共查询到20条相似文献，搜索用时 31 毫秒
1.
The non-parametric Mann–Whitney (MW) statistic test has been popularly used to assess the significance of a shift in median
or mean of hydro-meteorological time series. It has been considered that the test is more suitable for non-normally distributed
data and it may be not sensitive to the distribution type of sample data. However, no evidence has been provided to demonstrate
these. This study investigates the power of the test in various circumstances by means of Monte Carlo simulation. Simulation
results demonstrate that the power of the test is very sensitive to various properties of sample data. The power depends on
the pre-assigned significance level, magnitude of a shift, sample size, and its occurrence position within a time series;
and it is also strongly affected by the variation, skewness, and distribution type of a time series. The bigger the magnitude
of a shift, the more powerful the test is; the larger the sample size, the more powerful the test is; and the bigger the variation
within a time series, the less power the test has. The test has the highest power if a shift occurs at the midpoint of a time
series. For the samples with different distribution types, the power of the test is dramatically different. The test has the
highest power for time series with the extreme value type III (EV3) distribution while it indicates the lowest power for time
series with the lognormal distribution. 相似文献
2.
H. Korhonen 《Pure and Applied Geophysics》1973,103(1):259-265
Summary In this study spectral development of microseisms at Oulu is discussed by means of power spectra computed from the recordings of a long period vertical seismograph. North Atlantic storm microseisms are found to be distinguished at peak frequencies in band 90–130 mHz while the main peaks of Norwegian microseisms occur at frequencies higher than 130 mHz. In both of these cases a low frequency side maximum is found at nearly the same frequencies as the main maximum in corresponding sea wave spectra. Fast moving storms associate with rapid changes in microseism spectra. A shift of peaks towards higher frequencies is found in connection with storm approach. 相似文献
3.
AbstractA general linearized wave equation for a stratified rotating fluid is derived and applied to obtain a dispersion relation for waves of short latitudinal extent in a thin shell of fluid. Long period wave solutions in three ocean models are compared: (1) for a stratified ocean with both components of the rotation vector; (2) for a stratified ocean without the horizontal component of rotation, and finally, (3) for a homogeneous ocean without horizontal rotation. The inclusion of the horizontal component of the Earth's rotation is found to have no noticeable effect on the dispersion relation of long period waves; its only influence is the introduction of a vertical phase shift in the motions. The origin of this phase shift is found in the tendency of the motions to satisfy the Taylor-Proudman theorem. The phase shift is of possible oceanographic relevance only for bottom-trapped buoyancy waves in a relatively weak stratification. The differences between the three ocean models are also discussed with the help of graphs of the numerically integrated dispersion relations. The relative influences of shell thinness and stratification in inhibiting the influence of the horizontal component of the earth's rotation are also briefly discussed. 相似文献
4.
Physical mechanisms of man-made influences on the magnetosphere 总被引：3，自引：0，他引：3
Since the discovery of the Luxembourg effect in the 1930s, it is clear that man-made activities can perturb the ionosphere and the magnetosphere. The anthropogenic effects are mainly due to different kinds of waves coming from the Earth's surface. Acoustic-gravity waves are generated by large explosions, spacecraft launches, or flight of supersonic planes. Electromagnetic waves are active in different frequency ranges. Power line harmonic radiation which is radiated in the ELF range by electrical power systems can be observed over industrial areas. At VLF and HF, the ground-based transmitters used for communications or radio-navigation heat the ionosphere and change the natural parameters. A large variety of phenomena is observed: wave-particle interaction, precipitation of radiation belt electrons, parametric coupling of EM whistler waves, triggered emissions, frequency shift, and whistler spectrum broadening. This paper will review the different physical mechanisms which are relevant to such perturbations. The possibility of direct chemical pollution in the ionosphere due to gas releases is also discussed. 相似文献
5.
《Advances in water resources》2002,25(3):325-333
The non-parametric Mann–Whitney (MW) statistical test for assessing the significance of a shift in median or mean requires a tested series to be serially independent. However, hydrological time series such as water quality, streamflow, and others may frequently display serial correlation. In such cases, the existence of serial correlation might alter the ability of the test to detect a shift in mean. This study investigates this issue by means of the Monte Carlo simulation. Simulation results indicate that: (i) when there is no shift or a moderate shift in mean, the existence of positive serial correlation will increase the possibility to reject the null hypothesis of no shift while it might be true; and the existence of negative serial correlation will reduce the possibility to detect a shift; (ii) when a bigger shift occurs in a time series, for a series with smaller sample size, the influence of serial correlation on the test is similar to that in (i), but it is much less than that in (i); while for a series with larger sample size, the influence of serial correlation on the test is opposite to (i), i.e., positive serial correlation reduces the power of the test for detecting a shift while negative serial correlation slightly increases the power of the test for identifying a shift; and (iii) removal of serial correlation by pre-whitening can effectively remove the serial correlation and eliminate the influence of the serial correlation on the test. 相似文献
6.
