全文获取类型
收费全文 | 1836篇 |
免费 | 118篇 |
国内免费 | 30篇 |
专业分类
测绘学 | 74篇 |
大气科学 | 147篇 |
地球物理 | 382篇 |
地质学 | 668篇 |
海洋学 | 172篇 |
天文学 | 330篇 |
综合类 | 6篇 |
自然地理 | 205篇 |
出版年
2023年 | 12篇 |
2022年 | 7篇 |
2021年 | 38篇 |
2020年 | 47篇 |
2019年 | 65篇 |
2018年 | 70篇 |
2017年 | 73篇 |
2016年 | 93篇 |
2015年 | 75篇 |
2014年 | 74篇 |
2013年 | 145篇 |
2012年 | 82篇 |
2011年 | 113篇 |
2010年 | 95篇 |
2009年 | 118篇 |
2008年 | 97篇 |
2007年 | 96篇 |
2006年 | 85篇 |
2005年 | 60篇 |
2004年 | 65篇 |
2003年 | 54篇 |
2002年 | 39篇 |
2001年 | 32篇 |
2000年 | 38篇 |
1999年 | 27篇 |
1998年 | 27篇 |
1997年 | 15篇 |
1996年 | 22篇 |
1995年 | 14篇 |
1994年 | 12篇 |
1993年 | 9篇 |
1992年 | 12篇 |
1991年 | 13篇 |
1990年 | 13篇 |
1989年 | 13篇 |
1988年 | 9篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 13篇 |
1984年 | 15篇 |
1983年 | 13篇 |
1982年 | 13篇 |
1981年 | 9篇 |
1980年 | 4篇 |
1979年 | 7篇 |
1978年 | 11篇 |
1977年 | 6篇 |
1974年 | 4篇 |
1973年 | 3篇 |
1972年 | 3篇 |
排序方式: 共有1984条查询结果,搜索用时 15 毫秒
101.
Christopher Juhlin 《Geophysical Prospecting》1995,43(6):843-858
The velocity-stress formulation for propagation of elastic seismic waves through 2D heterogeneous transversely isotropic media of arbitrary orientation is presented. The equations are recast into a finite-difference scheme and solved numerically using fourth-order spatial operators and a second-order temporal operator on a staggered grid. Absorbing, free-surface and symmetry boundary conditions have been implemented. Test cases compare well with other published solutions. Synthetic seismograms are calculated over two idealized models: (i) vertical fractures in granite with a dolerite sill reflector and (ii) a dipping anisotropic shale. Comparisons with the isotropic counterparts show significant differences which may have to be accounted for in seismic processing in the future. 相似文献
102.
Christopher Morgan 《地球表面变化过程与地形》1983,8(4):323-338
Three high erosivity conditions (50 mm hr?1, 100 mm hr?1, and 200 mm hr?1) were generated in a laboratory using a rainfall simulator and coherent soil block samples from fourteen different soil erodibility conditions. The data acquired supports the theoretical contention that soil loss should not increase as a simple linear function of storm intensity. Rather, a variable relationship is caused by the rupturing of surface seals and the changing relative significance of splash, wash and rainwash processes. Slope angle appears to influence soil loss at the higher erosivity conditions of 100 mm hr?1 and 200 mm hr?1 on slopes that were either very steep (> 20°) or very shallow (< 3°), but on moderate slopes the relationship is unclear. Examination of the variation of soil loss with erosivity when soil loss for a specific high erosivity condition is known revealed that conversion and power factors are of doubtful value and little generality. A satisfactory predictive equation, a power curve, is seen to be of value only when comparing rainwash soil loss between the higher erosivity conditions. The relationship is most safely considered as soil and site specific. Where the influence of slope and soil erodibility are disregarded, a strong association between soil loss and rainfall intensity is found. That soil loss, and hence, soil erodibility varies non-uniformly with erosivity is clear. The findings indicate caution is required when comparing conclusions drawn from studies based upon different erosivity conditions. 相似文献
103.
