全文获取类型
收费全文 | 102篇 |
免费 | 10篇 |
国内免费 | 21篇 |
专业分类
测绘学 | 11篇 |
大气科学 | 11篇 |
地球物理 | 12篇 |
地质学 | 38篇 |
海洋学 | 27篇 |
综合类 | 14篇 |
自然地理 | 20篇 |
出版年
2024年 | 1篇 |
2023年 | 2篇 |
2022年 | 4篇 |
2021年 | 2篇 |
2020年 | 8篇 |
2019年 | 10篇 |
2018年 | 13篇 |
2017年 | 5篇 |
2016年 | 5篇 |
2015年 | 11篇 |
2014年 | 12篇 |
2013年 | 7篇 |
2012年 | 4篇 |
2011年 | 9篇 |
2010年 | 12篇 |
2009年 | 7篇 |
2008年 | 5篇 |
2007年 | 1篇 |
2006年 | 5篇 |
2005年 | 1篇 |
2004年 | 5篇 |
2003年 | 2篇 |
1992年 | 1篇 |
1987年 | 1篇 |
排序方式: 共有133条查询结果,搜索用时 281 毫秒
51.
The estimation of gear selectivity is a critical issue in fishery stock assessment and management. Several methods have been
developed for estimating gillnet selectivity, but they all have their limitations, such as inappropriate objective function
in data fitting, lack of unique estimates due to the difficulty in finding global minima in minimization, biased estimates
due to outliers, and estimations of selectivity being influenced by the predetermined selectivity functions. In this study,
we develop a new algorithm that can overcome the above-mentioned problems in estimating the gillnet selectivity. The proposed
algorithms include minimizing the sum of squared vertical distances between two adjacent points and minimizing the weighted
sum of squared vertical distances between two adjacent points in the presence of outliers. According to the estimated gillnet
selectivity curve, the selectivity function can also be determined. This study suggests that the proposed algorithm is not
sensitive to outliers in selectivity data and improves on the previous methods in estimating gillnet selectivity and relative
population density of fish when a gillnet is used as a sampling tool. We suggest the proposed approach be used in estimating
gillnet selectivity. 相似文献
52.
Steroid sex hormones, such as estradiol-17β (E2) and testosterone (T), are important regulators of sex change in fish. In
this study, we examined the effects of E2 treatment on the dynamics of E2 and T during gonadal differentiation in the olive
flounder Paralichthys olivaceus using histology and radioimmunoassay (RIA). Flounder larvae were divided into five groups (G0-G4), and fed with 0 (control),
0.2, 2, 20 and 100 mg E2/kg feed from 35 to 110 day post hatching (dph). Fish growth in the G1 and G2 groups was not significantly
different from that of the control group (P>0.05), while fish in the G3 and G4 groups were less active and showed growth depression and high mortality. The gonads of
fish in the G3 and G4 groups were smaller and surrounded by hyperplastic connective tissue. The frequency of females in the
G0-G4 groups was 54.5%, 75.0%, 100%, 100% and 93.3%, respectively. The RIA analyses of E2 and T showed that T levels decreased
during gonadal differentiation, and increased slightly at the onset of ovarian differentiation, while E2 levels increased
gradually and peaked at the onset of ovarian differentiation in the control group. In the E2-treated groups, T levels decreased
before the onset of ovarian differentiation. E2 levels were high on the 48 dph, but declined to a lower level on the 54 dph,
and then increased gradually during gonadal differentiation. And a sharp increase of E2 levels were observed in all E2-treated
groups at the onset of ovarian differentiation. The data suggest that T and E2 play important roles during gonadal differentiation,
and an E2 dose of 2 mg/kg feed could induce sex reversal in P. olivaceus. 相似文献
53.
54.