利用非均匀P-偏振电磁波（电场平行于入射面）在导电界面反射系数所产生的附加相角导出了非均匀电磁波在导电介质界面的横向偏移,并给出了一个低频电磁波算例,绘制了横向偏移随入射角的变化曲线. 计算结果表明,入射角在相移常数临界角、衰减常数临界角和90°处,横向偏移曲线存在三个间断点;当入射角等于这三个角时,电磁波将沿界面传播;当入射角在这三个角附近时,横向偏移效应影响较大;在其他角度入射时横向偏移与波长在一个数量级. 论文还讨论了横向偏移效应对电磁波测井测量结果所产生的误差,并给出了误差计算方法和算例. 相似文献
7.
Hyoung-Gi Kim Chang-Yeoul Choi Jung-Wouk Woo Yongrok Choi Kyonghwan Kim Desheng Dash Wu 《Stochastic Environmental Research and Risk Assessment (SERRA)》2011,25(3):305-322
The Kyoto Protocol calls for industrialized nations to cut greenhouse gas emissions by 5% from 1990 levels by 2008–2012, so
developed countries are presenting various policies to reduce greenhouse gas that is produced in transport fields. One of
those polices is a modal shift that changes from road freight to sea, inland waterways and railroad transportation that is
eco-friendly. The increase of road freight brings road congestion, accidents, logistic costs, air pollution and greenhouse
gases. Railroads are superior to the other modes of transportation in mass transportability, high speed, timeliness, safety
and environmental-friendliness, but the railway industry has been pushed behind in competition. Korean railroads were used
by passengers and for freight transport popularly until the middle of the 20th century. However, because of the sudden change
of logistics environments, having time efficiency being most important, railroads lost logistic competitive power against
the transportation by truck. This paper intends to examine the modal shift to railroad transportation, which enjoys high interest
as environmentally-friendly logistics, i.e., the modal shift strategy. Efficiency analysis is conducted using DEA and exploratory
factors are identified for the modal shift of the companies. This paper also proposes an alternative plan for green logistics
and environmentally-friendly logistics, by analyzing the characteristics of the railroad cargo transportation system and the
conditions of local railroad cargo transportation. 相似文献
8.
T. D. Borisova N. F. Blagoveshchenskaya I. M. Ivanova M. T. Rietveld 《Geomagnetism and Aeronomy》2013,53(1):32-42
The interaction between the Earth’s ionosphere and magnetosphere in a situation when artificial disturbances are generated in the F region of the auroral ionosphere with the EISCAT/Heating facility is studied. An experiment was performed in the daytime when the facility effective radiated power changed in a stepwise manner. Wavelike disturbances with periods of (130–140) s corresponding to Pc4 pulsations were simultaneously registered by the method of bi-static backscatter and with ground magnetometers. The variations in the Doppler frequency shift were correlated with the changes in the facility power. Incoherent scatter radar measurements at a frequency of 930 MHz (Tromsö) and numerical calculations were used in an analysis. It has been indicated that the ionospheric drift of small-scale artificial ionospheric irregularities was modulated by magnetospheric Alfvén waves. The possible effect of powerful HF radioemission on the Alfvén wave amplitude owing to the modification of the magnetospheric resonator ionospheric edge reflectivity and the generation of an outgoing Alfvén wave above the region where the ionospheric conductivity is locally intensified has been considered. 相似文献
9.
叙述了山西大同数字遥测地震台网建设过程中，对镇川台太阳能供电系统进行改进的情况，讨论了在地震观测系统中使用太阳能供电系统应考虑的问题，提出了相应的解决办法。指出，根据地域、气候等条件合理设计太阳能电池的容量，合理设置控制器的浮充电流，可使太阳能电源系统处于较佳的工作状态。 相似文献
10.
