全文获取类型
收费全文 | 90215篇 |
免费 | 1523篇 |
国内免费 | 995篇 |
专业分类
测绘学 | 2289篇 |
大气科学 | 7122篇 |
地球物理 | 18658篇 |
地质学 | 29782篇 |
海洋学 | 7975篇 |
天文学 | 20278篇 |
综合类 | 239篇 |
自然地理 | 6390篇 |
出版年
2020年 | 635篇 |
2019年 | 669篇 |
2018年 | 1348篇 |
2017年 | 1314篇 |
2016年 | 1969篇 |
2015年 | 1423篇 |
2014年 | 1966篇 |
2013年 | 4497篇 |
2012年 | 2166篇 |
2011年 | 3188篇 |
2010年 | 2734篇 |
2009年 | 4000篇 |
2008年 | 3618篇 |
2007年 | 3321篇 |
2006年 | 3303篇 |
2005年 | 2931篇 |
2004年 | 2979篇 |
2003年 | 2784篇 |
2002年 | 2654篇 |
2001年 | 2380篇 |
2000年 | 2340篇 |
1999年 | 2077篇 |
1998年 | 1999篇 |
1997年 | 1983篇 |
1996年 | 1745篇 |
1995年 | 1594篇 |
1994年 | 1467篇 |
1993年 | 1346篇 |
1992年 | 1265篇 |
1991年 | 1143篇 |
1990年 | 1292篇 |
1989年 | 1136篇 |
1988年 | 1050篇 |
1987年 | 1230篇 |
1986年 | 1112篇 |
1985年 | 1377篇 |
1984年 | 1565篇 |
1983年 | 1506篇 |
1982年 | 1384篇 |
1981年 | 1294篇 |
1980年 | 1140篇 |
1979年 | 1088篇 |
1978年 | 1145篇 |
1977年 | 1024篇 |
1976年 | 973篇 |
1975年 | 928篇 |
1974年 | 943篇 |
1973年 | 952篇 |
1972年 | 590篇 |
1971年 | 504篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Seismic and infrasonic observations of signals from a sequence of near-surface explosions at a site on the Kola Peninsula have been analyzed. NORSAR’s automatic network processing of these events shows a significant scatter in the location estimates and, to improve the automatic classification of the events, we have performed full waveform cross-correlation on the data set. Although the signals from the different events share many characteristics, the waveforms do not exhibit a ripple-for-ripple correspondence and cross-correlation does not result in the classic delta-function indicative of repeating signals. Using recordings from the ARCES seismic array (250 km W of the events), we find that a correlation detector on a single channel or three-component station would not be able to detect subsequent events from this source without an unacceptable false alarm rate. However, performing the correlation on each channel of the full ARCES array, and stacking the resulting traces, generates a correlation detection statistic with a suppressed background level which is exceeded by many times its standard deviation on only very few occasions. Performing f-k analysis on the individual correlation coefficient traces, and rejecting detections indicating a non-zero slowness vector, results in a detection list with essentially no false alarms. Applying the algorithm to 8 years of continuous ARCES data identified over 350 events which we confidently assign to this sequence. The large event population provides additional confidence in relative travel-time estimates and this, together with the occurrence of many events between 2002 and 2004 when a temporary network was deployed in the region, reduces the variability in location estimates. The best seismic location estimate, incorporating phase information for many hundreds of events, is consistent with backazimuth measurements for infrasound arrivals at several stations at regional distances. At Lycksele, 800 km SW of the events, as well as at ARCES, infrasound is detected for most of the events in the summer and for few in the winter. At Apatity, some 230 km S of the estimated source location, infrasound is detected for most events. As a first step to providing a Ground Truth database for this useful source of infrasound, we provide the times of explosions for over 50 events spanning 1 year. 相似文献
992.
