地磁巡测的几个问题

从变化磁场或内源磁场的变化来探索地震前兆,都是企图找到地震活动过程中的地磁场变化.近年来,流动地磁重复测量已成为配合固定地磁台研究地磁场空间分布及其变化特征的重要途径.目前在观测强震活动过程中所引起的地磁场变化、探索震磁关系、寻求地震孕育过程中的地磁场变化指标等方面做了一些工作.在实地观测中,假设地壳介质磁场的变化是地壳变异的一种反映,震源体或其应力场所展布的块体是不大的,因此可以认为它是放在地球基本磁场(?)_1和大陆磁异常场(?)_2中被磁化的一块磁体.从这个意义上讲,(?)_1和(?)_2的合成场是震源体的     

在角加速机制的假设下,地磁场与地球自转速度的变化有关

假设地球基本磁场可以由地球自转加速度激发。为了检验这种角加速机制是否符合实际,本文根椐角加速机制推测了所激发的磁场的各种预期性质,并且把它们和对地磁场的观测事实进行分析对比。检验结果显示,地磁场的变化和自转速度的变化之间具有共同的频谱,而且角加速机制所预期的磁场性质无论在结构上,还是在随时间变化上都与对地磁场的观测事实相符合,没有遇到一个反例,而且能够解释地球发电机假说所不能解释的某些现象,例如,地磁场倒转时间的不规则分布和磁极游移等。本文所建立起来的地磁场与地球自转速度变化之间的10种关系具有基本的意义,在研究地磁场的起源时应该思考和回答为什么会存在这些关系,这对于探索正确的地磁起源机制也许是有益的。     

中国大陆地区地磁长期变化及其特征

1.前言地球基本磁场随时、空有一个周期性的、长期缓慢的变化。一般认为,这种变化来源于地核内部或核幔边界,因此,利用地磁场的这种长期变化的规律,可以研究地球内部物质的     

空间和地下电流体系演化与地磁低点位移关系的研究

地磁场"日变化"是地球变化磁场的重要组成部分,是地球物理学地磁观测中的基本规律之一.起源于太阳辐射在电离层中产生的电流体系的影响,其中静日变化的规律性最强,它包含着空间和地球内部各种电磁过程丰富的信息.     

中国地磁观测

１　地磁观测的基本问题人类对地球磁场的认识是很早的,但对地球磁场的本质认识,却是在１９世纪中叶由麦克斯韦尔所确立的现代电磁场理论之后,并随着探测技术的不断发展而逐步深入的。高斯理论分析结果表明,地球表面地磁场的绝大部分来源于地球内部液体地核和地壳,来自地球外部的仅占万分之几到千分之几、有时可达百分之几,它们是由太阳、行星际空间、磁层及电离层活动所形成高空电流体系的变化及其在地下的感应变化所引起的,而且它们随时间的变化而变化。来源于地球内部的,只随时间缓慢变化,是基本稳定的,构成地磁场的主要部分（…     

地壳磁异常的全球模型

从全球磁场减去地核主磁场和变化磁场及其感应磁场后,就得到岩石圈磁场,又称地壳磁场,或地磁异常.地壳磁场是地磁场的重要组成部分,对导航、通信、地球物理勘探等有重要意义.本文简要综述近年来在地壳磁场基本信息获取方面取得的重大进展,特别是Oersted、CHAMP等卫星磁侧的最新成果,介绍在此基础上建立的全球磁异常模型,展望地壳磁场研究的发展趋势和应用前景.     

地磁绝对子夜均值异常识别方法研究

地磁绝对子夜均值数据,即台站地方时00～03时期间的地磁场绝对数据是地球主磁场和地壳磁场变化研究中常用的基础数据.该均值中基本去除了电离层电流体系的影响,但仍不可避免地存在着主磁场的变化和磁层磁场的变化,可能会存在与地震孕育有关的地壳磁场的变化、因台站观测环境干扰或绝对观测错误产生的不可弥补的错误数据以及因数据处理错误产生的可修正的错误数据.     

古代赤道极光揭示西太平洋磁异常演化

地球是一个多圈层耦合系统,从内部到表面、大气和空间的相互作用,共同塑造了地球的宜居环境. 偶极磁场的产生使地球环境逐渐演变为宜居,正如金星和火星偶极磁场的消失使这两颗行星的环境走向了另一个极端. 地球磁场起源于地核发电机过程,而地核热动力学和化学结构的变化则会导致地磁场的显著改变,导致地磁场的减弱甚至极性倒转,削弱对地...     

