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991.
Zhao-Yin WANG Yongsheng WU and Guangqian WANG Prof. Dept. of Hydraulic Engineering Tsinghua University & International Research Training Center on Erosion Sedimentation Beijing China Email: zywang@tsinghua.edu.cn or zywang@sun 《国际泥沙研究》2001,16(4)
1 mTRoorcnoxThe EnvirDment AgenCy of UK (l997) defins eUtIDPhication as "the enrichInnt of waters byinorgedc plant nUtrientS that result in the simulation of an mp Of syInPtOInati changes. These includethe inCrased PrOduCtiOn Of aigae or othe aquatic PlantS, affeChng the quallty of the water and distUIbingthe balance Of orpedsms Present within it. Such changes Inay be undesirable and intetw with wateuses." Sndth et al. (l999) indicated tha eopation of waters is mainly caused by h… 相似文献
992.
In a previous paper we presented a low-resolution (2°×2°) survey of radio recombination lines (RRLs) at 327 MHz in the longitude
rangel=330° to 0° to 89°. In this paper, we present the results of a higher resolution (2°×6′) survey of RRLs from seven 2°-wide
fields and two 6°-wide fields in the same longitude range. Observations were made using the Ooty Radio Telescope (ORT). A
total of 252 spectra that were obtained are presented. RRLs were detected in almost all the individual positions within the
fields withl<35° and at several individual positions within the fields in the longitude rangel=35° to 85°. Detailed analysis of the data towards the field centered at G45.5+0.0, shows that the line emission consists
of discrete zones of ionized gas. The angular extent of these zones are likely to be one degree or more corresponding to a
linear size of >110 pc at the kinematic distance. 相似文献
993.
High-Resolution Seismic Reflection Evidence for Middle Holocene Environmental Change, Owasco Lake, New York 总被引:2,自引:0,他引:2
Approximately 70 km of new decimeter-resolution seismic reflection profile data from Owasco Lake, New York define a middle Holocene (4600 14C yr B.P.) erosion surface in the north end of the lake at water depths as great as 26 m. Beneath the lake, post-glacial sediments are up to 9 m thick and represent about 10% of the total sediment fill. Early to middle Holocene sediments, 6 m thick, contain biogenic gas at the south end of the basin and a large (4 km×300 m×15 m) subaqueous slide deposit along the east-central portion of the lake. Late Holocene sediments are thinner or absent, particularly at the north end of the lake. The middle Holocene erosion surface may have been produced by a drop in lake level. Alternatively, it may represent a change in climate during the transition between the relatively warm Holocene hypsithermal and cool neoglacial. At this time (4600 14C yr B.P.) circulation in Owasco Lake appears to have evolved from sluggish to active. The increased circulation, which persists today, probably resulted from atmospheric cold fronts with strong southwesterly winds that piled up water at the north end of the lake. The increased water circulation may have been ultimately driven by decreasing insolation, which produced an increased pole-to-equator thermal gradient and, thus, stronger global winds that began at the transition between the hypsithermal and neoglacial. 相似文献
994.
A. M. Marotta R. Barzaghi A. Borghi E. Spelta 《Geophysical Journal International》2007,171(3):977-985
The thermomechanic evolution of the lithosphere–upper mantle system during Calabrian subduction is analysed using a 2-D finite element approach, in which the lithosphere is compositionally stratified into crust and mantle. Gravity and topography predictions are cross-checked with observed gravity and topography patterns of the Calabrian region. Modelling results indicate that the gravity pattern in the arc-trench region is shaped by the sinking of light material, belonging to both the overriding and subduction plates. The sinking of light crustal material, up to depths of the order of 100–150 km is the ultimate responsible for the peculiar gravity signature of subduction, characterized by a minimum of gravity anomaly located at the trench, bounded by two highs located on the overriding and subducting plates, with a variation in magnitude of the order of 200 mGal along a wavelength of 200 km, in agreement with the isostatically compensated component of gravity anomaly observed along a transect crossing the Calabrian Arc, from the Tyrrhenian to the Ionian Seas. The striking agreement between the geodetic retrieved profiles and the modelled ones in the trench region confirms the crucial role of compositional stratification of the lithosphere in the subduction process and the correctness of the kinematic hypotheses considered in our modelling, that the present-day configuration of crust–mantle system below the Calabrian arc results from trench's retreat at a rate of about 3 cm yr−1 , followed by gravitational sinking of the subducted slab in the last 5 Myr. 相似文献
995.
Centrifuge Modeling of Rock Slopes Susceptible to Block Toppling 总被引:2,自引:0,他引:2
Summary This paper presents the results of centrifuge tests that were aimed at validating the Goodman-Bray method for rock slope toppling
analysis. The Goodman-Bray method was extended by the authors to accommodate non-persistent basal planes of rock columns.
Two gypsum column models, with and without anchors were used to represent the failure modes. Measured critical centrifuge
accelerations were in agreement with the results obtained from numerical modeling. A background of the toppling slope failures
associated with a large hydropower project in China instigated the need for the centrifuge study. The centrifuge model tests
used an artificial rock. The observed failure mode did not follow a straight failure plane as proposed by Goodman and Bray.
