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91.
Matched-field inversion is used to, estimate geoacoustic properties from data obtained in an experiment with a vertical line array (VLA). The experiment was carried out using broad-band sources (shots) in water depths of about 200 m on the continental shelf off Vancouver Island. The data were processed to obtain spectral components of the field for frequencies near the bubble frequency for the shot. The ocean bottom in this region consists of a layer of mainly sandy sediments (about 100 m thick) overlying older consolidated material. Consequently, the inversion was designed to estimate the parameters of a two-layer elastic sediment model. In the inversion, an adaptive global search algorithm was used to investigate the multidimensional space of geoacoustic models in order to determine the set of values corresponding to the best replica field. Convergence is driven by adaptively guiding the search to regions of the parameter space associated with above-average values of the matched field correlation between the measured and replica fields. The geoacoustic profile estimated by the inversion consisted of a 125-m layer with compressional speed ~1700 m/s and shear speed ~400 m/s, overlying a layer with compressional speed ~1900 m/s. This model is consistent with the results from conventional seismic experiments carried out in the same region 相似文献
92.
Malaria (ague) was once common in many parts of Great Britain (GB). Here we identify areas currently at risk from vivax malaria
and examine how this pattern may change as a consequence of global warming during this century. We used a mathematical model
to describe how temperature affects the risk of vivax malaria, transmitted by a common British mosquito, Anopheles atroparvus. This model was linked with present-day temperature surfaces and future temperature change scenarios for GB, and used to map
areas suitable for malaria transmission. We found an excellent agreement between the present-day risk map for malaria and
historical records of ague distribution. This study demonstrates that many parts of GB are warm enough for malaria transmission
and the extent of these areas are likely to increase in the future. Health services need to remain aware of the possibility
of locally-transmitted malaria, particularly in marshland areas in southern England.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
93.
94.
Contaminated ground and contaminated estuary sediment illustrated by two case histories 总被引:1,自引:0,他引:1
Contaminated ground forms a problem in all of the industrialized countries of the world. Contaminated ground may give rise
to hazards and that implies a degree of risk which also involves a problem of definition. The investigation of a site which
is suspected of being contaminated differs somewhat from a routine site investigation. Sampling of soil, groundwater and gas-producing
material may be required. Various precautions may be necessary to do this and personnel may have to wear protective clothing.
The first case history considered involves a site investigation for a relief sewer in Glasgow. As the site investigation progressed
it ran into made-ground which contained chemical waste. The presence of this waste meant that the nature of the investigation
changed and much more stringent safety precautions had to be taken. It also meant that the initial location of the sewer tunnel
had to be repositioned at greater depth in uncontaminated sandstone rather than in the superficial deposits above.
The other case history considers the contamination of sediments in the Forth Estuary. When trace metals are released into
the water column they can be transferred rapidly to the sediment phase by adsorption onto suspended particulate matter, followed
by sedimentation. Intertidal flats may be considered as important trace metal sinks since they accumulate large amounts of
suspended matter. Hence, in polluted estuaries the deposition of suspended particles on intertidal flats may thus cause severe
contamination. The Forth Estuary has unique contamination for British estuaries; it is experiencing significant Hg pollution.
In addition, due to the presence of a nuclear submarine base in the Forth Estuary, 60Co is detectable in the intertidal sediments. Temporal and spatial contamination patterns were analysed in relation to historical
and present pollution point sources. The effect of fluvial and marine sediment mixing on trace metals and other processes
controlling contaminant levels is reviewed. Preliminary results on quantifying sediment accretion rates using Caesium levels
are discussed.
Received: 9 August 1996 · Accepted: 17 December 1996 相似文献
95.
