全文获取类型
收费全文 | 115604篇 |
免费 | 1878篇 |
国内免费 | 1248篇 |
专业分类
测绘学 | 2948篇 |
大气科学 | 8774篇 |
地球物理 | 23684篇 |
地质学 | 39054篇 |
海洋学 | 10393篇 |
天文学 | 25540篇 |
综合类 | 296篇 |
自然地理 | 8041篇 |
出版年
2021年 | 808篇 |
2020年 | 960篇 |
2019年 | 1021篇 |
2018年 | 2053篇 |
2017年 | 1955篇 |
2016年 | 2711篇 |
2015年 | 1807篇 |
2014年 | 2637篇 |
2013年 | 5807篇 |
2012年 | 2900篇 |
2011年 | 4208篇 |
2010年 | 3698篇 |
2009年 | 5170篇 |
2008年 | 4563篇 |
2007年 | 4372篇 |
2006年 | 4205篇 |
2005年 | 3653篇 |
2004年 | 3643篇 |
2003年 | 3459篇 |
2002年 | 3337篇 |
2001年 | 2945篇 |
2000年 | 2852篇 |
1999年 | 2494篇 |
1998年 | 2437篇 |
1997年 | 2409篇 |
1996年 | 2149篇 |
1995年 | 1966篇 |
1994年 | 1813篇 |
1993年 | 1659篇 |
1992年 | 1580篇 |
1991年 | 1455篇 |
1990年 | 1601篇 |
1989年 | 1442篇 |
1988年 | 1358篇 |
1987年 | 1552篇 |
1986年 | 1433篇 |
1985年 | 1741篇 |
1984年 | 1974篇 |
1983年 | 1866篇 |
1982年 | 1737篇 |
1981年 | 1659篇 |
1980年 | 1426篇 |
1979年 | 1387篇 |
1978年 | 1402篇 |
1977年 | 1298篇 |
1976年 | 1200篇 |
1975年 | 1171篇 |
1974年 | 1163篇 |
1973年 | 1191篇 |
1972年 | 738篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
The sub-iron-iron flux ratio in cosmic rays at an atmospheric depth of 9.8 g cm–2 has been estimated using a balloon-borne CR-39 (HCB) stack launched from Alice Springs for 32 hours. The recovered and chemically etched plates were analysed optically and the measured etch pit diameters yielded the sub-iron-iron flux ratio at the flight altitude. The sub-iron-iron flux ratio has been corrected for the top of the atmosphere by considering the loss of heavy ions due to nuclear interaction and fragmentation. The present result has been compared with the result expected from the source composition derived by Protheroeet al. as well as other authors. 相似文献
102.
Quinn B.G. Barrett R.F. Kootsookos P.J. Searle S.J. 《Oceanic Engineering, IEEE Journal of》1993,18(4):557-564
A hidden Markov model (HMM) technique for the estimation of the shape of a towed array is presented. It is assumed that there is a far-field source radiating sound containing possibly weak spectral lines. The technique uses either the Fourier coefficients at a given frequency computed from a single time block or the maximal eigenvector of a sample spectral covariance matrix. The technique is illustrated using several simulations. The results of these simulations indicate that the HMM technique yields shape and bearing estimates more accurate than those provided by a maximum-likelihood array shape estimation technique 相似文献
103.
104.
This paper considers the transient response of a pressurized long cylindrical cavity in an infinite poroelastic medium. To obtain transient solutions, Biot's equations for poroelastodynamics are specialized for this problem. A set of exact general solutions for radial displacement, stresses, pore pressure and discharge are derived in the Laplace transform space by using analytical techniques. Solutions are presented for three different types of prescribed transient radial pressures acting on the surface of a permeable as well as an impermeable cavity surface. Time domain solutions are obtained by inverting Laplace domain solutions using a reliable numerical scheme. A detailed parametric study is presented to illustrate the influence of poroelastic material parameters and hydraulic boundary conditions on the response of the medium. Comparisons are also presented with the corresponding ideal elastic solutions to portray the poroelastic effects. It is noted that the maximum radial displacement and hoop stress at the cavity surface are substantially higher than the classical static solutions and differ considerably from the transient elastic solutions. Time histories and radial variations of displacement, hoop stress, pore pressure and fluid discharge corresponding to a cavity in two representative poroelastic materials are also presented. 相似文献
105.
