全文获取类型
收费全文 | 394篇 |
免费 | 17篇 |
国内免费 | 6篇 |
专业分类
测绘学 | 8篇 |
大气科学 | 29篇 |
地球物理 | 131篇 |
地质学 | 109篇 |
海洋学 | 37篇 |
天文学 | 62篇 |
综合类 | 4篇 |
自然地理 | 37篇 |
出版年
2023年 | 2篇 |
2021年 | 5篇 |
2020年 | 6篇 |
2019年 | 7篇 |
2018年 | 11篇 |
2017年 | 8篇 |
2016年 | 18篇 |
2014年 | 9篇 |
2013年 | 13篇 |
2012年 | 9篇 |
2011年 | 25篇 |
2010年 | 8篇 |
2009年 | 18篇 |
2008年 | 27篇 |
2007年 | 25篇 |
2006年 | 28篇 |
2005年 | 19篇 |
2004年 | 23篇 |
2003年 | 13篇 |
2002年 | 13篇 |
2001年 | 4篇 |
2000年 | 11篇 |
1999年 | 5篇 |
1998年 | 7篇 |
1997年 | 5篇 |
1995年 | 8篇 |
1994年 | 4篇 |
1993年 | 8篇 |
1992年 | 3篇 |
1991年 | 5篇 |
1990年 | 6篇 |
1989年 | 9篇 |
1988年 | 8篇 |
1987年 | 2篇 |
1986年 | 5篇 |
1985年 | 3篇 |
1984年 | 7篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 5篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1971年 | 2篇 |
1970年 | 1篇 |
1969年 | 1篇 |
1968年 | 1篇 |
1967年 | 2篇 |
排序方式: 共有417条查询结果,搜索用时 15 毫秒
121.
A guided wave along a borehole is often observed in borehole radar measurements. These guided waves deform the antenna pattern and can cause artefacts in radar measurements. A water-filled borehole or a conducting logging cable can function as a waveguide for electromagnetic waves under some conditions. We describe the theoretical characteristics of such a guided wave in a borehole and compare them with our experiments. The measured signal discussed was obtained with a directional borehole radar. This radar uses a cylindrical conformal array antenna as receiver and is a model of a conducting structure in a borehole. The induced field around the borehole was compared with the theory. The most fundamental symmetrical and asymmetrical modes were TM01 and HE11, and they were identified in the measured signals using time–frequency distribution analysis and by observation of the azimuthal field distribution of the magnetic field. 相似文献
122.
—We investigate the distribution of partial melt in island arc using the seismic velocity structure of the mantle wedge beneath northeastern Japan. The comparison of the seismic tomography with laboratory velocity data on a partially-molten mantle rock yields estimates of melting zones in three dimensions. We employ experimental data on the degree of partial melt in hydrous peridotite to give constraints on the melt fraction and temperature. Melting and magma-rich zones derived from the velocity structure coincide with observed low Q zones. The results of the three-dimensional mapping indicate that the source of magma in island arc is diapir-like melting patches localized within the low velocity zones of the mantle wedge. Extensive volcanic activity along the volcanic front is due to the presence of vast magma-rich zones just beneath the Moho. Those melting zones in the uppermost mantle may, in turn, cause melting of lower crustal materials and produce felsic magma. Melt appears to stay at and beneath the Moho, where crystallization fractionation may proceed. Melt exists at greater depths in the back-arc region, which may correlate with across-arc variations of chemical compositions of the volcanic rocks observed in northeastern Japan. We suggest that magma migration in the ductile lower crust may cause low-frequency microearthquakes, and magma penetration into the brittle upper crust may produce mid-crustal S-wave reflectors. 相似文献
123.
