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排序方式: 共有162条查询结果,搜索用时 31 毫秒
51.
Optimizing statistical classification accuracy of satellite remotely sensed imagery for supporting fast flood hydrological analysis 总被引:1,自引:0,他引:1
Dimitrios D. Alexakis Athos Agapiou Diofantos G. Hadjimitsis Adrianos Retalis 《Acta Geophysica》2012,60(3):959-984
The aim of this study is to improve classification results of multispectral satellite imagery for supporting flood risk assessment
analysis in a catchment area in Cyprus. For this purpose, precipitation and ground spectroradiometric data have been collected
and analyzed with innovative statistical analysis methods. Samples of regolith and construction material were in situ collected and examined in the spectroscopy laboratory for their spectral response under consecutive different conditions
of humidity. Moreover, reflectance values were extracted from the same targets using Landsat TM/ETM+ images, for drought and
humid time periods, using archived meteorological data. The comparison of the results showed that spectral responses for all
the specimens were less correlated in cases of substantial humidity, both in laboratory and satellite images. These results
were validated with the application of different classification algorithms (ISODATA, maximum likelihood, object based, maximum
entropy) to satellite images acquired during time period when precipitation phenomena had been recorded. 相似文献
52.
Olga-Joan Ktenidou Francisco-J. Chávez-García Dimitrios Raptakis Kyriazis D. Pitilakis 《Bulletin of Earthquake Engineering》2016,14(3):623-645
We study site effects using 520 weak motion earthquake records from a vertical array in Aegion, Greece. The array is inside a basin, has four stations in soil, and one in bedrock (178 m depth). The site is marked by high seismicity and complex surface geology. We first use the records to establish the downhole accelerometer orientations and their evolution with time. Then we estimate site effects using empirical spectral ratios with and without a reference site (standard and horizontal-to-vertical spectral ratio). We find significant site amplification which cannot be accounted for by 1D model predictions, along with a significant difference in the amplification level between the two horizontal components. These are indications of 2D effects, namely surface waves generated at the basin edge. The difference in amplification between the horizontal components is maximised when these are rotated with respect to the orientation of the basin edge. The strongest amplification takes place in the direction parallel to the basin edge (SH, or out-of-plane motion), and is up to 2 times higher than in the perpendicular direction (SV, or in-plane motion). This directional effect on the amplification is corroborated by numerical 2D modelling using incident SH and SV waves, with the former possibly generating strong Love waves. In the records, the directionality is clear for windows containing the largest amplitudes of the records (S waves and strong surface waves), while it tends to vanish for coda-wave windows. This directionality is also observed when using response spectral ratios rather than Fourier ratios. We compute soil-to-rock amplification factors for peak ground acceleration (PGA) and find it is significantly higher than what is predicted by current design codes. We attribute this difference to the basin edge amplification, linear soil behaviour, and to the inability of simple scalar values like PGA to describe complex amplification effects. Finally, we analyse the earthquake records at a surface station near the slope crest and do not observe significant topographic amplification. 相似文献
53.
Imaging Pathways in Fractured Rock Using Three‐Dimensional Electrical Resistivity Tomography
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Judith Robinson Lee Slater Timothy Johnson Allen Shapiro Claire Tiedeman Dimitrios Ntarlagiannis Carole Johnson Frederick Day‐Lewis Pierre Lacombe Thomas Imbrigiotta John Lane 《Ground water》2016,54(2):186-201
Major challenges exist in delineating bedrock fracture zones because these cause abrupt changes in geological and hydrogeological properties over small distances. Borehole observations cannot sufficiently capture heterogeneity in these systems. Geophysical techniques offer the potential to image properties and processes in between boreholes. We used three‐dimensional cross borehole electrical resistivity tomography (ERT) in a 9 m (diameter) × 15 m well field to capture high‐resolution flow and transport processes in a fractured mudstone contaminated by chlorinated solvents, primarily trichloroethylene. Conductive (sodium bromide) and resistive (deionized water) injections were monitored in seven boreholes. Electrode arrays with isolation packers and fluid sampling ports were designed to enable acquisition of ERT measurements during pulsed tracer injections. Fracture zone locations and hydraulic pathways inferred from hydraulic head drawdown data were compared with electrical conductivity distributions from ERT measurements. Static ERT imaging has limited resolution to decipher individual fractures; however, these images showed alternating conductive and resistive zones, consistent with alternating laminated and massive mudstone units at the site. Tracer evolution and migration was clearly revealed in time‐lapse ERT images and supported by in situ borehole vertical apparent conductivity profiles collected during the pulsed tracer test. While water samples provided important local information at the extraction borehole, ERT delineated tracer migration over spatial scales capturing the primary hydrogeological heterogeneity controlling flow and transport. The fate of these tracer injections at this scale could not have been quantified using borehole logging and/or borehole sampling methods alone. 相似文献
54.
