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21.
Makoto Uyeshima 《Surveys in Geophysics》2007,28(2-3):199-237
There are several kinds of coupling mechanisms which can convert mechanical, chemical or thermal energies due to seismic or
volcanic activities into electromagnetic energies. As a result of concentrated efforts in laboratory and theoretical research,
the basic relationship between the intensity of electromagnetic sources and changes in mechanical, chemical and thermal state
is becoming established. Also with the progress of the electromagnetic simulation techniques, it has been possible to evaluate
in situ sensitivity. Based on this progress and also due to extensive improvement in measuring techniques, many field experiments
have been performed to elucidate subsurface geophysical processes underlying the preparation stage, onset, and subsequent
healing stage of earthquakes and volcanic eruptions. In volcanic studies, many studies have reported the measurement of electromagnetic
signals which were successfully interpreted in terms of various driving mechanisms. Although there have been numerous reports
about the existence of precursory electromagnetic signals in seismic studies, only a few of them could be successfully explained
by the proposed mechanisms, whereas coseismic phenomena are often consistent with those mechanisms including the absence of
detectable signals. In many cases, one or two orders of higher sensitivity were required, especially for precursory signals.
Generally, electromagnetic methods are more sensitive to near-surface phenomena. It will be necessary to discriminate electromagnetic
signals due to these near-surface sources, which often possess no relationship with the crustal activities. Further efforts
to enhance in situ sensitivity through improvements in observation techniques and in data processing techniques are recommended.
At the same time, multi-disciplinary confirmation against the validity of electromagnetic phenomena will inevitably be necessary.
A Network-MT observation technique has been developed to determine large-scale deep electrical conductivity structure. In
the method, a telephone line network or purpose-built long baseline cables are utilized to measure voltage differences with
long electrode separations. Because of the averaging effect of the electric fields, static shift problems due to small-scale,
near-surface lateral heterogeneities can be alleviated. Several field experiments revealed regional scale deep electrical
conductivity structures related to slab subduction or its stagnation, which enable us to elucidate underlying physical processes
caused by the slab motion. The technique can also be applied to monitor the electric potential field related to crustal activities.
The annual variation of the potential field and electrical conductivity in the French Alps were interpreted to be caused by
the annual variation of lake water level. The method was also used to monitor the regional scale spatio-temporal variation
of the SP field and electrical conductivity before and at the onset of earthquakes and volcanic eruptions. 相似文献
22.
Su Young Lee Masatsugu Ohnishi Yasuhiro Shibata Makoto Wwatanabe Kenichi Hoshino Mayumi Jige Xiangping Gu Hitoshi Chiba Tamotsu Oomori Kantaro Fujioka Peter A. Rona 《Resource Geology》2001,51(1):63-68
Abstract: Pyrite rich in Zn, up to 3.1 wt%, was found in the TAG active mound of the TAG hydrothermal field, the slow-spreading Mid-Atlantic Ridge at 26°08'N and 44°49'W. The Zn-rich pyrite is characterized by an optical homogeneity, a homogeneous distribution of Zn in the back-scattered electron images, both at a magnification of about 500, a negative correlation between Fe and Zn contents of the pyrite and a rather small unit cell edge (a0 = 5.4117 ± 0.0008Å), strongly indicating that the detected Zn is present in the pyrite in solid solution. Such Zn concentrations are observed exclusively in dendritic pyrite, suggesting that the Znrich pyrite grew from hydrothermal fluids of a high degree of supersaturation due to quenching on the seafloor. 相似文献
23.
Abstract Newly obtained radiolarian biostratigraphic age combined with previous isotopic age of the Troodos ophiolite shows that the ophiolite becomes systematically younger from east to west: Turonian, early Campanian, and late Campanian. The youngest late Campanian part of the ophiolite is directly covered by the volcaniclastic sediment derived from an active island arc, whereas the older part is covered by pelagic radiolarite. These facts constitute evidence that the Troodos ophiolite was probably emplaced during the subduction of an active spreading ridge. 相似文献
24.
Within a region of 5 km × 10 km on a downhill slope of the Yamato Mounties, in 1969 the Japanese Expedition Team collected many stones. 9 of them were recognized as meteorites. On 4 of these findings we determined the chemical composition and the rare gas content. According to the mineralogical and the chemical composition, Yamato (a) is an enstatite chondrite, Yamato (b) a Ca-poor achondrite, Yamato (c) seems to be a carbonations chondrite Type III and Yamato (d) a olivine bronzite chondrite. Exposure ages are 1.7, 31, 25 and 4.3 my respectively. 相似文献
25.
