An inversion of geomagnetic deep sounding data using simulated annealing

A maiden attempt on the use of the global optimization technique of Simulated Annealing (S.A.) inversion to model the conductivity structure derived from the geomagnetic deep sounding data of NW India is reported here. The location of the proposed model is now in agreement with the theory, since the conductive bodies are centered exactly below the center of the response function which was not feasible by the earlier linearized inversion. The central body is located at a depth of 19 km from the surface, suggesting thickness of 15 km and resistivity of 14 Ω m. The resistivity contrast of this ensemble of conductive bodies with the background is varying by a factor of 100 to 385. Existence of mid-crustal conductor (the conductivity estimate is marginally different) is clearly indicated which was also not detected in the earlier study. A likely explanation could be due to the presence of graphitic carbon at lower crustal depths, however, the role of electrolytic fluid present in the interconnected pore-spaces of rocks may be another explanation.     

Electrical structure of the tectonically active Kalabsha Fault, Aswan, Egypt

In this work, we use the magnetotelluric (MT) method to detect geoelectrical conductivity anomalies in the Earth's crust and link them to local seismic activity. This application affords the unusual opportunity to study the percolation of water from a lake into a fault system and its effect on the induced seismicity. MT measurements were carried out in the period range 0.0046–420 s at nine sites along a 15 km-long North–South profile crossing the Kalabsha Fault, on the western bank of Lake Aswan. Data were analysed by 2D simultaneous inversion of both polarisations. The resulting model is compared with the local seismicity map and reveals the conductive signature of the fault, as well as geological and tectonic stresses prevailing in the Aswan area. Our MT investigations show the following features:

The measured MT strike aligns with the seismic epicentre axis corresponding to the Kalabsha Fault.

While crossing the Fault, enhanced conductivity is found down to depths of 5 km on a 1–2 km profile segment.

At mid-crustal depths (20 km), a very high conductive body is found to coincide with the main seismic cluster in the Aswan area.

These observations indicate that seismic activity and high electrical conductivity are related. The link between them is the presence of crustal fluids which are presumably the cause of the high conductivity observed. Their presence is also required to trigger the observed seismicity. In addition, we explain the lower conductivity of the local upper crust in terms of stress-modulated rock porosity. We believe that these results are of general significance, as they could explain the mid-crustal seismicity of tectonically active zones.     

Conductivity anomalies: lower crust or asthenosphere?

The indication from surface measurements of a zone of relatively high conductivity (resistivity<200 ohm-m) at depths between 20 and 50 km has become so general over the Earth that regions without this zone can be considered anomalous. However, the depths indicated often span the lower crust and the uppermost mantle, so that before any effect can be definitely attributed to one or the other, the depth resolution in the electromagnetic measurements must be carefully considered. This paper applies the eigenvector decomposition of generalized linear inverse theory to soundings by Schlumberger resistivity, by magnetotellurics, by man-made electromagnetic fields formed by controlled current flow in grounded electric transmission lines, and by natural magnetic field variation studies to improve the bounds on depth, thickness and conductivity of a conductive layer. It is shown that many of the methods are capable of giving the depth to the top of a conductor with remarkable accuracy. Joint inversion of two or more of them offers an advantage in the separation of thickness and conductivity of both conductive and resistive layers. Natural geomagnetic field transfer functions, while accurately mapping the position of the edge of a conductor, do not provide the resolution of the other techniques, largely because the frequencies that can be practically measured at present are much too low.     

Geoelectromagnetic measurements across the southern Senegal basin (West Africa)

