全文获取类型
收费全文 | 1212篇 |
免费 | 35篇 |
国内免费 | 4篇 |
专业分类
测绘学 | 26篇 |
大气科学 | 59篇 |
地球物理 | 304篇 |
地质学 | 362篇 |
海洋学 | 102篇 |
天文学 | 192篇 |
综合类 | 2篇 |
自然地理 | 204篇 |
出版年
2021年 | 7篇 |
2020年 | 9篇 |
2019年 | 9篇 |
2018年 | 22篇 |
2017年 | 16篇 |
2016年 | 19篇 |
2015年 | 26篇 |
2014年 | 32篇 |
2013年 | 64篇 |
2012年 | 24篇 |
2011年 | 42篇 |
2010年 | 41篇 |
2009年 | 51篇 |
2008年 | 54篇 |
2007年 | 60篇 |
2006年 | 48篇 |
2005年 | 37篇 |
2004年 | 50篇 |
2003年 | 42篇 |
2002年 | 34篇 |
2001年 | 26篇 |
2000年 | 32篇 |
1999年 | 16篇 |
1998年 | 15篇 |
1997年 | 30篇 |
1996年 | 22篇 |
1995年 | 23篇 |
1994年 | 12篇 |
1993年 | 22篇 |
1992年 | 24篇 |
1991年 | 26篇 |
1990年 | 18篇 |
1989年 | 14篇 |
1988年 | 10篇 |
1987年 | 12篇 |
1986年 | 17篇 |
1985年 | 18篇 |
1984年 | 22篇 |
1983年 | 17篇 |
1982年 | 18篇 |
1981年 | 17篇 |
1980年 | 18篇 |
1979年 | 12篇 |
1978年 | 15篇 |
1977年 | 20篇 |
1976年 | 11篇 |
1975年 | 14篇 |
1974年 | 12篇 |
1973年 | 13篇 |
1972年 | 8篇 |
排序方式: 共有1251条查询结果,搜索用时 312 毫秒
971.
Scott C. Chapman † Douglas Scott Charles C. Steidel Colin Borys Mark Halpern Simon L. Morris Kurt L. Adelberger Mark Dickinson Mauro Giavalisco Max Pettini 《Monthly notices of the Royal Astronomical Society》2000,319(1):318-330
We have carried out targeted submillimetre observations as part of a programme to explore the connection between the rest-frame ultraviolet and far-infrared properties of star-forming galaxies at high redshift, which is currently poorly understood. On the one hand, the Lyman break technique is very effective at selecting galaxies. On the other, 'blank-field' imaging in the submillimetre seems to turn up sources routinely, amongst which some are star-forming galaxies at similar redshifts. Already much work has been done searching for optical identifications of objects detected using the SCUBA instrument. Here we have taken the opposite approach, performing submillimetre photometry for a sample of Lyman break galaxies, the ultraviolet properties of which imply high star formation rates. The total signal from our Lyman break sample is undetected in the submillimetre, at an rms level of ∼0.5 mJy, which implies that the population of Lyman break galaxies does not constitute a large part of the recently detected blank-field submillimetre sources. However, our one detection suggests that with reasonable SCUBA integrations we might expect to detect those few Lyman break galaxies that are far-infrared brightest. 相似文献
972.
Arne Krtzinger Ludger Mintrop Douglas W. R. Wallace Kenneth M. Johnson Craig Neill Bronte Tilbrook Philip Towler Hisayuki Y. Inoue Masao Ishii Gary Shaffer Rodrigo F. Torres Saavedra Eiji Ohtaki Eiji Yamashita Alain Poisson Christian Brunet Bernard Schauer Catherine Goyet Greg Eischeid 《Marine Chemistry》2000,72(2-4)
The ‘International Intercomparison Exercise of fCO2 Systems’ was carried out in 1996 during the R/V Meteor Cruise 36/1 from Bermuda/UK to Gran Canaria/Spain. Nine groups from six countries (Australia, Denmark, France, Germany, Japan, USA) participated in this exercise, bringing together 15 participants with seven underway fugacity of carbon dioxide (fCO2) systems, one discrete fCO2 system, and two underway pH systems, as well as systems for discrete measurement of total alkalinity and total dissolved inorganic carbon. Here, we compare surface seawater fCO2 measured synchronously by all participating instruments. A common infrastructure (seawater and calibration gas supply), different quality checks (performance of calibration procedures for CO2, temperature measurements) and a common procedure for calculation of final fCO2 were provided to reduce the largest possible amount of controllable sources of error. The results show that under such conditions underway measurements of the fCO2 in surface seawater and overlying air can be made to a high degree of agreement (±1 μatm) with a variety of possible equilibrator and system designs. Also, discrete fCO2 measurements can be made in good agreement (±3 μatm) with underway fCO2 data sets. However, even well-designed systems, which are operated without any obvious sign of malfunction, can show significant differences of the order of 10 μatm. Based on our results, no “best choice” for the type of the equilibrator nor specifics on its dimensions and flow rates of seawater and air can be made in regard to the achievable accuracy of the fCO2 system. Measurements of equilibrator temperature do not seem to be made with the required accuracy resulting in significant errors in fCO2 results. Calculation of fCO2 from high-quality total dissolved inorganic carbon (CT) and total alkalinity (AT) measurements does not yield results comparable in accuracy and precision to fCO2 measurements. 相似文献
973.