Diatom assemblages and sulfur content in sediments were analyzed to clarify changes in the sedimentary environment of Kushu Lake, a coastal lake on Rebun Island in Hokkaido, Japan. Salinity variations were assessed by means of a diatom-based index of paleosalinity and the sedimentary sulfur content. This paper discusses the Holocene development of the lake, in relation to Holocene relative sea-level change. For paleoenvironmental interpretation of the lake development, the rationale of the threshold method (Anundsen et al., 1994) was applied.At ca. 8000 yr BP, a coastal embayment (paleo-Kushu Bay) resulted from marine ingression. The threshold elevation at the mouth of the paleo-Kushu Bay kept pace with the rising sea-level, resulting in its enclosure at the culmination of Holocene marine transgression (ca. 6500–5000 yr BP). From predicted relative sea-level at ca. 6000 yr BP for Rebun Island (Nakada et al., 1991), the threshold may have been at least above –3 to –5 m altitude. A freshwater lake environment with strongly anoxic bottom conditions may have occurred from ca. 5500 to 5100 yr BP. After an important episode of marine ingression, the lake was isolated completely from the open sea at ca. 4900 yr BP. The diatom record suggests that the maximum lacustrine extent occurred at ca. 4900–3100 yr BP. Thereafter, water depth decreased at the lake margins.In Kushu Lake, the threshold elevation, due to a build-up of a coastal barrier, prevents us from determining the amplitude of sea-level changes, even though the age of isolation contacts corresponds to periods of regression and climatic deterioration. In spite of isostatic subsidence, the effective protection provided by the well-developed barrier did not allow registration of any relative sea-level fluctuations since its isolation. 相似文献
124.
125.
Recent crustal movements in eastern Taiwan were investigated based on geodetic and seismological data as a case study of the tectonics along a colliding plate boundary. Secular horizontal strain along the Longitudinal Valley, a probable arc-continent collision boundary between the Philippine Sea and Eurasion plates, is compressive with a large rate of 2 microstrains/year, and the maximum compression axis trends in NW-SE direction. This means that nearly two-thirds of the convergence rate between the two plates is consumed in the narrow zone along the valley and confirms the view that the valley is the plate boundary between the two. The amount of uplift in the Central Range, deduced from trigonometric leveling, reaches more than one meter during the past 60 years. Such active orogeny is caused by the high strain rate in this area. Horizontal displacement of the Coastal Range, a leading edge of the Philippine Sea plate, has a left-lateral component, suggesting that the boundary is a region of oblique collision and the range moves toward the north, producing subduction of the Philippine Sea plate off the Coast of northeastern Taiwan.An extensional strain pattern is found northeastern Taiwan near Hualien, in northern Taiwan near Ilan, and in southeastern Taiwan south Taitung. In northeastern Taiwan, the direction of the maximum extension axis is nearly parallel to the isodipth contours of deep-focus earthquakes that occurred under the southern part of the Ryukyu Trench. From a model calculation using a finite-element method, this pattern of strain is interpreted as an edge effect of the collision-subduction junction in northeastern Taiwan. The direction of the maximum extension in northern Taiwan forms high angles with the direction of the tectonic line near Ilan stretching from the Okinawa Trough. This fact is harmonious with the view that the Okinawa Trough is spreading. Extensional strain observed in southeastern Taiwan may be related to the eastward subduction since the area is situated on a transition zone, from collision to subduction. 相似文献
126.
Haruo Sato 《Pure and Applied Geophysics》1988,126(2-4):465-497
The study of coda waves has recently attracted increasing attention from seismologists. This is due to the fact that it is viewed as a new means by which the stress accumulation stage preceding a large earthquake can be measured, since the scattering paths nearly uniformly cover a fairly large region around the focus and observation stations, compared with the direct ray paths. To date, we have had many reports on the temporal variation of the relation between coda duration and amplitude magnitude, and that of the coda attenuationQ
c
–1
which is estimated from coda amplitude decay. Some of these have shown a precursor-like behavior; however, others seem to have shown a coseismic change. We have critically reviewed these reports, and discussed what these observational facts tell us about the change in the heterogeneous crust. We found significant temporal variations, not only in the mean but also in the scatter ofQ
c
–1
, associated with the mainshock occurrence. The formation of new cracks, the reopening and growing of existing cracks, the interaction of these cracks, and the pore water movement through these cracks might correspond to such variations. In addition, we may expect an inhomogeneous distribution of crack clusters in a fairly large region, compared with the aftershock region. The gradual appearance of such crack clusters seems to be the most plausible mechanism by which coda decay gradients are caused to largely scatter in the stress accumulation stage. 相似文献
127.
