时间域航空电磁系统瞬变全时响应正演模拟

近年来很多专家学者致力于时间域航空系统正反演的研究.本文针对一维均匀层状介质和三维模型进行正演.不仅计算垂直方向上的电磁响应,还计算了同线方向上的电磁响应,为航空电磁多分量观测提供理论依据.通过比较航空电磁系统的脉冲响应和阶跃响应特征,发现脉冲响应在早期时间存在奇异性,造成计算不稳定.然而,阶跃响应在早期时间没有奇异性,因而利用系统的阶跃响应可得到一种计算时间域航空电磁系统全时响应的稳定算法.该算法具有较高的精度,并很好地保持了磁场强度B和磁感应dB/dt关系的一致性.该算法推广到三维地质体的时间域正演模拟亦取得很好的效果.     

3D Modelling of the electromagnetic response of geophysical targets using the FDTD method1

A publicly available and maintained electromagnetic finite-difference time domain (FDTD) code has been applied to the forward modelling of the response of 1D, 2D and 3D geophysical targets to a vertical magnetic dipole excitation. The FDTD method is used to analyse target responses in the 1 MHz to 100MHz range, where either conduction or displacement currents may have the controlling role. The response of the geophysical target to the excitation is presented as changes in the magnetic field ellipticity. The results of the FDTD code compare favourably with previously published integral equation solutions of the response of 1D targets, and FDTD models calculated with different finite-difference cell sizes are compared to find the effect of model discretization on the solution. The discretization errors, calculated as absolute error in ellipticity, are presented for the different ground geometry models considered, and are, for the most part, below 10% of the integral equation solutions. Finally, the FDTD code is used to calculate the magnetic ellipticity response of a 2D survey and a 3D sounding of complicated geophysical targets. The response of these 2D and 3D targets are too complicated to be verified with integral equation solutions, but show the proper low- and high-frequency responses.     

核磁共振测井在火成岩地层应用的适应性分析

火成岩岩石具有高磁化率特征,与沉积岩差别明显,岩石的高磁化率会对核磁共振信号产生显著影响,开展核磁共振测井在火成岩地层应用的适应性分析研究,为广泛使用核磁共振测井用于火成岩储层评价提供参考.采用理论模拟、实验分析及实际测井资料相结合的方法,分析了高磁化率岩石的核磁共振响应特征.分析结果表明,火成岩岩石具有很高的磁化率,从酸性火成岩到基性火城岩,岩石的磁化率一般是逐渐增大的.高磁化率岩石孔隙内部会产生强梯度磁场,孔隙内部的磁场梯度越强,核磁共振的T2谱前移越明显,核磁信号衰减幅度也越大,核磁分析孔隙度与常规分析的孔隙度误差也越大;相同岩石磁化率情况下,岩石孔径越小,岩石孔隙内部磁场梯度越强,核磁信号衰减越大,核磁共振测井计算的孔隙度也越低.因此,核磁共振测井与岩性有关系,核磁共振测井会受到高磁化率岩石的显著影响.核磁共振测井在部分火成岩,尤其在中基性火成岩和小孔径火成岩储层中应用具有很大的局限性.     

Electromagnetic transient response of a small wire loop buried in a homogeneous conducting earth

Summary The analysis extends previously obtained results for the transient magnetic fields of small loops in the presence of a homogeneous conducting half-space. Here, the secondary scattering of pulses by a buried target loop is examined for the case where both the source and receiver loops are located at the surface. While the response of the buried or target loop is orders of magnitude below the primary response, there is a significant difference in the respective transient waveforms. Thus, the results have possible application to mine rescue using a passive detection technique.     

位于导电介质中三维导体的电磁响应

本文给出一种用积分方程法配合有限差分法计算位于导电介质中三维导体的电磁响应的新途径,在二层大地条件下给出了具体计算方法和结果。详细讨论了围岩介质的导电性对导体异常的影响。这对于开展低阻覆盖层地区的电磁法工作具有一定的指导意义。     

Fast approximate EM induction modeling of metallic and UXO targets using a permeable prism