Magnetotelluric and differential geomagnetic sounding surveys, consisting of nine soundings, were performed in 1984 along a 200-km profile across the southern Senegal basin. They were intended to obtain information concerning the resistivity structure of the crust and upper mantle and the distribution of the induced electric currents. Magnetotelluric data indicate that two-dimensional resistivity models are appropriate for the region. The zone above the basement is highly inhomogeneous in geoelectrical structure. Very conductive sediments (0.6-3 ohm m) appear in the Mesozoic-Cenozoic Senegal basin. These sediments lie at depths of up to 4500 m on the west end of the profile. Below this, a modest resistivity material (10–30 ohm m) extends to a maximum depth of about 3000 m. The material at depth on the cast part of the traverse line is thought to be Palaeozoic sediments of the Bove basin. The depth of the magnetotelluric basement lies between about 250 m (in the east) and 4800 m (in the west). The crust is characterised by a drop in electrical resistivity at a depth of 15 km below the east part of the profile. Considering the total section, we observe a general trend towards lower resistivities at depths in excess of 100 km, the transition from 2000 ohm m to about 2 ohm m occurs in the depth range 100 to 175 km. An analysis of the geomagnetic variation field has identified a concentration of telluric current flow beneath the deep basin. It appears that the additional currents flowing in the striking direction of the Senegal basin are largely controlled by sedimentary rocks of high conductivity lying at depths less than 5 km. Model studies show that the local conductivity distribution is able to explain the currents circulating in the thick well-conducting sediments.     

核磁共振与瞬变电磁三维联合解释方法

传统核磁共振地下含水量解释多采用基于均匀半空间或层状导电模型的一维反演,分层给出地下含水信息.然而,这些方法忽略了地下复杂电阻率分布信息对结果的影响,也不能很好地反映局部三维含水构造.本文从三维电介质中核磁共振响应的正演理论出发,提出首先利用瞬变电磁数据进行基于等效导电平面法的快速电阻率成像,然后将成像结果作为核磁共振三维反演的电性模型,进行联合解释.激发磁场的分布采用有限元法直接求解,通过引入伪δ源实现电流源的加载,并强加散度条件排除了三维磁场模拟中"弱解"的影响.针对核磁共振灵敏度矩阵的病态性和数据中存在的干扰信号,提出考虑罚项的非线性拟合目标函数,利用线性化方法进行核磁共振反演.模型数据表明该方法能较准确反映地下三维含水构造,实测算例进一步证明了方法的有效性.本研究将促使核磁共振方法在岩溶、裂隙水、孤立水体等复杂水文地质条件及隧道、矿井灾害水源探测等方面得到有效应用.     

内蒙古锡林浩特-东乌旗剖面壳幔电性结构研究

为研究二连-东乌旗贺根山一带成矿构造环境,提供矿产资源勘查、预测、评价的地质背景依据,跨贺根山和锡林浩特板块缝合带一线布设了26个超宽频带长周期大地电磁测深点,点距3～6 km,剖面长度100 km,在对获取的资料采用Robust变换、互参考处理的基础上,定性分析了视电阻率和相位曲线、二维偏离度、电性主轴,并采用二维共...     

On the Generation of ULF Magnetic Variations by Conductivity Fluctuations in a Fault Zone

v--vPrior to the October 18, 1989 Loma Prieta M_s 7.1 earthquake, Fraser-Smith et al. (1990) recorded a 10-100 fold increase in ultra-low frequency (ULF) magnetic fields near the earthquake epicenter. Several mechanisms for generation of these ULF fields by fluid flow in the earth have been advanced, but all appear to require unrealistic fluid velocities or hydraulic permeabilities to match the observations. As an alternative explanation, Merzer and Klemperer (1998) proposed that the increase in ULF magnetic fields could result from induced electric currents flowing in a fault-zone made temporarily much more electrically conductive by stress-induced reorganization of pore geometry. Using a numerical model we show that while this mechanism could produce a significant increase in ULF variations, mutual induction between the fault zone and the surrounding crust would probably limit the amplitude increase to levels well below those observed at Loma Prieta. We consider a variant on this quasi-static conductive fault zone model in which low frequency telluric currents are modulated by small higher frequency variations of bulk fault zone conductivity. We show that because the spectrum of natural EM variations is red, substantially larger relative increases in ULF magnetic fields could be produced by this mechanism with even small conductivity fluctuations at these frequencies. These variations would be easy to detect with a well-designed experiment, if they occurred. In principle this mechanism could explain the Loma Prieta ULF observations, however the magnitude of conductivity fluctuations that would be required to match the very large reported amplification factors still appears to be too large to be physically plausible.     

