首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   907篇
  免费   34篇
  国内免费   3篇
测绘学   31篇
大气科学   51篇
地球物理   221篇
地质学   346篇
海洋学   69篇
天文学   151篇
综合类   2篇
自然地理   73篇
  2021年   8篇
  2020年   14篇
  2019年   13篇
  2018年   17篇
  2017年   24篇
  2016年   22篇
  2015年   21篇
  2014年   22篇
  2013年   49篇
  2012年   24篇
  2011年   45篇
  2010年   38篇
  2009年   61篇
  2008年   37篇
  2007年   31篇
  2006年   39篇
  2005年   43篇
  2004年   36篇
  2003年   25篇
  2002年   22篇
  2001年   19篇
  2000年   19篇
  1999年   9篇
  1998年   13篇
  1997年   12篇
  1996年   7篇
  1995年   10篇
  1994年   11篇
  1993年   7篇
  1992年   8篇
  1991年   9篇
  1990年   8篇
  1989年   10篇
  1988年   9篇
  1987年   14篇
  1986年   8篇
  1985年   13篇
  1984年   20篇
  1983年   19篇
  1982年   9篇
  1981年   11篇
  1980年   7篇
  1979年   9篇
  1978年   5篇
  1977年   5篇
  1976年   6篇
  1975年   5篇
  1958年   6篇
  1950年   5篇
  1948年   6篇
排序方式: 共有944条查询结果,搜索用时 31 毫秒
871.
Zusammenfassung Stabilitätsberechnung ebener und räumlicher FelsböschungenFür die Bemessung der Ankerkräfte zur Sicherung einer gleitgefährdeten Felsmasse wird für das ebene Problem eine Grundformel hergeleitet. Der Einfluß der Kohäsion kommt dabei explizit zum Ausdruck, die Einflüsse der Geometrie, des Reibungswinkels und des Sicherheitsfaktors treten in zwei Faktoren zusammengefaßt in Erscheinung. Die Aufdeckung einer formalen Analogie zwischen dem Abgleiten auf einer Ebene und dem Abgleiten eines Felskeiles auf zwei Ebenen erlaubt es, die einfache Grundformel des ebenen Problems direkt für die Lösung des verwickelten räumlichen Problems anzuwenden. Die Bemessungsformel dient in beiden Fällen auch zur Durchführung einer umfangreichen Parameteranalyse um die Empfindlichkeit der Konstruktion auf die Variation der einzelnen Parameter zu ermitteln. Diese Untersuchungen können rasch und ohne langwierige Rechnungen selbst im Felde durchgeführt werden. Als Hilfsmittel hiezu dienen entweder die beiliegenden Diagramme oder das für den programmierbaren Taschenrechner Hewlett Packard HP-65 angegebene Programm. Anhand einiger Beispiele wird die Anwendung auf praktische Fälle des Felsbaues illustriert.
Summary Stability Analysis of Rock Slopes for Plane and Wedge FailureA simple formula for plane slope failure is derived, which may be used for the design of anchors to ensure the safety of rock masses endangered by slides. The cohesional strength component appears explicitly in the formula, while the geometrical parameters, the friction angle and the safety factor are included in only two factors,k 1 andk 2. The discovery of a formal analogy between the plane failure problem and that of wedge failure on two intersecting plane surfaces makes it possible to use the basic formula also for the solution of the complicated three-dimensional wedge problem. The basic formula may in both cases also be used to make a detailed parametric study to investigate the sensitivity of the rock slope construction to variations in individual parameters. This investigation may be carried out quickly, without time consuming calculations, even at the site. As an aid for this either the accompanying charts or the given program for the Hewlett Packard HP-65 pocket calculator may be used. The application of the method is illustrated in several practical examples of the design of rock slopes.

Résumé Calculs de stabilité de talus rocheux à deux et trois dimensionsDans le cas du problème plan, on a mis au point une formule fondamentale pour le dimensionnement des ancrages qui doivent retenir une masse rocheuse susceptible de glisser. L'influence de la cohésion y figure explicitement, celle de la géométrie, de l'angle de frottement et du coefficient de sécurité y apparaissent implicitement dans deux facteurs. On a découvert une analogie formelle entre le glissement sur un plan et le glissement d'un coin de roche le long de deux plans. Ceci permet d'appliquer directement la formule fondamentale, du problème plan, qui est simple, à la résolution du problème enchevêtré qui se présente dans l'espace. Dans les deux cas, la formule de dimensionnement permet aussi la réalisation d'une vaste analyse paramétrique pour déterminer la sensibilité de la construction à la variation des paramètres individuels. Ces investigations peuvent êtres faites à la fois rapidement et sans calculs fastidieux sur le terrain-même, si on se sert soit des diagrammes ci-joints, soit du programme indiqué pour la calculatrice de poche Hewlett Packard HP-65. Quelques exemples illustrent l'application à des cas concrets.