S.J. Turner P.A. Flindell D. Hendri I. Hardjana P.F. Lauricella R.P. Lindsay B. Marpaung G.P. White 《Journal of Geochemical Exploration》1994,50(1-3)
The Ratatotok district in the Minahasa Regency of North Sulawesi, Indonesia is an area of significant gold mineralisation. Gold has been mined in the district since at least the 1850s, and intensively by the Dutch between 1900 and 1921 with a recorded production of 5,060 kg of gold. Newmont began exploring the district in 1986, and has delineated a major sediment-hosted replacement-style deposit at Mesel, and other smaller deposits in an 8×5 km area. A total drill-indicated resource of over 60 metric tonnes of gold (+2 Moz) is reported for Mesel, and three of the smaller deposits. Approximately 80% of this resource is refractory. Silver grades are usually low (<10 g/t). The Mesel deposit is similar to many Carlin-type deposits in carbonate hostrocks, alteration, geochemical signature and ore mineralogy, but is distinct in tectonic setting. The discovery of replacement-style mineralisation at Mesel, in an impure limestone within a Tertiary island arc environment, demonstrates that deposits with outward characteristics similar to Carlin-type mineralisation are not restricted to a continental setting.Carbonate sediments in the Ratatotok district were deposited in a Late Miocene restricted basin. Later compressional tectonics caused uplift that resulted in karst development in the limestone and erosion of the adjacent volcanic arc with deposition of a thick epiclastic unit. This was followed by intrusion of shallow level pre-mineral andesite into the sequence. Mineralisation at Mesel, and probably elsewhere in the district, is synchronous with the late-stage reactivation of strike-slip faults. Mineralising fluids at Mesel were focussed along steep structures sympathetic to these faults, and trapped below a relatively impermeable andesite cap rock. Hydrothermal fluids caused decalcification of the silty, more permeable carbonate units with the formation of secondary dolomite, deposition of fine arsenian pyrite, silica veinlets and gold. Volume loss due to decalcification and dolomite formation caused collapse brecciation which enhanced fluid flow and further mineralisation. This locally culminated in total decarbonation and deposition of massive silica. Late-stage stibnite occurs in structural zones within the ore deposit, whereas arsenic (as realgar and orpiment) and mercury (as cinnabar) are concentrated on the periphery.Elsewhere in the Ratatotok district, gold mineralisation is restricted to replacement-style mineralisation in permeable zones along limestone-andesite contacts, open-space filling quartz-calcite veins and stockworks, and residual quartz-clay breccias. The residual breccias are developed in-situ, and are interpreted to form by dissolution of the wallrock limestone from around pre-existing mineralisation. This has resulted in widespread eluvial gold occurrences. 相似文献
96.
97.
Fitzpatrick Lindsay Titze Daniel Anderson Eric J. Beletsky Dmitry Kelley John G. W. 《Ocean Dynamics》2023,73(7):433-447
Ocean Dynamics - Generally, ports in the North American Great Lakes are not supported with navigational guidance (water level, water temperature, currents, ice) by NOAA’s Great Lakes... 相似文献
98.
Coordinated mineralogical and isotopic analyses of a cosmic symplectite discovered in a comet 81P/Wild 2 sample 下载免费PDF全文
Ann N. Nguyen Eve L. Berger Keiko Nakamura‐Messenger Scott Messenger Lindsay P. Keller 《Meteoritics & planetary science》2017,52(9):2004-2016
We have discovered in a Stardust mission terminal particle a unique mineralogical assemblage of symplectically intergrown pentlandite ((Fe,Ni)9S8) and nanocrystalline maghemite (γ‐Fe2O3). Mineralogically similar cosmic symplectites (COS) have only been found in the primitive carbonaceous chondrite Acfer 094 and are believed to have formed by aqueous alteration. The O and S isotopic compositions of the Wild 2 COS are indistinguishable from terrestrial values. The metal and sulfide precursors were thus oxidized by an isotopically equilibrated aqueous reservoir either inside the snow line, in the Wild 2 comet, or in a larger Kuiper Belt object. Close association of the Stardust COS with a Kool mineral assemblage (kosmochloric Ca‐rich pyroxene, FeO‐rich olivine, and albite) that likely originated in the solar nebula suggests the COS precursors also had a nebular origin and were transported from the inner solar system to the comet‐forming region after they were altered. 相似文献
99.
100.
Gavin M. Elliott Christian Berndt Lindsay M. Parson 《Marine Geophysical Researches》2009,30(3):207-214
The continental margin of SW Africa is typical of a volcanic rifted margin associated with a hotspot trail characterized by a large volcanic ridge, the Walvis Ridge, defining the hotspot migration, and extensive extrusive volcanism that produced seaward-dipping reflectors (SDR). Previously unpublished seismic data show two significant anomalies of the SW African Margin when compared to other typical volcanic rifted margins: (1) Hyaloclastitic outer highs are rare, and (2) the SDR in the North dip towards the Walvis Ridge. We explain these anomalies by a major transform segment close to the centre of volcanism combined with pulsed volcanism. The Walvis Ridge represents an east-west striking extrusive centre which produced a SDR sequence. Following break-up the northern boundary of the Walvis Ridge became a left lateral transform fault. Our data support the idea that a transform fault system interacting with a ridge jump were responsible for the accretion of the São Paulo Plateau to the American plate. 相似文献