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351.
Cryogenic drilling is a technique developed at the University of California (UC), Berkeley, for drilling in unstable sediments of environmental monitoring, for characterizing, and for remediation wells, The method uses standard air rotary drilling techniques, but with cold nitrogen rather than ambient air as the circulating fluid in order to freeze and stabilize the borehole wall. Several laboratory and full-scale field tests have been performed. A.52-foot-deep (16 m) soil boring and 24 foot (7 m) monitoring well have been drilled as part of the Lawrence Berkeley Laboratory Site Characterization Project. Continued testing and refinement of the equipment and operational method are in progress. The method has been proposed for use as part of the Department of Energy (DOE) weapons site cleanup at locations with unstable sediments such as Hanford, Sandia, and Idaho National Engineering Laboratory (INEL). 相似文献
352.
An Ashtech GPS 3DF, a commercially available system for measuring ship's heading from an array of GPS antennas, was installed on RRS Discovery in August 1992. Data were collected during 8 days of ship trials, including periods in port and while on station or steaming. Headings from the GPS system have been compared with headings from the ship's gyrocompass, and residuals calculated. While in port, residuals had a standard deviation of 0.066°, suggesting satisfactory performance by both instruments. While at sea, time-averaged residuals varied with a range of nearly 3°, most of which is attributed to systematic errors in the ship's gyro. After correcting the gyro using 10-min averages of the GPS minus gyro headings, the residual GPS minus gyro differences had a standard deviation of 0.17°. This represents a likely limit on the use of GPS heading data to improve conventional gyro measurements. 相似文献
353.
We investigated Oceanographer Canyon, which is on the southeastern margin of Georges Bank, during a series of fourteen dives in the “Alvin” and “Nekton Gamma” submersibles. We have integrated our observations with the results of previous geological and biological studies of Georges Bank and its submarine canyons. Fossiliferous sedimentary rocks collected from outcrops in Oceanographer Canyon indicate that the Cretaceous—Tertiary boundary is at 950 m below sea level at about 40°16′N where at least 300 m of Upper Cretaceous strata are exposed; Santonian beds are more than 100 m thick and are the oldest rocks collected from the canyon. Quaternary silty clay, deposited most probably during the late Wisconsin Glaciation, veneers the canyon walls in many places, and lithologically similar strata are present beneath the adjacent outer shelf and slope. Where exposed, the Quaternary clay is commonly burrowed by benthic organisms that cause extensive erosion of the canyon walls, especially in the depth zone (100–1300 m) inhabited by red crabs (Geryon) and/or jonah crabs (Cancer). Bioerosion is minimal on high, near-vertical cliffs of sedimentary rock, in areas of continual sediment movement, and where the sea floor is paved by gravel. A thin layer of rippled, unconsolidated silt and sand is commonly present on the canyon walls and in the axis; ripple orientation is most commonly transverse to the canyon axis and slip-faces point downcanyon. Shelf sediments are transported from Georges Bank over the eastern rim and into Oceanographer Canyon by the southwest drift and storm currents; tidal currents and internal waves move the sediment downcanyon along the walls and axis. Large erratic boulders and gravel pavements on the eastern rim are ice-rafted glacial debris of probable late Wisconsinan age; modern submarine currents prevent burial of the gravel deposits. The dominant canyon megafauna segregates naturally into three faunal depth zones (133–299 m; 300–1099 m; 1100–1860 m) that correlate with similar zones previously established for the continental slope epibenthos. Faunal diversity is highest on gravelly sea floors at shallow and middle depths. The benthic fauna and the fishes derive both food and shelter by burrowing into the sea floor. In contrast to the nearby outer shelf and upper slope, Oceanographer Canyon has not been extensively exploited by the fishing industry, and the canyon ecosystem probably is relatively unaltered. 相似文献
354.
355.
Beth ShapiroAlan Cooper 《Quaternary Research》2003,60(1):94-100
Thousands of Late Pleistocene remains are found in sites throughout Beringia. These specimens comprise an Ice Age genetic museum, and the DNA contained within them provide a means to observe evolutionary processes within populations over geologically significant time scales. Phylogenetic analyses can identify the taxonomic positions of extinct species and provide estimates of speciation dates. Geographic and temporal divisions apparent in the genetic data can be related to ecological change, human impacts, and possible landscape mosaics in Beringia. The application of ancient DNA techniques to traditional paleontological studies provides a new perspective to long-standing questions regarding the paleoenvironment and diversity of Late Pleistocene Beringia. 相似文献
356.