1690～2000年地磁场能量的三维分布及其长期变化

利用Bloxham & Jackson 地磁场模型和国际参考地磁场模型(IGRF),研究了1690～2000年地磁总能量及其北向、东向和垂直向分量的能量以及非偶极子磁场的能量在地球内部的分布及长期变化.结果表明,地表和地核以外地磁场总能量及其北向和垂直向的能量是持续衰减的,垂直向的磁场能量占总能量的64％以上,对总能量的贡献起主要作用;东向分量的能量随时间的变化以增加为主.地磁场的能量变化率存在56年的周期,主要是由偶极子磁场产生的.地表以外的非偶极子磁能从减小到增大转折出现在1770年,比地核以外滞后40年.地球内部磁能随时间的变化显示,偶极子磁能逐渐减小,非偶极子磁能增加,越靠近核幔边界增加越快;偶极子和非偶极子磁能的变化量相等的分界面在距地心3780km处.从核幔边界到地表,磁能变化的衰减非偶极子比偶极子快,表明偶极子磁场比非偶极子磁场有更深的场源.     

磁线圈对地磁场的物理模拟及对地磁导航研究的意义

利用磁线圈对地球基本磁场开展动态模拟,将地磁场随空间的变化转化为在实验室中随时间的变化,能够为地磁导航系统的匹配试验提供物理仿真的地磁场环境。本项研究依照虚拟仪器技术思路,对磁线圈、恒流电源、磁通门磁力仪(或Overhauser磁力仪)和工控机进行数字化连接,并采用LabVIEW编程进行驱动控制,通过自动调节线圈绕组中的电流大小来实现对磁场的物理模拟并进行实时监测,尝试组建了地磁场动态模拟系统,并分别开展了对地磁场总强度和三分量信号的动态模拟实验。模拟系统可以实现天然地磁场和人工编制两种动态信号的输出模拟,且拟合效果良好。地磁场动态模拟系统的初步建立为今后地磁匹配试验等研究提供了仿真磁场环境。     

On the synchronism in the events within the core and on the surface of the earth: the changes in the organic world and in the polarity of the geomagnetic field in the phanerozoic

The data on geomagnetic reversals are compared with the changes in the organic world and with the lower-mantle plumes. The times of the formation of plumes and the times of their appearance on the Earth’s surface relate to the intervals characterized by the different frequencies of geomagnetic reversals, i.e., there is no interrelation between the formation of plumes and the frequency of the changes in the geomagnetic field polarity. At the same time, a certain synchronism is observed between the frequency of the geomagnetic reversals and the boundaries of the biostratigraphic ages, i.e., the changes in the organic world in the long-period range. A hypothesis is proposed, which explains the change in the sign of the geomagnetic field by the combined effect of the irregular rotation of the internal core relative to the mantle and the changes in the slope angle of the axis of the Earth’s rotation, which, in turn, results in synchronous events on the Earth’s surface: the rates of changes in the organic world.     

Anomalies in the secular variations in the main geomagnetic field in the context of the hierarchic dipole model

The developed dynamic model of the main geomagnetic field (MGF) includes the sources of three levels, which have existed and continuously changed during the 100-year period. The spatial-temporal dynamics of the secular variations in MGF has been considered based on this model. It has been indicated that, at different times, the formation of anomalies (located in similar regions) depended on the total dynamics of different combinations of sources, including a change in the parameters of the dipoles of the secondorder smallness. Anomalies generated by the dynamics of the dipoles of the third-order smallness do not evidently manifest themselves in the total secular variations but are responsible for the specific shapes of isodynamic lines. Based on the stability and continuity of the constructed model, the conclusion is made that long-living turbulent flows of different scales can exist in the liquid core.     

The changes in the electric conductivity of the lithosphere in the source region of the strongest Olyutora earthquake in the Koryak highlands

The results of the magnetotelluric (MT) soundings before and after an earthquake are analyzed. The interpretation is based on the longitudinal and transverse MT curves (along and across the strike of the main tectonic elements, respectively). The MT curves are distorted by the ρ- and coast effect. The distortions due to the coast effect are estimated by the testing three-dimensional (3D) model. It is established that the coast effect distortion at the periods up to 1000 s is small and can be disregarded. The divergence of the longitudinal and transverse MT curves, which points to the presence of the deep faults, is thoroughly studied. The inversion of the MTsounding curves is carried out by the REBOCC program of the numerical two-dimensional modeling. This program implements the procedures of elimination of the ρ-effect and the joint inversion of the longitudinal and transverse MT sounding curves. The obtained geoelectrical cross sections provide an insight into the structure of electrical conductivity of the lithosphere before and after the earthquake. The more intense variations in the electric conductivity are observed in the zone of the deep faults. These variations are related to the changes in the porosity and saturation of the rocks by the highly mineralized fluids.     

