全文获取类型
收费全文 | 95250篇 |
免费 | 1893篇 |
国内免费 | 787篇 |
专业分类
测绘学 | 2623篇 |
大气科学 | 7331篇 |
地球物理 | 19743篇 |
地质学 | 31831篇 |
海洋学 | 8264篇 |
天文学 | 21692篇 |
综合类 | 233篇 |
自然地理 | 6213篇 |
出版年
2021年 | 613篇 |
2020年 | 766篇 |
2019年 | 882篇 |
2018年 | 1739篇 |
2017年 | 1672篇 |
2016年 | 2284篇 |
2015年 | 1493篇 |
2014年 | 2270篇 |
2013年 | 4668篇 |
2012年 | 2404篇 |
2011年 | 3432篇 |
2010年 | 3041篇 |
2009年 | 4296篇 |
2008年 | 3847篇 |
2007年 | 3620篇 |
2006年 | 3615篇 |
2005年 | 2925篇 |
2004年 | 3013篇 |
2003年 | 2822篇 |
2002年 | 2763篇 |
2001年 | 2479篇 |
2000年 | 2388篇 |
1999年 | 2084篇 |
1998年 | 2047篇 |
1997年 | 2054篇 |
1996年 | 1747篇 |
1995年 | 1662篇 |
1994年 | 1494篇 |
1993年 | 1377篇 |
1992年 | 1269篇 |
1991年 | 1154篇 |
1990年 | 1315篇 |
1989年 | 1204篇 |
1988年 | 1059篇 |
1987年 | 1274篇 |
1986年 | 1132篇 |
1985年 | 1380篇 |
1984年 | 1650篇 |
1983年 | 1498篇 |
1982年 | 1423篇 |
1981年 | 1337篇 |
1980年 | 1161篇 |
1979年 | 1132篇 |
1978年 | 1190篇 |
1977年 | 1066篇 |
1976年 | 998篇 |
1975年 | 945篇 |
1974年 | 935篇 |
1973年 | 979篇 |
1972年 | 636篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Q. J. Huggett A. K. Cooper M. L. Somers A. R. Stubbs 《Marine Geophysical Researches》1992,14(1):47-63
GLORIA side-scan sonographs from the Bering Sea Basin show a complex pattern of interference fringes sub-parallel to the ship's track. Surveys along the same trackline made in 1986 and 1987 show nearly identical patterns. It is concluded from this that the interference patterns are caused by features in the shallow subsurface rather than in the water column. The fringes are interpreted as a thin-layer interference effect that occurs when some of the sound reaching the seafloor passes through it and is reflected off a subsurface layer. The backscattered sound interferes (constructively or desctructively) with the reflected sound. Constructive/destructive interference occurs when the difference in the length of the two soundpaths is a whole/half multiple of GLORIA's 25 cm wavelength. Thus as range from the ship increases, sound moves in and out of phase causing bands of greater and lesser intensity on the GLORIA sonograph. Fluctuations (or wiggles) of the fringes on the GLORIA sonographs relate to changes in layer thickness. In principle, a simple three dimensional image of the subsurface layer may be obtained using GLORIA and bathymetric data from adjacent (parallel) ship's tracks. These patterns have also been identified in images from two other systems; SeaMARC II (12 kHz) long-range sonar, and TOBI (30 kHz) deep-towed sonar. In these, and other cases world-wide, the fringes do not appear with the same persistence as those seen in the Bering Sea. 相似文献
992.
993.
A simple sand trap is used to measure swash and backwash bedload transport rates on intertidal profiles. Data from sixty-eight beach experiments are used to calculate a mean value of 12.78 kg m?4s?2 for the calibration coefficient in the Bagnold beach equation. 相似文献
994.
Anaerobic salt marsh sediments were amended with a variety of organic pollutants and the effects on methanogenesis, sulfate reduction and carbon dioxide evolution were examined. Addition of 1000 μg g?1 (dry weight sediment) Arochlor 1221, lindane, endrin, benzene and phenanthrene resulted in no significant effects on the activities studied. Methanogenesis was inhibited by 1000 μg g?1toxaphene, PCP, chlordane, naphthalene, DDT, Kepone and heptachlor and by 100 μg g?1 PCP and toxaphene. At 1000 μg g?1 naphthalene and toxaphene and 100 μg g?1 PCP, a period of initial inhibition of methanogenesis was followed by stimulation relative to controls. Arochlor 1254 (1000 μg g?1) and Temik (500 and 10 μg g?1) stimulated methanogenesis from the outset. Temik at 500 μg g?1 gave the greatest stimulation of methanogenesis (900% of controls) of any of the compounds studied. Sulfate reduction was inhibited by 1000 μg g?1 PCP, toxaphene, naphthalene and chlordane and by 500 μg g?1 atrazine and 100 μg g?1 heptachlor. Sustained inhibition of sulfate reduction by naphthalene, toxaphene and PCP may have contributed to the stimulation of methanogenesis. Carbon dioxide evolution was not significantly affected by most of the compounds studied except for 100 μg g?1 PCP and 1000 μg g?1 aphthalene, each of which gave significant inhibition in only one of three experiments.Concentrations of individual organic pollutants required to cause observable effects were high. It is concluded that, except for highly polluted sediments, methanogenesis, sulfate reduction and CO2 evolution would not be affected by the compounds studied here at concentrations typically found in the environment. 相似文献
995.