Roy C. Bartholomay Brian V. Twining Peter E. Rose 《Ground Water Monitoring & Remediation》2014,34(1):79-88
Starting in 2008, a 4‐year tracer study was conducted to evaluate ambient changes in groundwater concentrations of a 1,3,6‐naphthalene trisulfonate tracer that was added to drill water. Samples were collected under open borehole conditions and after installing a multilevel groundwater monitoring system completed with 11 discrete monitoring zones within dense and fractured basalt and sediment layers in the eastern Snake River aquifer. The study was done in cooperation with the U.S. Department of Energy to test whether ambient fracture flow conditions were sufficient to remove the effects of injected drill water prior to sample collection. Results from thief samples indicated that the tracer was present in minor concentrations 28 days after coring, but was not present 6 months after coring or 7 days after reaming the borehole. Results from sampling the multilevel monitoring system indicated that small concentrations of the tracer remained in 5 of 10 zones during some period after installation. All concentrations were several orders of magnitude lower than the initial concentrations in the drill water. The ports that had remnant concentrations of the tracer were either located near sediment layers or were located in dense basalt, which suggests limited groundwater flow near these ports. The ports completed in well‐fractured and vesicular basalt had no detectable concentrations. 相似文献
993.
The seasonal nature of Australia’s tropical rivers means that connected groundwater aquifers are an important source of both
consumptive and non-consumptive water, particularly during the dry season. The management of these common pool groundwater
resources is one of the predominant water issues facing northern Australia. A national program of water reform stipulates
the expansion of water trading as a key instrument for water allocation. The effectiveness of new institutional arrangements
such as water markets will be determined mostly by how well they coordinate with local environmental requirements, local institutions
and local norms. This paper describes a novel application of combined field work, institutional analysis, experimental economics
and agent-based modeling to the analysis of a potential water market in the Katherine region of the Northern Territory, Australia.
The effectiveness of different versions of the policy instrument is assessed in light of local conditions. Instruments that
enable personal relationships and local institutions and norms to play a role in water management are found to be more effective
in terms of both farming income and environmental impact. 相似文献
994.
Laura Minjeaud Valerie D. Michotey Nicole Garcia Patricia C. Bonin 《Aquatic Sciences - Research Across Boundaries》2009,71(4):425-435
Seasonal patterns of di-nitrogen fluxes together with denitrification, anammox, and N-fixation rates were studied in sediment in an area subject to strong human pressure via waste water, tributaries and shellfish farming in the Mediterranean Sea (Carteau Cove, NW area of the Gulf of Lion). Ammonium concentration demonstrated no seasonal variation, however, a large increase in its concentration was observed over a 10 years period due to intense biodeposition of organic matter. In contrast, nitrate concentration demonstrated no seasonal or long-term (10 years) variation. The main processes affecting di-nitrogen flux magnitudes were denitrification and N-fixation. Anammox was only detected occasionally, nevertheless it represented at times up to 39% of the N2-flux. Nitrate reducing processes were variable and denitrification showed a 20-fold increase over the past 10 years and might actually have reached its potential maximal activity. Rates of N2 production (denitrification and anammox) were generally higher than those of N-fixation, leading to elimination of nitrogen from the ecosystem. 相似文献
995.
996.
The aim of this paper is to point out the advantages of multipoles for the exploration of the very near subsurface (0–3 m) by continuous profiling. We propose a new geometry with eight poles for a MUltipole Continuous Electrical Profiling (MUCEP) measuring system, where the array has a V-shape and is thus called ‘Vol-de-canards’. A series of criteria including 3D numerical simulations are performed (direct and inverse modelling) to determine the optimal geometry and to compare its performance (in terms of depth of investigation and resolution of the geometry of the targets) with the other arrays (quadrupoles or rectangular-type multipoles). This multipole was built together with a real-time acquisition system. The multidepth maps obtained confirm the characteristics predicted by numerical simulations. 相似文献
997.
