全文获取类型
收费全文 | 1624篇 |
免费 | 57篇 |
国内免费 | 10篇 |
专业分类
测绘学 | 78篇 |
大气科学 | 148篇 |
地球物理 | 336篇 |
地质学 | 608篇 |
海洋学 | 148篇 |
天文学 | 249篇 |
综合类 | 1篇 |
自然地理 | 123篇 |
出版年
2021年 | 21篇 |
2020年 | 21篇 |
2019年 | 21篇 |
2018年 | 48篇 |
2017年 | 29篇 |
2016年 | 46篇 |
2015年 | 38篇 |
2014年 | 49篇 |
2013年 | 63篇 |
2012年 | 65篇 |
2011年 | 80篇 |
2010年 | 69篇 |
2009年 | 107篇 |
2008年 | 94篇 |
2007年 | 51篇 |
2006年 | 70篇 |
2005年 | 74篇 |
2004年 | 57篇 |
2003年 | 61篇 |
2002年 | 48篇 |
2001年 | 31篇 |
2000年 | 39篇 |
1999年 | 35篇 |
1998年 | 40篇 |
1997年 | 26篇 |
1996年 | 24篇 |
1995年 | 24篇 |
1994年 | 17篇 |
1993年 | 24篇 |
1992年 | 16篇 |
1991年 | 15篇 |
1990年 | 10篇 |
1989年 | 12篇 |
1988年 | 8篇 |
1987年 | 23篇 |
1986年 | 9篇 |
1985年 | 9篇 |
1984年 | 16篇 |
1983年 | 9篇 |
1982年 | 14篇 |
1981年 | 12篇 |
1980年 | 11篇 |
1979年 | 11篇 |
1977年 | 6篇 |
1974年 | 10篇 |
1971年 | 7篇 |
1970年 | 8篇 |
1969年 | 8篇 |
1966年 | 5篇 |
1964年 | 5篇 |
排序方式: 共有1691条查询结果,搜索用时 31 毫秒
111.
The trends of malformation prevalence in embryos of dab, Limanda limanda, in the southern North Sea after the year 1990 mirrored the drop in major pollutants in the rivers draining into the German Bight. Despite this general decline, we detected a pollution event in the southern North Sea in winter 1995/1996 employing the prevalence of malformations in pelagic dab embryos as an indicator. An abrupt rise in malformation prevalence in the embryos of dab, corresponded to a dramatic increase in DDT levels in parent fish from the same area, indicating a hitherto unnoticed introduction of considerable quantities of DDT into the system. 相似文献
112.
Modelling the variability of midlatitude storm activity on decadal to century time scales 总被引:1,自引:0,他引:1
Irene Fischer-Bruns H. von Storch J. F. González-Rouco E. Zorita 《Climate Dynamics》2005,25(5):461-476
The output of several multi-century simulations with a coupled ocean–atmosphere general circulation model is examined with
respect to the variability of global storm activity in winter on time scales of decades and longer. The frequency of maximum
wind speed events within a grid box, using the lower limits on the Beaufort wind speed scale of 8 and 10 Bft as thresholds,
is taken as the characteristic parameter. Two historical climate runs with time-dependent forcing of the last five centuries,
one control simulation, and three climate change experiments are considered. The storm frequency shows no trend until recently.
Global maps for the industrially influenced period hardly differ from pre-industrial maps, even though significant temperature
anomalies temporarily emerge in the historical runs. Two indicators describing the frequency and the regional shift of storm
activity are determined. In historical times they are decoupled from temperature. Variations in solar and volcanic forcing
in the historical simulations as well as in greenhouse gas concentrations for the industrially influenced period are not related
to variations in storm activity. Also, anomalous temperature regimes like the Late Maunder Minimum are not associated with
systematic storm conditions. In the climate change experiments, a poleward shift of storm activity is found in all three storm
track regions. Over the North Atlantic and Southern Ocean, storm activity increases, while it decreases over the Pacific Ocean.
In contrast to the historical runs, and with the exception of the North Pacific storm frequency index, the storm indices parallel
the development of temperature, exceeding the 2 σ-range of pre-industrial variations in the early twenty-first century. 相似文献
113.
