全文获取类型
收费全文 | 114392篇 |
免费 | 1829篇 |
国内免费 | 776篇 |
专业分类
测绘学 | 2639篇 |
大气科学 | 7638篇 |
地球物理 | 22416篇 |
地质学 | 41159篇 |
海洋学 | 10176篇 |
天文学 | 26307篇 |
综合类 | 314篇 |
自然地理 | 6348篇 |
出版年
2022年 | 718篇 |
2021年 | 1216篇 |
2020年 | 1324篇 |
2019年 | 1448篇 |
2018年 | 3217篇 |
2017年 | 2988篇 |
2016年 | 3413篇 |
2015年 | 1802篇 |
2014年 | 3325篇 |
2013年 | 5882篇 |
2012年 | 3643篇 |
2011年 | 4747篇 |
2010年 | 4216篇 |
2009年 | 5449篇 |
2008年 | 4731篇 |
2007年 | 4843篇 |
2006年 | 4488篇 |
2005年 | 3613篇 |
2004年 | 3678篇 |
2003年 | 3329篇 |
2002年 | 3019篇 |
2001年 | 2589篇 |
2000年 | 2524篇 |
1999年 | 2046篇 |
1998年 | 2139篇 |
1997年 | 1979篇 |
1996年 | 1717篇 |
1995年 | 1720篇 |
1994年 | 1475篇 |
1993年 | 1378篇 |
1992年 | 1286篇 |
1991年 | 1326篇 |
1990年 | 1336篇 |
1989年 | 1183篇 |
1988年 | 1087篇 |
1987年 | 1292篇 |
1986年 | 1118篇 |
1985年 | 1394篇 |
1984年 | 1587篇 |
1983年 | 1515篇 |
1982年 | 1377篇 |
1981年 | 1335篇 |
1980年 | 1176篇 |
1979年 | 1102篇 |
1978年 | 1083篇 |
1977年 | 953篇 |
1976年 | 951篇 |
1975年 | 926篇 |
1974年 | 896篇 |
1973年 | 990篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Considerable water leakages from reservoirs make it difficult to attain the planned storage capacity. In some cases water leakages give rise to suffusion followed by catastrophes. Until recently methods for locating water leakages were extremely imperfect. Geophysical methods offer good prospects in this direction. For solving these problems, it is effective to use streaming potential measurements, water flow rate observations and thermometry. Laboratory experiments were carried out in connection with the fact that water leakages from reservoirs are characterized by negative anomalies of natural currents; the more filtration discharge, the higher these anomalies are. As a result, the relationship governing the intensity of streaming potential and sand granulometric composition, electrolyte concentrations and other factors were revealed. To determine the velocity of water flowing to leakage sites, a special device, based on the relationship between the temperature of a heated body and its resistance, is applied. This device simultaneously makes it possible to measure the water medium temperature. Observations at reservoir sites were effected by moving along the reservoir non-polarizable potential electrodes and water velocity devices. Recordings were carried out automatically by the recording device of the logging apparatus. Under the conditions of ice cover on water surfaces, measurements were made through separate points by digging holes in the ice cover. Practical field observations were carried out at reservoir sites located in regions where fissured massive rocks as well as loose sediments predominate. In the first case field experiments were carried out in alpine reservoirs, in Armenia. The major water leakages were found to be concentrated on the right bank of the reservoir. In this connection it was not only possible to locate water leakage sites, but also to evaluate their relative intensity. These data were used for planning antifiltration measures. In the second case water leakages from a reservoir located in Uzbekistan in the submontane part of the Pamirs were studied. Streaming potential anomalies and high benthonic flow rates made it possible to discover high filtrations in the base and walls of the dam. Further perfection of these methods should not only permit the determination of water leakage sites and their relative intensity, but also filtration discharges in absolute units. 相似文献
952.
953.
A high resolution Boomer system was used to investigate the bedrock configuration of part of Port Jackson, New South Wales. The continuous reflection profiling technique was successful in delineating a channel incised in bedrock under a thickness of up to 60 m. of unconsolidated sediment covered by up to 20 m. of water. 相似文献
954.
