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491.
Subsurface fluid flow and solute transport take place in a multiscale heterogeneous environment. Neither these phenomena nor their host environment can be observed or described with certainty at all scales and locations of relevance. The resulting ambiguity has led to alternative conceptualizations of flow and transport and multiple ways of addressing their scale and space–time dependencies. We focus our attention on four approaches that give rise to nonlocal representations of advective and dispersive transport of nonreactive tracers in randomly heterogeneous porous or fractured continua. We compare these approaches theoretically on the basis of their underlying premises and the mathematical forms of the corresponding nonlocal advective–dispersive terms. One of the four approaches describes transport at some reference support scale by a classical (Fickian) advection–dispersion equation (ADE) in which velocity is a spatially (and possibly temporally) correlated random field. The randomness of the velocity, which is given by Darcy’s law, stems from random fluctuations in hydraulic conductivity (and advective porosity though this is often disregarded). Averaging the stochastic ADE over an ensemble of velocity fields results in a space–time-nonlocal representation of mean advective–dispersive flux, an approach we designate as stnADE. A closely related space–time-nonlocal representation of ensemble mean transport is obtained upon averaging the motion of solute particles through a random velocity field within a Lagrangian framework, an approach we designate stnL. The concept of continuous time random walk (CTRW) yields a representation of advective–dispersive flux that is nonlocal in time but local in space. Closely related to the latter are forms of ADE entailing fractional derivatives (fADE) which leads to representations of advective–dispersive flux that are nonlocal in space but local in time; nonlocality in time arises in the context of multirate mass transfer models, which we exclude from consideration in this paper. We describe briefly each of these four nonlocal approaches and offer a perspective on their differences, commonalities, and relative merits as analytical and predictive tools. 相似文献
492.
A three-dimensional primitive-equation model is used to simulate the Long Island Sound (LIS) outflow for a 1-year (2001) period. The model domain includes LIS and New York Bight (NYB). Tidal and wind forcing are included, and seasonal salinity and temperature variations are assimilated. The model results are validated with the HF radar, moored acoustic Doppler current profiler (ADCP), and ferry-based ADCP observations. The agreement between simulated and observed flow patterns generally is very good. The difference in seasonal mean currents between the model and moored ADCP is about 0.01 m/s; the correlation of dominant velocity fluctuations between the model and HF radar is 0.83; and the difference in mean LIS transport between the model and shipboard ADCP is about 5%. However, the model predicts a prominent tidally generated headland eddy not supported by the HF radar observation. The model sensitivity study indicates that the tides, winds, and ambient coastal front all have important impact on the buoyant outflow. The tides and winds cause stronger vertical mixing, which reduces the surface plume strength. The ambient coastal front, on the other hand, tends to enhance the plume. 相似文献
493.
494.
河口沉积物在陆源砷向海洋输送过程中具有源和汇的双重作用,分析砷在河口沉积物中的存在形态和分布特征对于掌握砷的毒理效应和污染评价具有重要意义.通过野外调查和样品采集,对秦皇岛典型河流河口沉积物中砷的质量比进行了测定,并采用Tessier五步连续提取法对砷在沉积物中的存在形态进行了提取分析.结果表明,汤河河口表层沉积物中砷的质量比为35.73~46.63 mg/kg,戴河河口表层沉积物中砷的质量比为13.60~16.00 mg/kg,汤河河口沉积物砷含量明显高于戴河河口.总砷含量随沉积深度的增加而有降低的趋势.各形态砷在沉积物中的分布特征基本一致,即以残渣态为主,占其总量的87%以上,其他各形态相对含量较小,其中可利用性最强的可交换态含量最小,平均为1.04%.各形态砷分布与沉积物的pH和有机质含量有一定的相关关系, pH和有机质含量的改变将会影响砷的形态分布特征. 相似文献
495.
496.
497.