Mit 10 Abbildungen  相似文献   
872.
 Lavas erupted in the Tuxtla Volcanic Field (TVF) over the last 7 Ma include primitive basanites and alkali basalts, mildly alkaline Hy-normative mugearites and benmoreites, and calc-alkaline basalts and basaltic andesites. The primitive lavas are silica-undersaturated, with high concentrations of both incompatible and compatible trace elements, variable La/Yb with constant Yb at 6 to 8 times chondritic, and low Sr and O and variable Pb and Nd isotopic ratios. The primitive magmas originated by increasing degrees of melting with pressure decreasing from greater than 30 kbar to 20 kbar, in the garnet stability field. Another group of alkali basalts and hawaiites has lower Ni and Cr concentrations and higher Fe/Mg ratios, and was derived from the primitive group by crystal fractionation at pressures of several kbar. Incompatible trace elements in these silica undersaturated lavas show depletion in high field strength elements (HFSE) relative to large ion lithophile elements, similar to subduction-related basalts. Ba/Nb ratios are nearly constant and thus the HFSE depletion cannot be the result of a residual HFSE-bearing phase in the source, but could be the result of generation from a source contaminated by fluids or melts from the subducted lithosphere. The silica-saturated mugearites and benmoreites, and the calc-alkaline basalts and basaltic andesites, were erupted only between 3.3 and 1.0 Ma. These have incompatible element concentrations generally lower than in the silica-undersaturated lavas, and thus could not have been derived by crystal fractionation from the silica-undersaturated alkaline magmas. Magmas parental to the silica-saturated magmas originated by higher degrees of melting at lower pressures than the primitive magmas. Melting may have been promoted by an influx of fluid from the subducted lithosphere. Trace element and Sr, Nd, Pb and O isotopic data suggest that three components are involved in the generation of TVF magmas: the mantle, a fluid from the subducted lithosphere, and continental crust. TVF alkaline lavas are similar to those erupted in the back-arc region of the MVB and Japan, and show characteristics similar to alkaline magmas erupted in the southern Andean volcanic arc. These low degree melts reach the surface along with calc-alkaline lavas in the TVF due to an extensional stress field that allows their passage to the surface. Received: 15 September 1994/Accepted: 14 February 1995  相似文献   
873.
874.
The Casamance estuary, on the coast of Senegal, is an inverse hypersaline estuary: salinity increases landward, and dry season salinity values are up to 172 psu due to the evaporation of seawater. Dissolved inorganic carbon (DIC) concentrations decreased landward as a negative linear function of salinity. Thermodynamic modelling and the absence of CaCO3 in the sediments indicate that this loss of DIC was not due to calcite precipitation in the main water body. The innermost, almost landlocked, waters contained high phytoplankton biomass (50–300 μg chl I?1) and high concentrations of allochthonous dissolved organic carbon. Photosynthetic uptake of DIC and subsequent particulate organic carbon sedimentation is proposed as hypothetical explanation of the relationship between DIC and salinity; localized overheating in shallow waters might also be involved.  相似文献   
875.
Optical dating of tsunami-laid sand from an Oregon coastal lake   总被引:1,自引:0,他引:1  
Optical ages for five samples of tsunami-laid sand from an Oregon coastal lake were determined using an infrared optical-dating method on K-feldspar separates and, as a test of accuracy, compared to ages determined by AMS 14C dating of detrital plant fragments found in the same beds. Two optical ages were about 20% younger than calibrated 14C ages of about 3.1 and 4.3 ka. Correction of the optical ages using measured anomalous fading rates brings them into agreement with the 14C ages. The approach used holds significant promise for improving the accuracy of infrared optical-dating methods.Luminescence data for the other three samples result in optical age limits much greater than the 14C ages. These data provide a textbook demonstration of the correlation between scatter in the luminescence intensity of individual sample aliquots and their normalization values that is expected when the samples contain sand grains not adequately exposed to daylight just prior to or during deposition and burial. Thus, the data for these three samples suggest that the tsunamis eroded young and old sand deposits before dropping the sand in the lake.  相似文献   
876.