Urbanization and the ability to manage for a sustainable future present numerous challenges for geographers and planners in metropolitan regions. Remotely sensed data are inherently suited to provide information on urban land cover characteristics, and their change over time, at various spatial and temporal scales. Data models for establishing the range of urban land cover types and their biophysical composition (vegetation, soil, and impervious surfaces) are integrated to provide a hierarchical approach to classifying land cover within urban environments. These data also provide an essential component for current simulation models of urban growth patterns, as both calibration and validation data. The first stages of the approach have been applied to examine urban growth between 1988 and 1995 for a rapidly developing area in southeast Queensland, Australia. Landsat Thematic Mapper image data provided accurate (83% adjusted overall accuracy) classification of broad land cover types and their change over time. The combination of commonly available remotely sensed data, image processing methods, and emerging urban growth models highlights an important application for current and next generation moderate spatial resolution image data in studies of urban environments. 相似文献
974.
Investigating strong ground-motion variability using analysis of variance and two-way-fit plots 总被引:2,自引:1,他引:1
A statistical method to quantitatively assess the relative importance of unmodelled site and source effects on the observed
variability (σ) in ground motions is presented. The method consists of analysis of variance (ANOVA) using the computed residuals
with respect to an empirical ground-motion model for strong-motion records of various earthquakes recorded at a common set
of stations. ANOVA divides the overall variance (σ
2) into the components due to site and source effects (respectively σ
S
2 and σ
E
2) not modelled by the ground-motion model plus the residual variance not explained by these effects (σ
R
2). To test this procedure, four sets of observed strong-motion records: two from Italy (Umbria-Marche and Molise), one from
the French Antilles and one from Turkey, are used. It is found that for the data from Italy, the vast majority of the observed
variance is attributable to unmodelled site effects. In contrast, the variation in ground motions in the French Antilles and
Turkey data is largely attributable, especially at short periods, to source effects not modelled by the ground-motion estimation
equations used. 相似文献
975.
Time domain electromagnetic‐induced polarisation: extracting more induced polarisation information from grounded source time domain electromagnetic data
下载免费PDF全文
![点击此处可从《Geophysical Prospecting》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Electrical induced polarisation surveys are used to detect chargeable materials in the earth. For interpretation of time domain electrical‐induced polarisation data a common procedure is to first invert the direct current data (electric current on time) to recover conductivity and then invert the induced polarisation data (current off‐time) to recover chargeability. This direct current‐induced polarisation inversion procedure assumes that the off time data are free of secondary electromagnetic induction effects. To comply with this, early time data are often discarded or not recorded. For mid‐time data, an electromagnetic decoupling technique, which removes electromagnetic induction in the observations, needs to be implemented. Usually, responses from a half‐space or a layered earth are subtracted. Recent capability in three‐dimensional time domain electromagnetic forward modelling and inversion allows to revisit these procedures. In a Time domain electromagnetic‐induced polarisation survey, a high sampling rate allows early time channels of the electromagnetic data to be recorded. The recovery of chargeability then follows a three‐step workflow: (i) invert early time channel time domain electromagnetic data to recover the three‐dimensional conductivity; (ii) use that conductivity to compute the time domain electromagnetic response at later time channels and subtract this fundamental response from the observations to extract the induced polarisation responses, and (iii) invert the induced polarisation responses to recover a three‐dimensional chargeability. This workflow effectively removes electromagnetic induction effects in the observations and produces better chargeability and conductivity models compared with conventional approaches. In a synthetic example involving a gradient array, we show that the conductivity structure obtained from the early time channel data, which are usually discarded, is superior to that obtained from the steady state direct current voltages. This adds a further reason to collect these electromagnetic data. 相似文献
976.