The time-domain EM induction response of non-magnetic and magnetic targets can be approximated using a conductive permeable prism composed of six faces of conductive plates, each face being composed of a set of conductive ribbons. The effect of magnetic permeability is included by the use of two “apparent flux gathering” coefficients, and two “effective magnetic permeability” coefficients, in the axial and transverse directions. These four magnetic property coefficients are a function of physical properties and geometry of the target, but are independent of prism orientation relative to a transmitter. The approximation algorithm is computationally fast, allowing inversions for target parameters to be achieved in seconds. The model is tested on profiles acquired with a Geonics EM63 time-domain EM metal detector over a non-magnetic copper pipe target, and a steel artillery shell in horizontal and vertical orientations. Results show that this approximation to a permeable prism has a capability of fitting geometric, conductivity and magnetic parameters at both early and late sample times. The magnetic parameters show strong change from early to late times on the EMI decay curve, indicating that the magnetic properties of the target have non-linear characteristics. It is proposed that these magnetic parameters and the nature of their non-linearity may carry additional discrimination information for distinguishing between intact munitions and scrap in UXO studies.     

位于导电介质中三维导体的电磁响应

本文给出一种用积分方程法配合有限差分法计算位于导电介质中三维导体的电磁响应的新途径,在二层大地条件下给出了具体计算方法和结果。详细讨论了围岩介质的导电性对导体异常的影响。这对于开展低阻覆盖层地区的电磁法工作具有一定的指导意义。     

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.     

Analogue model measurements for a horizontal magnetic dipole embedded within a conducting medium

The validity of an analogue model method employing a horizontal magnetic dipole source situated within a conducting layer for the cases of a poorly conducting model earth and a highly conducting model ocean is studied by comparing model magnetic field measurements with theoretical calculations. The model is then used to study one example for each case; the response of a conducting cylinder simulating an ore body embedded in the earth, and the response of a conducting wedge simulating a shelving ocean.     

井中磁源瞬变电磁响应特征研究

井中瞬变电磁波勘探是一个全空间地球物理场问题.采用Gaver-Stehfest逆拉氏变换方法,正演计算了瞬变信号激励下接收线圈上的电磁场响应.分析了包含井眼泥浆、套管、水泥环和地层的轴对称多层介质模型的电磁场响应特征,考察了各层介质参数对井中瞬变电磁响应的影响.不同电导率井眼泥浆的电磁场响应衰减曲线表明,井眼泥浆电导率...     

The effects of anisotropy of marine magnetic anomalies on the Curie point depth estimates from spectral analysis

Azimuthally averaged power spectra are widely used in the Curie point depth (CPD) estimation with the implicit assumption that the magnetization distribution is random and uncorrelated. However, the marine magnetic anomalies are caused by bands of normal and reverse magnetization and show obvious trends. To investigate the effects of the anisotropy of marine magnetic anomalies on the CPD estimates, we develop 3D fractal striped magnetization models to produce lineated marine magnetic anomalies for the first time. We analyze the spectra anisotropy of the lineated magnetic anomalies of the synthetic fractal striped magnetization models and investigate its effects on the CPD estimates. The synthetic models and actual data show that the spectra of the lineated marine magnetic anomalies are directionally anisotropic. The amplitude response is strong and the slope of the logarithmic spectrum is large in a direction perpendicular to the stripes of magnetic anomalies, whereas the amplitude response is weak and the slope of the logarithmic spectrum is small in a direction parallel to the stripes of magnetic anomalies. The depth estimates in the perpendicular direction are close to the actual values, whereas the depths estimates in the parallel direction are significantly lower than the actual values. The actual marine magnetic anomalies of the South China Sea exhibit an anisotropic power spectrum that is consistent with the spectral anisotropy of magnetic anomalies of the synthetic fractal striped magnetization models.     

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.     

同步轨道磁场和地面磁场对太阳风动压变化事件的响应

本文对磁宁静时的123个动压变化事件(不包含激波事件)进行了统计研究.研究表明,在白天侧(9～15MLT)同步轨道磁场z分量对太阳风动压增大、减小事件具有较强的正响应,而在夜侧(21～3MLT)响应明显减弱,响应幅度具有明显的磁地方时分布.对动压增大事件的平均响应幅度在午前最大,而对动压减小事件的平均响应幅度在午后达到...     

Tropospheric vorticity responses to the solar magnetic sector structure and geomagnetic disturbances

The responses of the Vorticity Area Index (VAI) at 500 mb to large geomagnetic disturbances and to magnetic sector boundary crossings are evaluated for the periods 1947–57 and 1963–74, during which time the geomagnetic response to sector structure were known to be distinctly different. Results indicate that the nature of the VAI response to geomagnetic disturbances is markedly similar between the two subsets. The response does not vanish even when only those geomagnetic disturbances not related to sector boundary passage are used in the analysis, which suggests that enhanced geomagnetic activity can independently influence the lower atmosphere. Unlike the geomagnetic disturbance-related effects the sector-related effects have varied with time in a very complex manner. In view of this it is concluded that geomagnetic disturbances, whose effects have shown pronounced consistency, may prove a better solar signal in future sun-weather studies.     

