首页 | 官方网站   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   120580篇
  国内免费   1159篇
  完全免费   9376篇
地球科学   131115篇
  2018年   53篇
  2017年   312篇
  2016年   669篇
  2015年   2940篇
  2014年   5297篇
  2013年   4763篇
  2012年   5677篇
  2011年   6189篇
  2010年   5425篇
  2009年   6278篇
  2008年   6244篇
  2007年   5632篇
  2006年   5029篇
  2005年   4884篇
  2004年   4604篇
  2003年   4143篇
  2002年   4093篇
  2001年   3887篇
  2000年   3796篇
  1999年   3536篇
  1998年   3300篇
  1997年   3251篇
  1996年   3141篇
  1995年   2998篇
  1994年   3115篇
  1993年   2986篇
  1992年   2842篇
  1991年   2513篇
  1990年   2433篇
  1989年   2308篇
  1988年   1730篇
  1987年   1904篇
  1986年   1529篇
  1985年   1301篇
  1984年   1387篇
  1983年   1204篇
  1982年   1233篇
  1981年   1038篇
  1980年   806篇
  1979年   544篇
  1978年   389篇
  1977年   367篇
  1976年   310篇
  1975年   166篇
  1974年   199篇
  1973年   406篇
  1972年   322篇
  1971年   318篇
  1970年   286篇
  1969年   212篇
  1968年   109篇
  1967年   128篇
  1966年   144篇
  1965年   168篇
  1964年   175篇
  1963年   158篇
  1962年   147篇
  1961年   159篇
  1960年   172篇
  1959年   170篇
  1958年   135篇
  1957年   170篇
  1956年   123篇
  1955年   142篇
  1954年   108篇
  1953年   104篇
  1952年   142篇
  1951年   105篇
  1950年   115篇
  1949年   96篇
  1948年   82篇
  1947年   18篇
  1946年   21篇
  1945年   7篇
  1944年   25篇
  1943年   22篇
  1942年   40篇
  1941年   29篇
  1940年   31篇
  1939年   35篇
  1936年   3篇
  1932年   8篇
  1931年   6篇
  1928年   8篇
  1926年   4篇
  1924年   5篇
  1922年   6篇
  1920年   6篇
排序方式: 共有131115条查询结果,搜索用时 46 毫秒
1.
2.
—Careful observation has shown that mining-induced seismicity follows a multimodal distribution, which we assume to arise from many distinct physical processes. The two major modes however, arise from those seismic events that are associated in some way with geological features on the one hand, and those that are associated, among other things, with fracturing in the volume of extreme stress concentrations ahead of the stope faces, on the other. We call the former "genuine" events and the latter "spurious" events.¶Untangling these modes has been a major problem for those researchers wishing to work with unimodal seismic catalogs. Partial separation of the genuine events from a catalog can be obtained by a careful selection from a scatter diagram of log (radiated seismic energy) against log (scalar seismic moment) or equivalently by selecting a threshold value of magnitude say, from an inspection of the Gutenberg-Richter diagram. This threshold is usually considerably greater than the threshold of completeness that can be achieved by modern seismic networks on mines.¶The main objective of this paper will be the demonstration that a simple neural network can improve this separation. In this study, for example, simple elimination below the threshold of log (scalar seismic moment) = 9.5 resulted in 206 genuine events remaining in the catalog. After running the eliminated events through a trained neural network, an additional 72 genuine events were found, representing an increase of nearly 35%.¶This has important consequences for statistical hazard analysis and for the identification of active geological structures in mines.  相似文献   
3.
4.
5.
