Piezomagnetic fields produced by dislocation sources

Tectonomagnetic modeling based on the linear piezomagnetic effect is reviewed with special attention to dislocation models. Stacey's scheme was the prototype for such modeling, as proposed in his first seismomagnetic calculations in 1964. The linear piezomagnetic law is presented, in which the stress-induced magnetization is expressed as a linear combination of stress components. The Gauss law for magnetic field and the Cauchy-Navier equation for static elastic equilibrium are combined through linear piezomagnetism and the Hooke law to yield the basic equation for piezomagnetic potential. A representation theorem for its solution is given by surface integrals of the displacement and its normal derivative over the strained body.A Green's function method is developed to compute the piezomagnetic field produced by a dislocation surface in an elastic half-space. Volterra's formula for piezomagnetic potential is derived by modifying Stacey's scheme for tectonomagnetic modeling. The Green's functions for the problem are called elementary piezomagnetic potentials, which are defined as potentials produced by elementary dislocations. Special consideration is required to construct the elementary piezomagnetic potentials, because the stress field around a point dislocation has a singularity of orderr ^–3. The integral representing elementary piezomagnetic potentials is not uniformly convergent. Owing to inappropriate convergency, the Green's functions obtained in an earlier study led to a puzzling outcome. Revised Green's functions give consistent results with those obtained so far by numerical integrations. Generally the piezomagnetic field produced by dislocation sources is weak in the case of a homogeneous earth model. Two enhancement effects for piezomagnetic signals are suggested: one due to inhomogeneous magnetization and the other via bore-hole observations.     

The piezomagnetic computation of magnetic anomalies due to ground loading by a man-made lake

Summary A method has been developed to determine the stress distribution and resultant piezomagnetic surface anomaly field due to the load imposed on the earth's crust from filling man-made lakes of variable depths and arbitrary geometries but uniform elastic properties. The dam-magnetic effect observed byDavis andStacey [1] on Talbingo reservoir in the Snowy Mountains of SE Australia is adequately explained in terms of the piezomagnetic effect if more highly magnetic rock than that measured on the surface is present at depth.     

Depth calculation of piezomagnetic effect for earthquake prediction

Tectonic stresses acting in the lithosphere cause transitory changes in the magnetic properties of rock and thus of its observed magnetic field. Calculations have been made to estimate this piezomagnetic effect as a function of depth, by considering the relative stress sensitivity of magnetite with increasing temperature and hydrostatic pressure. The response of magnetization to seismotectonic stress is determined by an appropriate balancing of magnetocrystalline and magnetoelastic anisotropy energies. Magnetite becomes progressively more responsive to applied stress as depth increases, with the rate of change depending on the local geothermal gradient. The upper 15 km of the lithosphere is likely to be the most important in yielding observable piezomagnetic field anomalies for earthquake prediction.     

岩石剩余磁化强度的应力效应

为模拟构造应力对岩石剩余磁化强度的影响,本研究对六种不同类型的44块岩样进行了应力实验,发现岩石天然剩磁随应力的变化规律比迄今所估计的要复杂得多.作者指出,这些变化可归属于三种类型:在弱不可逆的Ⅰ型效应中,剩磁随应力呈现规则的减小,应力解除后剩磁部分恢复;而在强不可逆的Ⅱ型效应中,应力去除后剩磁大部不恢复;在Ⅲ型效应中,剩磁随应力呈极不规律的变化.进一步的岩石磁学分析表明,不同应力效应的重要原因,在于天然剩磁中的粘滞剩磁的比例不同,微观上取决于磁性矿物成分及其磁畴状态等因素.因此,不能用单一的岩石压磁模式来解释地震压磁效应,须考虑各震源区的不同岩石组成.      

从震源物理的角度讨论影响地壳应力的某些磁学问题

本文从压磁效应的另一个方面——磁化对应力的影响,讨论了震磁问题。利用磁畴理论和磁弹耦合理论,得出了地学尺度下包含磁致伸缩效应的弹性平衡方程。由此分析变化磁场对应力的影响及居里面的力学特殊性。对热应力的磁贡献部份进行了模型估算。结果表明,这个异常应力足以影响地震的发生。根据上述结果,作者结合震源模式讨论了几个地震预报问题。     

