全文获取类型
收费全文 | 314篇 |
免费 | 29篇 |
国内免费 | 3篇 |
专业分类
测绘学 | 8篇 |
大气科学 | 25篇 |
地球物理 | 84篇 |
地质学 | 102篇 |
海洋学 | 35篇 |
天文学 | 31篇 |
综合类 | 4篇 |
自然地理 | 57篇 |
出版年
2023年 | 7篇 |
2022年 | 3篇 |
2021年 | 10篇 |
2020年 | 14篇 |
2019年 | 24篇 |
2018年 | 19篇 |
2017年 | 11篇 |
2016年 | 21篇 |
2015年 | 10篇 |
2014年 | 13篇 |
2013年 | 18篇 |
2012年 | 16篇 |
2011年 | 22篇 |
2010年 | 16篇 |
2009年 | 18篇 |
2008年 | 23篇 |
2007年 | 18篇 |
2006年 | 11篇 |
2005年 | 9篇 |
2004年 | 9篇 |
2003年 | 10篇 |
2002年 | 13篇 |
2001年 | 4篇 |
2000年 | 2篇 |
1999年 | 3篇 |
1998年 | 5篇 |
1997年 | 3篇 |
1996年 | 2篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1985年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
排序方式: 共有346条查询结果,搜索用时 15 毫秒
111.
Rebecca A. Hodge 《ISPRS Journal of Photogrammetry and Remote Sensing》2010,65(2):227-240
Terrestrial Laser Scanning (TLS) is increasingly being used to collect mm-resolution surface data from a broad range of environments. When scanning complex surfaces, interactions between the surface topography, laser footprint and scanner precision can introduce errors into the point cloud. Quantification of these errors is, however, limited by the availability of independent measurement techniques. This research presents simulated TLS as a new approach to error quantification. Two sets of experiments are presented. The first set demonstrates that simulated TLS is able to reproduce real TLS data from a plane and a pebble. The second set uses simulated TLS to assess a methodology developed for the collection and processing of field TLS data. Simulated TLS data is collected from surfaces up to ~1 m2 created from regular arrays of uniform spheres (sphere diameters of 10 to 100 mm) and irregular arrays of mixed spheres (median sphere diameters of 16 to 94 mm). These data were analysed to (i) assess the effectiveness of the processing methodology at removing erroneous points; (ii) quantify the magnitude of errors in a digital surface model (DSM) interpolated from the processed point cloud; and (iii) investigate the extent to which the interpolated DSMs retained the geometric properties of the original surfaces. The processing methodology was found to be effective, especially on data from coarser surfaces, with the retained points typically having an inter-quartile range (IQR) of point errors of ~2 mm. DSM errors varied as a function of sphere size and packing, with DSM errors having an IQR of ~2 mm for the regular surfaces and ~4 mm for the irregular surfaces. Finally, whilst in the finer surfaces point and DSM errors were a substantial proportion of the sphere diameters, geometrical analysis indicated that the DSMs still reproduced properties of the original surface such as semivariance and some percentiles of the surface elevation distribution. 相似文献
112.
Marine protected areas (MPAs) are among the most widely accepted methods of marine management. MPAs are not, however, always placed such that they can maximize impact on conservation and livelihoods. Current MPA guidelines fall short in focusing primarily on biophysical criteria, overlooking interrelated socioeconomic factors. We identified 32 socioeconomic factors that influence whether MPA placement has an impact on biodiversity and/or livelihoods and weighted the quality of evidence using a novel “Evidence for Impact” Score. Results suggest that stakeholder engagement, poverty, population density, and strong leadership have most potential to positively impact biodiversity and/or livelihoods, but the direction of impact (i.e., positive or negative) can be context-dependent. We found a generally poor evidence base for impact evaluation of socioeconomic factors: though some factors were highly cited, few studies actually evaluate impact. Results indicate the need for a more interdisciplinary approach to MPA placement and more empirical studies that assess impact. 相似文献
113.