In a 3-yr study, late prespawning winter flounder were collected from various stations in Long Island Sound (three of them heavily urbanized) and spawned in the laboratory. For comparative purposes, flounder from two sites in the Boston Harbor area were similarly treated in 1987 and 1988. Of the stations in Long Island Sound, New Haven Harbor alone consistently produced low percent viable hatch and small larvae. Boston Harbor produced the smallest larvae of all sites. There were no significant station-to-station differences in lipid utilization during larval development; yolk reserves at stations showing a low percent viable hatch, small larvae, and low embryonic development rate were probably used in part for stress metabolism. No significant differences in concentrations of polychlorinated biphenyls for collections from Long Island Sound were found either in livers of spawned fish, in sediments, or in eggs of winter flounder. The very low metal concentrations in winter flounder eggs showed no relation to the degree of metal contamination found at stations in Long Island Sound and Boston Harbor.  相似文献   
877.
David R. Nelson 《Lithos》1989,22(4):265-274
Kimberlites which intruded the Sisimiut (formerly Holsteinsborg) region of central west Greenland during the Early Palaeozoic have initial 87Sr/86Sr between 0.7028 and 0.7033 and εNd between + 1.3 and + 3.9. Mid-Proterozoic potassic lamproites from the same region have initial 87Sr/86Sr between 0.7045 and 0.7060, εNd between −13 and −10 and unradiogenic initial Pb isotopic compositions. The isotopic data favour an asthenospheric mantle source for the kimberlite magmas, in common with “basaltic” kimberlites from other localities, whereas the lamproite magma sources evolved in isolation from the convecting mantle for > 1000 Ma, probably within the subcontinental lithospheric mantle of the Greenland craton, prior to emplacement of the lamproites.  相似文献   
878.
EDITORIAL   总被引:1,自引:0,他引:1  
  相似文献   
879.
David R. Nelson 《Lithos》1992,28(3-6):403-420
The potassic igneous rock suite (with molar K2O/Na2O > 1) can be divided into an “orogenic” subgroup that occur in subduction-related tectonic settings and an “anorogenic” sub-group that are confined to stable continental settings. Representatives of both sub-groups possess trace element and isotopic features consistent with the contamination of their magma sources by incompatible element rich and isotopically evolved “metasomatic” components. It is argued here that these metasomatic components are principally derived from subducted lithosphere, including subducted sediments. Most examples of orogenic potassic magmatism (e.g. Italian potassic rocks, Spanish lamproites, Sunda arc leucitites) have trace-element and Sr, Nd and Pb isotopic characteristics consistent with the contamination of their mantle sources by a component derived from marine sediments. Anorogenic sub-group potassic magmas have generally similar incompatible trace element and Sr and Nd isotopic characteristics to those of orogenic potassic magmas, but many examples have unusual Pb isotopic compositions with unradiogenic 206Pb/204Pb. Modern marine sediments characteristically have low U/Pb ratios and the unradiogenic 206Pb/204Pb of anorogenic potassic magmas may have evolved during long-term storage of subducted sediments (or components derived from them) within the subcontinental lithosphere. These unusual Pb isotopic compositions require substantial time periods (> 1 Ga) to have elapsed between the fractionation events lowering the U/Pb ratio (i.e. erosion and sedimentation at the Earth's surface) and subsequent potassic magmatism and it is therefore not surprising that most examples of anorogenic potassic magmatism are not associated with recent subduction processes. Although the eruption of potassic magmas is commonly related to rifting or hotspot activity, these processes do not necessarily play an important role in the genesis of the unusual sources from which potassic magmas are derived.  相似文献   
880.
The highly alkaline springs of the Maqarin area of northern Jordan are currently under examination as part of an international project testing the models used to analyse the safety of repositories for low and intermediate level radioactive waste (L/ILW). The Maqarin area contains a rock-groundwater system which is an ideal natural analogue of a concrete-filled L/ILW repository emplaced in a sedimentary host rock. The high pH (12.5) groundwaters at this site are the product of interaction with naturally occurring cement minerals and not of the alteration of ultramafic minerals. Stable isotope data for the hyperalkaline groundwaters lie below both the local meteoric water line and the eastern Mediterranean water line. At least part of the shift appears to be the result of groundwater interaction with the cement minerals. This paper introduces the site of the natural analogue study and, in presenting novel data on the chemistry and stable isotopic signature of the groundwater, provides background information which is used in a companion paper (Alexander et al., 1992) on testing the predictive capabilities of geochemical thermodynamic codes (and their associated databases) which will be utilised as part of the safety assessment of a L/ILW repository.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号