全文获取类型
收费全文 | 13894篇 |
免费 | 2623篇 |
国内免费 | 2735篇 |
专业分类
测绘学 | 624篇 |
大气科学 | 1002篇 |
地球物理 | 4062篇 |
地质学 | 6691篇 |
海洋学 | 1955篇 |
天文学 | 2017篇 |
综合类 | 692篇 |
自然地理 | 2209篇 |
出版年
2024年 | 70篇 |
2023年 | 250篇 |
2022年 | 475篇 |
2021年 | 488篇 |
2020年 | 528篇 |
2019年 | 602篇 |
2018年 | 497篇 |
2017年 | 517篇 |
2016年 | 553篇 |
2015年 | 590篇 |
2014年 | 690篇 |
2013年 | 633篇 |
2012年 | 751篇 |
2011年 | 796篇 |
2010年 | 653篇 |
2009年 | 954篇 |
2008年 | 875篇 |
2007年 | 943篇 |
2006年 | 930篇 |
2005年 | 808篇 |
2004年 | 806篇 |
2003年 | 810篇 |
2002年 | 648篇 |
2001年 | 601篇 |
2000年 | 576篇 |
1999年 | 502篇 |
1998年 | 557篇 |
1997年 | 329篇 |
1996年 | 310篇 |
1995年 | 291篇 |
1994年 | 259篇 |
1993年 | 227篇 |
1992年 | 191篇 |
1991年 | 117篇 |
1990年 | 79篇 |
1989年 | 92篇 |
1988年 | 60篇 |
1987年 | 65篇 |
1986年 | 39篇 |
1985年 | 20篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 6篇 |
1980年 | 6篇 |
1979年 | 10篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1976年 | 5篇 |
1954年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
基于神经网络的平底结构砰击压力预报 总被引:1,自引:0,他引:1
对利用神经网络预报平底结构入水砰击压力的方法进行了探讨。首先利用仿真软件计算各种情况下平底结构入水所产生的砰击压力,以此形成训练神经网络的数据集。其次利用数据集对三层反馈式网络进行了训练,讨论了不同隐含层节点数对该非线性系统的拟合能力,并且对梯度下降法、动量修正法和基于优化的LM算法的有效性和精度进行了比较,最后得出了适合平底结构入水砰击系统的网络结构。 相似文献
122.
123.
124.
苏振明 《中国海洋大学学报(自然科学版)》1992,(4)
对东白令海水域双线鲽(Lepidopsetta bilineata)、太平洋拟庸鲽(Hippoglosso-ides elassodon)和阿拉斯加鲽(Pleuronectes quadrituberculata)等三种经济鲽类的捕捞群体结构和生长特性进行比较。得出结论是:(1)捕捞群体结构复杂;(2)渐近体长和渐近体重较大、寿命长、K值小;(3)雌鱼比雄鱼个体大、生长快、寿命长;(4)三种鲽类的捕捞群体结构和生长特性有明显差异。 相似文献
125.
Japanese fisheries production in the Japan/East Sea between 1958 and 2003 increased to their peak (1.76 million tons) in the late 1980s and decreased abruptly with the collapse of Japanese sardine. Catch results for 58 fisheries and various environmental time-series data sets and community indices, including mean trophic level (MTL) and Simpson’s diversity index (DI), were used to investigate the impacts of fishing and climate changes on the structure of the fish community in the Tsushima warm current (TWC) region of the Japan/East Sea. The long-term trend in fisheries production was largely dependent on the Japanese sardine that, as a single species, contributed up to 60% of the total production in the Japanese waters of the Japan/East Sea during the late 1980s. Excluding Japanese sardine, production of the small pelagic species was higher during 1960s and 1990s but lower during 1970s and 1980s. This variation pattern generally corresponds with the trend in water temperature, warmer before early 1960s and after 1990s but colder during 1970s and 1980s. The warm-water, large predatory fishes and cold water demersal species show opposite responses to the water temperature in the TWC region, indicating the significant impact of oceanic conditions on fisheries production of the Japan/East Sea. Declines in demersal fishes and invertebrates during 1970s and 1980s suggested some impact of fishing. MTL and DI show a similar variation pattern: higher during 1960s and 1990s but lower during 1970s and 1980s. In particular, the sharp decline during the 1980s resulted from the abundant sardine catches, suggesting that dominant species have a large effect on the structure of the fish community in the Japan/East Sea. Principal component analysis for 58 time-series data sets of fisheries catches suggested that the fish community varied on inter-annual to inter-decadal scales; the abrupt changes that occurred in the mid-1970s and late 1980s seemed to correspond closely with the climatic regime shifts in the North Pacific. These results strongly suggest that the structure of the fish community in the Japan/East Sea was largely affected by climatic and oceanic regime shifts rather than by fishing. There is no evidence showing “fishing down food webs” in the Japan/East Sea. However, in addition to the impacts of abrupt shifts that occurred in the late 1980s, the large predatory and demersal fishes seem to be facing stronger fishing pressure with the collapse of the Japanese sardine. 相似文献
126.