陕西地区单台Z／H地磁测深研究

应用陕西地磁台网Ｓｑ资料和单台Ｚ／Ｈ地磁测深法对陕西地区地下电导率分布进行了研究，结果表明，理想代换导体视探ｈ约为５００ＫＭ，电导率σ为０．０７ｓ／ｍ（周期为１２小时），且存在２５０ＫＭ和７００ＫＭ深度附近的高导层，感应比例尺度实部Ｃ＾１ｒｅａｌ和Ｃ＾２ｒｅａｌ及内外场之比│Ｑ＾１│和│Ｑ＾２│似有１１年周期变化。各台测定的结果相当接近，和其他方法测定的范围结果基本一致。用冬夏季静日均值结果较春秋     

A regional strategy for geothermal exploration with emphasis on gravity and magnetotellurics

As part of the resource evaluation and exploration program conducted by Los Alamos Scientific Laboratory for the national Hot Dry Rock (HDR) Geothermal Program, a regional magnetotelluric (MT) survey of New Mexico and Arizona is being performed. The MT lines are being located in areas where the results of analysis of residual gravity anomaly maps of Arizona and New Mexico, integrated with other geologic and geophysical studies indicate the greatest potential for HDR resources.The residual gravity anomalies are derived by applying the concept of predicting gravity anomalies from topography. This can be accomplished by employing reductions similar to those used in some isostatic investigations, in which a regional topographic surface is used as the Bouguer reduction datum. The datum is derived by comparison of various harmonics of Bouguer anomalies and elevations of stations. Topography can be used to predict Bouguer anomalies because of isostatic compensation; the resultant anomalies can be considered high frequency residual anomalies or isostatic anomalies corrected for regional compensation. Such maps have been produced for Arizona, New Mexico, west Texas, and Chihuahua, Mexico.The main objective of the MT project is to produce a regional geoelectric contour map of the pervasive deep electrical conductor within the crust and/or upper mantle beneath the Colorado Plateau and the adjacent Basin and Range Province and Rio Grande Rift. The MT survey consists of 200 sites along several long profiles with site spacing of 15–20 km. Pre-existing available MT data are being integrated with the new data. After the data are processed, a one-dimensional inversion is applied to the sounding curve and used as a starting point for 2-D modeling. Such a project and ultimate map will be of major value in studying the regional geophysics and tectonics of the southwest United States as they now apply to HDR resources in particular and geothermal resources in general.Electrical conductivity anomalies of large areal extent are of particular interest in geothermal exploration. Correlation analysis of large conductive anomalies with other geophysical, geological, and geotectonic data is being performed. Preliminary analysis of the data has suggested several major regions of anomalously shallow high electrical conductivity. Among these is the Aquarius area of northwest Arizona which is the site of a longwavelength residual anomaly low, which when modeled and correlated with other geophysical data can be shown to be possibly related to low density and high temperature in the crust at depths of 20 km or less. Preliminary analysis of MT data indicates the possible existence of a mid-crustal high electrical conductivity anomaly in this same region.     

Modelling and inversion of electromagnetic data using an approximate plate model

This paper presents a computational method for the interpretation of electromagnetic (EM) profile data in the frequency domain using a thin plate model within a two-layer earth. The modelling method is based on an integral equation formulation, where the conductor is represented by a lattice structure composed of two-dimensional surface elements. Several approximations are used to simplify the theoretical basis and to decrease the computation time. The simple parametric model allows efficient use of optimization methods. We employ a linearized inversion scheme based on singular value decomposition and adaptive damping. The new forward computation method and the parameter optimization are combined in the computer program, emplates . The modelling examples demonstrate that the approximate method is capable of describing the characteristic behaviour of the EM response of a thin plate-like conductor in conductive surroundings. The efficacy of the inversion is demonstrated using both synthetic and field data. An optional depth compensation method is used to improve the interpreted values of the depth of burial. The results show that the method is cost effective and suitable for interactive interpretation of EM data.     

