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131.
级配碎石作为重载铁路基床表层的主要填料,其受列车荷载的影响最大。因此,研究级配碎石在循环荷载作用下的动力行为及累积塑性应变演化特征变得尤为重要。首先,通过制备不同细粒含量的级配碎石填料,开展一系列大型动三轴试验,探究细粒含量、围压及动应力幅值对循环荷载作用下试样累积塑性应变的耦合影响机制。其次,基于塑性安定理论,确定不同应力水平下试样的动力行为,得到考虑围压及细粒含量参数的塑性蠕变状态临界动应力计算模型。最后,结合试验数据,建立考虑应力水平及细粒含量参数的塑性蠕变动力行为累积塑性应变预测模型,并明确各参数的物理意义。其研究成果可为既有重载铁路路基健康状态评估及考虑强度、变形综合控制的路基结构设计提供参考。 相似文献
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133.
Kyle S. Boodoo Jakob Schelker Nico Trauth Tom J. Battin Christian Schmidt 《水文研究》2019,33(17):2279-2299
Gravel bars (GBs) contribute to carbon dioxide (CO2) emissions from stream corridors, with CO2 concentrations and emissions dependent on prevailing hydraulic, biochemical, and physicochemical conditions. We investigated CO2 concentrations and fluxes across a GB in a prealpine stream over three different discharge‐temperature conditions. By combining field data with a reactive transport groundwater model, we were able to differentiate the most relevant hydrological and biogeochemical processes contributing to CO2 dynamics. GB CO2 concentrations showed significant spatial and temporal variability and were highest under the lowest flow and highest temperature conditions. Further, observed GB surface CO2 evasion fluxes, measured CO2 concentrations, and modelled aerobic respiration were highest at the tail of the GB over all conditions. Modelled CO2 transport via streamwater downwelling contributed the largest fraction of the measured GB CO2 concentrations (31% to 48%). This contribution increased its relative share at higher discharges as a result of a decrease in other sources. Also, it decreased from the GB head to tail across all discharge‐temperature conditions. Aerobic respiration accounted for 17% to 36% of measured surface CO2 concentrations. Zoobenthic respiration was estimated to contribute between 4% and 8%, and direct groundwater CO2 inputs 1% to 23%. Unexplained residuals accounted for 6% to 37% of the observed CO2 concentrations at the GB surface. Overall, we highlight the dynamic role of subsurface aerobic respiration as a driver of spatial and temporal variability of CO2 concentrations and evasion fluxes from a GB. As hydrological regimes in prealpine streams are predicted to change following climatic change, we propose that warming temperatures combined with extended periods of low flow will lead to increased CO2 release via enhanced aerobic respiration in newly exposed GBs in prealpine stream corridors. 相似文献
134.
Previous stratigraphical investigations of Romney Marsh have tended to be local in scale, and this has hindered efforts to establish a unifying stratigraphical framework for this area. This paper addresses this problem, by describing the results of a 12-km transect across Romney Marsh, linking previously studied back- and fore-marsh sites (Horsemarsh Sewer and Broomhill respectively), and presenting additional pollen, diatom and radiocarbon data from an intermediate, mid-marsh site (Brookland). One main organic unit is recorded across much of Romney Marsh, although its age, altitude and composition varies. Microfossil and radiocarbon data from Brookland and elsewhere on Romney Marsh show that this organic unit accumulated under a general removal and return of marine conditions that took place between ca. 5100 and 2000 yr BP. A recently proposed model of barrier development is used to investigate the history of back-barrier sedimentation in Romney and Walland Marshes. This model suggests a three-phase life-history for gravel barriers, which consists of initiation, stability and breakdown. Although there are problems in relating back-barrier deposits directly to barrier dynamics, nevertheless the Romney Marsh data do, for the most part, agree with the expected number and sequence of sea-level tendencies predicted by this model. The back-barrier stratigraphical data suggest that initiation, stability and breakdown of the Dungeness foreland occurred between ca. 6000 and 5000 yr BP, 5000 and 2000 yr BP and 2000 yr BP and present, respectively. 相似文献
135.
