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821.
822.
823.
Walter Munk 《Progress in Oceanography》1997,40(1-4)
Topex/Poseidon (T/P) altimetry has reopened the problem of how tidal dissipation is to be allocated. There is now general agreement of a M2 dissipation by 2.5 Terawatts (1 TW = 1012 W), based on four quite separate astronomic observational programs. Allowing for the bodily tide dissipation of 0.1 TW leaves 2.4 TW for ocean dissipation. The traditional disposal sites since
(1920) have been in the turbulent bottom boundary layer (BBL) of marginal seas, and the modern estimate of about 2.1 TW is in this tradition (but the distribution among the shallow seas has changed radically from time to time). Independent estimates of energy flux into the marginal seas are not in good agreement with the BBL estimates.T/P altimetry has contributed to the tidal problem in two important ways. The assimilation of global altimetry into Laplace tidal solutions has led to accurate representations of the global tides, as evidenced by the very close agreement between the astronomic measurements and the computed 2.4 TW working of the Moon on the global ocean. Second, the detection by
and
(1996) of small surface manifestation of internal tides radiating away from the Hawaiian chain has led to global estimates of 0.2 to 0.4 TW of conversion of surface tides to internal tides. Measurements of ocean microstructure yields 0.2 TW of global dissipation by pelagic turbulence (away from topography). We propose that pelagic turbulence is maintained by topographic scattering of barotropic into baroclinic tidal energy, via internal tides and internal waves. Previous estimates by
(1974);
, (1982)) of this conversion along 150,000 km of continental coastlines gave a negligible 0.02 TW; evidently the important conversion takes place along mid-ocean ridges.The maintenance of the abyssal global stratification requires a much larger expenditure of power. 2 TW versus 0.2 TW. This is usually attributed to wind forcing. If tidal power is to play a significant role here, then the BBL estimates need to be reduced. The challenge is to estimate dissipation from the energy flux divergence in the T/P adjusted tidal models, without prior assumptions concerning the dissipation processes. 相似文献
824.
Possible modes of coral-reef development at Molokai, Hawaii, inferred from seismic-reflection profiling 总被引:1,自引:0,他引:1
Walter A. Barnhardt Bruce M. Richmond Eric E. Grossman Patrick Hart 《Geo-Marine Letters》2005,25(5):315-323
High-resolution, seismic-reflection data elucidate the late Quaternary development of the largest coral-reef complex in the
main Hawaiian Islands. Six acoustic facies were identified from reflection characteristics and lithosome geometry. An extensive,
buried platform with uniformly low relief was traced beneath fore-reef and marginal shelf environments. This highly reflective
surface dips gently seaward to ~130 m depth and locally crops out on the seafloor. It probably represents a wave-cut platform
or ancient reef flat. We propose alternative evolutionary models, in which sea-level changes have modulated the development
of reef systems, to explain the observed stratigraphic relationships. The primary difference between the models is the origin
of the underlying antecedent surface, which arguably could have formed during either regression/lowstand or subsequent transgression. 相似文献
825.
David A. H. Buckley Kazuhiro Sekiguchi Darragh O'Donoghue Walter W. Wargau 《Astrophysics and Space Science》1995,230(1-2):117-130
This paper discusses the first all-sky surveys of cosmic extreme ultra-violet and soft X-ray sources, discovered by ROSAT. Details of the surveys are presented, with comparisons made to previous selected surveys in the X-ray regime. The subsequent optical identification programs are described, and the major results summarized. We then discuss the main classes of EUV emitters: active chromosphere stars and hot white dwarfs, and describe the importance of EUV observations in understanding the astrophysics of these objects. Many bright, and relatively nearby, sources have been identified as hitherto unrecognized active stars, representing the extremes in chromospheric and coronal activity, be it binary or age related. Many new hot DA white dwarfs have also been indentified, and the most exciting result in this area is the discovery that significant traces of heavier elements (e.g. C, N, O, Si, Fe and Al) exist in their atmospheres, substantially increasing their EUV opacities. The importance of hot white dwarfs as standard candles in probing the local interstellar medium is also discussed. Miscellaneous counterparts (AGN, PNN, O-B stars and CVs) that make up the rest of the sample of EUV sources are also briefly mentioned. We finish with a discussion of the on-going ROSAT Galactic Plane Survey (RGPS) identification program. 相似文献
826.
Michael Owor Tina Hartwig Andrew Muwanga Dieter Zachmann Walter Pohl 《Environmental Geology》2007,51(6):1065-1075
The abandoned Kilembe copper mine in western Uganda is a source of contaminants, mobilised from mine tailings into R. Rukoki
flowing through a belt of wetlands into Lake George. Water and sediments were investigated on the lakeshore and the lakebed.
Metal associations in the sediments reflect the Kilembe sulphide mineralisation. Enrichment of metals was compared between
lakebed sediments, both for wet and dry seasons. Total C in a lakebed core shows a general increment, while Cu and Co decrease
with depth. The contaminants are predominant (> 65%) in the ≤ 63 μm sediment size range with elevated Cu and Zn (> 28%), while
Ni, Pb and Co are low (< 18%) in all the fractions. Sequential extraction of Fe for lakeshore sediment samples reveals low
Fe mobility. Relatively higher mobility and biological availability is seen for Co, Cu and S. Heavy metal contents in lake
waters are not an immediate risk to the aquatic environment. 相似文献
827.