Hydrological changes in the Ganges system in Bangladesh in the post-Farakka period

Abstract

The basin area of the Ganges River in Bangladesh is extremely dependent on a regular water supply from upstream to meet requirements for agriculture, fisheries, navigation, salinity control, and domestic and industrial sectors. In 1975, India commissioned a barrage on the Ganges River at Farakka to divert a significant portion of the dry season flow in order to make the Calcutta Port navigable. Statistical analyses of discharge and water level data have been carried out to determine if significant changes have occurred in the hydrology of the Ganges system in Bangladesh in the post-Farakka period. Siltation of the Gorai River (an offtake of the Ganges River) has also been examined using the stage-discharge relationship and regression analysis. The analyses show that the diversion has caused considerable hydrological changes in the Ganges system in Bangladesh. The water supply in the dry season has been reduced substantially, while siltation of the Gorai River has increased significantly.     

Changes in sediment transport in the Kuye River in the Loess Plateau in China

In this paper, the changes in sediment transport over 51 years from 1955 to 2006 in the Kuye River in the Loess Plateau in China are assessed. Key factors affecting sediment yield and sediment transport, such as precipitation depth, discharge, and human activities are studied. To investigate the changes in sediment yield in this watershed, a trend analysis on sediment concentration, precipitation depth, and discharge is conducted. Precipitation depths at 2 Climate Stations （CSs）, as well as discharge and sediment transport at 3 Gauging Stations （GSs） are used to assess the features of sediment transport in the Kuye River. The rtmoff modulus （defined as the annual average discharge per unit area, L/（s·km^2）） and the sediment transport modulus （defined as the annual suspended sediment transport per unit area, t/（yr km^2）） are introduced in this study to assess the changes in runoff and sediment yield for this watershed. The results show that the highest average monthly discharge during the study period in the Kuye River is 66.23 m^3/s in August with an average monthly sediment concentration of 88.9 kg/m^3. However, the highest average monthly sediment concentration during the study period in the Kuye River is 125.34 kg/m^3 and occurs in July, which has an average discharge of 42.6 m^3/s that is much less than the average monthly discharge in August. It is found that both the runoff modulus and sediment transport modulus at Wenjiachuan GS on the Kuye River has a clear downward trend. During the summer season from July to August, the sediment transport modulus at Wenjiachuan GS is much higher than those at Toudaoguai and Longmen GSs on the Yellow River. The easily erodible loess in the Kuye River watershed and the sparse vegetation are responsible for the extremely high sediment yield from the Kuye River watershed. The analyses of the grain size distribution of suspended load in the Kuye River are presented. The average monthly median grain size of suspended load in the Kuye River is largest in February and then decreases until June. In July, the average monthly median grain size of suspended load approaches another peak and decreases until September. Then, the median grain size of suspended load starts to increase until February of the following year. However, the average monthly median grain size of suspended load in the Yellow River at Toudaoguai and Longmen GSs is the smallest between early summer and late fall The median grain size in the Yellow River starts to increase in November and approaches the largest size in January.     

Trends in the characteristics of the annual and semiannual variations observed in the radio wave absorption in the lower ionosphere

The continuous increase in concentration of greenhouse gases in the atmosphere is expected to cool higher levels of the atmosphere. There is some direct and indirect experimental evidence of long-term trends in temperature and other parameters in the mesosphere and lower thermosphere (MLT). Here we look for long-term trends in the annual and semiannual variations of the radio wave absorption in the lower ionosphere, which corresponds to the MLT region heights. Data from central and southeastern Europe are used. A consistent tendency to a positive trend in the amplitude of the semiannual wave appears to be observed. The reality of a similar tendency in the amplitude of the annual wave is questionable in the sense that the trend in the amplitude of the annual wave is probably induced by the trend in the yearly average values of absorption. The phases of both the annual and semiannual waves display a forward tendency, i.e. shift to an earlier time in the year. A tentative interpretation of these results in terms of changes of the seasonal variation of temperature and wind at MLT heights does not contradict the trends observed in those parameters.     

