全文获取类型
收费全文 | 114549篇 |
免费 | 1901篇 |
国内免费 | 1204篇 |
专业分类
测绘学 | 2947篇 |
大气科学 | 8824篇 |
地球物理 | 23317篇 |
地质学 | 42253篇 |
海洋学 | 9607篇 |
天文学 | 23775篇 |
综合类 | 384篇 |
自然地理 | 6547篇 |
出版年
2021年 | 723篇 |
2020年 | 821篇 |
2019年 | 873篇 |
2018年 | 4280篇 |
2017年 | 4065篇 |
2016年 | 3743篇 |
2015年 | 1684篇 |
2014年 | 2388篇 |
2013年 | 4867篇 |
2012年 | 3524篇 |
2011年 | 5781篇 |
2010年 | 5303篇 |
2009年 | 6364篇 |
2008年 | 5327篇 |
2007年 | 5715篇 |
2006年 | 3459篇 |
2005年 | 3387篇 |
2004年 | 3147篇 |
2003年 | 3101篇 |
2002年 | 2880篇 |
2001年 | 2384篇 |
2000年 | 2301篇 |
1999年 | 2013篇 |
1998年 | 1918篇 |
1997年 | 1949篇 |
1996年 | 1717篇 |
1995年 | 1565篇 |
1994年 | 1436篇 |
1993年 | 1310篇 |
1992年 | 1268篇 |
1991年 | 1240篇 |
1990年 | 1236篇 |
1989年 | 1166篇 |
1988年 | 1115篇 |
1987年 | 1268篇 |
1986年 | 1215篇 |
1985年 | 1412篇 |
1984年 | 1608篇 |
1983年 | 1507篇 |
1982年 | 1402篇 |
1981年 | 1356篇 |
1980年 | 1208篇 |
1979年 | 1173篇 |
1978年 | 1123篇 |
1977年 | 1075篇 |
1976年 | 964篇 |
1975年 | 969篇 |
1974年 | 971篇 |
1973年 | 1005篇 |
1972年 | 617篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Harold D. Palmer 《Geo-Marine Letters》1982,2(3-4):185-190
Geophysical surveys on the upper continental slope and shelf-break area off North Carolina confirm previous suggestions that
this highly dissected region is characterized by deposition of sediment swept from the adjacent shelf. Seismic reflection
profiling, echo sounding and side-scan sonar studies failed to reveal conclusive evidence for slumping or other gravity processes
which have been described for similar areas to the north. Outcrops appear to be lacking and the upper slope is a region of
sediment accumulation and/or periodic bypassing. 相似文献
992.
The asteroid Patiria miniata was used to develop a static bioassay for various crude oil water-soluble fractions (WSF). The criterion used is the size of 48-h exposed embryos. The reasons for choosing this species and the growth criterion are discussed. Experiments show that the body length of 48-h exposed embryos is inversely proportional to hydrocarbon concentration of the WSF and the time of exposure. It is also shown that the toxicity of the WSF decreases with the age of the preparation and that the first 12 h following fertilisation are most sensitive to the effects of WSF. 相似文献
993.
994.
Like any other engineering structure, the dynamic aspects of semisubmersible offshore platforms require serious consideration. The free vibrations of the semisubmersible structures have been investigated in the present work, in which the effect of the variation of the length, draft and hull spacings on the natural frequencies and mode shapes has been studied and the nature of the variations and their reasons have been discussed. 相似文献
995.
分析表观耗氧量、滴定碱度及总二氧化碳量等资料来研判红海及亚丁湾间之海水交换。结果显示,红海深层水的方解石及霰石饱和度均比亚丁湾和阿拉伯海深层水的饱和度高。红海全水柱之方解石和霰石都处於过饱和状态,亚丁湾和阿拉伯海中各深度之方解石亦呈过饱和状态,但霰石的饱和探度则大约在500m左右。分析深层水之生物体无机碳与有机碳的分解比值,可以发现此地区深层水中,大约有25%的总二氧化碳增加量是由无机碳酸钙溶解而来。 相似文献
996.
Single-channel seismic reflection profiles show evidence for areas of significant gas accumulation at the head of the Cape Fear Slide on the continental rise off North Carolina. Gas accumulation appears to occur beneath a gas hydrate seal in landward-dipping strata and in domed strata associated with diapirism. In addition, gas venting may have occurred near diapirs located at the head of the slide. 相似文献
997.
A large number of papers and technical reports are available on the technically important problem of structural elements executing transverse vibrations and carrying concentrated masses. In general it is assumed that the attachment is perfectly rigid. On the other hand, a rather limited amount of published work is available when the mass is elastically connected to the structure. The system then exhibits a more complex behavior than in the case of rigid attachment. The present study deals with the solution of the title problem using the well-known normal mode, sinusoidal eigenfunction expansions. 相似文献
998.