层状导电介质中地面核磁共振响应特征理论研究

水平层状导电大地中层状含水层地面核磁共振响应的数值计算技术涉及到导电介质中回线源 磁场的计算以及地下含水层中质子磁矩在线圈中产生感应信号的体积积分. 文中采用直接数 值积分技术,对具有振荡核函数的Hankel变换进行积分,以求取回线源磁场的径向与垂向分 量,计算并研究了回线产生的径向与垂向磁场分量随空间位置的变化规律. 基于磁场空间分 布特点,利用不等间距空间剖分技术计算地面核磁共振的体积积分,模拟了不同模型的地面 核磁共振响应并讨论了其影响因素. 结果表明,结合能对任意层状导电介质中磁场进行稳定 快速计算的直接数值积分技术与不等距空间模型剖分技术,可正确模拟地面核磁共振响应. 导电性是产生地面核磁共振信号相位的先决条件,但影响响应振幅强度与相位的因素还有含 水层的埋深、厚度以及装置大小等.     

Response of the convecting high-latitude F layer to a powerful HF wave

A numerical model of the high-latitude ionosphere, which takes into account the convection of the ionospheric plasma, has been developed and utilized to simulate the F-layer response at auroral latitudes to high-power radio waves. The model produces the time variations of the electron density, positive ion velocity, and ion and electron temperature profiles within a magnetic field tube carried over an ionospheric heater by the convection electric field. The simulations have been performed for the point with the geographic coordinates of the ionospheric HF heating facility near Tromso, Norway, when it is located near the midnight magnetic meridian. The calculations have been made for equinox, at high-solar-activity, and low-geomagnetic-activity conditions. The results indicate that significant variations of the electron temperature, positive ion velocity, and electron density profiles can be produced by HF heating in the convecting high-latitude F layer.     

基于站间天然电磁场单位脉冲响应的大地电磁时间序列去噪方法

针对天然大地电磁场信号在人文活动密集地区易受噪声干扰的问题,本文提出利用两个同步测点天然电磁场时间序列之间的单位脉冲响应,合成本地点受干扰时段的数据,从而去除大地电磁噪声.首先,选择高信噪比时段的数据,采用最小二乘法,估算本地点与参考点之间的单位脉冲响应,再根据卷积定律,结合参考磁场合成本地点的磁场和电场.最后用合成数据替换含噪声时段数据,实现时间域去噪.实测高信噪比数据和含噪数据的处理结果表明,该方法可以高精度合成本地点磁场与电场信号,有效去除本地点电场和磁场噪声,包括相关噪声,提高大地电磁数据质量.     

Ring current asymmetry during a magnetic storm

The ring current dynamics during the magnetic storm has been studied in the work. The response of the magnetospheric current systems to the external influence of the solar wind, specifically, resulting in the development of the asymmetric ring current component, has been calculated using the magnetic field paraboloid model. The asymmetric ring current has been considered as a family of spatial current circuits in the Northern and Southern hemispheres, composed of the zones of the partial ring current in the geomagnetic equator plane, which close through the system of field-aligned currents into the ionosphere. The value of the total partial ring current has been estimated by comparing the calculated asymmetry of the magnetospheric magnetic field at the geomagnetic equator with the value of the Asym-H geomagnetic index. The variations in the symmetric and asymmetric components of the ring current magnetic field have been calculated for the magnetic storm of November 6–14, 2004. The contributions of the magnetospheric current systems to the Dst and AU geomagnetic indices have been calculated.     

海底表面磁源瞬变响应建模及海水影响分析

根据电磁场理论,推导了磁偶源和接收点均位于海水中时层状海底模型的频域电磁场响应一般表达式,并通过此式,得到了海水为均匀半空间和有限海水深度两种情况下,垂直磁偶极装置、中心回线和重叠回线分别置于均匀半空间海底表面时的瞬变电磁响应（磁场和感应电压）表达式. 这些表达式将瞬变响应和海底的电导率等参数有机联系在一起,为海底瞬变电磁法的正演计算和反演解释提供了理论基础. 仿真计算表明,海水的存在不仅使得瞬变响应曲线形态发生变化,而且影响其对海底电导率的分辨能力.