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321.
The December 26, 2004 Sumatra tsunami caused severe damage at the coasts of the Indian ocean. We report results of a sedimentological
study of tsunami run-up parameters and the sediments laid down by the tsunami at the coast of Tamil Nadu, India, and between
Malindi and Lamu, Kenya. In India, evidence of three tsunami waves is preserved on the beaches in the form of characteristic
debris accumulations. We measured the maximum run-up distance at 580 m and the maximum run-up height at 4.85 m. Flow depth
over land was at least 3.5 m. The tsunami deposited an up to 30 cm thick blanket of moderately well to well-sorted coarse
and medium sand that overlies older beach deposits or soil with an erosional unconformity. The sand sheet thins inland without
a decrease of grain-size. The deposits consist frequently of three layers. The lower one may be cross-bedded with foresets
dipping landward and indicating deposition during run-up. The overlying two sand layers are graded or parallel-laminated without
indicators of current directions. Thus, it remains undecided whether they formed during run-up or return flow. Thin dark laminae
rich in heavy minerals frequently mark the contacts between successive layers. Benthic foraminifera indicate an entrainment
of sediment by the tsunami from water depths less than ca. 30 m water depth. On the Indian shelf these depths are present
at distances of up to 5 km from the coast. In Kenya only one wave is recorded, which attained a run-up height of 3 m at a
run-up distance of ca. 35 m from the tidal water line at the time of the tsunami impact. Only one layer of fine sand was deposited
by the tsunami. It consists predominantly of heavy minerals supplied to the sea by a nearby river. The sand layer thins landward
with a minor decrease in grain-size. Benthic foraminifera indicate an entrainment of sediment by the tsunami from water depths
less than ca. 30 m water depth, reaching down potentially to ca. 80 m. The presence of only one tsunami-related sediment layer
in Kenya, but three in India, reflects the impact of only one wave at the coast of Kenya, as opposed to several in India.
Grain-size distributions in the Indian and Kenyan deposits are mostly normal to slightly positively skewed and indicate that
the detritus was entrained by the tsunami from well sorted pre-tsunami deposits in nearshore, swash zone and beach environments. 相似文献
322.
Wen-chao Su Christoph A. Heinrich Thomas Pettke Xing-chun Zhang Rui-zhong Hu Bin Xia 《矿物岩石地球化学通报》2006,25(Z1):192
The Shuiyindong and Yata Carlin-type gold deposits, located in Southwestern Guizhou, China, are hosted by Permianbioclastic limestone in the form of stratabound mineralization and Middle Triassic calcareous clastic rocks as fault-controlled mineralization, respectively. In these deposits, quartz crystals in the veins or veinlets associated with mineralization have contained several populations of fluid inclusions with relatively quite big size and clear paragenetic relationship of entrapment. Petrography, microthermometry, Raman and LA-ICPMS of fluid inclusions analyses are used to characterize fluids chemistry and their evolution of Carlin-type system in Guizhou. 相似文献
323.
324.
Stratigraphic data from petroleum wells and seismic reflection analysis reveal two distinct episodes of subsidence in the southern New Caledonia Trough and deep‐water Taranaki Basin. Tectonic subsidence of ~2.5 km was related to Cretaceous rift faulting and post‐rift thermal subsidence, and ~1.5 km of anomalous passive tectonic subsidence occurred during Cenozoic time. Pure‐shear stretching by factors of up to 2 is estimated for the first phase of subsidence from the exponential decay of post‐rift subsidence. The second subsidence event occured ~40 Ma after rifting ceased, and was not associated with faulting in the upper crust. Eocene subsidence patterns indicate northward tilting of the basin, followed by rapid regional subsidence during the Oligocene and Early Miocene. The resulting basin is 300–500 km wide and over 2000 km long, includes part of Taranaki Basin, and is not easily explained by any classic model of lithosphere deformation or cooling. The spatial scale of the basin, paucity of Cenozoic crustal faulting, and magnitudes of subsidence suggest a regional process that acted from below, probably originating within the upper mantle. This process was likely associated with inception of nearby Australia‐Pacific plate convergence, which ultimately formed the Tonga‐Kermadec subduction zone. Our study demonstrates that shallow‐water environments persisted for longer and their associated sedimentary sequences are hence thicker than would be predicted by any rift basin model that produces such large values of subsidence and an equivalent water depth. We suggest that convective processes within the upper mantle can influence the sedimentary facies distribution and thermal architecture of deep‐water basins, and that not all deep‐water basins are simply the evolved products of the same processes that produce shallow‐water sedimentary basins. This may be particularly true during the inception of subduction zones, and we suggest the term ‘prearc’ basin to describe this tectonic setting. 相似文献
325.
U. Beyerle W. Aeschbach-Hertig M. Hofer D. M. Imboden H. Baur R. Kipfer 《Journal of Hydrology》1999,220(3-4):169-185
Noble gas isotopes (3He, 4He, Ne, Ar, Kr, Xe), tritium (3H), chlorofluorocarbons (CFCs) and dissolved oxygen (O2) were seasonally measured in a small groundwater system recharged by infiltration of river water at Linsental, northeastern Switzerland. All Groundwater samples contained an excess of atmospheric noble gases (‘excess air’) usually with an elemental composition equal to air. The concentrations of atmospheric noble gases in the groundwater were used to calculate the excess air component and the water temperature at recharge. The noble gas temperatures (NGTs) in the boreholes close to the river vary seasonally, however, the average NGT of all samples lies close to the mean annual temperature of the river water. Groundwater ages were calculated using the tritium/helium-3 (3H/3He) dating method. The water ages of the samples obtained near the river depend on the amount of recently infiltrated river water and are young during times of active river discharge. In contrast, the mean water age of about 3 years of the deep aquifer remained nearly constant over the sampling period. The observed CFC-11 (CFCl3) and CFC-12 (CF2Cl2) concentrations are significantly higher than the atmospheric equilibrium concentrations and therefore CFCs do not provide any direct information on the residence time of the groundwater. Nevertheless, the CFC excess in the groundwater shows a linear increase with the 3H/3He age. Additionally, both accumulation of radiogenic He (4Herad) and O2 consumption are strongly correlated with residence time. All these correlations can be interpreted either in terms of mixing of recently infiltrated river water with older groundwater or in terms of accumulation/consumption rates. 相似文献
326.