Hyung Rae Kim Ralph R. B. von Frese Patrick T. Taylor Alexander V. Golynsky Luis R. Gaya-Piqué Fausto Ferraccioli 《Geophysical Journal International》2007,171(1):119-126
The Antarctic magnetic anomaly map compiled marine and airborne surveys collected south of 60°S through 1999 and used Magsat data to help fill in the regional gaps between the surveys. Ørsted and CHAMP satellite magnetic observations with greatly improved measurement accuracies and temporal and spatial coverage of the Antarctic, have now supplanted the Magsat data. We combined the new satellite observations with the near-surface survey data for an improved magnetic anomaly map of the Antarctic lithosphere. Specifically, we separated the crustal from the core and external field components in the satellite data using crustal thickness variations estimated from the terrain and the satellite-derived free-air gravity observations. Regional gaps in the near-surface surveys were then filled with predictions from crustal magnetization models that jointly satisfied the near-surface and satellite crustal anomalies. Comparisons in some of the regional gaps that also considered newly acquired aeromagnetic data demonstrated the enhanced anomaly estimation capabilities of the predictions over those from conventional minimum curvature and spherical harmonic geomagnetic field models. We also noted that the growing number of regional and world magnetic survey compilations involve coverage gaps where these procedures can contribute effective near-surface crustal anomaly estimates. 相似文献
114.
Yongjun Gao Jochen Hoefs Eric Hellebrand Anette von der Handt Jonathan E. Snow 《Contributions to Mineralogy and Petrology》2007,153(4):429-442
Major and trace element profiles of clinopyroxene grains in oceanic gabbros from ODP Hole 735B have been investigated by a
combined in situ analytical study with ion probe, and electron microprobe. In contrast to the homogeneous major element compositions,
trace elements (REE, Y, Cr, Sr, and Zr) show continuous core to rim zoning profiles. The observed trace element systematics
in clinopyroxene cannot be explained by a simple diffusive exchange between melts and gabbros along grain boundaries. A simultaneous
modification of the melt composition is required to generate the zoning, although Rayleigh fractional crystallization modelling
could mimic the general shape of the profiles. Simultaneous metasomatism between the cumulate crystal and the porous melt
during crystal accumulation is the most likely process to explain the zoning. Deformation during solidification of the crystal
mush could have caused squeezing out of the incompatible element enriched residual melts (interstitial liquid). Migration
of the melt along grain boundaries might carry these melt out of the system. This process named as synkinematic differentiation
or differentiation by deformation (Natland and Dick in J Volcanol Geotherm Res 110(3–4):191–233, 2001) may act as an important magma evolution mechanism in the oceanic crust, at least at slow-spreading ridges. 相似文献
115.
Seismic exploration underneath highly heterogeneous layers such as basalt flows is possible by lowering the principal source frequency. Unfortunately this also reduces resolution. Wave‐localization theory is a multiple scattering theory that can be used to study stratigraphic filtering in chaotic lithologies. It predicts the apparent attenuation due to scattering of a plane wave traversing a layer with high velocity fluctuations. It can therefore predict the optimum principle source frequency in the trade‐off between loss of resolution and increased penetration depth. We show how this can be done with the help of a few statistical parameters derived from a well‐log analysis; namely, the average background velocity, the expected standard deviation in the velocity fluctuations, the typical scale length of the heterogeneities and the thickness of the basalt layer. In the likely situation that no local well logs exist, a multitude of scenarios can easily be examined at low cost. 相似文献
116.
117.
Aoibheann A. Kilfeather Jeffrey J. Blackford Jaap J. M. van der Meer 《Pure and Applied Geophysics》2007,164(2-3):509-525
Tsunami deposits are provisionally distinguished in the field on the basis of anomalous sand horizons, fining-up and fining-landward,
coupled with organic-rich, fragmented `backwash' sediments. In this paper, micromorphological features of a sediment sequence
previously interpreted as being of tsunami origin are described. These characteristics are shown to be consistent with the
macro-scale features used elsewhere, but show additional details not seen in standard stratigraphies, including possible evidence
for individual waves, possibly wave-magnitude progression, organic fragment alignment and intraclast microstructures. Although
replication and more complete studies are needed, this analysis confirms the identification of a tsunami in Willapa Bay in
ca.1700 AD, while demonstrating a widely applicable technique for confirming or refuting possible tsunami deposits. 相似文献
118.