--A distribution-free statistical approach is proposed for tackling the problem of regionalization devoted to the study of magnitude-intensity relationships in the Mediterranean area. The training-set constitutes a compilation of more than 2000 earthquakes which occurred in the Mediterranean region since the end of the nineteenth century, when instrumental data became available, with epicentral or maximum intensity S VI and MS magnitude values. Firstly an empirical magnitude-intensity correspondence has been computed for each intensity class by using the entire data set in the assumption of homogeneity at the regional scale. Residuals of such relation have been analyzed by a distribution-free statistical approach in order to evaluate the opportunity of a regionalization able to locally improve the performances of magnitude-intensity relationships. The analysis indicates that data concerning larger earthquakes (intensity S VII) do not suggest the opportunity of zonation, and that unbiased estimates of macroseismic magnitude can also be obtained in the assumption that magnitude-intensity correspondences are uniform over the entire Mediterranean area. Therefore, better constrained relations determined for the entire Mediterranean region should be preferred to ill-defined local ones. As concerns smaller events (intensity VI), the procedure suggests that medium/small-scale lateral variations (on a wavelength lower than 102 km) should be taken into account if an efficient estimate of magnitudes from maximum observed intensity is pursued, but that data presently available are not sufficient to suggest any reliable zonation of the area under study.  相似文献   
6.
7.
--An attempt has been made to use the global optimization technique of Simulated Annealing (S.A.) inversion to interpret the conductivity structure derived from the geomagnetic deep sounding data of N-W India. The present results supersede the earlier result obtained by 2-D conventional linearized inversion. The earlier linearized inversion, following an iterative gradient search technique on the same data set, has been re-evaluated and further constrained through an exhaustive search of the parameter space. The conductive response of an hypothesized conductor located between Ujjain and Guna, India, has been modelled by this random search tool. The location of the proposed model is now in agreement with the theory, since the conductive bodies are centered exactly below the center of the response function. In an earlier attempt by linearized inversion, this was not feasible. The central body is located at a depth of 19 km from the surface, suggesting a thickness of 15 km and resistivity of 14 ohm.m. The resistivity contrast of this ensemble of conductive bodies with the background varies by a factor of 100 to 385. This low conductivity contrast with respect to the background is in conformity to the low temperature as inferred from the available heat-flow data in the region. A marginally different estimate of the conductivity (normally mid-crustal conductors are assigned conductivities of the order of 50 to 200 ohm.m) for the mid-crustal conductor has been found. Existence of a mid-crustal conductor is clearly indicated which was also not detected in the earlier study. Low geo-temperature gradients existing in the region rule out the possibility of a thermal origin for this mid-crustal conductor. A likely explanation could be due to the presence of graphitic carbon at lower crustal depths. However, the role of electrolytic fluid present in the interconnected pore--spaces of rocks may be another tangible explanation.  相似文献   
8.
—Changes of the primary strain-stress state (caused by interaction between natural conditions and mining activity) can result, under special circumstances, to the origin of seismic induced events. The question of induced seismic activity prediction was treated as a problem of time series extrapolation of maximum cumulative amplitudes and numbers of seismic events recorded per day. The treatment was carried out by means of Multilayered Perceptron Neural Networks (MLP NN). The application to mining tremor prediction has been tested and methodological conditions have been obtained. It was proved that the prediction of the number of mining tremors per day is more precise than the prediction of future energy (maximum amplitudes). Further advance, based on the processing of seismo-acoustic activity series, is introduced.  相似文献   
9.
--A tensiometric method (Wilhelmy plate) is used to study Wood's metal dynamic wettability on specially prepared surfaces of quartz single crystals at low velocities of immersion (2 to 16 wm/s). After correction for buoyancy, the force exerted on the plate by the molten metal is used to derive the unit work of wetting n cos Š where n is the Wood's metal surface tension and Š is the interfacial contact angle. Tests at different temperatures (85, 120 and 200°C) show that below 120°C, viscosity effects cannot be neglected. At an immersion/emersion rate of 2 wm/s, n cos Š is in the range 0.417-0.444 N/m at 120°C, and 0.432-0.458 N/m at 200°C. These figures allow the conversion of injection pressures into capillary diameters during Wood's metal injection tests. The method is promising since it may be used in porous materials to check the sensitivity of the unit work of wetting n cos Š to parameters such as roughness and mineralogy, which are known to vary in a wide range within the pores and cracks of rocks.  相似文献   
10.
Seismotectonic Model and CN Earthquake Prediction in Italy   总被引:4,自引:0,他引:4  
  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-23

京公网安备 11010802026262号