The electric field produced in the mantle by the dynamo in the core

A rigorous singular perturbation theory is developed to estimate the electric field E produced in the mantle M by the core dynamo when the electrical conductivity σ in M depends only on radius r, and when |r?_rln σ| ? 1 in most of M. It is assumed that σ has only one local minimum in M, either (a) at the Earth's surface ?V, or (b) at a radius b inside the mantle, or (c) at the core-mantle boundary ?K. In all three cases, the region where σ is no more than e times its minimum value constitutes a thin critical layer; in case (a), the radial electric field E_r ≈ 0 there, while in cases (b) and (c), E_r is very large there. Outside the critical layer, E_r ≈ 0 in all three cases. In no case is the tangential electric field E_S small, nearly toroidal, or nearly calculable from the magnetic vector potential A as ??_tA_S. The defects in Muth's (1979) argument which led him to contrary conclusions are identified. Benton (1979) cited Muth's work to argue that the core-fluid velocity u just below ?K can be estimated from measurements on ?V of the magnetic field B and its time derivative $\text{?}{}_{\mathrm{t}}\text{B}$. A simple model for westward drift is discussed which shows that Benton's conclusion is also wrong.In case (a), it is shown that knowledge of σ in M is unnecessary for estimating E_S on ?K with a relative error |r?_r 1nσ|^?1from measurements of E_S on ?V and knowledge of ?_tB in M (calculable from ?_tB on ?V if σ is small). Then, in case (a), u just below ?K can be estimated as $\text{?}\text{r}\text{×}\text{E}{}_{\mathrm{S}}\text{/B}{}_{\mathrm{r}}$. The method is impractical unless the contribution to E_S on ?V from ocean currents can be removed.The perturbation theory appropriate when σ in M is small is considered briefly; smallness of σ and of |r?_r ln σ|^?1 a independent questions. It is found that as σ → 0, B approaches the vacuum field in M but E does not; the explanation lies in the hydromagnetic approximation, which is certainly valid in M but fails as σ → 0. It is also found that the singular perturbation theory for |r?_r ln σ|^?1 is a useful tool in the perturbation calculations for σ when both σ and |r?_r ln σ|^?1 are small.     

Variations in the natural X-ray background in the polar atmosphere

X-ray intensity variations in the surface layer of the atmosphere during precipitation in Apatity and Spitsbergen were analyzed. Based on a comprehensive system of monitoring over the radiation background, X-ray radiation variations were detected. These variations were found to be associated with meteorological processes in the lower atmosphere. X-ray energy spectra were obtained under good weather and precipitation conditions. These spectra have no characteristic lines inherent to radionuclides. Additional studies make it possible to suggest that the growth in X-ray radiation during precipitation is mainly caused by braking radiation of energetic electrons additionally accelerated by electric fields in rain clouds.     

Site response in the Qionghai Basin in the Wenchuan earthquake

Amplification effects of soil site response can significantly impact ground motions, and must be considered in the seismic fortification of buildings/structures to prevent or mitigate this potential seismic hazard. Utilizing acceleration time histories from the main shock of the Wenchuan earthquake recorded at four stations (i.e., one on bedrock and three on soil) in the Qionghai Basin, the site responses from three soil sites are studied by using the traditional spectral ratio method. The bedrock site is selected as a reference site. This study found that peak ground accelerations (PGAs) on the soil sites are much larger than on bedrock, with EW, NS and UD components of 3.96–6.58, 6.27–10.98, and 3.17–6.66 times those of the bedrock site, respectively. The amplification effects of the soil sites on ground motions in the frequency range of 0.1 Hz to 10 Hz are significant, depending on the thickness of the soil layer and the frequency content of the site. A significant amplification occurs with high frequency components of ground motion at shallow soil sites, and low and high frequency components of ground motion at intermediate soil sites.     

Variations in the Caspian Sea Level in the Historic Epoch

Reconstruction of variations in the Caspian Sea level is proposed based on the results of investigations of deposits in the Agrakhan sand bar and bays (or former bays) of the eastern sea coast. The history of sedimentation in particular regions is reconstructed by the radiocarbon dating. Generalized data on the age of deposits are used to construct the most likely temporal course of variations in the sea level within the historic epoch.