Cheng Hong Fang Yi Douglas J. Sherman Liu Chenchen Zou Xueyong Zhang Kaidi Kang Liqiang 《地球表面变化过程与地形》2018,43(6):1252-1258
Aeolian sand transport is a widespread physical phenomenon on the surface of Earth, as well as on Mars and Titan. Accurate measurements of the components of the transport system are necessary if we are to understand the nature of the physical processes. Sand traps are typically used to measure sediment transport rates, and issues associated with the sampling efficiency of traps and the development of reliable traps have received considerable attention in recent decades. In this study, we measured aeolian transport rate at five distances from a wind tunnel sidewall using a vertically‐segmented sand trap. Total transport rates were determined by weighing the bed sediment before and after each experiment, and with and without a trap installed. The following results were obtained: (1) sand transport increased linearly with the distance away from the sidewall, and the appropriate location to measure maximum transport is within the central 20% of the wind tunnel; (2) current methods overestimate the sampling efficiency of sand traps when comparing trap data to transport rate data obtained by weighing sand moved through the entire tunnel because the effects of the sidewalls in decreasing total transport are neglected; (3) the efficiency of the vertically‐segmented trap that we tested ranged from 11.57% to 31.68% using our revised methods, whereas standard methods caused efficiency to be overestimated by 32–72% of the efficiency; (4) using either method, the efficiency of the trap increased exponentially with shear velocity for the range we used. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
977.
978.
High-resolution analysis of macroscopic charcoal and pollen ratios were used to reconstruct a 10,000 yr history of fire and vegetation change around Dog Lake, now in the Montane Spruce biogeoclimatic zone of southeastern British Columbia. Lake sediment charcoal records suggest that fire was more frequent in the early Holocene from 10,000 to 8200 calendar yrs BP, when climate was warmer and drier than today and forest fuels were limited. Fire frequency increased and reached its maximum during the early to mid-Holocene from 8200 to 4000 calendar yrs BP, corresponding to the dry and warm Hypsithermal period in the Rocky Mountains. During the Hypsithermal period forests around Dog Lake were dominated by Pseudotsuga/Larix,Pinus and open meadows of Poaceae that were subject to frequent fire. From 4000 calendar yrs BP to present, fires became less frequent with the onset of cooler and wetter Neoglacial climate and an increase in wet-closed Picea and Abies forests in the valley. Changes in fire frequency are supported by dry-open/wet-closed pollen ratio data indicating that forest type and disturbance regimes vary with changing climate. The fire frequency and forest cover reconstructions from Dog Lake are a first attempt at defining a range of natural variability for Montane Spruce forests in southeastern British Columbia. Fire and vegetation management in Kootenay National Park can now use this century to millennial-scale range of variability to define the context of current forest conditions and potential changes under global warming scenarios. 相似文献
979.
We report on a detailed palaeomagnetic study of the Miocene Farellones volcanic formation in the Chilean Andes near Santiago (two sections, 37 sites, about 400 orientated cores). Petrological observations show evidence of low-grade metamorphism increasing downwards through the volcanic sequence. Optical observations of opaque minerals and magnetic experiments suggest that in many cases maghemitization is associated with hydrothermal alteration. However, thermal demagnetization data indicate that the low-grade metamorphism did not significantly modify the direction of the primary remanent magnetization recorded at the time of emplacement of the volcanic lava flows. Four intervals of polarity with two intermediate palaeodirections were observed in the ~650-m-thick composite section. According to the dispersion of flow average directions, palaeosecular variation was slightly larger than that observed in general during the Upper Cenozoic. The site mean directions obtained in this study differ significantly from the expected Miocene direction. Clockwise rotations of up to 20° of small blocks are probably associated with the deformation of the Andean Cordillera since middle Miocene times. Geomagnetic palaeointensity data were obtained, using the Thellier method, on 24 samples from eight distinct lava flows. The flow mean VDM varies from 1.4 to 4.0 × 1022 A m−2 . Altogether, our data seem to suggest the existence of a relatively low geomagnetic field undergoing large fluctuations. Although a linear relationship was observed between the natural remanent magnetization and the thermal remanent magnetization acquired during the Thellier–Thellier experiments, undetected chemical alteration of the magnetic minerals during hydrothermalism may also explain the unusually low palaeointensity obtained. 相似文献
980.
Douglas M. Barker Damian M. Lawler Donald W. Knight David G. Morris Helen N. Davies Elizabeth J. Stewart 《地球表面变化过程与地形》2009,34(2):280-290
Stream power can be an extremely useful index of fluvial sediment transport, channel pattern, river channel erosion and riparian habitat development. However, most previous studies of downstream changes in stream power have relied on field measurements at selected cross‐sections, which are time consuming, and typically based on limited data, which cannot fully represent important spatial variations in stream power. We present here, therefore, a novel methodology we call CAFES (combined automated flood, elevation and stream power), to quantify downstream change in river flood power, based on integrating in a GIS framework Flood Estimation Handbook systems with the 5 m grid NEXTMap Britain digital elevation model derived from IFSAR (interferometric synthetic aperture radar). This provides a useful modelling platform to quantify at unprecedented resolution longitudinal distributions of flood discharge, elevation, floodplain slope and flood power at reach and basin scales. Values can be resolved to a 50 m grid. CAFES approaches have distinct advantages over current methodologies for reach‐ and basin‐scale stream power assessments and therefore for the interpretation and prediction of fluvial processes. The methodology has significant international applicability for understanding basin‐scale hydraulics, sediment transport, erosion and sedimentation processes and river basin management. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献