The failures revealed a bi-planar slip surface with a deep-seated portion near the toe of the slope. The outcomes of the centrifuge
tests illustrated the need to search for the critical failure surface when performing a toppling analysis. The search technique
is similar to that usually performed in a conventional sliding analysis. 相似文献
996.
A Combined Three-Dimensional Geological-Geostatistical-Numerical Model of Underground Excavations in Rock 总被引:1,自引:1,他引:1
Summary. This paper exploits geological and borehole geotechnical data obtained in the exploratory phase of a tunneling project to
investigate in a first place if the kriging interpolation scheme may effectively reproduce the spatial variability of rock
mass quality (Rock Mass Rating, RMR) in the vicinity of tunnels. For this purpose a quick solver in Fortran has been developed
that performs variography analysis of 3D spatial data, fast kriging estimations of RMR between borehole sampling locations
at the centroids of the elements of the numerical model, and model validation. For the purpose of an integrated underground
excavation design, a step further is made by incorporating into the 3D mechanical numerical model of the rock mass, the three-dimensional
(3D) solid geological model, thus coupling the geology with the ground (geotechnical) model (i.e. each element of the numerical
model is assigned a geological material). The mechanical properties of each finite difference cell (or Representative Elementary
Volume) of the ground model were then prescribed according to its geological type, the spatial heterogeneity of the rock mass
expressed quantitatively with the kriging model, and the upscaling calculations of the mechanical properties of the intact
rocks determined in the laboratory, based on the size-effect (strength dependence on size) and Damage Theory. Furthermore,
a preliminary numerical simulation of the advance of unsupported tunnels in the model of the heterogeneous rock mass was performed
for illustration purposes. 相似文献
997.
Summary. Conventional blasting causes cracks and fractures in the rock. Controlled blasting techniques produce the macrocrack in a
desired direction and eliminate microcrack in the remaining rock. Macrocrack development in desired direction is required
for extraction of dimensional stone and at the same time there is need to reduce microcrack development in the block and remaining
rock. To achieve the objectives, experimental work in the quarries was carried out for separating marble block from the in
situ strata as practiced in some of the Indian mines by using detonating cord of 30 to 50 g/m by varying hole spacing, hole
diameter, air cushioning, water and sand filled blast-holes. Blasthole notching was carried out. Further, tests were carried
out by using various liners inside the blasthole to determine the damages in the extracted block and remaining rock. The designed
experimental work was undertaken and rock samples were collected by coring before and after blasting for quantification of
microcrack in the rock. P-wave velocity and microscopic studies were conducted for quantification of damages. Experiments
were also conducted at laboratory scale for the quantification of damages in single circular and notched holes with variation
of stemming and liners. The P-wave velocity close to hole always reduces after blast and in case of NG-based charge and detonating
cord it decreases up to 1/3rd. With PVC pipe and paper tube liners decrease is negligible. Thus, by using notched hole with paper tube, decrease in P-wave
is minimum indicating least damage. 相似文献
998.
G. M. E. Kamh 《Environmental Geology》2007,52(8):1539-1547
Al-hambra is an immense and valuable archaeological site in Spain built on Sabika hill with red brick and natural sandy limestone.
It exhibits weathering features indicating salt weathering process. The main aim of this study is to examine weathering processes
and intensity acting on Al-hambra. Rock petrography and mineralogical composition have been examined using thin sections,
scanning electron microscope, X-ray diffraction and X-ray fluorescence; limits of rock’s physical parameters using ultrasonic
waves and mercury porosimeter; rock salt content through hydrochemical analysis. Salts attacking this structure are mainly
from wet deposition of air pollutants on the long term chemical alteration of rock’s carbonate content to its equivalent salts.
The salts’ concentration limit within the examined rock samples is considerably low but it is effective on the long run through
hydration of sulphate salts and/or crystallization of chloride salts. Rock texture type and its silica as well as clay content
reduces its resistance to internal stresses by salts as well as wetting and drying cycles at such humid area. The recession
in limits of physical parameters examined for deep seated and weathered limestone samples quantitatively reflects weathering
intensity on Al-hambra. 相似文献
999.
1000.
Digital Image Based Approach for Three-Dimensional Mechanical Analysis of Heterogeneous Rocks 总被引:2,自引:0,他引:2
Summary This paper presents a digital image based approach for three-dimensional (3-D) numerical simulation and failure analysis of
rocks by taking into account the actual 3-D heterogeneity. Digital image techniques are adopted to extract two-dimensional
(2-D) material heterogeneity from material surface images. The 2-D image mesostructures are further extrapolated to 3-D cuboid
mesostructures by assuming the material surface as a representation of the inner material heterogeneity within a very small
depth. The iterative milling and scanning system is set up to generate the 3-D rock mesostructures. A Hong Kong granite specimen
is used as an example to demonstrate the procedure of 3-D mesostructure establishment. The mechanical responses and failure
process under the conventional Brazilian tensile test condition are examined through numerical analyses. The stress distribution,
crack propagation process and failure model of heterogeneous material cases are simulated with a finite difference software.
The numerical results indicate that material heterogeneity plays an important role in determining the failure behavior of
rocks under external loading. 相似文献