The first map of the sea bed morphology and sedimentary features within the RMSTitanic search area is proposed from the interpretation of SAR side-scan sonar images. Downslope sedimentary features such as erosional
furrows and crown scarps constitute a 7 km wide instability corridor. A large field (15 km2) of asymmetrical sediment waves indicating a downslope transport is identified. Current-induced features corresponding to
associated sand ribbons and barchan dunes resulting from the Western Boundary Undercurrent action are mapped. The morphology
of theTitanic Canyon is also precised from the SAR images. Finally, the origin of the sea bed features is discussed in an attempt to link
each bed form to a sedimentary process. 相似文献
996.
Seismic reflection profiling demonstrates the importance of mass movement in the north-central California Borderland. Bulk
properties of surficial hemipelagic sediments in the Santa Barbara, Santa Cruz, and San Nicolas basins show that different
types of mass-transported sediments have different properties, although their general sedimentologic characteristics are very
similar. Slump deposits have moderate water contents (48% wet weight basis), high plasticity indices (46%), and moderate activities
(1.6). For debris flows these values are low: 28%, 5%, and 0.3, respectively, and for other types (liquefied flow, mudflows,
and turbidities) the values are high: 54 to 63%, 49 to 58%, and 1.2 to 1.5, respectively. 相似文献
997.
P. Cobo C. Ranz A. Fernández M. Cuesta D.K. Anthony M. Siguero 《Marine Geophysical Researches》2005,26(2-4):87-95
Vertical resolution is of fundamental importance in sonar exploration and is directly related to the duration of the acoustic
pulse generated by the transducer. The shorter the radiated pulse, the higher the vertical resolution. Many sub-bottom profiling
sonar systems use piezoelectric transducers because they are reversible and well understood. Piezoelectric projectors are
normally resonant transducers, which are intrinsically narrowband. A piezoelectric transducer is usually driven by a tone-burst.
However, it is possible to use Fourier techniques to find a pre-compensated electrical driving function so that the transducer
radiates a prescribed wider band acoustic waveform. This technique can be applied to synthesize zero-phase cosine-magnitude,
Gaussian, and bionic pulses, with a conventional sandwich transducer. Zero-phase cosine-magnitude waveforms provide minimum
length pulses (and therefore maximum resolution) within a prescribed frequency band.The aim of this paper is to illustrate
the synthesis of wideband acoustic pulses using an underwater piezoelectric projector. The conventional acoustic waveform
radiated when a Tonpiltz transducer is transiently excited using a “click” and allows its frequency response function to be
measured. This function is used to design the electrical signal which then drives the transducer so that it radiates the shortest
pulse compatible with its mechanical response. The significant resolution enhancement of the waveform shaping process is illustrated
by its application to a sediment wedge model. 相似文献
998.
In this study, we examined the distribution of polycyclic aromatic hydrocarbons (PAHs) in a contaminated coastal area and the characteristics of the natural organic matter in tandem. We present a detailed study of PAH concentration, distribution, and organic matter characteristics of three core samples from Pensacola Bay, Florida. Solid-state 13C Nuclear Magnetic Resonance (NMR), pyrolysis gas chromatography coupled with mass spectrometry (GC-MS), and tetramethyl ammonium hydroxide (TMAH) thermochemolysis GC-MS were applied to obtain structural details about the sedimentary organic matter. Elemental compositions (carbon and nitrogen) and estimates of black carbon contents are also reported. These coastal sediments were found to contain more PAHs in the upper 15 cm layers than in the bottom 15-25 cm samples. The samples that contained the most PAHs also contained the least amount of aromatic carbon and contained a significant amount of paraffinic carbon. Lignin-derived pyrolysis and TMAH thermochemolysis products were abundant and generally higher in all of the samples in comparison to those reported for modern coastal sediments, indicating a large flux of terrestrial carbon. The black carbon contents were found to range from 4.3% to 6.8%, which are significantly lower than other reports of black carbon in sediments, which represent as much as 65% of the total organic carbon content. The low black carbon content suggests that this type of refractory carbon may not be as responsible for regulating PAH distribution as indicated by other researchers. 相似文献
999.
1000.
Sabra K.G. Roux P. Thode A.M. D'Spain G.L. Hodgkiss W.S. Kuperman W.A. 《Oceanic Engineering, IEEE Journal of》2005,30(2):338-347
Estimates of the travel times between the elements of a bottom hydrophone array can be extracted from the time-averaged ambient noise cross-correlation function (NCF). This is confirmed using 11-min-long data blocks of ambient noise recordings that were collected in May 1995 near the southern California coast at an average depth of 21 m in the 150-700 Hz frequency range. Coherent horizontal wavefronts emerging from the time derivative of the NCF are obtained across the array's aperture and are related to the direct arrival time of the time-domain Green's function (TDGF). These coherent wavefronts are used for array element self-localization (AESL) and array element self-synchronization (AESS). The estimated array element locations are used to beamform on a towed source. 相似文献