Analytical modelling of near‐source pulse‐like seismic demand for multi‐linear backbone oscillators
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Nonlinear static procedures, which relate the seismic demand of a structure to that of an equivalent single‐degree‐of‐freedom oscillator, are well‐established tools in the performance‐based earthquake engineering paradigm. Initially, such procedures made recourse to inelastic spectra derived for simple elastic–plastic bilinear oscillators, but the request for demand estimates that delve deeper into the inelastic range, motivated investigating the seismic demand of oscillators with more complex backbone curves. Meanwhile, near‐source (NS) pulse‐like ground motions have been receiving increased attention, because they can induce a distinctive type of inelastic demand. Pulse‐like NS ground motions are usually the result of rupture directivity, where seismic waves generated at different points along the rupture front arrive at a site at the same time, leading to a double‐sided velocity pulse, which delivers most of the seismic energy. Recent research has led to a methodology for incorporating this NS effect in the implementation of nonlinear static procedures. Both of the previously mentioned lines of research motivate the present study on the ductility demands imposed by pulse‐like NS ground motions on oscillators that feature pinching hysteretic behaviour with trilinear backbone curves. Incremental dynamic analysis is used considering 130 pulse‐like‐identified ground motions. Median, 16% and 84% fractile incremental dynamic analysis curves are calculated and fitted by an analytical model. Least‐squares estimates are obtained for the model parameters, which importantly include pulse period Tp. The resulting equations effectively constitute an R ? μ ? T ? Tp relation for pulse‐like NS motions. Potential applications of this result towards estimation of NS seismic demand are also briefly discussed. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
55.
The availability of digital elevation databases representing the topographic and bathymetric relief with global homogeneous coverage and increasing resolution permits the computation of crust-related Earth gravity models, the so-called topographic/isostatic Earth gravity models (henceforth T/I models). Although expressing the spherical harmonic content of the topographic masses, the interpretation purpose of T/I models has not been given the attention it deserves, apart from the fact that they express some degree of compensation to the observed spectrum of the topographic heights, depending on the kind of the applied compensation mechanism. The present contribution attempts to improve the interpretation aspects of T/I Earth gravity models. To this end, a rigorous spectral assessment is performed to a standard Airy/Heiskanen T/I model against different CHAllenging Minisatellite Payload (CHAMP), Gravity Recovery and Climate Experiment (GRACE), Gravity field and steadystate Ocean Circulation Explorer (GOCE) satellite-only, and combined gravity models. Different correlation bandwidths emerge for these four groups of satellite-based gravity models. The band-limited forward computation of the models using these bandwidths reproduces nicely the main features of the applied T/I model. 相似文献
56.
Acta Geotechnica - A multi-disciplinary approach is adopted in the present work towards investigating bio-cemented geo-materials which extends from sample preparation, to microstructural inspection... 相似文献
57.
