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31.
We report results of ultra wide-band radar sea spike experiments using steep and weakly breaking non-linear water surface features in a wave tank. To generate these features we used a 1 s paddle wave and wind waves for a sequence of wind speeds. A scanning laser was used to measure synchronously the surface slope profile across 12 cm along the wave propagation direction once per radar pulse. A time domain reflectometer (TDR) radar transmitted short horizontally polarized pulses at X-band, several hundred picoseconds long, to give a range resolution of 10 cm. A radar range of 36 cm was digitally sampled so that surface feature echoes could be tracked through the area continuously with 5 ms temporal resolution with each instrument. We report results considering the wave slope component in the propagation direction and the corresponding curvature component. For the conditions studied, two types of features which produce sea spike radar echoes were generated–a non-linear feature near the crest front of the wind wave, caused by extreme steepening as a result of the passage of the paddle wave, and a steepened blocked wind wave in the trough of the paddle wave, caused by the local orbital current of the 1 s wave being nearly equal to and opposite the phase velocity of the wind wave. 相似文献
32.
33.
The formal solutions of displacement field to the problem of elastic wave scattering and diffraction due to an infinitely long rigid cylinder embedded in an infinite elastic medium by an impulsive point source have been obtained in the integral form. The integrals for the reflected and the diffracted waves both in the shadow zone and in the illuminated zone are evaluated asymptotically for the early time motion by the Reisdue-Cagniard method and the Saddle-point-Cagniard method.Numerical results of the diffractedP, S andPS waves at a fixed circum-distance from the surface of the rigid cylinder show noticeably that (1) the energy partition for the diffractedS wave is small in comparison with that for the diffractedP wave, (2) the wave form of the diffractedS wave is broader and more diffused than that of the diffractedP wave, (3) the direction of the radial motions of the diffractedP andS waves varies as a function of the observational point, and (4) the energy partition for the diffractedP wave is much smaller than that for the direct or the reflectedP waves.This paper has been presented at the 46th Annual International Meeting of Society of Exploration Geophysicists in Houston, Texas, Oct. 28, 1976. 相似文献
34.
High-frequency (HF) radar observations of surface currents were conducted for 3 months during summer 2002 in the Keum River estuary. A comparison between HF radar-derived currents and directly measured ones form a buoy showed that the regression slope is close to 1 and the correlation coefficient greater than 0.86, with an RMS difference less than 13 cm/s which is less than 17% of the tidal current. This fairly good agreement allows us to use HF radar observation in investigating the surface flow and circulation in this tidal-current-dominant coastal-plume area. To examine the spatial variation in tidal current characteristics, as well as currents associated with non-tidal forcing, the HF radar-derived currents were separated into tidal and sub-tidal frequency currents. The overall pattern of M2-current ellipse distribution in the study area showed a counterclockwise rotation, with the offshore maximum current direction to the northeast. Eccentricity, the direction of maximum current, and the phase of net motion of the ellipse changed near the estuary mouth and near the gap of the Saemangeum reclamation tide dyke due to the complex coastal geometry and the out-flowing jet during the ebb period. 相似文献
35.
Increased human activities since discovery of gold in northern California have changed the structure and function of many ecosystems. To reconstruct the changes in watershed environmental conditions, sediment cores were collected from three montane marshes in northern Sierra Nevada, CA. Pollen analysis was conducted, and water content, bulk density, ash, carbon, nitrogen, phosphorus, major cations, and lead concentrations of sediments were determined. Cores were dated by the 210Pb method. Pollen analyses showed changes in plant communities in this region due to severe logging in the late-1800s and moderate logging in the 1900s. The local changes were more clearly recorded in small marsh pollen profiles than regional changes. The water milfoil (Myriophyllum spicatum L.) introduction into Tahoe Basin was inferred from the pollen record and 210Pb dating. Road construction and maintenance activities were recorded in physical and chemical characteristics, such as increases in sodium, calcium, and magnesium concentrations. Pollen and physical and chemical records also documented the time line of the expansion of dry meadow, and the decrease of pine forest in one of the watersheds. This study showed that sediments in small marshes were especially useful to reconstruct local disturbance by human activities. 相似文献
36.