岩石破裂电磁辐射（EMR）现象实验研究

岩石破裂电磁辐射现象是客观存在的物理现象.随着电磁辐射观测技术在地震研究、冲击矿压预测等领域的应用，极大地推动了岩石破裂电磁辐射的实验研究.本文对岩石破裂电磁辐射的影响因素以及相伴生的现象，以及有关的物理解释进行了概括介绍.由于不同研究者使用实验设计、实验参数、实验条件的不同，使得观测和研究结果同样难以统一认识.不同的研究者根据各自的试验提出了不同的物理机制.同时对已发现现象的重复性、证实性研究岩石试验缺乏.严重匮乏利用数值模拟以及建立模型定量研究岩石破裂的电磁辐射.此外，岩样实验系统不同于实际的地震系统，进行模拟震源环境的实验研究，发展大尺度的标本和原岩现场实验的基础上，如何建立室内室外岩石实验与地震观测事实之间的联系是问题的关键.     

磁性层状介质的CSAMT电场分量响应特征

在传统的勘探电磁学理论中,往往把岩层磁导率近似看成空气中的磁导率,从而得到简化后的电磁场响应式,主要用于指导非磁性介质情况下的电磁探测.本文开展了磁性介质的电磁响应理论、数值模拟和特征分析等研究,以便实现对磁性矿体的电磁精细探测.首先推导出磁性层情况下的电磁响应表达式;然后,计算了几种典型磁性介质地质断面的响应曲线,并对其响应特征进行了分析;研究结果表明:当磁性层位于地表时,当表层相对磁导率μr1.2,磁性层对电磁测深曲线具体明显的影响;当岩层为中间层时,只要μr1.2,对H型、A型曲线中段会有影响.对H型曲线,主要表现在使极小值处的曲线抬高、变宽,以致使曲线形态发生变化;当磁性层处于最后一层时,不论岩层的磁导率是多少,对各类二、三层曲线来讲,不会影响曲线的形态,只是使渐近线前第一个极值点的位置右移,极值点至渐近线之间的线段变得较为陡峭.     

地震前的电磁效应

伴随着孕震后期加剧的岩石应变与破裂过程,孕震区及其附近岩层出现机-电能量转换,并相应地激发了某些电磁效应。文中主要分析了地震前的电离层电磁效应,提出了附加电离区的VLF电波检测等监测手段,对强震的短临预测具有较为重要的意义     

露天煤矿地下水电磁探测与类型判别

为准确确定露天煤矿地下水分布与类型，根据不同赋存条件电磁差异，基于磁场强度与电阻率参数进行异常划分与类型判别。以新疆某露天煤矿为例，采用高精度磁法通过磁异常强度确定烧变岩边界，并采用高密度电法和瞬变电磁法通过电阻率圈定低阻异常区，结合异常响应差异分别推断地下水分布范围与赋存类型。地面钻探验证探测成果的准确性。结果表明:露天煤矿烧变岩水与砂岩裂隙水电磁响应差异明显，基于磁场强度与电阻率的双参数综合电磁法能准确圈定其分布范围，并判别其赋存类型。      

唐山地震与震磁效应的最佳观测

根据作者与Hastie和Stacey提出的三维压磁效应模式计算方法,围绕1976年唐山大地震,计算了七种不同断层模式的压磁效应,比较了在不同磁纬处,走向滑动断层的不同走向、倾角和埋深在地面上所引起的压磁异常场。结果表明,震磁效应的可观测性同地震的破裂机制有着紧密的关系。断层倾角45——60对应着最佳观测条件,其压磁异常峰值可高达12nT;在最差观测条件时压磁异常量几乎无法测出,这时断层位于磁纬0处,或走向为E-W,或埋深为10公里左右。在大多数模式中,垂直分量的异常略大于总强度异常,但在磁赤道附近可高达三倍左右。对唐山大地震的计算表明,压磁异常量约3——4nT,其异常大于1nT区域的线性尺度与断层长度之比仅为1.04——1.10。由于压磁场是地震过程的一种弱效应场,目前使用仪器的观测精度不够和磁测点的震中距过大,可能是未测出该地震明显的磁异常的原因。      

单轴压力下岩石磁化率的变化

利用感应法测定了北京、唐山地区五类强磁性火成岩在单轴压力下的压磁曲线。实验发现,曲线可划分为两个形态不同的阶段。在加压的初始阶段,压磁曲线表现为上升;尔后,几乎以不变的速率直线下降。曲线形态与岩样的种类、循环次数有一定的关系。压磁系数变化范围为-(0.9—3.0)×10~(-4)cm~2kg。从加压开始到岩石破坏,磁化率一般下降20—30%。还发现,磁化率的变化与岩样的体膨胀关系不太明显。运用铁磁学中的磁畴理论对上述压磁曲线作了初步的解释。     