Jeffrey P. Chanton Christopher S. Martens George W. Kipphut 《Geochimica et cosmochimica acta》1983,47(10):1791-1804
Sediment accumulation rate studies utilizing excess 210Pb and 137Cs were conducted as part of recent investigations of biogeochemical cycling at a single site in Cape Lookout Bight, a rapidly changing coastal basin on the Outer Banks of North Carolina (U.S.A.). Cores three meters in length reveal a depositional history for the bight interior characterized by a gradual transition in texture from coarse-grained to fine-grained material over the period 1946–1979. This transition is controlled by progressive enclosure of the bight by an active northerly migrating recurved spit. The textural gradation is periodically interrupted by layers of well-sorted sand associated with major storm events. Lead-210 data indicate that the upper meter of the sediment has accumulated at a rate of 3.35 to 4.71 g · cm?2 · yr?1 or approximately 8.4 to 11.8 cm · yr?1 (at ø = 0.84). Below 120 cm depth, dilution of clay and silt by low activity sand necessitates correction of the 210Pb profile in order to establish a geochronology. Grain size 210Pb distribution measurements at three depths reveal that the specific activity (dpm · g?1) of clay is 3.2 times that of silt and 24.7 times that of sand. Corrections of bulk sediment excess 210Pb activities based on these measurements lead to dates for textural changes which are consistent with charted changes in basin morphology and major storm events.Sixteen 137Cs measurements between 33–241 cm depth reveal a peak activity at 105–115 cm and indicate a minimum sedimentation rate of approximately 2.7 g · cm?2 · yr?1. 相似文献
104.
Gilberto Saccorotti Ivan Lokmer Christopher J. Bean Giuseppe Di Grazia Domenico Patan 《Journal of Volcanology and Geothermal Research》2007,160(3-4):340-354
Following the installation of a broadband network on Mt. Etna, sustained Long-Period (LP) activity was recorded accompanying a period of total quiescence and the subsequent onset of the 2004–2005 effusive episode. From about 56000 events detected by an automatic classification procedure, we analyse a subset of about 3000 signals spanning the December 17th, 2003–September 25th, 2004, time interval. LP spectra are characterised by several, unevenly-spaced narrow peaks spanning the 0.5–10 Hz frequency band. These peaks are common to all the recording sites of the network, and different from those associated with tremor signals. Throughout the analysed time interval, LP spectra and waveforms maintain significant similarity, thus indicating the involvement of a non-destructive source process that we interpret in terms of the resonance of a fluid-filled buried cavity. Polarisation analysis indicates radiation from a non-isotropic source involving large amounts of shear. Concurrently with LP signals, recordings from the summit station also depict Very-Long-Period (VLP) pulses whose rectilinear motion points to a region located beneath the summit craters at depths ranging between 800 and 1100 m beneath the surface. Based on a refined repicking of similar waveforms, we obtain robust locations for a selected subset of the most energetic LP events from probabilistic inversion of travel-times calculated for a 3D heterogenous structure. LP sources cluster in a narrow volume located beneath the summit craters, and extending to a maximum depth of ≈ 800 m beneath the surface. No causal relationships are observed between LP, VLP and tremor activities and the onset of the 2004–2005 lava effusions, thus indicating that magmatic overpressure played a limited role in triggering this eruption. These data represent the very first observation of LP and VLP activity at Etna during non-eruptive periods, and open the way to the quantitative modelling of the geometry and dynamics of the shallow plumbing system. 相似文献
105.
It is well known that terrain may vary markedly over small areas and that statistics used to characterise spatial variation in terrain may be valid only over small areas. In geostatistical terminology, a non-stationary approach may be considered more appropriate than a stationary approach. In many applications, local variation is not accounted for sufficiently. This paper assesses potential benefits in using non-stationary geostatistical approaches for interpolation and for the assessment of uncertainty in predictions with implications for sampling design. Two main non-stationary approaches are employed in this paper dealing with (1) change in the mean and (2) change in the variogram across the region of interest. The relevant approaches are (1) kriging with a trend model (KT) using the variogram of residuals from local drift and (2) locally-adaptive variogram KT, both applied to a sampled photogrammetrically derived digital terrain model (DTM). The fractal dimension estimated locally from the double-log variogram is also mapped to illustrate how spatial variation changes across the data set. It is demonstrated that estimation of the variogram of residuals from local drift is worthwhile in this case for the characterisation of spatial variation. In addition, KT is shown to be useful for the assessment of uncertainty in predictions. This is shown to be true even when the sample grid is dense as is usually the case for remotely-sensed data. In addition, both ordinary kriging (OK) and KT are shown to provide more accurate predictions than inverse distance weighted (IDW) interpolation, used for comparative purposes. 相似文献
106.
