全文获取类型
收费全文 | 2515篇 |
免费 | 147篇 |
国内免费 | 31篇 |
专业分类
测绘学 | 88篇 |
大气科学 | 289篇 |
地球物理 | 650篇 |
地质学 | 770篇 |
海洋学 | 214篇 |
天文学 | 353篇 |
综合类 | 6篇 |
自然地理 | 323篇 |
出版年
2022年 | 13篇 |
2021年 | 48篇 |
2020年 | 59篇 |
2019年 | 53篇 |
2018年 | 66篇 |
2017年 | 80篇 |
2016年 | 106篇 |
2015年 | 88篇 |
2014年 | 97篇 |
2013年 | 182篇 |
2012年 | 118篇 |
2011年 | 157篇 |
2010年 | 119篇 |
2009年 | 140篇 |
2008年 | 140篇 |
2007年 | 139篇 |
2006年 | 132篇 |
2005年 | 109篇 |
2004年 | 92篇 |
2003年 | 92篇 |
2002年 | 81篇 |
2001年 | 51篇 |
2000年 | 56篇 |
1999年 | 38篇 |
1998年 | 34篇 |
1997年 | 30篇 |
1996年 | 28篇 |
1995年 | 30篇 |
1994年 | 17篇 |
1993年 | 19篇 |
1992年 | 12篇 |
1991年 | 15篇 |
1990年 | 15篇 |
1989年 | 27篇 |
1988年 | 10篇 |
1987年 | 20篇 |
1986年 | 11篇 |
1985年 | 22篇 |
1984年 | 14篇 |
1983年 | 14篇 |
1982年 | 12篇 |
1981年 | 18篇 |
1980年 | 10篇 |
1979年 | 13篇 |
1978年 | 9篇 |
1977年 | 7篇 |
1976年 | 6篇 |
1975年 | 7篇 |
1974年 | 8篇 |
1972年 | 5篇 |
排序方式: 共有2693条查询结果,搜索用时 100 毫秒
991.
Depositional and post-depositional controls on magnetic signals from saltmarshes on the north-west coast of Ireland 总被引:3,自引:0,他引:3
Magnetic measurements were carried out on samples from two saltmarshes deposited during the last century on the north-west coast of Ireland. Based on profiles from cliffed saltmarsh edge, mid-marsh and supratidal locations, the processes that affect the generation and persistence of magnetic properties in environments were investigated. This was done to help in understanding the geochemical processes affecting north-west Irish coast saltmarshes and to determine the frequency and provenance of sediment-laden flooding events; hence, disentangling the importance of clastic and organic contributions to saltmarsh growth. Magnetic properties indicate depositional and post-depositional effects varying on both temporal and spatial scales. The interpretations presented here suggest that both biogenic and diagenetic processes have a strong effect on the magnetic properties recorded: in the former case, the growth of magnetotactic bacteria is the main process; in the latter case, it is reductive diagenesis, associated with organic matter decomposition, that is important. The biogenic and diagenetic effects overprint and degrade the detrital magnetic signatures respectively. This suggests that magnetic measurements within the context of north-west Irish coast saltmarshes can provide limited information on clastic sediment input to these environments. Comparisons between sites on the north-west coast of Ireland show similar magnetic properties, despite differences in deposition rates, implying that significant post-depositional alteration of magnetic properties occurs within the first 20–40 years after deposition. Magnetic properties of north-west coast Irish saltmarshes are put into a regional context by comparison with magnetic profiles from similar sites. 相似文献
992.
On the design of formal theories of geographic space 总被引:1,自引:0,他引:1
This paper discusses the design of formal theories of geographic space for the application in Geographic Information Systems.
GIS software is an implementation of formal theories of geographic space. The notions of formal theories are introduced and
discussed in the context of examples from the GIS field.
Our approach is an application of the general framework of formal theories to the special class of theories of geographic
space, in particular to the geometry of geographic space. A framework is introduced for characterizing and evaluating formal
theories of geographic space and the process of their design. This is used to provide (1) a classification of formal theories
of geographic space, (2) criteria of their adequacy, and (3) an evaluation of design decisions in the process of formalization.