密云水库的构造磁实验

为探讨水库地区的构造磁效应,在密云水库周围布设了221个测点,测点间距为2——5km,测点离水库堤岸的距离为几十米至15km不等,使用G-816型与G-826型质子旋进磁力仪,在1983——1987年期间,每季度观测一次地磁场总强度.地磁与密云水库蓄水的资料分析得到,地磁变化与水库的水位变化、水容量变化的关系系数分别为-(0.280.22)nT/m与-(0.350.31)10-8nT/m3,表明地磁变化与该水库蓄水变化有着较强的负相关.这种较强的负相关可能是密云水库地区地下较强磁性岩石的压磁效应的反映.由于水库构造磁实验是地震磁现象的较好模拟,因此,根据实验结果可以相信,在地震活动区布设加密的高精度地磁观测,是能够捕捉到震磁前兆信息的.      

单轴应力作用下两种不同方法研究岩石磁化率变化的结果

本文介绍了对北京——唐山地区八种强磁性岩石标本的压磁实验结果,观察了它们在高压应力下,直至破坏前后的磁化率变化特征。发现磁化率的变化只是其应力值的函数,与体积变化过程、微裂膨胀过程关系甚微。磁化率的压力系数取值范围为——(0.4——3.0)10-3MPa-1。同时,本文还得到了标本中出现破裂后岩石的压磁曲线。 运用铁磁学理论,对上述压磁实验结果作了初步的分析解释。      

The volcano-magnetic effect

Summary A geometrically simple volcano is considered, havig a spherical magma chamber of 2.5 km radius centred at 10 km depth. The Curie point isotherm is assumed to be a plane at 20 km depth, except for the spherical volume which is also non-magnetic. The stress pattern in the vicinity of the spherical chamber, due to regional stress of sufficient intensity to cause an eruptions, is used to calculate the change in magnetization which results from the piezomagnetic effect through the volume of solid rock. The consequent magnetic field anomaly at the surface is then obtaied by numerical integration of the dipole law of force over the stressed volume. For rocks of the type found on the volcanic island of St. Vincent (West Indies), this model gives a maximum local volcano-magnet c effect of about 7 gammas.     

2022年青海门源6.9级地震前岩石圈磁场异常变化分析

基于青海及甘肃部分地区2020—2021年2期流动地磁测点的观测资料,获得该地区的岩石圈磁场异常变化,通过研究岩石圈磁场各个分量的变化特征,系统分析2022年1月8日青海门源6.9级地震前岩石圈磁场的异常变化特征。结果表明：(1)门源地震前,震中区域岩石圈磁场各个分量具有较为明显的异常显示。具体表现为：震中区域附近岩石圈磁场变化的水平矢量具有方向转向及幅值弱化的异常变化现象;垂直矢量方向出现明显的南向、北向对冲的异常现象;磁偏角在震中附近具有“0”变线和高梯级带分布;总强度和垂直分量在震中附近具有“0”变线分布,并发生弯曲。(2)依据压磁效应分析,冷龙岭断裂北部岩石圈磁场增大,应力变化主要以释放为主,应力变化应处于较低水平;冷龙岭断裂南部岩石圈磁场强度减小,应力变化主要以积累为主,应力变化应处于较高水平。门源地震前,冷龙岭断裂构造应力调整引起的压磁效应可能是造成震中区域岩石圈磁场异常变化的主要原因。     

The influencing factors and mechanisms of the electromagnetic radiation during rock fracture

ＴｈｅｉｎｆｌｕｅｎｃｉｎｇｆａｃｔｏｒｓａｎｄｍｅｃｈａｎｉｓｍｓｏｆｔｈｅｅｌｅｃｔｒｏｍａｇｎｅｔｉｃｒａｄｉａｔｉｏｎｄｕｒｉｎｇｒｏｃｋｆｒａｃｔｕｒｅＹＵ－ＺＨＯＮＬＩＵ１）（刘煜洲）ＹＩＮＬＩＵ１）（刘因）ＹＩＮ－ＳＨＥＮＧＷＡＮＧ２...     

