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101.
Derecke Palmer 《中国地质大学学报(英文版)》2009,20(3)
The 1D τ-p inversion algorithm is widely employed to generate starting models with most computer programs that implement refraction tomography. However, this algorithm emphasizes the vertical resolution of many layers, and as a result, it frequently fails to detect even large lateral variations in seismic velocities, such as the decreases that are indicative of shear zones. This study presents a case that demonstrates the failure of the 1D τ-p inversion algorithm to define or even detect a major shear zone that is 50 m or ten stations wide. Furthermore, the majority of refraction tomography programs parameterize the seismic velocities within each layer with vertical velocity gradients. By contrast, the 2D generalized reciprocal method (GRM) inversion algorithms emphasize the lateral resolution of individual layers. This study demonstrates the successful detection and definition of the 50-m wide shear zone with the GRM inversion algorithms. The existence of the shear zone is corroborated by a 2D analysis of the head wave amplitudes and by numerous closely spaced orthogonal seismic profiles carried out as part of a later 3D refraction investigation. Furthermore, a 1D analysis of the head wave amplitudes indicates that a reversal in the seismic velocities, rather than vertical velocity gradients, occurs in the weathered layers. While all seismic refraction operations should aim to provide as accurate depth estimates as is practical, the major conclusion reached in this study is that refraction Inversion algorithms that emphasize the lateral resolution of individual layers generate more useful results for geotechnical and environmental applications. The advantages of the Improved lateral resolution are obtained with 2D profiles in which the structural features can be recognized from the magnitudes of the variations in the seismic velocities. Furthermore, the spatial patterns obtained with 3D investigations facilitate the recognition of structural features that do not display any intrinsic variation or "signature" in seismic velocities. 相似文献
102.
Ian Palmer 《International Journal of Coal Geology》2009,77(1-2):119-126
Permeability changes can be very large during depletion of coalbed methane wells: up to 100 times in the San Juan basin. Analytic models of permeability increase during depletion are accessible, easy to use, and practical. They are more transparent, and less complicated, than coupled numerical models. However, there are minor differences between the analytic models, and these are reviewed here. There are also discrepancies in attempting to match field data, which is the final test of a model, and these are discussed. Two different sets of San Juan data have been modeled, after suppressing the pressure-dependent permeability, with reasonably consistent reservoir parameters. The matching parameters appear to be justifiable, the small porosities do not seem to be ruled out by reservoir experts, and the changes of elastic parameters (including pore–volume compressibility) with depletion do not seem unreasonable. The analytic models are used in reservoir simulators to model and predict injection of greenhouse gases in coal seams. Only when we can match the permeability increase of coalbed methane wells during depletion, will we have the confidence to use the analytic models in reservoir simulators to match or predict sequestration of greenhouse gases such as CO2. 相似文献
103.
The energy flux in internal waves generated at the Celtic Sea shelf break was estimated by (i) applying perturbation theory to a week-long dataset from a mooring at 200 m depth, and (ii) using a 2D non-hydrostatic circulation model over the shelf break. The dataset consisted of high resolution time-series of currents and vertical stratification together with two 25-h sets of vertical profiles of the dissipation of turbulent kinetic energy. The observations indicated an average energy flux of 139 W m−1, travelling along the shelf break towards the northwest. The average energy flux across the shelf break at the mooring was only 8 W m−1. However, the waves propagating onshelf transported up to 200 W m−1, but they were only present 51% of the time. A comparison between the divergence of the baroclinic energy flux and observed dissipation within the seasonal thermocline at the mooring showed that the dissipation was at least one order of magnitude larger. Results from a 2D model along a transect perpendicular to the shelf break showed a time-averaged onshelf energy flux of 153–425 W m−1, depending on the magnitude of the barotropic forcing. A divergence zone of the energy flux was found a few kilometre offshore of the location of the observations in the model results, and fluxes on the order of several kW m−1 were present in the deep waters further offshelf from the divergence zone. The modelled fluxes exhibited qualitative agreements with the phase and hourly onshelf magnitudes of the observed energy fluxes. Both the observations and the model results show an intermittent onshelf energy flux of 100–200 W m−1, but these waves could only propagate ∼20–30 km onshore before dissipating. This conclusion was supported by a 25-h dataset sampled some 180 km onto the shelf, where a weak wave energy flux was found going towards the shelf break. We therefore conclude that shelf break generated internal waves are unlikely to be the main source of energy for mixing on the inner part of the shelf. 相似文献
104.
Lewis Matt J. Palmer Tamsin Hashemi Resa Robins Peter Saulter Andrew Brown Jenny Lewis Huw Neill Simon 《Ocean Dynamics》2019,69(3):367-384
Ocean Dynamics - The combined hazard of large waves occurring at an extreme high water could increase the risk of coastal flooding. Wave-tide interaction processes are known to modulate the wave... 相似文献
105.