Long-range sidescan sonar can be used to map sediment distributions over wide expanses of deep ocean floor. Seven acoustic facies that arise from differing sediment or rock types have been mapped over the low-relief Saharan continental rise and Madeira abyssal plain. These have been calibrated with sampling, profiling and camera studies and the facies can be traced confidently on a regional scale using the sidescan data. The mapping of the sediment distribution shows that a complex interplay of turbidity current and debris flow processes can occur at a continental rise/abysaal plain transition over 1000 km from the nearest continental slope. 相似文献
106.
C. Marchal 《Chinese Astronomy and Astrophysics》1985,9(3):214-220
We investigate the condition for the third body of the problem of three bodies to be an “isolated body”, and the properties of its motion, particularly in the case where the system has a negative total angular momentum. We give the permitted regions for the acceleration and radius vector of the isolated body, the law of variation of its velocity and the region of escape of the isolated body. 相似文献
107.
A. Arjannikova C. Larroque J. -F. Ritz J. Déverchère J. F. Stéphan S. Arjannikov V. San'kov 《地学学报》2004,16(5):265-272
We used satellite imagery and field data to investigate the south‐westernmost Baikal rift zone. We focus our study in the Mondy and Ikhe Ukhgun valleys, site of an Mw = 6.9 seismic event in 1950. Surface deformations are observed along the E–W‐trending Mondy strike‐slip fault and along the Ikhe Ukhgun thrust. The Mondy fault system is 80 km long and is composed of four segments 10–15 km long. These segments are characterized by subvertical planes with left‐lateral movements. The Ikhe Ukhgun thrust is 20 km long, dips 40° to the south and shows reverse movement with a left‐lateral component. These observations are consistent with the present‐day regional NNE–SSW compression and with the focal mechanism of the 1950 Mondy earthquake that was recently re‐evaluated. These features, like those observed in the Tunka basin, demonstrate a recent change of regional strain regime from transtension to transpression that we place before the Late Pleistocene. 相似文献
108.
Autotrophic biomass and productivity as well as nutrient distributions and phytoplankton cell populations in the James River estuary, Virginia, were quantified both spatially and temporally over a 17-month period. Emphasis was placed on the very low salinity region of the estuary in order to gain information on the fate of freshwater phytoplankters. Differing amounts of freshwater plant biomass are advected into the estuary as living material, DOC or POC and the demonstrated variability of this input must play an important role in marine biogeochemical cycling.Late summer and fall maxima in both chlorophyll a and the photosynthetic production of particulate organic carbon in very low salinity regions were inversely correlated with river discharge.During periods of low river discharge greater than 50% of the chlorophyll a biomass measured at 0‰ disappeared within a narrow range of salinity (0–2‰). Cell enumeration data suggest that species introduced from the freshwater end-member tend to comprise the bulk of the biomass removed. Confounding factors, which may contribute to the regulation of both the abundance and species of phytoplankters mid-river, include the flocculation of colloidal material with phytoplankton cells, the presence of the turbidity maximum and the growth of endemic phytoplankton populations.An inverse relationship exists between the phytoplankton abundance in very low salinity waters and the abundance of biomass measured in the lower portion of the river (estuary). Thus, autotrophic production in the fresh and very low salinity areas may indirectly regulate the onset on the spring bloom in the estuary by controlling the amount of nutrients available. 相似文献
109.
Berry Head, a limestone headland in Torbay, southwest England, exhibits a series of subaerial marine erosion platforms and raised beaches spanning an altitude range of 97 m. Solution caves on the headland show preferred horizontal development at elevations that are correlated with the marine erosion platforms, and developed in a marine/freshwater mixing zone whose position was controlled by high sea-level still-stands. Corbridge Cave in Berry Head Quarry lies below the raised beaches in Torbay, and contains evidence of three marine transgressions in the form of fine-grained marine ponding deposits with a marine microfauna. Uranium-series dating of intercalated speleothems indicates that a transgression during oxygen isotope stage 5e reached an elevation of 5.8 m OD, while an earlier transgression (probably during stage 7) reached at least 7.2 m OD. These findings are used to constrain possible interpretations of the aminostratigraphy of raised beaches in Southwest Britain, and a correlation of the Unnamed Stage of Bowen, Sykes, Reeves, Miller, Andrews, Brew and Hare with oxygen isotope stage 5e is proposed. 相似文献
110.
Universal cokriging is used to obtain predictions when dealing with multivariate random functions. An important type of nonstationarity is defined in terms of multivariate random functions with increments which are stationary of orderk. The covariance between increments of different variables is modeled by means of the pseudo-cross-covariance function. Criteria are formulated to which the parameters of pseudo-cross-covariance functions must comply so as to ensure positive-definiteness. Cokriging equations and the induced cokriging equations are given. The study is illustrated by an example from soil science. 相似文献