The paper demonstrates the choices in the design of GIS and the dependencies between these choices. Considering the design
space for theories underlying a GIS, we can see that current GIS are based on one choice: analytical geometry. Other designs
are possible and a systematic exploration of alternative types of GIS, for example, based on constraints or based on stored
spatial relations, becomes necessary.
Received: 30 April 1997/Accepted: 8 March 1999 相似文献
993.
994.
995.
Steep‐faced glaciodeltaic progradational successions are often studied in order to reconstruct the behaviour of the glacial feeder system, or changes in the sediment sink. This paper analyses the magnitude and frequency of depositional events associated with steep‐faced glacier‐fed progradational successions recorded in Scandinavia and Ireland. The successions exhibit depositional patterns that may be interpreted as a function of underlying non‐linear dynamics. A number of the sequences display fractal scaling in the frequency and thickness of foreset units. Other successions demonstrate chaotic patterns and strong relationships between delta‐front angle and bed thicknesses, suggesting that the progradation of such sequences is self‐organized, and to an extent occurs independently of forcing by the feeder system that provides sediment to the delta front. These patterns of sedimentation appear to be a function of the steepness of the delta front and/or the textural characteristics of the sediment. This paper provides further evidence for the simultaneous presence of order and chaos in Earth surface processes and calls into question the extent to which palaeoenvironmental reconstructions may be made from steep‐faced progradational successions. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
996.
Sediment accumulation in embayments controlled by bathymetric slope and wave energy: Implications for beach formation and persistence 下载免费PDF全文
John Preston Martin D. Hurst Simon M. Mudd Guillaume C.H. Goodwin Anthony J. Newton Andrew J. Dugmore 《地球表面变化过程与地形》2018,43(11):2421-2434
High energy, rocky coastlines often feature sandy beaches within headland‐bound embayments. Not all such embayments have beaches however, and beaches in embayments can be removed by storms and may subsequently reform. What dictates the presence or absence of an embayed beach and its resilience to storms? In this paper, we explore the effect of offshore slope and wind conditions on nearshore sediment transport within idealised embayments to give insight into nearshore sediment supplies. We use numerical simulations to show that sand can accumulate near shore if the offshore slope is >0.025 m/m, but only under persistent calm conditions. Our modelling also suggests that if sediment in an embayment with an offshore gradient steeper than 0.025 m/m is removed during a period of persistent stormy conditions, it will be unlikely to return in sub‐decadal timescales. In contrast, sediment located in embayments with shallower gradients can reform swiftly in both calm and stormy conditions. Our findings have wide implications for contemporary coastal engineering in the face of future global climate change, but also for Quaternary environmental reconstruction. Our simple method to predict beach stability based on slope can be used to interpret differing responses of embayments to periods of changing coastal storminess such as the medieval climate anomaly‐little ice age (MCA‐LIA) transition. © 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. 相似文献
997.
998.