内蒙古东部古生代块体碰撞区的大地电磁测深研究——Ⅱ.二维解释

本文对内蒙古东部地区赤峰-东乌旗剖面上12个测点的MT资料进行了二维解释.针对测区具体情况,在计算中主要强调拟合有效视电阻率ρeff,ρaz和有效相位Φeff,Φaz.二维模型给出,地壳上地幔在西乌旗以北和翁牛特旗以南地区有5层结构,壳内高导层埋深20-25km,上地幔软流层顶面埋深约100km.大兴安岭火山岩区有6层或7层结构,上地壳10km左右和下地壳30-40km深度处分别有一高导层,软流层顶面埋深约75km.结合地质等资料得出:1.翁牛特旗以南地区至少在深部仍然属于中朝块体北缘;2.翁牛特旗-巴林右旗之间是中朝块体北缘古生代以前的一个增生体,西拉木伦河断裂是由于后期的构造活动在增生体内产生的一个逆冲断层;3.上地幔隆起和下地壳高导层（岩浆房?）的存在是产生大兴安岭火山岩的深部原因;4.大兴安岭地区可能存在推覆构造,推覆面即为上地壳高导低速层;5.二维模型没有反映出贺根山地区地下有岩浆通道或深断裂;6.中朝和西伯利亚块体早古生代和晚古生代的缝合带分别位于林西一带和西乌旗一带.     

3-D HLEM Modeling of Tropical Weathering: A Trial from a Granitic Area of N-W Rajasthan, India

--In a sand-covered granite terrain of northwestern Rajasthan, India, a five-frequency HLEM survey along a 10-km traverse line generated several clear and strong anomalies. Subjected to a joint constrained 1-D layered earth inversion, the IP and OP frequency soundings at each point of observation yields either no solution or one which is inconsistent with the adjacent point. On the other hand, a 3-D model fitting of these anomalies with a tabular body in a layered host leads to a more meaningful interpretation, suggesting the presence of (1) pockets of weathered conductive material and (2) resistive intruding dykes, embedded in a semi-weathered layer, as the cause of the observed anomalies. The locations of weathered pockets are probably determined by pre-existing weak structures such as joints, fractures and faults, which facilitated movement of groundwater and hence weathering. Thus covered features deep in the bedrock such as intrusives, joints, fractures and faults, which are not directly detectable by the HLEM method, being under a more conductive overburden, are indirectly detected through their imprints left in the overlying weathering profile.     

Electrical conductivity investigation of the Corso-Sardinian microplate area

The analysis of magnetovariational data from an array quasi- simultaneously covering Sardinia (Italy) and Corsica (France), from summer 1988 to spring 1991, indicates the existence of two major conducting bodies, one north of Corsica, the other south of Sardinia. Fourier maps also show local conductivity anomalies along the Campidano Graben (Sardinia), across the Bonifacio Straits and in the northernmost part of Corsica. Two-dimensional (2D) modelling and inversion procedures have been carried out in order to find the optimum conductivity and geometrical characteristics of the conductive bodies deduced from Fourier maps, induction arrows and pseudosections studies. As a result of the 2D inversion, two zones with enhanced electrical conductivity are found, respectively, in the Sardinia Channel and in the Bonifacio Straits. The former can be related to the thermal characteristics of the area; the latter, shallower, can be ascribed to an accumulation of sediments. Some short-period magnetovariational anomalies in northern Corsica can be related to recent reheating processes that occurred there, as well as to the crustal thinning in the Ligurian sea.     