With our meshfree numerical code SPARC (Soft PARticle Code), which is based on strong solutions of the equations of equilibrium, we were able to derive vortex patterns ("turbulence") in deformations hitherto believed to be homogeneous. The formation of such vortices demonstrates the nonuniqueness of the corresponding boundary value problem. We present some evidence that such vortices can be related with ptygmatic folds, which are observed in rock. 相似文献
58.
Turbidite stacking patterns in salt‐controlled minibasins: Insights from integrated analogue models and numerical fluid flow simulations
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Xiaoxi Wang Stefan M. Luthi David M. Hodgson Dimitrios Sokoutis Ernst Willingshofer Remco M. Groenenberg 《Sedimentology》2017,64(2):530-552
The sea floor of intraslope minibasins on passive continental margins plays a significant role in controlling turbidity current pathways and the resulting sediment distribution. To address this, laboratory analogue modelling of intraslope minibasin formation is combined with numerical flow simulations of multi‐event turbidity currents. This approach permits an improved understanding of evolving flow–bathymetry–deposit interactions and the resulting internal stacking patterns of the infills of such minibasins. The bathymetry includes a shelf to slope channel followed by an upper minibasin, which are separated by a confining ridge from two lower minibasins that compares well with analogous bathymetries reported from natural settings. From a wider range of numerical flow experiments, a series of 100 consecutive flows is reported in detail. The turbidity currents are released into the channel and upon reaching the upper minibasin follow a series of stages from short initial ponding, ‘filling and spilling’ and an extended transition to long retrogradational ponding. Upon reaching the upper minibasin floor, the currents undergo a hydraulic jump and therefore much sediment is deposited in the central part of the minibasin and the counterslope. This modifies the bathymetry such that in the fill and spill stage, flow stripping and grain‐size partitioning cause some finer sediment to be transported across the confining ridge into the lower minibasins. Throughout the basin infill process, the sequences retrograde upstream, accompanied by lateral switching into locally formed depressions in the upper minibasin. After the fill and spill stage, significant deposition occurs in the channel where retrograding cyclic steps with wavelengths of 1 to 2 km develop as a function of pulsating flow criticality. These results are at variance with conventional schemes that emphasize sequential downstream minibasin filling through ponding dominated by vertical aggradation. Comparison of these results with published field and experimental examples provides support for the main conclusions. 相似文献
59.
Stylianos Bitharis Christos Pikridas Aristeidis Fotiou Dimitrios Rossikopoulos Harald Schuh 《Marine Geodesy》2017,40(5):297-314
In this study, we show how the Global Navigation Satellite System (GNSS)-derived vertical velocities contribute to the correction of tide gauge (TG) measurements used for the sea level rise estimation in Greece. Twelve sites with records of local sea level heights are processed in order to estimate their trend. Certain error sources related to TGs, e.g. equipment changes, data noise, may lead to biased or erroneous estimations of the sea level height. Therefore, it would be preferred to follow a robust estimation technique in order to detect and reduce outlier effects. The geocentric sea level rise is estimated by taking into account the land vertical motion of co-located GNSS permanent stations at the Hellenic area. TGs measure the height of the water relative to a monitored geodetic benchmark on land. On the other hand, using GNSS-based methods the vertical land motion can be derived. By means of extended models fitted to the GNSS time-series position, obtained from seven years of continuous data analysis, periodic signals are well described. The synergy of the two co-located techniques results in the correction of TG relative sea level heights taking into account the GNSS vertical velocities and consequently obtaining the conversion to absolute (geocentric) sea level trend. 相似文献
60.
Kinematics of shear bands 总被引:2,自引:2,他引:0
Dimitrios Kolymbas 《Acta Geotechnica》2009,4(4):315-318
Shear bands appear at limit states of soil bodies. They are analysed as thin zones of localised deformation that takes place
as simple (i.e. dilatant) shear. It can be observed, however, that shear bands are discontinuous and also may be “reflected”
at rigid boundaries. These phenomena appear as incompatible with the assumed shear deformation. The analysis in this paper
reveals the kinematics of such “incompatibilities” in terms of continuous deformation fields. 相似文献