全文获取类型
收费全文 | 1639篇 |
免费 | 480篇 |
国内免费 | 325篇 |
专业分类
测绘学 | 52篇 |
大气科学 | 71篇 |
地球物理 | 679篇 |
地质学 | 919篇 |
海洋学 | 403篇 |
天文学 | 39篇 |
综合类 | 94篇 |
自然地理 | 187篇 |
出版年
2024年 | 7篇 |
2023年 | 17篇 |
2022年 | 54篇 |
2021年 | 56篇 |
2020年 | 64篇 |
2019年 | 91篇 |
2018年 | 64篇 |
2017年 | 73篇 |
2016年 | 71篇 |
2015年 | 60篇 |
2014年 | 91篇 |
2013年 | 92篇 |
2012年 | 74篇 |
2011年 | 118篇 |
2010年 | 56篇 |
2009年 | 112篇 |
2008年 | 114篇 |
2007年 | 122篇 |
2006年 | 130篇 |
2005年 | 108篇 |
2004年 | 115篇 |
2003年 | 90篇 |
2002年 | 82篇 |
2001年 | 60篇 |
2000年 | 63篇 |
1999年 | 55篇 |
1998年 | 59篇 |
1997年 | 65篇 |
1996年 | 57篇 |
1995年 | 35篇 |
1994年 | 46篇 |
1993年 | 25篇 |
1992年 | 27篇 |
1991年 | 16篇 |
1990年 | 18篇 |
1989年 | 13篇 |
1988年 | 15篇 |
1987年 | 4篇 |
1986年 | 6篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 4篇 |
1978年 | 1篇 |
1954年 | 4篇 |
排序方式: 共有2444条查询结果,搜索用时 109 毫秒
761.
从河流移流扩散二维简化方程的解析解出发,考虑两岸边界反射作用,通过离岸排放条件下的系列计算实验、浓度图谱绘制、数学归纳和分析整理,提出了以相对离岸距离a'(=排放口离近侧河岸的距离a/河宽B)为自变量的宽阔与中宽河段的分类线x1.2'方程;提出了断面最大浓度轴线的定义、分段方程以及简化分段条件,给出了最大浓度轴线x轴段与弯曲段的分段线xe'方程、弯曲段与靠岸段的临界线xk'方程以及断面最大相对浓度沿程分布的分段简化计算公式,提出了河流中心线和两岸线上相对浓度沿程分布的计算公式,为河流水环境影响预测与评价提供了理论支持. 相似文献
762.
The seismic stratigraphy and sedimentary architecture of the Amundsen Gulf Trough and adjacent slope, Canadian Beaufort Sea margin, are investigated using a grid of 2-D seismic reflection data. The inner-shelf of the Amundsen Gulf Trough is interpreted to be composed predominantly of exposed or near-surface bedrock, overlain by a spatially-discontinuous veneer of glacimarine to open-marine sediment. There is a seaward transition from exposed bedrock on the inner-shelf to a thick (up to 500 m) outer-shelf prograding wedge of acoustically semi-transparent sediment. Eight seismic sequences, divided into four megasequences, are described from the outer-shelf stratigraphy. Eight till sheets are identified from Megasequences A to C, providing evidence for at least eight Quaternary ice-stream advances through the Amundsen Gulf Trough to the shelf break. A trough-mouth fan with a minimum volume of about 10,000 km3 is present on the adjacent slope. The Amundsen Gulf ice stream probably represented the most northwesterly marine-terminating ice stream of the Laurentide Ice Sheet through much of the Quaternary, providing a major route for ice and sediment transfer to the Arctic Ocean. The youngest till sheet within the Amundsen Gulf Trough, Megasequence D, was probably deposited by a subsidiary ice stream, the Anderson ice stream, subsequent to retreat of the last, Late Wisconsinan Amundsen Gulf ice stream. This provides evidence of dynamic ice-sheet behaviour and the reorganisation of the northwest Laurentide Ice Sheet margin during the last deglaciation. A number of buried glacigenic landforms, including palaeo-shelf break gullies and a grounding-zone wedge with a volume of 90 km3, are described from the Amundsen Gulf Trough stratigraphy. Lateral grounding-zone wedges are identified at the northern and southern lateral margins of the Amundsen Gulf and M'Clure Strait troughs, respectively, and are interpreted to have been formed roughly contemporaneously by ice streams in Amundsen Gulf and M'Clure Strait. 相似文献
763.