A. Chakrabarti 《Journal of Earth System Science》2005,114(3):353-368
Sedimentary structures of some coastal tropical tidal flats of the east coast of India, and inner estuarine tidal point bars
located at 30 to 50 kilometers inland from the coast, have been extensively studied under varying seasonal conditions. The
results reveal that physical features such as flaser bedding, herringbone cross-bedding, lenticular bedding, and mud/silt
couplets are common to both the environments. In fact, flaser bedding and lenticular bedding are more common in the point
bar facies during the monsoon months than in the coastal tidal flat environments. Interference ripples, though common in both
the environments, show different architectural patterns for different environmental domains. Interference ripples with thread-like
secondary set overriding the earlier ripple-form, resembling wrinkle marks, are the typical features in estuarine point bars
near the high water region. Because structures which are so far considered as key structures for near-coastal tidal flats
are common to both the environments, caution should be exercised for deciphering palaeo-environments, particularly for Proterozoic
rocks, where one has to depend only on physical sedimentary structures. 相似文献
498.
Hans Burchard Hans Ulrich Lass Volker Mohrholz Lars Umlauf Jürgen Sellschopp Volker Fiekas Karsten Bolding Lars Arneborg 《Ocean Dynamics》2005,55(5-6):391-402
In this study, the dynamics of medium-intensity inflow events over Drogden Sill into the Arkona Sea are investigated. Idealised
model simulations carried out with the General Estuarine Transport Model suggest that most of the salt transport during such
inflow events occur north of Kriegers Flak, a shoal with less than 20 m water depth surrounded by water depths of more than
40 m. This assumption about the pathway is supported by recent ship-based observations in the Arkona Sea during a medium-intensity
inflow event. The propagation of a saline bottom plume could be observed during several days after having passed Drogden Sill.
In the area north of Kriegers Flak the plume was about 10 m thick, and propagated with more than 0.5 m s−1 and a salinity of up to 20 psu (with ambient water salinity being 8 psu) eastwards. Although the model simulations were idealised,
the structural agreement between the observation and model result was good. The structure and pathways of these medium-intensity
inflow events are of specific interest due to the plans for erecting extensive offshore wind farms in the Arkona Sea which
may under certain circumstances lead to increased entrainment of ambient water into the bottom plumes. 相似文献
499.
根据磨刀门2003年12月9~15日的大、中潮同步水文观测资料,分析了磨刀门枯季的潮汐、潮流、余流、悬移质含沙量、盐度等水文特征,并对枯季河口动力过程,如咸淡水混合、河口射流等进行了初步研究。在枯季由于径流较弱,潮流成为主要动力。表层由于受径流和风的影响余流基本上沿河道走向向下游,中层以下有稳定的向上的余流存在。枯季磨刀门含沙量较小(<1 kg/m3),盐度在平面上和垂向上均有一定变化。磨刀门枯季咸淡水混合类型为缓混合型,各站盐度分层参数均在0.01~1.0。从实测流速的分布情况来看,河口下层有反向的水流,存在明显的因密度差而形成的密度环流。根据枯季实测资料计算所得的密度弗劳德数,磨刀门枯季以浮力射流为主。 相似文献
500.
Major part of the Holocene Ganges-Brahmaputra delta occupies the southern and southwestern part of Bangladesh with a smaller
part extending beyond the international boundary in the west. Five facies assemblages are documented in the lower deltaic
plain in five different depositional environments: levee or levee complex, bil or depression, abandoned meander belt, interdistributary
plain and estuarine plain. The thickness of the Holocene sediments ranges from 30 m to 70 m in the deltaic plain, usually
floored by the Pleistocene stiff clays, with the exception of the abandoned meander belt deposit where Holocene channel sand
deposited directly on the Pleistocene sand. Radiocarbon dates indicate that low-rate sedimentation has occurred in the northern
part, where 4-6 m thick sediments were deposited since the mid-Holocene, whereas 10-30 mthick sediments were deposited in
the southern part during the same span of time. In addition, significant coastal subsidence (3 mm/a on average), added by
sea-level rise (1.5 mm/a, conservative rate) occurs in the study area, which serves as a negative factor in degrading the
coastal plain of Bangladesh in the future, while taking into consideration the weaker sedimentation in the area. 相似文献