The purpose of this study is to quantify the dispersivity in the longitudinal direction by upscaling pore scale mixing over a network domain and to verify the dispersivity with that obtained through the more rigorous upscaling technique, the Brownian particle tracking model (BPTM). We model a porous medium with a network of pore-units that are comprised of pore bodies and bonds of finite volume. Such a pore-unit is assumed to be a mixing cell with the steady state flow condition for a single fluid. Dispersivity can be obtained by solving the mixing cell model (MCM) for the concentration in each pore-unit and by averaging the concentrations for a large number of pore units (as a function of time and space). A minimal size of network that ascertains an asymptotic value of dispersivity was determined and verified with large size pore networks. This numerically computed dispersivity is compared with the results from the BPTM for the same porous medium and flow conditions. We show that the dispersivity obtained from the MCM is equally reliable for the heterogeneous pore-networks and can be estimated as a function of pore size heterogeneity. For homogeneous networks with the MCM, the iteration time step plays an important role. On the other hand, for networks with the BPTM, the assumption of intra-bond velocity profile affects the results. 相似文献
119.
The 3D shear experiment over the Natih field in Oman: the effect of fracture-filling fluids on shear propagation 总被引:1,自引:0,他引:1
This is the final paper in a series on the 3D multicomponent seismic experiment in Oman. In this experiment a 3D data set was acquired using three-component geophones and with three source orientations. The data set will subsequently be referred to as the Natih 9C3D data set. We present, for the first time, evidence demonstrating that shear waves are sensitive to fluid type in fractured media. Two observations are examined from the Natih 9C3D data where regions of gas are characterized by slow shear-wave velocities. One is that the shear-wave splitting map of the Natih reservoir exhibits much larger splitting values over the gas cap on the reservoir. This increase in splitting results from a decrease in the slow shear-wave velocity which senses both the fractures and the fracture-filling fluid. Using a new effective-medium model, it was possible to generate a splitting map for the reservoir that is corrected for this fluid effect. Secondly, an anomaly was encountered on the shear-wave data directly above the reservoir. The thick Fiqa shale overburden exhibits a low shear-wave velocity anomaly that is accompanied by higher shear reflectivity and lower frequency content. No such effects are evident in the conventional P-wave data. This feature is interpreted as a gas chimney above the reservoir, a conclusion supported by both effective-medium modelling and the geology.
With this new effective-medium model, we show that introduction of gas into vertically fractured rock appears to decrease the velocity of shear waves (S2), polarized perpendicular to the fracture orientation, whilst leaving the vertical compressional-wave velocity largely unaffected. This conclusion has direct implications for seismic methods in exploration, appraisal and development of fractured reservoirs and suggests that here we should be utilizing S-wave data, as well as the conventional P-wave data, as a direct hydrocarbon indicator. 相似文献
With this new effective-medium model, we show that introduction of gas into vertically fractured rock appears to decrease the velocity of shear waves (S2), polarized perpendicular to the fracture orientation, whilst leaving the vertical compressional-wave velocity largely unaffected. This conclusion has direct implications for seismic methods in exploration, appraisal and development of fractured reservoirs and suggests that here we should be utilizing S-wave data, as well as the conventional P-wave data, as a direct hydrocarbon indicator. 相似文献
120.
In this paper we describe the transport of pollution in groundwater in the neighbourhood of a well in a uniform background flow. We compute the rate at which contaminated particles reach the well as a function of the place of the source of pollution. The motion of a particle in a dispersive flow is seen as a random walk process. The Fokker-Planck equation for the random motion of a particle is transformed using the complex potential for the advective flow field. The resulting equation is solved asymptotically after a stretching transformation. Finally, the analytical solution is compared with results from Monte Carlo simulations with the random walk model. The method can be extended to arbitrary flow fields. Then by a numerical coordinate transformation the analytical results can still be employed. 相似文献