In a paper by Koefoed and Kegge (1968), which was based on previous work of Wesley (1958), the electrical current pattern has been derived that is induced by an oscillating magnetic dipole in a semi-infinite thin plate of infinitesimal resistivity. In the present paper, the range of validity of the assumptions, on which the work of Wesley is based, is subjected to a theoretical analysis. It is found that the decisive factor for the validity of Wesley's derivation is the quotient of the square of the penetration depth of the electrical current over the product of the thickness of the plate and a distance that is indicative of the size of the current loops in the plate. Wesley's derivation is shown to be valid only when this factor is negligible. It is also shown that in this condition the imaginary component of the anomaly must be negligible. Model experiments are described in which the electrical current pattern is studied also in the range in which the derivation of Wesley is not valid. The procedure used in these model experiments was to measure the tangential component of the magnetic field strength very close to the metal plate that simulated the conductive dyke. In order to express the results of the measurements in terms of the imaginary to real ratio, these results are compared with an interpretation graph for field measurements that was published by Hedström and Parasnis (1958). It is found that the current pattern in the plate is essentially the same as that which follows from Wesley's derivation, provided that the imaginary to real ratio is less than one third. The measurements do not permit to draw conclusions regarding the current pattern in the plate in conditions that correspond to larger values of the imaginary to real ratio. 相似文献
955.
H. G. Dawe 《The Photogrammetric Record》1969,6(34):335-347
This paper describes the planning and implementation of a very extensive aerial survey contract undertaken by Hunting Surveys on behalf of the Hong Kong Government. From photography carried out during January and February 1963, photogrammetric plotting at 1 : 600 and 1 : 1200 proceeded steadily over a period of 5 years to the point where all but the highest land in the Colony had been surveyed at one or other of these two large scales. The problems arising from the rather unique terrain conditions and the urban and industrial development of Hong Kong are dealt with mainly from the photographic and photogrammetric aspect but reference is made to the contribution of the Crown Lands and Survey Office of the Public Works Department of Hong Kong, who provided all ground control and undertook field verification. 相似文献
956.
957.
G. R. Cooper G. Petrie A. D. N. Smith M. Sowton K. B. Atkinson 《The Photogrammetric Record》1969,6(33):239-258
The International Society for Photogrammetry held the Xlth International Congress at Lausanne during July 1968. The authors review the activities of five of the seven I.S.P. Commissions. 相似文献
958.
Pole-to-pole moisture conditions for the IGY 总被引:1,自引:0,他引:1
Summary A study of the mean atmospheric humidity conditions on a planetary scale during the IGY covering the calendar year 1958 is presented. The fields of mean precipitable water content and of the zonal and meridional transports of water vapor are analyzed for the entire globe. Zonally averaged values of the various quantities at several levels are presented in tabular form and compared whenever possible with previous results or indirectly are analyzed on the basis of information obtained from different sources. The structure of these fields is studied and the corresponding implications for the general circulations of the atmosphere are discussed. Finally the zonal water balance for all the globe is discussed and its implications analyzed. 相似文献
959.
V. A. Williams 《The Photogrammetric Record》1968,6(31):18-23
Topics of immediate practical interest formed the basis of the Address. Particular emphasis was placed upon problems of point transfer, pre-marking of ground control points, aerial triangulation by observation of independent models, and the status of photogrammetric operators. 相似文献
960.
M. G. LAIRD 《Sedimentology》1968,10(2):111-120
Curved or planar discordant surfaces occurring within a limited stratigraphic range immediately beneath shallow marine deposits represent penecontemporaneous shear surfaces along which slumps have moved. Rotated packets of strata frequently retained within the curved discordant surfaces show both internal and external evidence of having moved laterally. Movement is considered to have been triggered off by a sudden shock or shocks which may have been either of sedimentary or tectonic origin. The slumping occurred at the change in gradient between slope and shallow marine shelf deposits. 相似文献