Structural and Hydrogeological Controls on Hydrocarbon and Brine Migration into Drinking Water Aquifers in Southern New York 下载免费PDF全文
Rebecca L. Kreuzer Thomas H. Darrah Benjamin S. Grove Myles T. Moore Nathaniel R. Warner William K. Eymold Colin J. Whyte Gautam Mitra Robert B. Jackson Avner Vengosh Robert J. Poreda 《Ground water》2018,56(2):225-244
Environmental concerns regarding the potential for drinking water contamination in shallow aquifers have accompanied unconventional energy development in the northern Appalachian Basin. These activities have also raised several critical questions about the hydrogeological parameters that control the naturally occurring presence and migration of hydrocarbon gases in shallow aquifers within petroliferous basins. To interrogate these factors, we analyzed the noble gas, dissolved ion, and hydrocarbon gas (molecular and isotopic composition) geochemistry of 98 groundwater samples from south‐central New York. All samples were collected ?1km from unconventional drilling activities and sample locations were intentionally targeted based on their proximity to various types of documented fault systems. In agreement with studies from other petroliferous basins, our results show significant correlations between elevated levels of radiogenic [4He], thermogenic [CH4], and dissolved ions (e.g., Cl, Br, Sr, Ba). In combination, our data suggest that faults have facilitated the transport of exogenous hydrocarbon‐rich brines from Devonian source rocks into overlying Upper Devonian aquifer lithologies over geologic time. These data conflict with previous reports, which conclude that hydrodynamic focusing regulates the occurrence of methane and salt in shallow aquifers and leads to elevated levels of these species in restricted flow zones within valley bottoms. Instead, our data suggest that faults in Paleozoic rocks play a fundamental role in gas and brine transport from depth, regulate the distribution of their occurrence in shallow aquifers, and influence the geochemistry of shallow groundwater in this petroliferous basin. 相似文献
114.
Matthew J. Westoby David R. Rounce Thomas E. Shaw Catriona L. Fyffe Peter L. Moore Rebecca L. Stewart Benjamin W. Brock 《地球表面变化过程与地形》2020,45(14):3431-3448
There exists a need to advance our understanding of debris-covered glacier surfaces over relatively short timescales due to rapid, climatically induced areal expansion of debris cover at the global scale, and the impact debris has on mass balance. We applied unpiloted aerial vehicle structure-from-motion (UAV-SfM) and digital elevation model (DEM) differencing with debris thickness and debris stability modelling to unravel the evolution of a 0.15 km2 region of the debris-covered Miage Glacier, Italy, between June 2015 and July 2018. DEM differencing revealed widespread surface lowering (mean 4.1 ± 1.0 m a-1; maximum 13.3 m a-1). We combined elevation change data with local meteorological data and a sub-debris melt model, and used these relationships to produce high resolution, spatially distributed maps of debris thickness. These maps were differenced to explore patterns and mechanisms of debris redistribution. Median debris thicknesses ranged from 0.12 to 0.17 m and were spatially variable. We observed localized debris thinning across ice cliff faces, except those which were decaying, where debris thickened. We observed pervasive debris thinning across larger, backwasting slopes, including those bordered by supraglacial streams, as well as ingestion of debris by a newly exposed englacial conduit. Debris stability mapping showed that 18.2–26.4% of the survey area was theoretically subject to debris remobilization. By linking changes in stability to changes in debris thickness, we observed that slopes that remain stable, stabilize, or remain unstable between periods almost exclusively show net debris thickening (mean 0.07 m a-1) whilst those which become newly unstable exhibit both debris thinning and thickening. We observe a systematic downslope increase in the rate at which debris cover thickens which can be described as a function of the topographic position index and slope gradient. Our data provide quantifiable insights into mechanisms of debris remobilization on glacier surfaces over sub-decadal timescales, and open avenues for future research to explore glacier-scale spatiotemporal patterns of debris remobilization. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd 相似文献
115.