Underwater sounds near a fuel receiving facility in western Hong Kong: relevance to dolphins 总被引:1,自引:0,他引:1
Western Hong Kong is home to two species of marine mammals: Indo-Pacific humpbacked dolphins (Sousa chinensis) and finless porpoises (Neophocaena phocaenoides). Both are threatened in many parts of their range in southeast Asia [for example, International Biological Research Institute Reports 9 (1997), 41; Asian Marine Biology 14 (1997) 111]. In 1998, when the new Hong Kong International Airport opened in western Hong Kong, small tankers (about 100 m long, cargo capacity about 6300 metric tons) began delivering fuel to the Aviation Fuel Receiving Facility (AFRF) just off Sha Chau Island, north of the airport. Calibrated sound recordings were taken over a 4-day period from a quiet, anchored boat at distances 80-2000 m from aviation fuel delivery activities at the AFRF. From the recordings, 143 sections were selected for analysis. Narrowband spectral densities on the sound pressures were computed, and one-third octave band levels were derived for center frequencies from 10 to 16,000 Hz. Broadband levels, viz. 10-20,000 Hz. were also computed. The results showed that the Sha Chau area is normally noisy underwater, with the lowest broadband levels measured corresponding to those expected during a storm at sea (sea state 6). This background noise is believed to come largely from heavy vessel traffic in the Urmston Road to the north and east of Sha Chau and from vessels in the Pearl River Estuary to the West. The sound levels from the AFRF tankers are comparable to the levels measured from similar- and smaller-sized supply vessels supporting offshore oil exploration. The strongest sounds recorded were from a tanker leaving the AFRF at distance 100 m from the hydrophone, for which the one-third octave band level at 100 Hz was 141 dB re 1 microPa (spectrum level 127 dB re 1 microPa2/Hz) and the 10-20,000 Hz broadband level was 146 dB. At distances of 100 m or more and frequencies above 300 Hz, the one-third octave band levels were less than 130 dB (spectrum level 111 dB re 1 microPa2/Hz) and decreased with increasing frequency and distance. At distances greater than about 500 m, AFRF-associated sounds were negligible, masked by the generally high noise level of the area and attenuated by poor transmission in the very shallow water (<10 m). Because it is believed that humpbacked dolphins and finless porpoises are not very sensitive to sounds below 300 Hz, the Airport Authority Hong Kong (AA) stipulated that dedicated terminal vessels not radiate underwater sounds at spectrum levels greater than 110 dB re 1 microPa2/Hz at frequencies above 300 Hz and distances greater than 300 m. The spectrum levels at 300 Hz and higher frequencies of sounds from the tankers arriving, departing, or off-loading at AFRF were less than 110 dB re 1 microPa2/Hz even at distances of 200 m or less. The AA stipulation was met. However, it is presently unknown whether the generally strong noise levels of western Hong Kong inhibit acoustically based feeding and communication, or result in increased stress or permanent shifts in hearing thresholds. 相似文献
999.
The Karman Vortex Street generated by a circular cylinder is investigated by the numerical solution of the compressible Navier–Stokes equations in the incompressible Mach number range (Mach<0.3) using the Beam and Warming implicit scheme. The agreement with the fully incompressible projection method (Chorin, 1968) is fairly good while convergence time is very much better. The investigation suggests that the compressible Navier–Stokes equations may be used as an efficient alternative to study incompressible flows as well. Mach numbers just below 0.3 are enough to simulate incompressible flow behavior and at the same time do not cause numerical ill-conditioning in the solution. In addition, some relevant features of the vortices generated and carried by the wake of the cylinder could be fairly well captured. 相似文献
1000.
Spatial variability in the primary productivity in the East China Sea and its adjacent waters 总被引:7,自引:0,他引:7
Primary productivity in the East China Sea and its adjacent area was measured by the13C tracer method during winter, summer and fall in 1993 and 1994. The depth-integrated primary productivity in the Kuroshio
Current ranged from 220 to 350 mgC m−2d−1, and showed little seasonal variability. High primary productivity (above 570 mgC m−2d−1) was measured at the center of the continental shelf throughout the observation period. The productivity at the station nearest
to the Changjiang estuary exhibited a distinctive seasonal change from 68 to 1,500 mgC m−2d−1. Depth-integrated primary productivity was 2.7 times higher in the shelf area than the rates at the Kuroshio Current. High
chlorophyll-a specific productivity (mgC mgChl.-a−2d−1) throughout the euphotic zone was mainly found in the shelf area rather than off-shelf area, probably due to higher nutrient
availability and higher activity of phytoplankton at the subsurface layer in the shelf area. 相似文献