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51.
Experiments were carried out on an intermittent estuary during its closed (summer) and open (winter) states to identify the physical processes responsible for vertical mixing across the halocline, and to quantify vertical fluxes of oxygen and salt between water layers. During the blocked phase a two-layer structure was observed, with a brackish surface layer overlying old seawater. Within a deep basin the wind-driven turbulent mixing was consistent with the measured surface-layer turbulent dissipation, but the dissipation in the bottom layer appeared to be driven by internal seiching. In the shallow regions of the estuary vertical fluxes of dissolved oxygen were indicative of oxygen demand by respiration and remineralization of organic material in bottom water and sediments. During the estuary's open phase a three-layer structure was observed, having a fresh, river-derived surface layer, a middle layer of new seawater, and a bottom layer of old seawater. In the shallower regions surface-layer turbulent diffusion was consistent with the strong, gusty winds experienced at the time. The dissolved oxygen of the incoming seawater decreased to very low values by the time it reached the upstream deep basin as a result of the low cross-pycnocline oxygen flux being unable to compensate for the oxygen utilization. At least 50 % of the cross-pycnocline salt fluxes in the shallow reaches of the open estuary are suggested to be driven by Holmboe instabilities.
Responsible Editor: Hans Burchard 相似文献
52.
K. David Newell John H. Doveton Daniel F. Merriam Barbara Sherwood Lollar William M. Waggoner L. Michael Magnuson 《Natural Resources Research》2007,16(3):277-292
In late 2005 and early 2006, the WTW Operating, LLC (W.T.W. Oil Co., Inc.) #1 Wilson well (T.D. = 5772 ft; 1759.3 m) was drilled
for 1826 ft (556.6 m) into Precambrian basement underlying the Forest City Basin in northeastern Kansas. Approximately 4500
of the 380,000 wells drilled in Kansas penetrate Precambrian basement. Except for two previous wells drilled into the arkoses
and basalts of the 1.1-Ga Midcontinent Rift and another well drilled in 1929 in basement on the Nemaha Uplift east of the
Midcontinent Rift, this well represents the deepest penetration into basement rocks in the state to date. Granite is the typical
lithology observed in wells that penetrate the Precambrian in the northern Midcontinent. Although no cores were taken to definitively
identify lithologies, well cuttings and petrophysical logs indicate that this well encountered basement metamorphic rocks
consisting of schist, gneiss, and amphibolitic gneiss, all cut by aplite dikes.
The well was cased and perforated in the Precambrian, and then acidized. After several days of swabbing operations, the well
produced shows of low-Btu gas, dominated by the non-flammable component gases of nitrogen (20%), carbon dioxide (43%), and
helium (1%). Combustible components include methane (26%), hydrogen (10%), and higher molecular-weight hydrocarbons (1%).
Although Coveney and others [Am. Assoc. Petroleum Geologists Bull., v. 71, no, 1, p. 39–48, 1987] identified H2-rich gas in two wells located close to the Midcontinent Rift in eastern Kansas, this study indicates that high levels of
H2 may be a more widespread phenomenon than previously thought. Unlike previous results, the gases in this study have a significant
component of hydrocarbon gas, as well as H2, N2, and CO2. Although redox reactions between iron-bearing minerals and groundwater are a possible source of H2 in the Precambrian basement rocks, the hydrocarbon gas does not exhibit the characteristics typically associated with proposed
abiogenic hydrocarbon gases from Precambrian Shield sites in Canada, Finland, and South Africa. Compositional and isotopic
signatures for gas from the #1 Wilson well are consistent with a predominantly thermogenic origin, with possible mixing with
a component of microbial gas. Given the geologic history of uplift and rifting this region, and the major fracture systems
present in the basement, this hydrocarbon gas likely migrated from source rocks and reservoirs in the overlying Paleozoic
sediments and is not evidence for abiogenic hydrocarbons generated in situ in the Precambrian basement. 相似文献
53.
Steven Sherwood 《Climatic change》2007,80(3-4):205-212
54.
Sherwood B. Idso 《Climatic change》1992,21(4):429-433
There are a number of ways by which the biosphere may counter any impetus for global warming that might be produced by the rising CO2 content of earth's atmosphere. Evidence for one of these phenomena, the DMS-cloud feedback effect, is discussed in light of recent claims that it is not of sufficient strength to be of much importance. 相似文献
55.
F. Ghassemzadeh J. Sherwood M. C. Geddes W. D. Williams 《International Journal of Salt Lake Research》1997,6(3):269-278
The analysis of nitrate in seawater and hypersaline waters should take account of a significant “salt effect”. Procedures
developed for fresh water and marine waters must be applied cautiously to highly saline waters. The most widely used standard
method for the determination of nitrate-nitrogen (NO3-N) in fresh and marine waters involves the quantitative reduction of nitrate by the Cd column technique followed by colorimetric
procedures. In our study, three approaches to estimate NO3-N in highly saline waters were examined. The first involved dilution. This approach overcame the salt effect but dilution
limited the detection of low concentrations of nitrate in highly saline waters. The second involved the use of standard nitrate
solutions in saline water. This method is not recommended because of nitrate impurities in AR grade salts. The third- and
preferred approach-involved the use of standard additions. “Spikes” of a known volume of NO3-N standard solution were added to natural saline waters. Nitrate values estimated by the stadard addition method were used
to calculate an equation for salt error correction at different salinities applicable to waters with the same relative ionic
composition as seawater. This could then be used to correct nitrate determinations in highly saline waters where standards
made in distilled water were used for calibration. Many previously published data for NO3-N in saline water used methods of analysis which do not take account of salt error and are therefore probably in error. 相似文献
56.
57.
