全文获取类型
收费全文 | 302篇 |
免费 | 22篇 |
国内免费 | 7篇 |
专业分类
测绘学 | 29篇 |
大气科学 | 25篇 |
地球物理 | 78篇 |
地质学 | 122篇 |
海洋学 | 4篇 |
天文学 | 43篇 |
自然地理 | 30篇 |
出版年
2023年 | 2篇 |
2021年 | 3篇 |
2020年 | 6篇 |
2019年 | 9篇 |
2018年 | 9篇 |
2017年 | 16篇 |
2016年 | 14篇 |
2015年 | 11篇 |
2014年 | 16篇 |
2013年 | 21篇 |
2012年 | 14篇 |
2011年 | 15篇 |
2010年 | 15篇 |
2009年 | 24篇 |
2008年 | 19篇 |
2007年 | 16篇 |
2006年 | 21篇 |
2005年 | 11篇 |
2004年 | 8篇 |
2003年 | 7篇 |
2002年 | 10篇 |
2001年 | 8篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1998年 | 4篇 |
1997年 | 8篇 |
1995年 | 1篇 |
1993年 | 3篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1989年 | 2篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1976年 | 1篇 |
1975年 | 3篇 |
1974年 | 1篇 |
1967年 | 1篇 |
1965年 | 3篇 |
1964年 | 1篇 |
1963年 | 2篇 |
1962年 | 3篇 |
1961年 | 2篇 |
1949年 | 1篇 |
排序方式: 共有331条查询结果,搜索用时 25 毫秒
41.
Investigating groundwater flow components in an Alpine relict rock glacier (Austria) using a numerical model 总被引:1,自引:0,他引:1
Marcus Pauritsch Thomas Wagner Gerfried Winkler Steffen Birk 《Hydrogeology Journal》2017,25(2):371-383
Relict rock glaciers are complex hydrogeological systems that might act as relevant groundwater storages; therefore, the discharge behavior of these alpine landforms needs to be better understood. Hydrogeological and geophysical investigations at a relict rock glacier in the Niedere Tauern Range (Austria) reveal a slow and fast flow component that appear to be related to the heterogeneous structure of the aquifer. A numerical groundwater flow model was used to indicate the influence of important internal structures such as layering, preferential flow paths and aquifer-base topography. Discharge dynamics can be reproduced reasonably by both introducing layers of strongly different hydraulic conductivities or by a network of highly conductive channels within a low-conductivity zone. Moreover, the topography of the aquifer base influences the discharge dynamics, which can be observed particularly in simply structured aquifers. Hydraulic conductivity differences of three orders of magnitude are required to account for the observed discharge behavior: a highly conductive layer and/or channel network controlling the fast and flashy spring responses to recharge events, as opposed to less conductive sediment accumulations sustaining the long-term base flow. The results show that the hydraulic behavior of this relict rock glacier and likely that of others can be adequately represented by two aquifer components. However, the attempt to characterize the two components by inverse modeling results in ambiguity of internal structures when solely discharge data are available. 相似文献
42.
A New Appraisal of Sri Lankan BB Zircon as a Reference Material for LA‐ICP‐MS U‐Pb Geochronology and Lu‐Hf Isotope Tracing 下载免费PDF全文
Maristella M. Santos Cristiano Lana Ricardo Scholz Ian Buick Mark D. Schmitz Sandra L. Kamo Axel Gerdes Fernando Corfu Simon Tapster Penelope Lancaster Craig D. Storey Miguel A.S. Basei Eric Tohver Ana Alkmim Herminio Nalini Klaus Krambrock Cristiano Fantini Michael Wiedenbeck 《Geostandards and Geoanalytical Research》2017,41(3):335-358
A potential zircon reference material (BB zircon) for laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) U‐Pb geochronology and Hf isotope geochemistry is described. A batch of twenty zircon megacrysts (0.5–1.5 cm3) from Sri Lanka was studied. Within‐grain rare earth element (REE) compositions are largely homogeneous, albeit with some variation seen between fractured and homogeneous domains. Excluding fractured cathodoluminescence bright domains, the variation in U content for all analysed crystals ranged from 227 to 368 μg g?1 and the average Th/U ratios were between 0.20 and 0.47. The Hf isotope composition (0.56–0.84 g/100 g Hf) is homogeneous within and between the grains – mean 176Hf/177Hf of 0.281674 ± 0.000018 (2s). The calculated alpha dose of 0.59 × 1018 g?1 for a number of BB grains falls within the trend of previously studied, untreated zircon samples from Sri Lanka. Aliquots of the same crystal (analysed by ID‐TIMS in four different laboratories) gave consistent U‐Pb ages with excellent measurement reproducibility (0.1–0.4% RSD). Interlaboratory assessment (by LA‐ICP‐MS) from individual crystals returned results that are within uncertainty equivalent to the TIMS ages. Finally, we report on within‐ and between‐grain homogeneity of the oxygen isotope systematic of four BB crystals (13.16‰ VSMOW). 相似文献
43.