Cheinway Hwang 《Journal of Geodesy》1995,70(1-2):110-116
The product of two associated Legendre functions can be represented by a finite series in associated Legendre functions with unique coefficients. In this study a method is proposed to compute the coefficients in this product-sum formula. The method is of recursive nature and is based on the straightforward polynomial form of the associated Legendre function's factor. The method is verified through the computation of integrals of products of two associated Legendre functions over a given interval and the computation of integrals of products of two Legendre polynomials over [0,1]. These coefficients are basically constant and can be used in any future related applications. A table containing the coefficients up to degree 5 is given for ready reference. 相似文献
37.
Horizontal and vertical distribution of PCBs and chlorinated pesticides in sediments from Masan Bay,Korea 总被引:1,自引:0,他引:1
Horizontal and vertical distributions of organochlorine compounds (OCs) were determined in sediments from Masan Bay. The concentrations of polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethanes (DDTs), HCB, hexachlorocyclohexanes (HCHs) and chlordane related compounds (CHLs) in sediments were in the range of 1.24-41.4, 0.28-89.2, 0.02-0.59, nd-1.03, and nd-2.56 ng/g, respectively. The spatial distribution of OCs showed a negative gradient from the inner of the bay to outer part of the bay, indicating that the source of OCs was probably located inside the bay. Compositional pattern of PCB congeners showed a relatively high concentration of high-chlorinated congeners in the inner part of the bay and a relatively low concentration of low-chlorinated congeners in the outer part. In sediment core from Masan Bay maximum concentrations of PCBs and DDTs are observed in the subsurface samples and correspond to an age of early 1980s and late 1960s. The concentration profiles of PCBs and DDTs in sediments of Masan Bay appear to correspond to use of PCBs and DDTs in Korea. 相似文献
38.
The distinct element method (DEM) has been used successfully for the dynamic analysis of rigid block systems. One of many
difficulties associated with DEM is modeling of damping. In this paper, new procedures are proposed for the damping modeling
and its numerical implementation in distinct element analysis of rigid multi-block systems. The stiffness proportional damping
is constructed for the prescribed damping ratio, based on the non-zero fundamental frequency effective during the time interval
while the boundary conditions remain essentially constant. At this time interval, the fundamental frequency can be estimated
without complete eigenvalue analysis. The damping coefficients will vary while the damping ratio remains the same throughout
the entire analysis. A new numerical procedure is developed to prevent unnecessary energy loss that can occur during the separation
phases. These procedures were implemented in the development of the distinct element method for the dynamic analyses of piled
multi-block systems. The analysis results for the single-block and two-block systems were in a good agreement with the analytic
predictions. Applications to the seismic analyses of piled fourblock systems revealed that the new procedures can make a significant
difference and may lead to much-improved results. 相似文献
39.
40.
On the interannual variability of the Bonin high associated with the East Asian summer monsoon rain 总被引:1,自引:0,他引:1
In order to assess how the Bonin high affects interannual variability of the East Asian summer monsoon (EASM) around the Korean Peninsula, the pulsation of the Bonin high and its association with teleconnection patterns was examined. The major factor for the interannual intensity of the EASM is the center position of the Bonin high rather than its center pressure. Up to 12 harmonics over time can be used to reconstruct the Bonin high, demonstrating its intraseasonal variation. The interannual variability of the Bonin high correlates with the Tibet high. This correlation is dominant for the EASM onset time, though not its retreat. The primary teleconnection pattern, reliant up on the interannual variability of the Bonin high, is the Western Pacific oscillation (WPO) in April. In relation to long-term variability, the correlation between the WPO and the Bonin high appears to contribute to the retreat stage of the EASM, which has itself increased since the mid-1970s. Furthermore, the WPO in May and the Tibet correlation has marked the onset rather than the retreat of the EASM since the 1970s. This highly correlated pattern since the mid-1970s may be the result of El Niño. 相似文献