127.
Jos Rafael García‐March Antonio Manuel García‐Carrascosa lvaro Luís Pea 《Marine Ecology》2002,23(3):207-217
Abstract. Pinna nobilis Linnaeus 1758 is an endemic bivalve mollusc in the Mediterranean Sea, where it inhabits seagrass meadows, especially Posidonia oceanica (L.) Delile. It is the largest bivalve in the Mediterranean, reaching lengths up to 120 cm. In its natural habitat, P. nobilis lives with the anterior part of the valve buried in the seabed, attached to Posidonia rhizomes by byssus threads. This habit makes it impossible to measure its total length directly in situ. As the only way to determine the individual age is the relationship between age and total length, several equations have been proposed to estimate total length by relating it to the unburied parts of the shell. Such measurements are essential to ecological studies that consider age, growth, and population dynamics, and that evaluate the environmental factors that affect this species.Accurately estimating total length depends on the accuracy and precision of the method employed to measure the unburied shell parts. In this paper, we point out the lack of precision of the instruments and methods used until now; we also demonstrate the reason for this imprecision. A new device to measure unburied parts of Pinna nobilis with a precision comparable to that obtained when measuring extracted valves is described. This device is unaffected by substratum type and reduces measurement time. The latter is a very important feature, because these procedures are usually performed whilst SCUBA diving. Finally, a growth equation has been fitted to the measurements obtained with the new device from a population located in Moraira (Alicante, western Mediterranean). 相似文献
128.
129.
The upper layer (above 140 m depth) temperature in the western Philippine Sea near Taiwan was sampled using a coastal monitoring
buoy (CMB) with 15 attached thermistors during July 28–August 7, 2005. The data were collected every 10 min at 1, 3, 5, 10,
15, and 20 m using the CMB sensors, and every 15 sec at 15 different depths between 25 m and 140 m. Internal waves and solitons
were identified from the time-depth plot of the temperature field. Without the internal waves and solitons, the power spectra,
structure functions, and singular measures (representing the intermittency) of temperature field satisfy the power law with
multi-scale characteristics at all depths. The internal waves do not change the basic characteristics of the multifractal
structure. However, the internal solitons change the power exponent of the power spectra drastically, especially in the low
wave number domain; they also break down the power law of the structure function and increase the intermittency parameter.
The physical mechanisms causing these different effects need to be explored further. 相似文献
130.
Ingi Olafsson Eirik Sundvor Olav Eldholm Kjersti Grue 《Marine Geophysical Researches》1992,14(2):137-162
Analysis in both the x—t and —p domains of high-quality Expanded Spread Profiles across the Møre Margin show that many arrivals may be enhanced be selective ray tracing and velocity filtering combined with conventional data reduction techniques. In terms of crustal structure the margin can be divided into four main areas: 1) a thicker than normal oceanic crust in the eastern Norway Basin; 2) expanded crust with a Moho depth of 22 km beneath the huge extrusive complex constructed during early Tertiary breakup; 3) the Møre Basin where up to 13–14 km of sediments overlie a strongly extended outer part with a Moho depth at 20 km west of the Ona High; and 4) a region with a 25–27 km Moho depth between the high and the Norwegian coast. The velocity data restricts the continent-ocean boundary to a 15–30 km wide zone beneath the seaward dipping reflector wedges. The crust west of the landward edge of the inner flow is classified as transitional. This region as well as the adjacent oceanic crust is soled by a 7.2–7.4 km s–1 lower crustal body which may extend beneath the entire region that experienced early Tertiary crustal extension. At the landward end of the transect a 8.5 km s–1 layer near the base of the crust is recognized. A possible relationship with large positive gravity anomalies and early Tertiary alkaline intrusions is noted. 相似文献