Karst aquifers represent dual flow systems consisting of a highly conductive conduit system embedded in a less permeable rock matrix. Hybrid models iteratively coupling both flow systems generally consume much time, especially because of the nonlinearity of turbulent conduit flow. To reduce calculation times compared to those of existing approaches, a new iterative equation solver for the conduit system is developed based on an approximated Newton–Raphson expression and a Gauß–Seidel or successive over‐relaxation scheme with a single iteration step at the innermost level. It is implemented and tested in the research code CAVE but should be easily adaptable to similar models such as the Conduit Flow Process for MODFLOW‐2005. It substantially reduces the computational effort as demonstrated by steady‐state benchmark scenarios as well as by transient karst genesis simulations. Water balance errors are found to be acceptable in most of the test cases. However, the performance and accuracy may deteriorate under unfavorable conditions such as sudden, strong changes of the flow field at some stages of the karst genesis simulations. 相似文献
44.
We dispute the claim by Alexeev (2010) that the numerous fossil meteorites found in mid-Ordovician sediments in southern Sweden
can be explained by a single large meteorite shower some 470 million years ago, and—in particular—that the reported variable
cosmic ray exposure ages derived from cosmogenic 21Ne in chromite grains extracted from the fossil meteorites actually reflect variable contributions of nucleogenic 21Ne instead. 相似文献
45.
46.
This work examines the simulation of stream–aquifer interactions as grids are refined vertically and horizontally and suggests that traditional methods for calculating conductance can produce inappropriate values when the grid size is changed. Instead, different grid resolutions require different estimated values. Grid refinement strategies considered include global refinement of the entire model and local refinement of part of the stream. Three methods of calculating the conductance of the Cauchy boundary conditions are investigated. Single- and multi-layer models with narrow and wide streams produced stream leakages that differ by as much as 122% as the grid is refined. Similar results occur for globally and locally refined grids, but the latter required as little as one-quarter the computer execution time and memory and thus are useful for addressing some scale issues of stream–aquifer interactions. Results suggest that existing grid-size criteria for simulating stream–aquifer interactions are useful for one-layer models, but inadequate for three-dimensional models. The grid dependence of the conductance terms suggests that values for refined models using, for example, finite difference or finite-element methods, cannot be determined from previous coarse-grid models or field measurements. Our examples demonstrate the need for a method of obtaining conductances that can be translated to different grid resolutions and provide definitive test cases for investigating alternative conductance formulations. 相似文献
47.
48.
Christian Hirt Martin Schmitz Uwe Feldmann-Westendorff Gerhard Wübbena Cord-Hinrich Jahn Günter Seeber 《GPS Solutions》2011,15(2):149-159
The method of geometric-astronomical leveling is presented as a suited technique for the validation of GNSS (Global Navigation
Satellite System) heights. In geometric-astronomical leveling, the ellipsoidal height differences are obtained by combining
conventional spirit leveling and astronomical leveling. Astronomical leveling with recently developed digital zenith camera
systems is capable of providing the geometry of equipotential surfaces of the gravity field accurate to a few 0.1 mm per km.
This is comparable to the accuracy of spirit leveling. Consequently, geometric-astronomical leveling yields accurate ellipsoidal
height differences that may serve as an independent check on GNSS height measurements at local scales. A test was performed
in a local geodetic network near Hanover. GPS observations were simultaneously carried out at five stations over a time span
of 48 h and processed considering state-of-the-art techniques and sophisticated new approaches to reduce station-dependent
errors. The comparison of GPS height differences with those from geometric-astronomical leveling shows a promising agreement
of some millimeters. The experiment indicates the currently achievable accuracy level of GPS height measurements and demonstrates
the practical applicability of the proposed approach for the validation of GNSS height measurements as well as the evaluation
of GNSS height processing strategies. 相似文献
49.
Modest diatom responses to regional warming on the southeast Tibetan Plateau during the last two centuries 总被引:1,自引:0,他引:1
Juliane Wischnewski Anson W. Mackay Peter G. Appleby Steffen Mischke Ulrike Herzschuh 《Journal of Paleolimnology》2011,46(2):215-227
A general mean annual temperature increase accompanied with substantial glacial retreat has been noted on the Tibetan Plateau
during the last two centuries but most significantly since the mid 1950s. These climate trends are particularly apparent on
the southeastern Tibetan Plateau. However, the Tibetan Plateau (due to its heterogeneous mountain landscape) has very complex
and spatially differing temperature and precipitations patterns. As a result, intensive palaeolimnological investigations
are necessary to decipher these climatic patterns and to understand ecological responses to recent environmental change. Here
we present palaeolimnological results from a 210Pb/137Cs-dated sediment core spanning approximately the last 200 years from a remote high-mountain lake (LC6 Lake, working name)
on the southeastern Tibetan Plateau. Sediment profiles of diatoms, organic variables (TOC, C:N) and grain size were investigated.