Investigating a time‐shift extended imaging condition in a Kirchhoff pre‐stack depth migration algorithm 下载免费PDF全文
Gareth Shane O'Brien Sean Joseph Delaney Michael Igoe John Doherty Andrew Colhoun 《Geophysical Prospecting》2018,66(4):688-706
Extracting true amplitude versus angle common image gathers is one of the key objectives in seismic processing and imaging. This is achievable to different degrees using different migration techniques (e.g., Kirchhoff, wavefield extrapolation, and reverse time migration techniques) and is a common tool in exploration, but the costs can vary depending on the selected migration algorithm and the desired accuracy. Here, we investigate the possibility of combining the local‐shift imaging condition, specifically the time‐shift extended imaging condition, for angle gathers with a Kirchhoff migration. The aims are not to replace the more accurate full‐wavefield migration but to offer a cheaper alternative where ray‐based methods are applicable and to use Kirchhoff time‐lag common image gathers to help bridge the gap between the traditional offset common image gathers and reverse time migration angle gathers; finally, given the higher level of summation inside the extended imaging migration, we wish to understand the impact on the amplitude versus angle response. The implementation of the time‐shift imaging condition along with the computational cost is discussed, and results of four different datasets are presented. The four example datasets, two synthetic, one land acquisition, and a marine dataset, have been migrated using a Kirchhoff offset method, a Kirchhoff time‐shift method, and, for comparison, a reverse time migration algorithm. The results show that the time‐shift imaging condition at zero time lag is equivalent to the full offset stack as expected. The output gathers are cleaner and more consistent in the time‐lag‐derived angle gathers, but the conversion from time lag to angle can be considered a post‐processing step. The main difference arises in the amplitude versus offset/angle distribution where the responses are different and dramatically so for the land data. The results from the synthetics and real data show that a Kirchhoff migration with an extended imaging condition is capable of generating subsurface angle gathers. The same disadvantages with a ray‐based approach will apply using the extended imaging condition relative to a wave equation angle gather solution. Nevertheless, using this approach allows one to explore the relationship between the velocity model and focusing of the reflected energy, to use the Radon transformation to remove noise and multiples, and to generate consistent products from a ray‐based migration and a full‐wave equation migration, which can then be interchanged depending on the process under study. 相似文献
999.
In granite aquifers, fractures can provide both storage volume and conduits for groundwater. Characterization of fracture hydraulic conductivity (K) in such aquifers is important for predicting flow rate and calibrating models. Nuclear magnetic resonance (NMR) well logging is a method to quickly obtain near-borehole hydraulic conductivity (i.e., KNMR) at high-vertical resolution. On the other hand, FLUTe flexible liner technology can produce a K profile at comparable resolution but requires a fluid driving force between borehole and formation. For three boreholes completed in a fractured granite, we jointly interpreted logging NMR data and FLUTe K estimates to calibrate an empirical equation for translating borehole NMR data to K estimates. For over 90% of the depth intervals investigated from these boreholes, the estimated KNMR are within one order of magnitude of KFLUTe. The empirical parameters obtained from calibrating the NMR data suggest that “intermediate diffusion” and/or “slow diffusion” during the NMR relaxation time may occur in the flowing fractures when hydraulic aperture are sufficiently large. For each borehole, “intermediate diffusion” dominates the relaxation time, therefore assuming “fast diffusion” in the interpretation of NMR data from fractured rock may lead to inaccurate KNMR estimates. We also compare calibrations using inexpensive slug tests that suggest reliable KNMR estimates for fractured rock may be achieved using limited calibration against borehole hydraulic measurements. 相似文献
1000.
Extreme flood‐driven fluvial bank erosion and sediment loads: direct process measurements using integrated Mobile Laser Scanning (MLS) and hydro‐acoustic techniques 下载免费PDF全文
Julian Leyland Christopher R. Hackney Stephen E. Darby Daniel R. Parsons James L. Best Andrew P. Nicholas Rolf Aalto Dimitri Lague 《地球表面变化过程与地形》2017,42(2):334-346
This methods paper details the first attempt at monitoring bank erosion, flow and suspended sediment at a site during flooding on the Mekong River induced by the passage of tropical cyclones. We deployed integrated mobile laser scanning (MLS) and multibeam echo sounding (MBES), alongside acoustic Doppler current profiling (aDcp), to directly measure changes in river bank and bed at high (~0.05 m) spatial resolution, in conjunction with measurements of flow and suspended sediment dynamics. We outline the methodological steps used to collect and process this complex point cloud data, and detail the procedures used to process and calibrate the aDcp flow and sediment flux data. A comparison with conventional remote sensing methods of estimating bank erosion, using aerial images and Landsat imagery, reveals that traditional techniques are error prone at the high temporal resolutions required to quantify the patterns and volumes of bank erosion induced by the passage of individual flood events. Our analysis reveals the importance of cyclone‐driven flood events in causing high rates of erosion and suspended sediment transport, with a c. twofold increase in bank erosion volumes and a fourfold increase in suspended sediment volumes in the cyclone‐affected wet season. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献