前兆观测干扰信号频谱特征分析 总被引:2,自引:2,他引:0
地震前兆观测数据中干扰信号频繁出现,大大增加了地震前兆异常信息提取难度。本文选用基于傅里叶变换的功率谱、傅里叶谱以及基于希尔伯特-黄变换的边际谱方法,针对黑龙江地区数字化水位和竖直摆倾斜测项中的典型干扰信号进行分析,提取了水位高频干扰、周期干扰以及竖直摆倾斜的风扰、同震应变等常见干扰信号的频谱特征。通过对比3种频谱分析方法的差异性,发现功率谱法和傅里叶谱对提取干扰和噪声等平稳信号更加有效,而边际谱法对提取同震应变波等非平稳信号的效果更加显著。 相似文献
55.
新疆塔河流域洪水量级、频率及峰现时间变化特征、成因及影响 总被引:2,自引:0,他引:2
采用塔里木河流域(塔河流域)8个水文站及相应气象站数据,全面分析了洪水发生量级、频率和峰现时间等特征,研究洪水发生成因及其影响。结果表明:1980s中后期塔河流域气温与降水持续增加,整个塔河流域年及季节洪峰流量普遍呈上升趋势,大部分在1980s中后期发生突变。1980s中期以后塔河流域年及季节洪峰流量呈持续增加或者显著增加趋势,量级位于整个观测时期均值之上,处于洪水“丰富”期。“丰富”期暴雨型和升温型洪水发生次数及造成的灾害损失均呈显著增加趋势,引起严重洪灾损失的洪水也集中在这一时期,且多由暴雨型洪水引发。大量级洪水(最大三场洪水及重现期大于10年的洪水)多集中发生在1990年之后,并且易引发多个水文站点同时出现。 相似文献
56.
研究表明,地震波穿过构造活动相对稳定地区时,能量衰减不明显且具有高Q值;而穿过构造活动地区时,能量会发生强烈衰减且具有低Q值。本文利用Sato模型对依舒断裂带北段萝北-通河地区15个数字化地震台站记录到的266条M_L≥2.0地震尾波Q_c值进行了分析和研究,发现黑龙江萝北、通河附近地区Q_c值具有明显的复杂性和差异性,且萝北地区Q_c值远低于通河地区Q_c值,其原因一方面与区域构造活动有关,另一方面与区域地壳构造复杂、地下介质破碎、区域应力不断变化有关。本文还讨论了萝北、通河附近地区Q值随着频率变化的关系及Q_0值空间分布特征,有效地分析了该区域介质状态的变化过程,对萝北地区地震活动性的研究和预测有指导意义。 相似文献
57.
58.
59.
在海上风电单桩基础水平承载力的设计中,风荷载和波浪荷载是两个最主要的常规水平荷载,需要考虑在波浪荷载和风荷载的不同荷载组合下的桩土相互作用。利用有限元软件ABAQUS构建桩土相互作用模型,对桩基施加不同组合的环境荷载,研究桩土的相互作用。在固定环境荷载的情况下,将土体分层,研究不同土质条件下桩基水平承载力的差异。分析可得极端海况下桩身泥面位移约是正常海况下的5倍,且桩身水平位移主要由风荷载引起。桩周土体所受水平应力与桩体的摆动幅度相关,且桩基摆动对桩周土体水平变形的影响范围有限,以桩基为中心1.7倍桩径范围内土体所受影响显著。海床上层土体的强度对桩基水平承载力起关键性作用,上软下硬海床与纯软土海床相比水平承载力大约提高25%,而上硬下软海床与纯软土海床相比水平承载力约提高3倍。 相似文献
60.
Grain boundary plays a key role in electromigration process of polycrystal interconnection. We take a free volume to represent a 'vacancy--ion complex' as a function of grain boundary specific resistivity, and develop a new characterisation model for grain boundary noise. This model reveals the internal relation between the boundary scattering section and electromigration noise. Comparing the simulation result with our experimental result, we find the source as well as the form of noise change in the electromigration process. In order to describe the noise enhancement at grain boundary quantitatively, we propose a new parameter—grain boundary noise enhancement factor, which reflects that the grain boundary noise can characterise the electromigration damage sensitively. 相似文献