Formation and failure of volcanic debris dams in the Chakachatna River valley associated with eruptions of the Spurr volcanic complex, Alaska 总被引:1,自引:0,他引:1
Christopher F. Waythomas 《Geomorphology》2001,39(3-4)
The formation of lahars and a debris avalanche during Holocene eruptions of the Spurr volcanic complex in south-central Alaska have led to the development of volcanic debris dams in the Chakachatna River valley. Debris dams composed of lahar and debris-avalanche deposits formed at least five times in the last 8000–10,000 years and most recently during eruptions of Crater Peak vent in 1953 and 1992. Water impounded by a large debris avalanche of early Holocene (?) age may have destabilized an upstream glacier-dammed lake causing a catastrophic flood on the Chakachatna River. A large alluvial fan just downstream of the debris-avalanche deposit is strewn with boulders and blocks and is probably the deposit generated by this flood. Application of a physically based dam-break model yields estimates of peak discharge (Qp) attained during failure of the debris-avalanche dam in the range 104<Qp<106 m3 s−1 for plausible breach erosion rates of 10–100 m h−1. Smaller, short-lived, lahar dams that formed during historical eruptions in 1953, and 1992, impounded smaller lakes in the upper Chakachatna River valley and peak flows attained during failure of these volcanic debris dams were in the range 103<Qp<104 m3 s−1 for plausible breach erosion rates.Volcanic debris dams have formed at other volcanoes in the Cook Inlet region, Aleutian arc, and Wrangell Mountains but apparently did not fail rapidly or result in large or catastrophic outflows. Steep valley topography and frequent eruptions at volcanoes in this region make for significant hazards associated with the formation and failure of volcanic debris dams. 相似文献
107.
We used growth rates of juvenile winter flounderPseudopleuronectes americanus to assess anthropogenic influence on habitat quality at three sites in Narragansett Bay, Rhode Island. The upper bay site,
Gaspee Point, had the highest population density and concentration of total nitrogen; human inputs decreased down bay. Growth
rates of individually marked fish were measured in three 15-d experiments from June 8 to July 6, 1998 in 1-m2 cages placed at upper, middle, and lower bay sites. Water temperature, salinity, dissolved oxygen (DO), and benthic food
were also measured. Stable isotopes of nitrogen and carbon were measured in experimental fish as possible indicators of nutrient
enrichment and to identify organic carbon sources. Growth rates were 0.22–0.60 mm d−1, with the highest average at the mid-bay site. Growth was initially fastest at Gaspee Point, but dropped off as DO concentrations
fell. Step-wise multiple regression indicated that location (upper, middle, or lower bay) explained most of the variability
in fish growth (40%). Coefficients of other significant variables indicated that fish grew faster at lower salinities, smaller
sizes, and with decreased time that DO was below 2.3 mg l−1. Benthic prey varied among sites and there was significantly less food and fewer species at Gaspee Point.Polydora cornuta was a favored food at all sites and was found in over half the stomachs. Values of δ15N in fish and sediments did not reflect differences in total nitrogen concentrations recorded near the sites. We suggest that
anthropogenic influences, such as nutrients and sewage, affected habitat quality by reducing DO, which lowered fish growth
rates. 相似文献
108.
Christopher Green 《Climatic change》1992,22(4):265-291
There is growing scientific and public concern that increasing concentrations of greenhouse gases in the atmosphere will produce global warming and other climatic changes. Although economic activity is the main source of greenhouse gas emissions, information and incentive problems make it difficult to translate concern about global warming into economic behaviour and policy conducive to reducing emissions. The paper considers a set of near term (carbon tax), intermediate term (afforestation, energy efficiency) and long term (new non-fossil fuel technologies) strategies for reducing CO2 in the atmosphere. Each strategy has useful attributes, but shortcomings or limitations too. While the near term and intermediate term strategies can slow and perhaps reverse the growth of CO2 emissions, only a successful long term strategy of fostering the development of some promising non-fossil fuel technologies, such as solar and solar-hydrogen, can eventually halt the build-up of CO2 in the atmosphere. Moreover, public investment in the development of new non-fossil fuel technologies would largely obviate the information and incentive problems that currently stand in the way of an economically viable greenhouse policy. 相似文献
109.
110.
Chester David K. Duncan Angus M. Dibben Christopher Guest John E. Lister Philip H. 《Natural Hazards》1999,19(1):29-46
The 1928 eruption of Etna, Sicily, although the largest such event this century, has not been studied in detail. In this paper the nature of the eruption, the destruction it caused – including the complete devastation of the town of Mascali (pre-eruption population 2,000) – and emergency responses of the authorities to it are reviewed in the context of fascist politics and planning priorities. It is contended that, although at one level the response to the 1928 eruption was successful, at another fascism merely continued and enhanced a reactive, propitiatory approach to hazard mitigation. We argue that this legacy was not successfully overcome until the middle of the nineteen eighties. Finally contemporary Italian moves towards a more proactive approach to disaster planning, both generally and in the context of Etna, are discussed. 相似文献