD) impacted tributary of Little Raccoon Creek, drains a sub-watershed area of 3.67 km2 (2.28 square miles). Averaging 58.7 kg/day (129.4 lbs/day) at its mouth, demonstrated aluminum loads from Middleton Run are particularly severe. A preliminary study of two previously unmonitored tributaries was conducted to justify future treatment projects. Monthly chemical water quality data were collected for 6 months. Soil leachate samples collected on five strip mining sites within the sub-watershed were analyzed for acidity, alkalinity, pH, aluminum and iron. Soil leachate tests have shown that one of the pits has a much larger pollution production potential and should be targeted for remediation. 相似文献
89.
GPS Solutions - Observations of moderate night time amplitude scintillation on the GPS L1C/A signal were recorded at the midlatitude station of Nicosia, corresponding geographic latitude and... 相似文献
90.
Vargas Marcelle Martins Beskow Samuel de Mello Carlos Rogério de Moura Maíra Martim Nunes Maria Cândida Moitinho Faria Lessandro Coll Aquino Leandro Sanzi 《Natural Hazards》2021,109(1):593-614
Natural Hazards - Process-based hydrological models are of great importance to understand hydrological processes and support decision making. The LImburg Soil Erosion Model (LISEM) requires... 相似文献