We present a remotely-sensed degradation assessment technique based on the measurement of landscape heterogeneity (a key determinant of degradation status). The moving standard deviation index (MSDI) is derived by passing a moving standard deviation filter across the Landsat TM band 3 (red band). Degraded/unstable landscapes exhibit higher MSDI values than their undisturbed/stable counterparts. Significant differences in MSDI were detected (p<0·001) across four fence-lines which separated rangeland of contrasting condition. The relationship of the index to normalized difference vegetation index (NDVI) was tested in five different ecosystems and significant correlations were obtained in all cases (p<0·001). We propose the MSDI as a powerful adjunct to vegetation indices. 相似文献
106.
Outlines of soft-bodied sessile epibionts that grew attached to the surfaces of calcareous shells in ancient seas are sometimes preserved by a process termed ‘Epibiont Shadowing’. Processes that altered the surface of the shell were prevented from happening immediately beneath the attached epibiont, leaving a shadow of its attachment site following its death and decay. Microboring around the perimeter of the epibiont by presumed endolithic cyanobacteria gave rise to Endolithic Shadows, and dissolution of the calcareous substrate, maybe beneath larger smothering organisms, produced Solution Shadows of smaller organisms that protected their sites of attachment from such etching effects. Recognition of this type of preservation allows the stratigraphic range of certain soft-bodied groups to be extended. Details of the shadows may yield information about the morphological construction of the groups in question. 相似文献
107.
D. L. Band S. Ryder L. A. Ford J. L. Matteson D. M. Palmer B. J. Teegarden M. S. Briggs W. S. Paciesas G. N. Pendleton R. D. Preece 《Astrophysics and Space Science》1995,231(1-2):203-206
We evaluated the significance of the line candidates identified by the visual search of burst spectra from BATSE's spectroscopy detectors. None of the candidates satisfy the detection criteria: anF-test probability less than 10–4 for a feature in one detector and consistency among the detectors which viewed the burst. Most of the candidates are not very significant, and are likely to be fluctuations. Because of the expectation of finding absorption lines, the search was biased towards absorption features. We do not have a quantitative measure of the completeness of the search which would form the basis of a comparison with previous missions. Therefore a more objective computerized search has begun. 相似文献
108.
R. D. D’Arrigo E. R. Cook M. J. Salinger J. Palmer P. J. Krusic B. M. Buckley R. Villalba 《Climate Dynamics》1998,14(3):191-199
Distinct periods of warmth have been identified in instrumental records for New Zealand and the surrounding southwest Pacific
over the past 120 years. Whether this warming is due to natural climate variability or the effects of increasing greenhouse
gases is difficult to determine given the limited length of instrumental record. Longer records derived from tree rings can
help reduce uncertainties in detection of possible causes of climatic change, although relatively few such records have been
developed for the Southern Hemisphere. In this work, we describe five temperature-sensitive tree-ring width chronologies for
New Zealand which place the recent warming trend into a long-term (pre-anthropogenic) context. Included are three pink pine
(Halocarpus biformis) chronologies, two for Stewart Island and one for the North Island of New Zealand. Two silver pine (Lagarostrobus colensoi) series, one each from the North and South Islands, are updated from previous work. The length of record ranges from AD 1700
for Putara, North Island to AD 1400 for Ahaura, South Island. The pink and silver pine are different species from those used
previously to reconstruct temperatures for New Zealand. All five chronologies are positively and significantly correlated
with warm-season (November-April) individual station temperature records, a New Zealand-wide surface air temperature index
and gridded land/marine temperatures for New Zealand and vicinity. The highest 20 and 40-year growth periods in all five tree-ring
series coincide with the New Zealand temperature increase after 1950. An exception is found for the 40-year interval at Ahaura,
the least temperature-sensitive of the five sites. A t-test comparison indicates that these recent growth intervals are significantly
higher (0.01 to 0.0001 level) than any of those prior to the twentieth century for three of the five sites, dating as far
back as AD 1500. The results suggest that the recent warming has been distinctive, although not clearly unprecedented, relative
to temperature conditions inferred from tree-ring records of prior centuries.
Received: 18 February 1997/Accepted: 11 September 1997 相似文献
109.
110.
The anisotropy of the particle distribution and its variation with time at 1 AU early in a solar cosmic ray event can provide information on the pitch-angle scattering of the particles in the interplanetary medium. The proton event of 20 April 1971 is described in which the anisotropy of the 7.6–55 MeV energy channel remained large (? 100%) and field-aligned well into the decay phase of the event. A Monte Carlo technique, which gives the pitch-angle distribution, is employed to investigate two models put forward to explain this sustained anisotropy. It is shown that the observed event is consistent with one model in which the injection of particles at the Sun decayed with ane-folding time of 7 hr. In this model the parallel propagation is determined by small-angle scattering in a diverging field equivalent to a uniform diffusion coefficient of 2.1 × 1022 cm2 s?1 (the corresponding classical mean free path is 0.90 AU). A model with impulsive injection and in whichκ(r) increases strongly with distance from the Sun cannot satisfactorily explain the observations. 相似文献