利用SV地震波（偏振化方向在入射面内的横波）在自由表面反射系数的附加相角导出了SV波Goos-Hänchen效应所引起的横向偏移和横向偏移渡越时间,给出了Goos-Hänchen效应正常时差动校正量,讨论了Goos-Hänchen效应对反射SV波正常时差的影响,绘出了横向偏移、Goos-Hänchen效应正常时差及Goos-Hänchen效应正常时差校正量曲线.曲线表明：反射地震波存在负横向偏移效应,在大多数入射角度范围,横向偏移（横向偏移渡越时间）与波长（周期）为同一个数量级.对掠入射波或入射角在临界角附近的入射波,Goos-Hänchen效应对正常时差有较大的测量误差,对反射P、SV波的传播走时产生了不可忽略的影响,因此在实际的地震资料处理中应进行横向偏移效应误差校正. 相似文献
11.
人类活动的加强导致湖泊生态系统发生"突变",造成生物多样性下降、藻类暴发、水质恶化等等环境和生态问题.中国许多湖泊已经发生"突变"或面临着突变风险.获悉湖泊生态系统发生突变的时空差异对于区域湖泊的保护,预防湖泊突变的发生以及制定合适的修复策略至关重要.本研究收集了中国55个不同区域湖泊的古湖沼学数据,探讨了湖泊突变的区域特点.研究确定了湖泊生态系统发生突变时间和区域差异,并揭示了空间差异的原因.结果表明,中国湖泊生态系统突变时间的区域分异特征为:长江中下游湖泊最早出现突变;东部和东北湖区湖泊突变时间明显早于西南、内蒙古和新疆湖泊;各湖区内,城郊湖泊突变时间早于乡村湖泊.人类活动强度是造成湖泊突变时空差异的主要驱动力.研究认为,区域人类活动强度影响了湖泊生态系统的演化进程,造成了中国湖泊生态系统突变的区域差异.本研究从生态系统突变的角度,利用古湖沼学综述了中国湖泊生态系统在人类活动下的演化进程,更深刻地认识了中国湖泊现状,为湖泊保护提供了有力的科学依据. 相似文献
12.
13.
14.
W.A. Mulder 《Geophysical Prospecting》2014,62(1):17-33
Migration velocity analysis with the constant‐density acoustic wave equation can be accomplished by the focusing of extended migration images, obtained by introducing a subsurface shift in the imaging condition. A reflector in a wrong velocity model will show up as a curve in the extended image. In the correct model, it should collapse to a point. The usual approach to obtain a focused image involves a cost functional that penalizes energy in the extended image at non‐zero shift. Its minimization by a gradient‐based method should then produce the correct velocity model. Here, asymptotic analysis and numerical examples show that this method may be too sensitive to amplitude peaks at large shifts at the wrong depth and to artefacts. A more robust alternative is proposed that can be interpreted as a generalization of stack power and maximizes the energy at zero‐subsurface shift. A real‐data example is included. 相似文献
15.
16.
讨论了叠前地震记录中随机噪声的模拟方法,采用定性分析与Monte Carlo仿真相结合的方法,研究了在物理小波为基本小波的变换域该类随机噪声局部小波功率谱的统计特性,得出：其局部小波功率谱服从χ22分布.在此基础上,利用假设检验给出了在给定置信度下确定有效信号在小波变换域内能量分布空间的方法,通过在有效信号能量分布空间重建信号来压制噪声.给出了模型及实际地震资料算例,证明了本文提出的方法的有效性. 相似文献
17.
18.
19.
利用SH地震波（偏振化方向垂直入射面的横波）在地层界面反射系数的附加相角导出了SH波Goos-Hänchen效应所引起的横向偏移和横向偏移渡越时间,给出了Goos-Hänchen效应正常时差公式,讨论了Goos-Hänchen效应对反射SH波正常时差的影响,绘出了横向偏移、横向偏移渡越时间、Goos-Hänchen效应正常时差及Goos-Hänchen效应正常时差校正量曲线.数值算例表明：对掠入射波或入射角在临界角附近的入射波,SH反射波的横向偏移、横向偏移渡越时间非常大,Goos-Hänchen效应对正常时差会产生较大的测量误差,在其他角度的入射波,横向偏移（横向偏移渡越时间）与波长（周期）为同一个数量级.横向偏移效应对SH反射波的传播走时影响是不可忽略的,因此在实际的地震资料处理中应进行横向偏移效应误差校正. 相似文献
20.
By means of a simple one-dimensional model some basic features of the spreading of cooling water are discussed. Based on the idea of equilibrium temperature, a numerical technique for the simulation of diffusion and cooling processes is presented. The technique is applied to a two-dimensional time-dependent model of the tide in the lower river Elbe. The measurements at the outlet of an existing nuclear power plant (Stade) are reproduced quantitatively. 相似文献