Stable isotope analysis of some representative fish and invertebrates of the Newfoundland and Labrador continental shelf food web 总被引:3,自引:1,他引:3
We examined stable carbon and nitrogen isotopic signatures of 17 fish and 16 invertebrate taxa common to the Newfoundland and Labrador (NL) continental shelf food web. Particular sampling emphasis was placed on Atlantic cod (Gadus morhua) and related prey species (e.g. shrimp, Pandalus borealis, and capelin, Mallotus villosus). We found highly significant (p < 0.0001) differences between near-shore (bays) and offshore (shelf edge) δ15N signatures for cod, ‘other fish’ (pooled) and invertebrates (pooled). In contrast, there were only minor differences in δ13C signatures of ‘other fish’ (p < 0.05) and no difference for cod and invertebrates among the two habitats. We sampled at two times of the year (January and June) and found no systematic effect of season on both δ13C and δ15N in cod, ‘other fish’ and invertebrates. We calculated isotopic fractionation factors for cod from the entire shelf (mixed diet) and for cod with diets composed mainly of capelin or shrimp. These values ranged between 2.2‰ and 3.9‰ for δ15N and −0.4‰ and 0.8‰ for δ13C and, for δ15N, may reflect diet-related differences in bioenergetic status. We discuss potential mechanisms for near-shore versus offshore enrichment of δ15N signatures, and demonstrate the implications of this spatial variation on δ15N-derived trophic position estimates. 相似文献
58.
59.
Sediment transport and the potential for erosion or deposition have been investigated on the Palos Verdes (PV) and San Pedro shelves in southern California to help assess the fate of an effluent-affected deposit contaminated with DDT and PCBs. Bottom boundary layer measurements at two 60-m sites in spring 2004 were used to set model parameters and evaluate a one-dimensional (vertical) model of local, steady-state resuspension, and suspended-sediment transport. The model demonstrated skill (Brier scores up to 0.75) reproducing the magnitudes of bottom shear stress, current speeds, and suspended-sediment concentrations measured during an April transport event, but the model tended to underpredict observed rotation in the bottom-boundary layer, possibly because the model did not account for the effects of temperature–salinity stratification. The model was run with wave input estimated from a nearby buoy and current input from four to six years of measurements at thirteen sites on the 35- and 65-m isobaths on the PV and San Pedro shelves. Sediment characteristics and erodibility were based on gentle wet-sieve analysis and erosion-chamber measurements. Modeled flow and sediment transport were mostly alongshelf toward the northwest on the PV shelf with a significant offshore component. The 95th percentile of bottom shear stresses ranged from 0.09 to 0.16 Pa at the 65-m sites, and the lowest values were in the middle of the PV shelf, near the Whites Point sewage outfalls where the effluent-affected layer is thickest. Long-term mean transport rates varied from 0.9 to 4.8 metric tons m−1 yr−1 along the 65-m isobaths on the PV shelf, and were much higher at the 35-m sites. Gradients in modeled alongshore transport rates suggest that, in the absence of a supply of sediment from the outfalls or PV coast, erosion at rates of ∼0.2 mm yr−1 might occur in the region southeast of the outfalls. These rates are small compared to some estimates of background natural sedimentation rates (∼5 mm yr−1), but do not preclude higher localized rates near abrupt transitions in sediment characteristics. However, low particle settling velocities and strong currents result in transport length-scales that are long relative to the narrow width of the PV shelf, which combined with the significant offshore component in transport, means that transport of resuspended sediment towards deep water is as likely as transport along the axis of the effluent-affected deposit. 相似文献
60.
H.I. Petersen N. Sherwood A. Mathiesen M.B.W. Fyhn N.T. Dau N. Russell J.A. Bojesen-Koefoed L.H. Nielsen 《Marine and Petroleum Geology》2009
Several exploration wells have intersected a Cenozoic coal-bearing, fluvial-deltaic mudstone and sandstone succession in the northeastern Vietnamese part of the Malay Basin, and have successfully tested seismically identified direct hydrocarbon indicators (DHIs). The oil and gas/condensate discovery well 46-CN-1x encountered a ∼55 m thick section of lacustrine mudstones having considerable potential as an oil source. Vitrinite reflectance (VR) measurements from these alginite-bearing rocks introduce several problems in thermal maturity evaluation, including associated VR suppression and delineation of cavings and bitumens. Reliable thermal maturity gradients, however, may be established using a combination of conventional VR measurements and ‘equivalent VR’ (EqVR) values derived from the fluorescence alteration of multiple macerals (FAMM) technique. These measurements, performed on dispersed organic matter (DOM) in cuttings from 46-CN-1x, allow separation of low-reflecting bitumens and vitrinite in cavings from indigenous vitrinite and the FAMM results indicate VR suppression of 0.14% in an alginite-bearing mudstone with a high Hydrogen Index value. On the basis of available ‘raw’ VR data, a highly irregular maturity trend is determined, with the deepest sample (2675–2680 m) having a VR of ∼0.4%Ro. The EqVR value, however, for the deepest sample is 0.70%. The maturity trend determined from the FAMM data (and VR data, omitting samples having suppressed VR) indicates that the top of the oil window (VR of 0.75%Ro) is located at about 2800 m depth. Modelling the geothermal gradient using the EASY%Ro algorithm yields ∼40 °C/km for both of the two maturity profiles; this is in the low end of the range for the Malay Basin. Modelled temperature histories indicate onset of hydrocarbon generation for the uppermost Oligocene source rocks between 2 Ma and present-day, which post-dates trap formation. Seismic facies patterns suggest that lacustrine oil-prone units are in the oil window in the same graben complex a few km NW of the investigated well, and these rocks are likely to be the source of the hydrocarbons found in the well. A more widespread occurrence of hydrocarbons sourced from this kitchen is indicated by other discoveries and mapping of DHIs in the area. 相似文献