干密度是评价软土固化质量的重要指标,一般通过室内击实试验获得。以氯氧镁水泥(MOC)为结合料,砾石土为固化对象,研究了原料配比对MOC固化砾石土击实性能的影响。通过室内击实试验,考察了轻烧镁粉掺量、卤水浓度、卤水掺量和活性MgO/MgCl_2物质的量比(Mg/Cl比)影响混合料干密度的规律。结果表明:MOC固化砾石土击实料的干密度随卤水掺量增加先增加后降低,随轻烧镁粉掺量及Mg/Cl比的增加而先增加后降低,随卤水浓度及轻烧镁粉掺量的增加而增加,随卤水浓度增加及Mg/Cl比降低而增加。 相似文献
136.
Determination of the in situ engineering properties of foundation materials has always been a challenge for geotechnical engineers and, thus, several methods have been developed so far. Dynamic Cone Penetration (DCP) test is one of the most versatile amongst them. In the present research, a light weight simple DCP device was developed and used for evaluation of the engineering properties of sandy soils in laboratory conditions. The device consisted of an 8-kg hammer that drops over a height of 575 mm, and drives a 60° cone tip with 20 mm base diameter into the ground. To control the validation of the results, laboratory direct shear and plate load tests were used as reference tests. The soil sample was a poorly graded sandy soil (SP) taken from alluvial deposits of the Tehran plain. All DCP tests and PLTs were undertaken on compacted soil in a mould with 700 mm diameter and 700 mm height. Based on the results of the experiments, the relationships between Dynamic Penetration Index (DPI), relative density (Dr), modulus of elasticity (E), shear modulus (G), modulus of subgrade reaction (KS), and the friction angle of the soil were obtained with a high coefficient of determination (> 90%). The repeatability of the test results was also evaluated by calculating the coefficient of variations (Cv), which was less than 30% for all tests. 相似文献
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138.
A guide is provided to the minimum sample masses required to obtain reproducible measures of the particle-size distributions of coarse sediments. This is based on studies of the actual particle-size distributions of a range of clastic deposits. Procedures are given to enable representative bulk samples of tills, fluvial gravels and beach gravels to be taken. 相似文献
139.
青弋江上游泾县段阶地砾石层砾组结构及其沉积环境研究 总被引:1,自引:1,他引:0
通过对青弋江上游泾县段阶地砾石层进行砾组分析,讨论阶地砾石层的沉积环境及其对青弋江发育的启示。结果表明:① 砾径以中砾和粗砾为主,砾石沉积时水动力条件较强,流速基本为2 m/s左右,最大可达到3.5 m/s,特别是T3砾石层形成时期;② 砾向在T3和T2砾石层形成时期分别为南南西(SSW)和南西西(SWW)方向,古流向变化不大,呈自南而北的基本流向;③ 砾态以次圆和圆为主,其总含量超过70%,较高的磨圆度暗示砾石经历了较远距离的搬运;④ 砾性主要有石英砂岩、砂岩、脉石英和石英岩,其总含量达到90%以上,且砾石物源区变化不显著;⑤ T2砾石层和T1砾石层是典型的河流沉积,而T3砾石层可能是河流沉积和泥石流沉积叠加作用的产物,并且T3砾石层的沉积特征对于分析古青弋江的发育有一定的启示作用。 相似文献
140.
钱塘江下切河谷充填及其层序地层学特征 总被引:20,自引:0,他引:20
钱塘江口是世界著名的强潮型河口湾。平面上呈典型的喇叭状,具明显的三分性:上部的正常河流段、中部的河口湾漏斗、下部的口门带。据沉积作用和沉积相组合,钱塘江河口湾的形成及其发育可以分为四个阶段:(1)末次冰期(20000-15000aB,R)──下切河谷形成;(2)冰后期早期海侵(15000-7500aB,P.)─-河口湾充填;(3)最大海侵(7500-6000aB.P.)──海湾形成;(4)海面相对稳定期(6000aB.P.至今)──河口湾发育。其间形成了一套完整的海退-海侵进积旋回沉积层序。下切河谷底部的厚度异常的河床相砂砾层,从形成阶段上看,可以分为两个阶段,早期是河流下切的滞留沉积,晚期是基面抬升河流加积作用形成,它们的界限即为初次海泛面。加积作用形成的河床相砂砾层,有别于滞留沉积作用形成的河床相砂砾层,前者具加积副层序组合、向上变细的正粒序,且形成的地质年代较晚。 相似文献