NWC通信台在电离层中激发电磁响应的时变特征

本文利用DEMETER卫星VLF频段电场和磁场频谱数据对DEMETER卫星运行期间2005年至2009年澳大利亚甚低频(Very Low Frequency)通信台NWC发射的通信信号造成的电离层电磁响应的日变化、季节变化及年变化特征进行了统计分析,统计结果表明电磁响应日变化显著,夜间电场强度明显增强可达40dB,磁场变化略小也可为15dB左右,而季节变化不显著,年变化主要受太阳活动的影响,太阳活动越强,电磁响应越小.为解释数据分析结果,对地-电离层电磁波传播过程采用传递矩阵方法进行了模拟计算,模拟结果与数据分析的结果一致.我们认为这种随时间变化的特点可能由250km以下电离层电子密度分布特征导致,因此研究250km以下的电离层电子密度变化可能对寻找地震电离层电磁异常有重要意义.     

Effect of Stress on the Magnetic Memory of Induced Magnetic Anisotropy of Rocks and Its Mathematical Model

When re-heated to temperatures below the Curie temperature and subsequently cooled in a constant magnetic field (H _T), rock samples which contain magnetic minerals can acquire an induced magnetic anisotropy (IMA). As the result of acquiring the IMA, a constriction develops in the hysteresis loop of the magnetization of these rocks at the values of the magnetizing field close or equal to the HT. Thus the IMA is capable of retaining the information on the palaeointensity of the geomagnetic field, i.e., if IMA was created in a rock in the geomagnetic field in a past geological epoch, it preserves the information on the intensity of that field. Investigations have shown, that when IMA is created in a rock under external stress, the stress has an impact on the magnetic memory. Here we also deal with the issue of how stress affects the magnetic memory of IMA. A mathematical model for the effect of stress on magnetic memory phenomena related to induced magnetic anisotropy in rocks containing multidomain magnetite and titanomagnetite grains is proposed herewith. The effect of temperature on the magnetic memory of rocks is discussed also.     

强磁扰对视电阻率观测的影响

基于地球天然交变电磁场的基本理论,研究了磁静日、磁暴时的视电阻率的变化特征。结果显示：磁暴时ρｓ有明显变化,一般大于或等于２倍的磁静日的标准差。同时发现,由于地下介质的不均匀性,天然交变电磁场各种频率成份对地下电性介质的贡献不同,集肤效果也不同,从而使得在同一地电台、相同观测环境、相同观测条件下,磁暴时地电阻率的测道的差异,其干扰水平显著出明显的方向性。最后对研究中所发现的一些问题尽可能地给予了物     

Review of Electric and Magnetic Fields Accompanying Seismic and Volcanic Activity

New observations of magnetic, electric and electromagnetic field variations, possibly related to recent volcanic and seismic events, have been obtained on Mt. Unzen in Japan, Reunion Island in Indian Ocean, the Long Valley volcanic caldera in California, and for faults in China and Russia, California and several other locations. For volcanic events, contributions from different physical processes can be identified during the various eruption stages. Slow processes (weeks to months) include near-surface thermal demagnetization effects, piezomagnetic effects, and effects from rotation/displacement of magnetized material. Rapid processes (seconds to days) include piezomagnetic effects from instantaneous stress redistribution with explosive eruptions and electrokinetic effects from rupture of high pressure fluid compartments commonly encountered in volcanic regions. For seismic events, the observed coseismic offsets are instantaneous, provided care has been taken to ensure sensors are insensitive to seismic shaking and are in regions of low magnetic field gradient. Simple piezomagnetic dislocation models based on geodetically and seismically determined fault parameters generally match the observed signals in size and sign. Electrokinetic effects resulting from rupture of fluid filled compartments at hydrostatic to lithostatic pore pressures can generate transient signals in the frequency band 100 Hz to 0.01 Hz. However, large-scale fluid driven processes are not evident in near-field measurements in the epicentral region minutes to weeks before large earthquakes. The subset of ionospheric disturbances generated by trapped atmospheric pressure waves (also termed gravity waves and/or acoustic waves, traveling ionospheric disturbances or TID's) that are excited by earthquakes and volcanic eruptions are common and propagate to great distances. These are known and expected consequences of earthquakes, volcanic explosions (and other atmospheric disturbances), that must be identified and their effects removed from VLF/ULF electromagnetic field records before associating new observations of ionospheric disturbances with earthquake activity.