3D modelling of near-surface, environmental effects on AEM data

This study considers the three-dimensional (3D) modelling of compact, at-surface conductive bodies on frequency domain airborne electromagnetic (AEM) survey data. The context is the use of AEM data for environmental and land quality applications. The 3D structures encountered are typically conductive, of limited thickness (<20 m) and form ‘point’ source locations carrying potential environmental risk. The scale of such bodies may generate single-profile, ‘bulls-eye’ anomalies. In attempts to recover geological information, such anomalies may be considered to represent noise. In environmental AEM, the correct interpretation of such features is important. The study uses a combination of theoretical models and trial-fixed-wing survey data obtained in populated areas of the UK. Scale issues are discussed in terms of the volumetric footprints of the induced electric field generated by systems flown at both low and high elevation. One of the primary uses of AEM survey data lies in the assessment of conductivity maps. These are typically obtained using one-dimensional (1D) conductivity models at individual measurement points. In order to investigate the limitations of this approach, 3D modelling of conductive structures with dimensions less than 350×350 m and thicknesses extending to 20 m has been carried out. A 1D half space inversion of the data obtained at each frequency is then used to assess the behaviour of the spatial information. The results demonstrate that half space conductivity values obtained over compact 3D targets generally provide only apparent conductivity results. For thin, at-surface bodies, conductivity values are biased to lower values than the true conductivity except at high frequency. The spatial perturbation to both coupling ratios and 1D conductivity models can be laterally extensive. The results from 3D modelling indicate that the use of horizontal derivatives applied to the conductivity models offers enhanced edge detection. The practical application of such derivatives to both regional- and local-scale survey data is presented.. The special case of a near-surface, metallic pipeline has been modelled. The problem constitutes an inductive limit (current gathering) response in which the perturbation is largely confined to the in-phase coupling ratios. The main perturbations, in data and conductivity models, are within about 40 m of each side of the pipeline. The maximum perturbation to the conductivity model is only a factor of 1.5 above background. Detailed survey data across a former compact landfill (about 100×100 m) are used to compare the model behaviour predicted by the 3D modelling with survey results. The survey, conducted at two separate altitudes, provides a demonstration of 3D effects on 1D survey models as a function of frequency and elevation. Although the nature of the landfill materials and their location are not known precisely, the mapping information appears realistic.     

基于印模法的地面磁电阻率数据三维非线性共轭梯度反演

本文提出了能提高异常体分辨能力,同时得到绝对电导率的地面磁电阻率数据三维反演方法.磁电阻率响应用准直流的低频磁场代替;数值模拟由频率域电场满足的Helmholtz方程出发,采用三维交错网格有限差分法;长直导线源作为发射源,其中源的计算包含在背景场中;结合地面磁电阻率数据各分量的特点,选择y分量进行反演研究;反演采用三维非线性共轭梯度反演技术,为了提高异常体的深度分辨能力,进行迭代重构反演;用印模法对初始模型进行重构,采用的是辅模型在浅部,元模型在深部的组合方式.从合成数据和实际数据的反演结果可以得到以下的认识:(1)由频率域麦克斯韦方程组出发,低频磁场数据反演可以直接得到电导率,而不是相对电导率之比;(2)采用印模法组合初始模型,进行迭代重构反演,可以提高地面磁电阻率数据反演对异常体的分辨能力,确定埋深位置,同时不会丧失对于浅部异常体的分辨能力;(3)在结合印模法的地面磁电阻率数据三维反演中,深部异常体的分辨能力受地表不均匀导电体影响较小;(4)确定印模深度可以采用上一次重构反演结束时的模型变化量,通过相邻两次重构反演结束时的模型变化量之差来确定迭代重构是否终止.因为静磁场与重力场在数学上的相似性,本文的反演方法可以被运用到重力场等位场的地面数据的反演中.     

Mise-à-la-masse interpretation using a perfect conductor in a piecewise uniform earth

A useful analysis of the mise-à-la-masse problem can be made by considering a perfectly conducting orebody in a piecewise uniform conducting earth. While the use of a perfect conductor is clearly an idealization of the true geological conditions it provides several advantages for the present purpose.
The electric field associated with the above model can be expressed in terms of a surface integral of the normal potential gradient over the boundary of the conductor, where the normal gradient satisfies a well-posed Fredholm integral equation of the first kind. This integral equation formulation remains unchanged when the conductor is arbitrarily located in the conducting earth, including the important case when it crosses surfaces of conductivity discontinuity. Moreover, it is readily specialized to the important case of a thin, perfectly conductive lamina.
Consideration of the boundary value problem relevant to a conductive body fed by a stationary current source suggests that under certain circumstances, equivalent mise-à-la-masse responses will result from any perfect conductor confined by the equipotential surfaces of the original problem. This type of equivalence can only be reduced by extending the potential measurements into or on to the conductor itself.
This ambiguity in the interpretation of mise-à-la-masse surveys suggests a simple if approximate integral solution to the mise-à-la-masse problem. The solution is suitable for modelling the responses of perfect conductors and could possibly be used as the basis of a direct inversion scheme for mise-à-la-masse data.     