The Arctic as a trigger for glacial terminations 总被引:1,自引:0,他引:1
We propose a hypothesis to explain the very abrupt terminations that end most of the glacial episodes. During the last glaciation,
the buildup and southerly expansion of large continental ice-sheets in the Northern Hemisphere and extensive cover of sea
ice in the N. Pacific and the N. Atlantic imposed a much more zonal climatic circulation system than exists today. We hypothesize
that this, in combination with the frigid (dry) polar air led to a significant decrease in freshwater runoff into the Arctic
Ocean. In addition the freshwater contribution of the fresher Pacific water was completely eliminated by the emergence of
the Bering Strait (sill depth 50 m). As the Arctic freshwater input was depleted, regions of the Arctic Ocean lost surface
stability and eventually overturned, bringing warmer deep water to the surface where it melted the overlying sea ice. This
upwelled water was quickly cooled and sank as newly formed deep water. For sustained overturn events, such as might have occurred
during the peak of very large glacial periods (i.e. the last glacial maximum), the voluminous deep water formed would eventually
overflow into the Nordic Seas and North Atlantic necessitating an equally voluminous rate of return flow of warmer surface
waters from the North Atlantic thus breaking down the Arctic's zonal isolation, melting the expansive NA sea ice cover and
initiating oceanic heating of the atmosphere over the ice-sheets bordering the NA. We suggest that the combined effect of
these overturn-induced events in concert with a Milankovitch warming cycle, was sufficient to drive the system to a termination.
We elaborate on this proposed sequence of events, using the model for the formation of the Weddell Sea polynya as proposed
by Martinson et al. (1981) and various, albeit sparse, data sets from the circum-Arctic region to apply and evaluate this
hypothesis to the problem of glacial terminations. 相似文献
828.
829.
Timothy G. Fisher Walter L. Loope William Pierce Harry M. Jol 《Journal of Paleolimnology》2007,37(3):365-382
We reconstruct postglacial lake-level history within the Lake Michigan basin using soil stratigraphy, ground-penetrating radar
(GPR), sedimentology and 14C data from the Silver Lake basin, which lies adjacent to Lake Michigan. Stratigraphy in nine vibracores recovered from the
floor of Silver Lake appears to reflect fluctuation of water levels in the Lake Michigan basin. Aeolian activity within the
study area from 3,000 years (cal yr. B.P.) to the present was inferred from analysis of buried soils, an aerial photograph
sequence, and GPR. Sediments in and around Silver Lake appear to contain a paleoenvironmental record that spans the entire
post-glacial history of the Lake Michigan basin. We suggest that (1) a pre-Nipissing rather than a Nipissing barrier separated
Silver Lake basin from the Lake Michigan basin, (2) that the Nipissing transgression elevated the water table in the Silver
Lake basin about 6,500 cal yr. B.P., resulting in reestablishment of a lake within the basin, and (3) that recent dune migration
into Silver Lake is associated with levels of Lake Michigan.
This is the fourth in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were
presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University
of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M. Lewis were guest editors of this special issue. 相似文献
830.
Steve W. Lyon Arthur J. Lembo Jr. M. Todd Walter Tammo S. Steenhuis 《Advances in water resources》2006
In humid, well-vegetated areas, such as in the northeastern US, runoff is most commonly generated from relatively small portions of the landscape becoming completely saturated, however, little is known about the spatial and temporal behavior of these saturated regions. Indicator kriging provides a way to use traditional water table data to quantify probability of saturation to evaluate predicted spatial distributions of runoff generation risk, especially for the new generation of water quality models incorporating saturation excess runoff theory. When spatial measurements of a variable are transformed to binary indicators (i.e., 1 if above a given threshold value and 0 if below) and the resulting indicator semivariogram is modeled, indicator kriging produces the probability of the measured variable to exceed the threshold value. Indicator kriging gives quantified probability of saturation or, consistent with saturation excess runoff theory, runoff generation risk with depth to water table as the variable and the threshold set near the soil surface. The probability of saturation for a 120 m × 180 m hillslope based upon 43 measurements of depth to water table is investigated with indicator semivariograms for six storm events. The indicator semivariograms show high spatial structure in saturated regions with large antecedent rainfall conditions. The temporal structure of the data is used to generate interpolated (soft) data to supplement measured (hard) data. This improved the spatial structure of the indicator semivariograms for lower antecedent rainfall conditions. Probability of saturation was evaluated through indicator kriging incorporating soft data showing, based on this preliminary study, highly connected regions of saturation as expected for the wet season (April through May) in the Catskill Mountain region of New York State. Supplementation of hard data with soft data incorporates physical hydrology of the hillslope to capture significant patterns not available when using hard data alone for indicator kriging. With the need for water quality models incorporating appropriate runoff generation risk estimates on the rise, this manner of data will lay the groundwork for future model evaluation and development. 相似文献