The 210Pb record suggests a period of rapid sedimentation, which might be linked to major tectonic events in the region ca. 1950.
Furthermore, unusually high 210Pb supply rates over the last 50 years suggest that the lake has possibly been subjected to increasing precipitation rates,
sediment focussing and/or increased spring thaw. The majority of diatom taxa encountered in the core are typical of slightly
acidic to circumneutral, oligotrophic, electrolyte-poor lakes. Diatom species assemblages were rich, and dominated by Cyclotella sp., Achnanthes sp., Aulacoseira sp. and fragilarioid taxa. Diatom compositional change was minimal over the 200-year period (DCCA = 0.85 SD, p = 0.59); only a slightly more diverse but unstable diatom assemblage was recorded during the past 50 years. The results indicate
that large-scale environmental changes recorded in the twentieth century (i.e. increased precipitation and temperatures) are
likely having an affect on the LC6 Lake, but so far these impacts are more apparent on the lake geochemistry than on the diatom
flora. Local and/or regional peculiarities, such as increasing precipitation and cloud cover, or localized climatic phenomena,
such as negative climate feedbacks, might have offset the effects of increasing mean surface temperatures. 相似文献
50.
Banded iron formation (BIF)-hosted iron ore deposits in the Windarling Range are located in the lower greenstone succession of the Marda–Diemals greenstone belt, Southern Cross domain, Yilgarn Craton and constitute a total hematite–martite–goethite ore resource of minimum 52 Mt at 60 wt.% Fe (0.07 P). Banded iron formation is interlayered with high-Mg basalts at Windarling and precipitated during episodes of volcanic quiescence. Trace element content and the rare earth element (REE) ratios Y/Ho (42 to 45), Sm/Yb (1.5), together with positive La and Gd anomalies in ‘least-altered’ hematite–magnetite–metachert–BIF indicate the precipitation from Archean seawater that was fertilised by hydrothermal vent fluids with a basaltic HREE-Y signature. Hypogene iron ore in sub-greenschist facies metamorphosed BIF formed during three distinct stages: ore stage 1 was a syn- to post-metamorphic, syn-D1, Fe–Ca–Mg–Ni–Co–P–REE metasomatism that produced local Ni–REE-rich Fe–dolomite–magnetite alteration in BIF. Hydrothermal alteration was induced by hot fluid flow controlled by brittle–ductile reactivation of BIF-basalt margins and crosscutting D1 faults. The Ni–Co-rich content of dolomite and a shift in REE ratios in carbonate-altered BIF towards Archean mafic rock signature (Y/Ho to 31 to 40, Sm/Yb to 1 to 2 and Gd/Gd* to 1.2 to 1.4) suggest that high-Mg basalts in the Windarling Range were the primary source of introduced metals. During ore stage 2, a syn-deformational and likely acidic and oxidised fluid flow along BIF-basalt margins and within D1 faults leached carbonate and precipitated lepidoblastic and anhedral/granoblastic hematite. High-grade magnetite–hematite ore is formed during this stage. Ore stage 3 hydrothermal specular hematite (spcH)–Fe–dolomite–quartz alteration was controlled by a late-orogenic, brittle, compressional/transpressional stage (D4; the regional-scale shear-zone-related D3 is not preserved in Windarling). This minor event remobilised iron oxides, carbonate and quartz to form veins and breccia but did not generate significant volumes of iron ore. Ore stage 4 involved Mesozoic(?) to recent supergene oxidation and hydration in a weathering environment reaching down to depths of ~100 to maximum 200 m below surface. Supergene ore formation involved goethite replacement of dolomite and quartz as well as martitisation. Important ‘ground preparation’ for supergene modification and upgrade were mainly the formation of steep D1 to D4 structures, steep BIF/basalt margins and particularly the syn-D1 to syn-D2 carbonate alteration of BIF that is most susceptible to supergene dissolution. The Windarling deposits are structurally controlled, supergene-modified hydrothermal iron ore systems that share comparable physical, chemical and ore-forming characteristics to other iron ore deposits in the Yilgarn Craton (e.g. Koolyanobbing, Beebyn in the Weld Range, Mt. Gibson). However, the remarkable variety in pre-, syn- and post-deformational ore textures (relative to D1 and D2) has not been described elsewhere in the Yilgarn and are similar to the ore deposits in high-strain zones, such as of Brazil (Quadrilátero Ferrífero or Iron Quadrangle) and Nigeria. The overall similarity of alteration stages, i.e. the sequence of hydrothermal carbonate introduction and hypogene leaching, with other greenstone belt-hosted iron ore deposits supports the interpretation that syn-orogenic BIF alteration and upgrade was crucial in the formation of hypogene–supergene iron ore deposits in the Yilgarn Craton and possibly in other Archean/Paleoproterozoic greenstone belt settings worldwide. 相似文献