Penetration of electric field from the near ground atmospheric layer to the ionosphere

A mathematical model has been proposed for describing quasi-stationary atmospheric electric fields with approximate, but fairly accurate allowance for ionospheric conductivity. It is shown that some well-known models of electric field penetration from the Earth into the ionosphere have been deemed inadequate, though they work well in the atmosphere below 50 km. In these models, the arbitrarily specified boundary condition in the upper boundary of the atmosphere omits the existing good conductor or adds not existent conductor. The maximum possible field in our model is far less than in models where ionospheric conductivity is not taken into account, but vastly larger than in models based on the approximation with infinite Pedersen conductivity in the upper ionosphere.     

SOTEM数据一维OCCAM反演及其应用于三维模型的效果

本文基于垂直磁场分量研究了SOTEM数据的一维OCCAM反演方法,并将其应用于理论三维数据及野外实测数据的反演.对于大部分一维模型,OCCAM反演可取得较好的反演效果,且反演结果不依赖于偏移距;噪声对SOTEM数据的OCCAM反演具有较大影响,但当信号含噪水平不超过5%时,反演结果仍具有较好的准确性;若浅层存在较厚的低阻层,OCCAM反演结果对下部地层的分辨能力下降,仅能获得具有平均效应的电阻率.将一维算法应用于SOTEM三维数据的反演,会产生较大的误差,尤其是在异常体边缘地带影响最为严重.该影响程度与异常体和背景电阻率之间的差异有关,对于大多数电性近似呈连续变化的真实大地而言,一维OCCAM反演算法仍可获得较好的效果.最后通过陕西某煤田深部富水性调查的实测SOTEM数据反演验证了本文的研究成果.     

基于大地电磁测深的藏南雅鲁藏布江缝合带电性结构研究

为研究日喀则市活动断裂深浅部构造关系及深部孕震机制,跨雅鲁藏布江谢通门—日喀则段部署了48个宽频大地电磁测深点,剖面长度为108 km。在二维反演的基础上对壳幔200 km深度范围内的电性结构进行了探测研究。剖面自南向北依次经过喜马拉雅地块、雅鲁藏布江缝合带和拉萨—冈底斯地块。喜马拉雅地块地壳表现为高阻特性,其北侧的仲巴—郎杰学陆缘移置混杂地体发育了深达上地幔盖层的巨厚的北倾低阻体;雅鲁藏布江主缝合带表现为喇叭状低阻通道,宽约10 km,存在深浅部两处低阻体,浅部南倾深部北倾,低阻通道南部发育近似直立或南倾的高阻日喀则蛇绿岩,北部发育近直立的高阻冈底斯花岗岩体,整体表现为两个高阻异常体中间夹一个连通壳幔的带状低阻通道;拉萨—冈底斯地块以高阻为主,中下地壳普遍发育低阻体。缝合带附近因板块俯冲作用导致壳幔局部增厚或减薄,表现为电性的梯度变化,表现为低阻特性的部分则是壳幔物质的运移通道。      

地面可控源频率测深三维非线性共轭梯度反演

讨论了地面可控源电磁勘探三维非线性共轭梯度反演的可行性以及反演过程中考虑场源的必要性.反演采用非线性共轭梯度反演方法.反演过程中,模型响应利用交错网格有限差分技术计算.反演数据采用与发射源平行的电场x分量E_x.利用层状导电模型作为背景,设计了两个理论模型进行数值试验:第一个模型中包含两个电阻率异常,以检验反演的有效性;第二个模型中,在测区外设置了一个低阻异常,以考察源的信息在反演中的作用.两个模型的反演分别从层状背景模型开始,迭代120次后终止.数值试验结果表明,(1)非线性共轭梯度反演所获得的电阻率分布和理论模型吻合较好;(2)非线性共轭梯度算法收敛速度较慢,需要较多的迭代次数完成反演;(3)对于可控源频率电磁勘探,必须考虑源位置信息.因此,本文采用考虑场源信息的地面可控源非线性共轭梯度反演方法能完成真正意义上的可控源频率电磁测深数据的反演.