全文获取类型
收费全文 | 4369篇 |
免费 | 179篇 |
国内免费 | 93篇 |
专业分类
测绘学 | 158篇 |
大气科学 | 309篇 |
地球物理 | 1145篇 |
地质学 | 1529篇 |
海洋学 | 389篇 |
天文学 | 716篇 |
综合类 | 21篇 |
自然地理 | 374篇 |
出版年
2022年 | 30篇 |
2021年 | 48篇 |
2020年 | 48篇 |
2019年 | 64篇 |
2018年 | 86篇 |
2017年 | 105篇 |
2016年 | 122篇 |
2015年 | 111篇 |
2014年 | 136篇 |
2013年 | 264篇 |
2012年 | 132篇 |
2011年 | 232篇 |
2010年 | 168篇 |
2009年 | 234篇 |
2008年 | 245篇 |
2007年 | 218篇 |
2006年 | 197篇 |
2005年 | 174篇 |
2004年 | 139篇 |
2003年 | 166篇 |
2002年 | 139篇 |
2001年 | 86篇 |
2000年 | 91篇 |
1999年 | 63篇 |
1998年 | 71篇 |
1997年 | 59篇 |
1996年 | 66篇 |
1995年 | 55篇 |
1994年 | 55篇 |
1993年 | 43篇 |
1992年 | 50篇 |
1991年 | 43篇 |
1990年 | 35篇 |
1989年 | 37篇 |
1988年 | 37篇 |
1987年 | 41篇 |
1986年 | 47篇 |
1985年 | 60篇 |
1984年 | 48篇 |
1983年 | 51篇 |
1982年 | 53篇 |
1981年 | 59篇 |
1980年 | 48篇 |
1979年 | 39篇 |
1978年 | 50篇 |
1977年 | 28篇 |
1976年 | 30篇 |
1975年 | 25篇 |
1974年 | 31篇 |
1973年 | 29篇 |
排序方式: 共有4641条查询结果,搜索用时 15 毫秒
801.
Sudip Dey Sushmita Paul Chandrani Debbarma Prasamita Sarkar 《Journal of the Geological Society of India》2011,77(4):367-376
The present work concentrates on microstructure imaging for visualising the changes of depositional environment during Tertiary
period. For that purpose Tipam layers of Gajalia fold area in southern Tripura was selected which was deposited during late
Tertiary period under marine-coastal environment. Thin sections of eleven selected samples of the study area were prepared
in the laboratory for microstructure analysis. For visualising the crystalline particles or quartz brightness and contrast
of the image was increased up to maximum level. Surface conditions of the samples were analysed within RGB combination. The
grain size and shapes of the eleven samples tested prove that the depositional environment remained very dynamic through Tertiary
period in the study area. The microstructure and surface conditions also strongly support this view. 相似文献
802.
Paul D. Taylor Consuelo Sendino 《Proceedings of the Geologists' Association. Geologists' Association》2011,122(3):492-500
When first described in 1922, Dinocochlea ingens, an Early Cretaceous (Valanginian) fossil from the Wadhurst Clay Formation (Wealden) of Hastings in Sussex, England, was regarded as the steinkern of a huge gastropod over 2 m in length and claimed to be the largest gastropod ever to have lived. Most subsequent researchers have doubted the identity of D. ingens as a gastropod, usually believing it to be a pseudofossil, but no convincing explanation has been proposed for its origin. Restudy of the type material leads to the new hypothesis that Dinocochlea originated as a horizontal, corkscrew-shaped burrow resembling Helicodromites that acted as a nucleus for concretion growth. Sectioned specimens of Dinocochlea are filled with concentrically banded sediment interpreted to be growth rings of a concretion. Modification of a computer simulation designed to illustrate gastropod shell growth shows how the development of a concretion around a corkscrew-shaped burrow could lead to the morphology seen in Dinocochlea. The most likely tracemaker for Dinocochlea was a capitellid polychaete (‘threadworm’) with a body just a few mm in diameter. 相似文献
803.
Gemma M. Byrne Richard H. Worden David M. Hodgson David A. Polya Paul R. Lythgoe Craig D. Barrie Adrian J. Boyce 《Applied Geochemistry》2011
Fluvial dissolved Fe concentrations decrease upon mixing with seawater, resulting in the formation of Fe-floccules. However, a clear understanding of the fate of these floccules has yet to be established. Assessing how tidal processes affect the formation of Fe-colloids in the Leirárvogur estuary, SW Iceland, is an important step in understanding the formation and potential deposition of estuarine Fe-rich minerals within this estuarine system. The Leirárvogur estuary drains predominately Fe-rich basalt, increasing the likelihood of detecting changes in Fe-phases. Fluvial waters and local lake waters that drain into the estuary were compared and the effects of seasonal changes were considered, in an attempt to understand how varying end-members and external factors play a role in Fe-rich mineral formation. Aqueous and colloidal Fe concentrations were found to be greater towards the head of the Leirárvogur estuary, suggesting that potential Fe-rich minerals and complexes are forming at sites of fluvial input. Increasing suspended colloidal Fe towards the estuary mouth suggests that Fe-colloids are readily transported seaward. 相似文献
804.