Michael O. Garcia Ken H. Rubin Marc D. Norman J. Michael Rhodes David W. Graham David W. Muenow Khalil Spencer 《Bulletin of Volcanology》1998,59(8):577-592
Samples of basalt were collected during the Rapid Response cruise to Loihi seamount from a breccia that was probably created
by the July to August 1996 Loihi earthquake swarm, the largest swarm ever recorded from a Hawaiian volcano. 210Po–210Pb dating of two fresh lava blocks from this breccia indicates that they were erupted during the first half of 1996, making
this the first documented historical eruption of Loihi. Sonobuoys deployed during the August 1996 cruise recorded popping
noises north of the breccia site, indicating that the eruption may have been continuing during the swarm. All of the breccia
lava fragments are tholeiitic, like the vast majority of Loihi's most recent lavas. Reverse zoning at the rim of clinopyroxene
phenocrysts, and the presence of two chemically distinct olivine phenocryst populations, indicate that the magma for the lavas
was mixed just prior to eruption. The trace element geochemistry of these lavas indicates there has been a reversal in Loihi's
temporal geochemical trend. Although the new Loihi lavas are similar isotopically and geochemically to recent Kilauea lavas
and the mantle conduits for these two volcanoes appear to converge at depth, distinct trace element ratios for their recent
lavas preclude common parental magmas for these two active volcanoes. The mineralogy of Loihi's recent tholeiitic lavas signify
that they crystallized at moderate depths (∼8–9 km) within the volcano, which is approximately 1 km below the hypocenters
for earthquakes from the 1996 swarm. Taken together, the petrological and seismic evidence indicates that Loihi's current
magma chamber is considerably deeper than the shallow magma chamber (∼3–4 km) in the adjoining active shield volcanoes.
Received: 21 August 1997 / Accepted: 15 February 1998 相似文献
998.
Truncation of the distribution of ground-motion residuals 总被引:1,自引:3,他引:1
Recent studies to assess very long-term seismic hazard in the USA and in Europe have highlighted the importance of the upper
tail of the ground-motion distribution at the very low annual frequencies of exceedance required by these projects. In particular,
the use of an unbounded lognormal distribution to represent the aleatory variability of ground motions leads to very high
and potentially unphysical estimates of the expected level of shaking. Current practice in seismic hazard analysis consists
of truncating the ground-motion distribution at a fixed number (ε
max) of standard deviations (σ). However, there is a general lack of consensus regarding the truncation level to adopt. This paper investigates whether
a physical basis for choosing ε
max can be found, by examining records with large positive residuals from the dataset used to derive one of the ground-motion
models of the Next Generation Attenuation (NGA) project. In particular, interpretations of the selected records in terms of
causative physical mechanisms are reviewed. This leads to the conclusion that even in well-documented cases, it is not possible
to establish a robust correlation between specific physical mechanisms and large values of the residuals, and thus obtain
direct physical constraints on ε
max. Alternative approaches based on absolute levels of ground motion and numerical simulations are discussed. However, the choice
of ε
max is likely to remain a matter of judgment for the foreseeable future, in view of the large epistemic uncertainties associated
with these alternatives. Additional issues arise from the coupling between ε
max and σ, which causes the truncation level in terms of absolute ground motion to be dependent on the predictive equation used. Furthermore,
the absolute truncation level implied by ε
max will also be affected if σ is reduced significantly. These factors contribute to rendering a truncation scheme based on a single ε
max value impractical. 相似文献
999.
1000.
Modelling peak accelerations from earthquakes 总被引:1,自引:0,他引:1
This paper deals with the prediction of peak horizontal accelerations with emphasis on seismic risk and insurance concerns. Non‐linear mixed effects models are used to analyse well‐known earthquake data and the consequences of mis‐specifying assumptions on the error term are quantified. A robust fit of the usual model, using recently developed robust weighted maximum likelihood estimators, is presented. Outlying data are automatically identified and subsequently investigated. A more appropriate model accounting for the extreme value nature of the responses, is also developed and implemented. The implication on acceleration predictions is demonstrated. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献