Toshifumi Yanagisawa Yasushi Muraki Yutaka Matsubara Fumio Abe Kimiaki Masuda Sachiyo Noda Takahiro Sumi Yuji Kato Mitsuaki Fujimoto Shuji Sato Ian Bond Nicholas Rattenbury Philip Yock Pam Kilmartin John Hearnshaw Micheal Reid Denis Sullivan Brian Carter Richard Dodd Garry Nankivell Noiman Rumsey Morihiro Honda Maki Sekiguchi Masanori Yoshizawa Takashi Nakamura Humitaka Sato Seiji Kabe Makoto Kobayashi Yoshiyuki Watase Jun Jugaku Toshiharu Saito Barbel Koribalsky 《Experimental Astronomy》2000,10(4):519-535
We have constructed a large, mosaic CCD camera called MOA-cam2 which has 4096 × 6144-pixelsto search for gravitational microlensing events. MOA-cam2 has three4096 × 2048-pixel SITe CCD chips, which have a very high quantum efficiency (nearly 80% in the wave region 500 to 800 nm),and three buttable sides. We have placed the threechips side by side with 100 m dead space. MOA-cam2 has been installed on the 61 cm Boller and Chivens telescope of the MOA collaboration at the Mt. John University Observatory (MJUO) in NewZealand since July 1998. The field coverage is 0.92° × 1.38° per exposure. The technical details of MOA-cam2 and the first images obtained with the Boller and Chivens telescope are presented. MOA-cam2 introduces a second phase of research on gravitational microlensing by the MOA collaboration. 相似文献
26.
Koji Kameo Makoto Okada Moamen El-Masry Toshio Hisamitsu Saneatsu Saito Hiroomi Nakazato Naohiko Ohkouchi Minoru Ikehara Hisato Yasuda Hiroshi Kitazato Asahiko Taira 《Island Arc》2006,15(3):366-377
Abstract A continuous, well-preserved core was obtained from the Choshi area, on the Pacific side of Japan, to investigate paleoceanographic and paleoclimatic changes around the northwestern Pacific region during the middle Pleistocene. Siliciclastic sequences in the core are divided into five formations – the Obama, Yokone, Kurahashi and Toyosato Formations in the Inubo Group and the Katori Formation, in ascending order. Examination of calcareous nannofossils and magnetic polarities detected four datums in the core sediments of the Inubo Group: the top of Reticulofenestra asanoi , the base of Helicosphaera inversa , the top of Pseudoemiliania lacunosa and the Brunhes–Matuyama boundary. Fourteen marine isotope stages (MIS24–MIS11) were identified in the δ18 O and δ13 C records based on detected datums and the graphic correlation with the standard stack oxygen isotope curve. Magnetic susceptibility and gamma-ray attenuation porosity evaluator density were also measured and low values characterize the glacial intervals. Biogenic sedimentation by primary production may be larger during the glacial periods because of invasions of nutrient-rich northern surface-waters related to the southward shift of the Kuroshio front in the Choshi area. 相似文献
27.
Abstract The late Quaternary calcareous nannofossil assemblages from Ocean Drilling Program Holes 807A and 846B, located in the western and eastern equatorial Pacific Ocean, respectively, were analyzed to clarify changes in surface-water conditions during the last 500 000 years. The uppermost Quaternary sediments in both holes contain abundant nannofossils, and their assemblages are characterized by high species diversity. The absolute abundances of coccoliths (specimens/g) and relative numbers of small reticulofenestrids decreased drastically in both holes between 0.3 and 0.2 Ma, whereas the relative abundance of warm-water species and Florisphaera profunda increased suddenly at this time. These data indicate that upwelling around the western and eastern equatorial Pacific regions weakened after 0.2 Ma, which was caused by a decrease in trade-wind strength. 相似文献
28.
Wataru Ootani Makoto Minowa Takayuki Watanabe Yutaka Ito Yasuhiro Kishimoto Kentaro Miuchi Yoshizumi Inoue Youiti Ootuka 《Astroparticle Physics》1998,9(4):325-329
The performance of a 21-g lithium fluoride bolometer is presented. The background spectrum was measured in the surface laboratory. We derive an exclusion plot for the spin-dependent coupled Weakly Interacting Massive Particles (WIMPs) cross section. 相似文献
29.
In three polished thin sections of Yamato 791717 (CO3). fifty-five Ca, Al-rich inclusions were found, which include two hibonite-bearing,
eight melilite-rich and forty-five spinel-pyroxene inclusions. Based on the petrography and mineral chemistry of the inclusions,
it is proposed that the melilite-rich inclusions and spinel-pyroxene inclusions condensed in the solar nebula, and the hibonite-bearing
inclusions crystallized from melts. The heavy alteration of the inclusions in Yamato 791717, which probably took place under
a very oxidizing condition in the solar nebular, is also confirmed.
Project supported by the National Natural Science Foundation of China (Grant No. 49673200). and by the Japanese Society for
Promotion of Sciences (JSPS). 相似文献
30.