A model study is conducted to examine the role of Pacific water in the dramatic retreat of arctic sea ice during summer 2007. The model generally agrees with the observations in showing considerable seasonal and interannual variability of the Pacific water inflow at Bering Strait in response to changes in atmospheric circulation. During summer 2007 anomalously strong southerly winds over the PaCific sector of the Arctic Ocean strengthen the ocean circulation and bring more Pacific water into the Arctic than the recent (2000-2006) average. The simulated summer (3 months ) 2007 mean Pacific water inflow at Bering Strait is 1.2 Sv, which is the highest in the past three decades of the simulation and is 20% higher than the recent average. Particularly, the Pacific water inflow in September 2007 is about 0.5 Sv or 50% above the 2000-2006 average. The strengthened warm Pacific water inflow carries an additional 1.0 x 1020 Joules of heat into the Arctic, enough to melt an additional 0.5 m of ice over the whole Chukchi Sea. In the model the extra summer oceanic heat brought in by the Pacific water mainly stays in the Chukchi and Beaufort region, contributing to the warming of surface waters in that region. The heat is in constant contact with the ice cover in the region in July through September. Thus the Pacific water plays a role in ice melting in the Chukchi and Beaufort region all summer long in 2007, likely contributing to up to O. 5 m per month additional ice melting in some area of that region. 相似文献
116.
As part of a long-term study of Saturn's rings, we have used the Hubble Space Telescope's (HST) Wide Field and Planetary Camera (WFPC2) to obtain several hundred high resolution images from 1996 to 2004, spanning the full range of ring tilt and solar phase angles accessible from the Earth. Using these multiwavelength observations and HST archival data, we have measured the photometric properties of spokes in the B ring, visible in a substantial number of images. We determined the spoke particle size distribution by fitting the wavelength-dependent extinction efficiency of a prominent, isolated spoke, using a Mie scattering model. Following Doyle and Grün (1990, Icarus 85, 168-190), we assumed that the spoke particles were sub-micron size spheres of pure water ice, with a Hansen-Hovenier size distribution (Hansen and Hovenier, 1974, J. Atmos. Sci. 31, 1137-1160). The WFPC2 wavelength coverage is broader than that of the Voyager data, resulting in tighter constraints on the nature of spoke particles. The effective particle size was reff=0.57±0.05 μm, and the size distribution was quite narrow with a variance of b=0.09±0.03, very similar to the results of Doyle and Grün (1990, Icarus 85, 168-190), and consistent with predictions of plasma cloud models for spoke production from meteoritic impacts (Goertz and Morfill, 1983, Icarus 53, 219-229; Goertz, 1984, Adv. Space Res. 4, 137-141). In all, we identified 36 spokes or spoke complexes, predominantly on the morning (east) ansa. The photometric contrast of the spokes is strongly dependent on effective ring opening angle, Beff. Spokes were clearly visible on the north face of the rings in 1994, just prior to the most recent ring plane crossing (RPX) epoch, and on the south face shortly after RPX. However, spokes were both less abundant and fainter as the rings opened up, and no spokes were detected after 18 October 1998 (Beff=−15.43°), when a single faint spoke was seen on the morning ansa. The high resolution and photometric quality of the WFPC2 images enabled us to set a detection limit of ?1% in fractional brightness contrast for spokes for the post-1998 observations. We compare the observed trend of spoke contrast with Beff to radiative transfer calculations based on three models of the distribution of spoke material. In the first, the spoke “haze” is uniformly mixed with macroscopic B ring particles. No variation in spoke contrast is predicted for single-scattering, in this case, and only a modest decrease in contrast with Beff is predicted when multiple scattering is taken into account. In the second model, the spoke dust occupies an extended layer that is thicker than the B ring, which gives virtually identical results to a third case, when the haze layer lies exclusively above the ring. Multiple-scattering Monte Carlo calculations for these two extended haze models match the trend of spoke contrast exceptionally well. We compute the predicted spoke contrast for a wide variety of viewing geometries, including forward- and backscattering. Based on these results, spokes should be easily detectable during the Cassini mission when the rings are viewed at relatively small (|B|?10°) ring opening angles. 相似文献
117.
118.