Shoichiro Fukao Manabu D. Yamanaka Hiromasa Matsumoto Toru Sato Toshitaka Tsuda Susumu Kato 《Pure and Applied Geophysics》1989,130(2-3):463-479
Vertical and temporal variations of three-dimensional wind velocity associated with an upper-tropospheric cold vortex-tropopause funnel system were observed by an MST radar in Japan (the MU radar). Marked changes of vertical velocity and horizontal wind direction were found between the inside and outside of the cold vortex. The vertical velocity activity outside the vortex was asymmetric; it was most active in a sector before the vortex. Unsaturated internal gravity waves in their generation stage contribute predominantly to the vertical velocity activity, suggesting that tropospheric occluded cyclones may be a possible source of middle-atmospheric gravity waves through the geostrophic adjustment process. 相似文献
128.
Manabu D. Yamanaka Shoichiro Fukao Hiromasa Matsumoto Toru Sato Toshitaka Tsuda Susumu Kato 《Pure and Applied Geophysics》1989,130(2-3):481-495
Marked wavelike variations of the lower stratospheric wind observed on 7–10 May, 1985 by an MST radar in Japan (by the MU radar) are analyzed assuming that they are induced by monochromatic internal inertio-gravity waves. These variations are mainly composed of two modes (periods: 22 and 24 hours), both of which have zonal phase velocities (C
X
) slower than the mean westerly wind (). A statistical analysis of the zonal phase velocity shows thatC
X
above andC
X
below the tropopause jet stream, which is considered to be a vivid proof of wave selection due to the tropospheric mean flow and upward wave emission from the tropopause jet. A comparison between the MU radar results and routine meteorological observations leads to the conclusion that the marked waves appear when the jet stream takes a maximum wind speed. 相似文献
129.
Electrical impedance of partially molten gabbro was measured as a function of the frequency of the applied electric field. The impedance of the partially molten gabbro was mainly controlled by the volume and geometry of partial melt, which were observed with an optical microscope study of the quenched sample. The experimental results are interpreted by a theory that formulates the electrical property of rocks containing partial melt, taking ionic diffusion in the melt into account. The theory evaluates the contributions from two types of melt-geometry distributed in the rock matrix, namely:
- 1. (a) nearly isolated melt pockets, and
- 2. (b) connected melt along grain boundaries.
130.
Pablo Diego GonzAlez Ana Maria Sato Eduardo Jorge Llambias Miguel A.S. Basei Silvio R.F. Vlach 《Gondwana Research》2004,7(4):1157-1170
Structural, metamorphic and isotopic data obtained from the Nogoli Metamorphic Complex of western Sierra de San Luis indicate that the Early Paleozoic Famatinian Orogeny overprinted an already structured and metamorphosed older basement. The older geological features are relict NW trending fabric associated with high-grade (amphibolite facies) regional metamorphism preserved within thin strips of schists and paragneisses and in the core of mafic to ultramafic lenses. Arc magmatism, medium P (Barrovian type)/high T (amphibolite to granulite facies) regional metamorphism and penetrative NNE to NE trending foliation are related to the building of the Famatinian orogenic belt. The P-T conditions of the Famatinian prograde metamorphism reached a pressure peak of ca. 8 kb, with a thermal peak from -750°C up to -820°C. U-Pb conventional and chemical dating and Ar-Ar plateau ages constrain the peak of the main orogenic phase related to the Famatinian belt to 470–457 Ma (Early to Mid-Ordovician). Greenschist facies retrograde metamorphism closely associated with shear zones and secondary Ar-Ar plateau and Sm-Nd ages suggest that a late to post-orogenic phase of the Famatinian belt was active at least since -445 Ma. This phase continued during the Silurian to Late Devonian times through multiple reactivation of early shear zones. The Famatinian Orogeny reset a previous thermal history and therefore, the timing of the relict fabric could not be constrained conclusively with radiometric dates. Despite this difficulty, a range of 520 to 490 Ma suggests some inheritance from Pampean events registered by the older NW-SE fabric. The Early to Mid-Ordovician regional metamorphism and ductile deformation of the western Sierra de San Luis is interpreted as the orogenic effects of the collision of the allochthonous Cuyania terrane with the autochthonous proto-Pacific margin of Gondwana during the Famatinian Orogeny. 相似文献