Gayle Hough Jennifer Green Paul Fish Andy Mills Roger Moore 《Marine Geophysical Researches》2011,32(1-2):151-162
Exploration and development of offshore hydrocarbon resources has advanced into remote deepwater regions over the last decade and poses significant technical challenges for the design and installation of wells and facilities at extreme water depths. Seafloor and shallow subsurface processes and conditions in these areas are complex and generally poorly understood, and the geohazards to development are larger scale and fundamentally different to those encountered onshore; consequently the geohazard risk to deepwater development projects is potentially significant and requires careful evaluation and mitigation during the front-end planning and engineering design stages of projects. There are no established industry standards or methods for the assessment of geohazards and engineering-quality geophysical data at the scale of development. The paper describes an integrated and systematic map-based approach for the assessment and mitigation of seabed geohazards and risk to proposed deepwater development. The approach employs a multi-disciplinary team working with engineering-quality field calibrated data to accurately map and assess seafloor ground conditions and ensure that development proposals are not exposed to intolerable geohazard risk. The approach taken is very similar to the practice of establishing geological models for land-based engineering projects, in which the complete geological history of the site is used to characterise and predict the performance of the ground. Such an approach is routine for major projects on land but so far does not seem to be common practice in the offshore industry. The paper illustrates the seafloor geomophological mapping approach developed. The products are being used to optimise development layouts to avoid geohazards where possible and to support site-specific engineering design of facilities based on a detailed understanding of the potential geohazard loadings and associated risk. 相似文献
805.
Paul Thorn 《Hydrogeology Journal》2011,19(2):445-461
Elevated salinity in groundwater (over 250?mg/l chloride) in a fractured chalk aquifer in the municipality of Greve, Denmark, has been attributed to seawater intrusion from the Baltic Sea, resulting in the closure of a number of wells nearest to the coastline. However, a recent study in eastern Denmark shows that the salinity could also be from ancient connate pore waters. Historical chemistry data from bulk-water samples collected from wells have been used to determine the source. The sample data were studied with respect to their historical changes and temporal trends in chloride, carbonate, sulfate, sodium, potassium, magnesium, and calcium. Wells in the southern third of the municipality are relatively low in sodium and carbonate and high in chloride and calcium/magnesium, indicating waters undergoing saline intrusion. Wells in the northern two-thirds of the municipality are high in carbonate and sodium, but low in chloride and calcium/magnesium, indicating a freshening of the aquifer. This is confirmed by the temporal trends in chloride, where chloride levels in the northern wells remain constant, whereas in the south, chloride increases as abstraction continues. Therefore close monitoring for seawater intrusion should be conducted in the southern third of the municipality. 相似文献
806.
Matrix-diffusion parameters deduced from an infiltration tracer test at Idaho National Laboratory (INL), USA, are combined with other site information in an analysis involving two dimensionless lumped parameters to assess the effects of matrix diffusion on contaminant transport at the INL over longer distance and time scales than were evaluated in the test. Matrix diffusion was interrogated in the test by comparing, in three different observation wells, the breakthrough curves of two simultaneously injected nonsorbing solutes that have different diffusion coefficients. The matrix-diffusion parameters deduced from the different breakthrough curves were in good agreement, suggesting that the parameters may be broadly applicable at the INL. With this in mind, the uncertainties in the individual parameters that make up the two lumped parameters were estimated, and the resulting ranges of parameter values were used to assess matrix diffusion over larger scales. Assessments of the effects of flow transients, spatial heterogeneity in transport parameters, and sorption on solute transport in the shallow subsurface flow system were also conducted. The methods presented here should be generally applicable to other settings for making bounding assessments of the effects of matrix diffusion while honoring the information obtained from tracer tests and other supporting data. 相似文献
807.