Tsegaye Tadesse Donald A. Wilhite Sherri K. Harms Michael J. Hayes Steve Goddard 《Natural Hazards》2004,33(1):137-159
Drought has an impact on many aspects of society. To help decision makers reduce the impacts of drought, it is important to
improve our understanding of the characteristics and relationships of atmospheric and oceanic parameters that cause drought.
In this study, the use of data mining techniques is introduced to find associations between drought and several oceanic and
climatic indices that could help users in making knowledgeable decisions about drought responses before the drought actually
occurs. Data mining techniques enable users to search for hidden patterns and find association rules for target data sets
such as drought episodes. These techniques have been used for commercial applications, medical research, and telecommunications,
but not for drought. In this study, two time-series data mining algorithms are used in Nebraska to illustrate the identification
of the relationships between oceanic parameters and drought indices. The algorithms provide flexibility in time-series analyses
and identify drought episodes separate from normal and wet conditions, and find relationships between drought and oceanic
indices in a manner different from the traditional statistical associations. The drought episodes were determined based on
the Standardized Precipitation Index (SPI) and Palmer Drought Severity Index (PDSI). Associations were observed between drought
episodes and oceanic and atmospheric indices that include the Southern Oscillation Index (SOI), the Multivariate ENSO Index
(MEI), the Pacific/North American (PNA) index, the North Atlantic Oscillation (NAO) Index, and the Pacific Decadal Oscillation
(PDO) Index. The experimental results showed that among these indices, the SOI, MEI, and PDO have relatively stronger relationships
with drought episodes over selected stations in Nebraska. Moreover, the study suggests that data mining techniques can help
us to monitor drought using oceanic indices as a precursor of drought. 相似文献
357.
Oxygen isotope evidence for the origin of enriched mantle beneath the mid-Atlantic ridge 总被引:1,自引:0,他引:1
Kari M. Cooper John M. Eiler Paul D. Asimow Charles H. Langmuir 《Earth and Planetary Science Letters》2004,220(3-4):297-316
Geochemical variations in mid-ocean ridge basalts have been attributed to differing proportions of compositionally distinct mantle components in their sources, some of which may be recycled crust. Oxygen isotopes are strongly fractionated by near-surface interactions of rocks with the hydrosphere, and thus provide a tracer of near-surface materials that have been recycled into the mantle. We present here oxygen isotope analyses of basaltic glasses from the mid-Atlantic ridge south of and across the Azores platform. Variations in δ18O in these samples are subtle (range of 0.47‰) and may partly reflect shallow fractional crystallization; we present a method to correct for these effects. Relatively high fractionation-corrected δ18O in these samples is associated with geochemical indices of enrichment, including high La/Sm, Ce/Pb, and 87Sr/86Sr and low 143Nd/144Nd. Our results suggest two first-order conclusions about these enriched materials: (1) they are derived (directly or indirectly) from recycled upper oceanic crustal rocks and/or sediments; and (2) these materials are present in the north Atlantic MORB sources in abundances of less than 10% (average 2–5%). Modeling of variations of δ18O with other geochemical variables further indicates that the enriched component is not derived from incorporation of sediment or bulk altered oceanic crust, from metasomatism of the mantle by hydrous or carbonate-rich fluids, or from partial melting of subducted sediment. Instead, the data appear to require a model in which the enriched component is depleted mantle that has been metasomatized by small-degree partial melts of subducted, dehydrated, altered oceanic crust. The age of this partial melting is broadly constrained to 250 Ma. Reconstructed plate motions suggest that the enriched component in the north Atlantic mantle may have originated by subduction along the western margin of Pangea. 相似文献
358.
Rb/Sr geochronology on a folded greenstone-granitoid complex in the Agnew area, Western Australia, yields four distinct ages of igneous activity that conform with stratigraphic and intrusive relationships. They are (using , NBS 70A = 522 ppm Rb and 65.3 ppm Sr):
Ma | 87Sr/86Sr initial (IR) | |
(1) Differentiated gabbro-granophyre from a stratigraphically old (Kathleen Valley) greenstone sequence | ||
(2) Voluminous tonalite, the Lawlers Tonalite | ||
(3) A less voluminous leucogranite, and a large complex pegmatite cutting the Perseverance nickel orebody | ||
(4) Aplitic leucotonalite (very minor volumes but widespread) |