The sandy beachface at Cape Henlopen, Delaware, receives dissolved nutrient inputs from fresh upland groundwater and estuarine
seawater and exports nutrients through intertidal and submarine groundwater discharge. The discharge of brackish beachface
waters indicates that there must be additional diagenetic nutrient sources and sinks within the porous beachface aquifer.
At some times of the year, diagenetic processes within the beachface remove nitrate from waters draining the beachface consistent
with the stoichiometry of either denitrification or dissimilatory nitrate reduction to ammonium. Up to 50–100% of the nitrate
load from the upland groundwaters is apparently reduced to N2O/N2 gas or ammonium during these periods. At other times, ammonium and nitrate are added to beachface waters consistent with
the stoichiometry of organic matter remineralization and nitrification. Dissolved reactive phosphorus and silica are similarly
consumed and produced by beachface processes at different times of the year, presumably by adsorption or desorption. Infiltration
of reactive estuarine particles may be an additional source of nutrients and carbon that supports the diagenetic activity
in the beachface aquifer. These observations suggest that sandy beachfaces are biogeochemically reactive systems that can
serve as sources, sinks, and temporary reservoirs of nutrients to support the primary and secondary production of the adjacent
intertidal zone. 相似文献
119.
A review of water column processes influencing hypoxia in the northern Gulf of Mexico 总被引:1,自引:0,他引:1
Michael J. Dagg James W. Ammerman Rainer M. W. Amon Wayne S. Gardner Rebecca E. Green Steven E. Lohrenz 《Estuaries and Coasts》2007,30(5):735-752
In this review, we use data from field measurements of biogeochemical processes and cycles in the Mississippi River plume
and in other shelf regions of the northern Gulf of Mexico to determine plume contributions to coastal hypoxia. We briefly
review pertinent findings from these process studies, review recent mechanistic models that synthesize these processes to
address hypoxia-related issues, and reinterpret current understanding in the context of these mechanistic models. Some of
our conclusions are that both nitrogen and phosphorus are sometimes limiting to phytoplankton growth; respiration is the main
fate of fixed carbon in the plume, implying that recycling is the main fate of nitrogen; decreasing the river nitrate loading
results in less than a 1:1 decrease in organic matter sinking from the plume; and sedimenting organic matter from the Mississippi
River plume can only fuel about 23% of observed coastal hypoxia, suggesting significant contributions from the Atchafalaya
River and, possibly, coastal wetlands. We also identify gaps in our knowledge about controls on hypoxia, and indicate that
some reinterpretation of our basic assumptions about this system is required. There are clear needs for improved information
on the sources, rates, and locations of organic matter sedimentation; for further investigation of internal biogeochemical
processes and cycling; for improved understanding of the rates of oxygen diffusion across the pycnocline; for identification
and quantification of other sources of organic matter fueling hypoxia or other mechanisms by which Mississippi River derived
organic matter fuels hypoxia; and for the development of a fully coupled physical-biogeochemical model. 相似文献
120.
Rebecca E. Eager Sethu Raman Teddy R. Holt Douglas Westphal Jeffrey Reid Jason Nachamkin Ming Liu Abdulla Al Mandoos 《Pure and Applied Geophysics》2007,164(8-9):1747-1764
A statistical evaluation of the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®) was performed over the Arabian Gulf region for the period, 1 August to 5 October, 2004. Verification skill scores of bias and root-mean-square error were estimated for surface variables and for vertical profiles to investigate any diurnal variations. The model predictions of boundary-layer heights are compared with the observations at Abu Dhabi, United Arab Emirates. The Middle East presents challenges to numerical weather prediction due to complex land-ocean-land mesoscale processes. An independent data set of surface measurements from 50 stations in the UAE was available from the Department of Water Resources Studies, Abu Dhabi for model verification. The results indicate a diurnal variation in the model errors. The errors are small considering the magnitudes of the observed variables. Errors in the coastal region can be attributed to the differences in the timing of the onset of sea and land breeze circulations in the simulations as compared to the observations. Errors are relatively smaller in the offshore locations. 相似文献