Reproductive patterns of two benthic bathyal octopods, Pteroctopus tetracirrhus and Scaeurgus unicirrhus have been studied in extremely nutrient-poor environment of the deep-sea Eastern Mediterranean. Both species were found to exhibit a reproductive tactics of producing eggs much larger than in the western part of the sea which likely results in larger hatchlings with higher viability. P. tetracirrhus exhibited a typical “deep-sea” spawning strategy of simultaneous maturation of a single batch of large eggs with atresia of excessive oocytes, whereas reproductive strategy of S. unicirrhus is particular for shelf octopodids: asynchronous maturation of numerous batches of small eggs with no obvious regulatory atresia. Existence of these two types of ovary development and utilisation of fecundity are closely related to two types of evolutionary stable reproductive strategies based on existence of either very large or very small eggs with a few species occupying the “intermediate” position. 相似文献
764.
Eirik Drift lies on the slope south of Greenland, where it has been formed under the influence of the Western Boundary Undercurrent (WBUC) closely downstream of its origin in the deep-water formation centres of the Nordic Seas. Therefore, the sediment record at Eirik Drift documents modifications in pathways and intensity of the WBUC. These modifications reflect alterations in deep-water formation in the Nordic Seas and are therefore coupled with climate changes. Based on the seismostratigraphic analysis of sedimentary architecture identified in a set of high-resolution seismic reflection data, we have reconstructed the palaeocirculation of the WBUC at Eirik Drift since the early Miocene. We revealed a strong WBUC during warm climate conditions, and in phases of climate cooling with enhanced sea-ice extent we interpreted weak WBUC influence. We suggest a southward shift of the deep-water formation regions along with a shift of the deep current system during the cool phases. This shift implies that the main North Atlantic Deep Water route affected Eirik Drift only during warm phases and that during cool phases weak branches of the circulation system flowed over Eirik Drift. 相似文献
765.
计算了由于形状、姿态和表面性质不准确导致火星探测器行星际定轨存在较大光压模型误差时,两种不同定轨方案(方案1只估计光压系数,方案2同时估计光压系数和随机加速度)对光压模型误差的补偿效果。在方案2中建立了高斯马尔科夫模型表征光压模型导致的加速度误差,并选取了合适的随机模型参数。仿真分析表明方案2可以有效补偿光压模型误差,避免滤波发散,提高定轨精度。 相似文献
766.
767.
768.
769.
Spatially averaged temperature and salinity profiles from individual cruises between 1990 and 2009 were analysed to outline the temporal evolution of water mass properties in the deep convection site in the South Adriatic Pit (SAP). The long-term variability in thermohaline conditions has been explained and related to a close feedback mechanism between the Ionian and the Adriatic Sea. Prominent influences of the Eastern Mediterranean Transient are manifested in changes in the vertical temperature and salinity patterns in the South Adriatic, and the whole studied period was divided into three stages according to the main thermohaline characteristics: 1990-1995, 1995-2004 and the last period from 2005 onwards.Particular attention was given to data collected during 2006-2009, which permitted us to situate the actual thermohaline properties in the context of the decadal variability. This last period was characterised by a very low production of dense water in the northern basin during 2007, while from winter 2008 high production of North Adriatic Deep Water (NAdDW) and Adriatic Deep Water (AdDW) in the northern and southern basins, respectively, was observed. Finally, we used the Optimum Multiparameter Analysis (OMP) to identify the percentages of the different water masses contained in the SAP, and this highlighted some differences between two recent periods studied (2007 and 2008) and the production of dense waters. 相似文献
770.
The behaviour of an embankment built on normally consolidated soft soil reinforced with deep mixing columns is studied using a coupled soil–water formulation. The numerical predictions are analysed in terms of settlements, increments in vertical effective stresses and excess pore pressures. Firstly, the effectiveness of the use of deep mixing columns is studied. Afterwards, a parametric analysis is performed to study the influence on the soil-columns system of the replacement ratio (columns’ spacing), the deformability of the embankment and columns, and the coefficient of vertical permeability of the columns. 相似文献