Syam Sundar De Goutami Chattopadhyay Bijoy Bandyopadhyay Suman Paul 《Comptes Rendus Geoscience》2011,343(10):664-676
The association between the monthly total ozone concentration and monthly maximum temperature over Kolkata (22.56° N, 88.30° E), India, has been explored in this paper. For this, the predictability of monthly maximum temperature based on the total ozone as predictor is investigated using Artificial Neural Network. The presence of persistence and similar cyclic patterns are revealed through autocorrelation and cross-correlation coefficients. Common cycles of length 12 and 6 have been identified through periodogram. Hence, a predictive model has been generated by Artificial Neural Network in the form of Multi Layer Perceptron (MLP) using scaled conjugate gradient learning with sigmoid non-linearity. After training and testing the network, an MLP with total ozone of month n as predictor and maximum temperature of month (n + 1) as the target output is found as the best model. Performance of the model has been judged statistically. Finally, the MLP model has been compared with linear and non-linear regressions and the efficiency of MLP has been established over the regression models. 相似文献
808.
Ice-core samples from Upper Fremont Glacier (UFG), Wyoming, were used as proxy records for the chemical composition of atmospheric
deposition. Results of analysis of the ice-core samples for stable isotopes of nitrogen (δ15N, ) and sulfur (δ34S, ), as well as and deposition rates from the late-1940s thru the early-1990s, were used to enhance and extend existing National Atmospheric
Deposition Program/National Trends Network (NADP/NTN) data in western Wyoming. The most enriched δ34S value in the UFG ice-core samples coincided with snow deposited during the 1980 eruption of Mt. St. Helens, Washington.
The remaining δ34S values were similar to the isotopic composition of coal from southern Wyoming. The δ15N values in ice-core samples representing a similar period of snow deposition were negative, ranging from -5.9 to -3.2 ‰ and
all fall within the δ15N values expected from vehicle emissions. Ice-core nitrate and sulfate deposition data reflect the sharply increasing U.S.
emissions data from 1950 to the mid-1970s. 相似文献
809.
810.
Patrick Drupp Eric Heinen De Carlo Fred T. Mackenzie Paul Bienfang Christopher L. Sabine 《Aquatic Geochemistry》2011,17(4-5):473-498
The marine shelf areas in subtropical and tropical regions represent only 35% of the total shelf areas globally, but receive a disproportionately large amount of water (65%) and sediment (58%) discharges that enter such environments. Small rivers and/or streams that drain the mountainous areas in these climatic zones deliver the majority of the sediment and nutrient inputs to these narrow shelf environments; such inputs often occur as discrete, episodic introductions associated with storm events. To gain insight into the linked biogeochemical behavior of subtropical/tropical mountainous watershed-coastal ocean ecosystems, this work describes the use of a buoy system to monitor autonomously water quality responses to land-derived nutrient inputs and physical forcing associated with local storm events in the coastal ocean of southern Kaneohe Bay, Oahu, Hawaii, USA. The data represent 2.5 years of near-real time observations at a fixed station, collected concurrently with spatially distributed synoptic sampling over larger sections of Kaneohe Bay. Storm events cause most of the fluvial nutrient, particulate, and dissolved organic carbon inputs to Kaneohe Bay. Nutrient loadings from direct rainfall and/or terrestrial runoff produce an immediate increase in the N:P ratio of bay waters up to values of 48 and drive phytoplankton biomass growth. Rapid uptake of such nutrient subsidies by phytoplankton causes rapid declines of N levels, return to N-limited conditions, and subsequent decline of phytoplankton biomass over timescales ranging from a few days to several weeks, depending on conditions and proximity to the sources of runoff. The enhanced productivity may promote the drawing down of pCO2 and lowering of surface water column carbonate saturation states, and in some events, a temporary shift from N to P limitation. The productivity-driven CO2 drawdown may temporarily lead to air-to-sea transfer of atmospheric CO2 in a system that is on an annual basis a source of CO2 to the atmosphere due to calcification and perhaps heterotrophy. Storms may also strongly affect proximal coastal zone pCO2 and hence carbonate saturation state due to river runoff flushing out high pCO2 soil and ground waters. Mixing of the CO2-charged water with seawater causes a salting out effect that releases CO2 to the atmosphere. Many subtropical and tropical systems throughout the Pacific region are similar to Kaneohe Bay, and our work provides an important indication of the variability and range of CO2 dynamics that are likely to exist elsewhere. Such variability must be taken into account in any analysis of the direction and magnitude of the air?Csea CO2 exchange for the integrated coastal ocean, proximal and distal. It cannot be overemphasized that this research illustrates several examples of how high frequency sampling by a moored autonomous system can provide details about ecosystem responses to stochastic atmospheric forcing that are commonly missed by traditional synoptic observational approaches. Finally, the work exemplifies the utility of combining synoptic sampling and real-time autonomous observations to elucidate the biogeochemical and physical responses of coastal subtropical/tropical coral reef ecosystems to climatic perturbations. 相似文献