Controlled source electromagnetic deep sounding: Theory,results and correlation with natural source results

Controlled source electromagnetic (EM) methods represent a unique set of geophysical experiments which can be used to determine the properties and state of the deep continental crust. Unlike natural source EM methods, an artificial EM source technique can be designed to optimize resolution and minimize interactions with local structure. The major drawbacks include restricted depth penetration, insufficient data sets and a limited number of modelling algorithms. Information about the electrical conductivity at lower crustal depths can be obtained but at the expense of requiring large moment sources, sophisticated processing techniques and data redundancy. Moreover, EM data are sufficiently complicated that numerical modelling is often necessary for quantitative interpretation. It is therefore essential to record enough information to justify the choice of interpretation algorithms, particularly since controlled source EM forward modelling routines are generally very restrictive and not widely available. As most interpretations are based on layered earth models, observations of all the EM field components can be useful for testing the dimensionality of the data and for justifying interpretations.     

浅部频率域电磁勘探方法综述

适用于近地表(2000m以内)勘探的频率域电磁法主要有音频大地电磁法(audio-frequency magnetotellurics,AMT),无线电大地电磁法(radio-magnetotellurics,RMT),可控源音频大地电磁法(controlled source audiofrequency magnetotellurics,CSAMT),广域电磁法(Wide Field Electromagnetic Method,WFEM).本文拟从最新的数据采集技术、数据处理技术、正反演算法、实例等四个方面,论述适用于浅部勘探的AMT,RMT,CSAMT和WFEM方法的国内外最新进展,总结目前AMT,RMT,CSAMT和WFEM方法遇到的困难,对潜在的发展方向提出建议.综述表明:(1)张量测量、多站阵列、多站叠加可提高AMT、RMT和CSAMT数据的质量.利用近区数据WFEM法可获得良好的效果.国产与国外仪器在质量方面的差距正在逐步缩小.(2)数学形态滤波技术、Hilbert-Huang变换等可有效分离出有用的数据,局部畸变仍然是亟待解决的难题,需要更为深入的研究.(3)矢量有限元与非结构网格的出现大幅度提高了有限元处理复杂电磁问题模拟的精度与应用范围,成为目前电磁正演的首选工具.完全非线性反演算法仍然局限于1D、2D问题,共轭梯度法和高斯牛顿算法等为解决3D问题的发展趋势.地质约束的引入和多数据联合反演可以减小反演的非唯一性.各向异性的反演为目前反演研究的热点之一.(4)野外数据解释的正确性严重依赖于对地下结构先期的维性判别,在2D特性不明显、3D特性明显时,需要采用3D进行反演解释.     

Radioactivity and geochemistry of granulites and some related rocks from the saxonian granulite complex

Summary The distribution of radioactive(Th, U, K), major and selected trace(Rb, Sr, Ba, Y, Zr, V, Cr, Ni) elements of granulites from the Saxonian Granulite Complex was studied. Similarly to the South Bohemian granulites, the Saxonian granulites can be divided according to the contents of their major and trace elements into two main groups, groupA containing mostly acid and subacid granulites (K ₂ O>2.5%, SiO ₂ >68%), and groupB containing mostly intermediate and basic granulites (K ₂ O<2.5%, SiO ₂ <68%). Statistically significant differences between groupsA andB were found for all major oxides and several trace elements(Rb, V, Cr, Ni). The Saxonian granulites follow the same calc-alkaline trend as the South Bohemian, granulitesA being placed mostly in the rhyolite field and granulitesB mostly in the dacite, andesite and basalt fields of this trend. The investigated granulites are characterized by a considerable scatter ofTh andU contents accompanied by very variableTh/U ratios; theTh andU concentrations of granulitesA are substantially lower than is usual for rocks of corresponding acidity.

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2.5维井间电磁反演在中国东部孤岛油田的应用

In this study, we present a practical technique of transforming cross-hole EM data into the inter-well resistivity distribution. The a priori information constraint is incorporated into an iterative regularized inversion procedure and a variable roughness is added into the inversion process. Finite element approximation based on a two and a half-dimensional （2.5D） model has been developed for the forward problem and the ＂pseudo-forward＂ problem needed for constructing the sensitivity matrix and synthetic data set. The regularized least-squares inversion scheme, constrained with the a priori information obtained from well logs, was adopted to reconstruct the inter-well resistivity profile from two synthetic electromagnetic data sets and field data acquired in the Gudao Oil Field, East China. The partial derivatives of the sensitivity matrix were computed by the adjoint equation based on the reciprocity principle. Inversion results of the synthetic and field data examples suggest that our method is robust and stable in the presence of random noise in the field data and can be used for cross-hole EM field data interpretation.     

Mining,Environmental, Petroleum,and Engineering Industry Applications of Electromagnetic Techniques in Geophysics

Electromagnetic (EM) techniques are extremely important as a direct detection geophysical tool utilized in the base metal industry. They were developed in countries such as Canada, whose thin conductive weathering overburden did not hamper the penetration of EM signals and enabled exploration to depths on the order of 300 m. As a result, EM techniques were used widely in North America and Scandinavia for many years before they became common in countries with a thick conductive overburden, such as Australia. The 1980s and 1990s have seen the use of EM methods move from anomaly finding to mapping, as well as the development of better, faster and more accurate computer modelling algorithms. A review of EM papers, for the years 1998 to 2002, showed that most dealt with EM techniques as mapping tools. Airborne, ground and marine EM techniques are still being developed, as are data processing and interpretation software. The advent of robust 2-D and 3-D computer modelling and inversion algorithms has led to the acceptance of EM methods as a mapping tool for many environmental and petroleum industry applications, a trend which is expected to increase.     

Accuracy of integral migration transformation in dependence on space-time sampling

Summary The accuracy of wave field extrapolation is studied with respect to the discretization of field data and integral extrapolator. Assuming a far-field approximation of the Rayleigh-Sommerfeld solution for a two-dimensional scalar wave equation, the minimum and the maximum transmitted frequency are expressed as functions of the sampling intervals t, x, and the half-width x₀ and angle _a of the migration aperture. The theoretical limitation of the transmitted frequency band is tested on numerical examples.

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A Grid-Spectral Method of the Solution of the 3D Kinematic Geodynamo with the Inner Core

A 3D kinematic geodynamo model in a sphere with the conductive solid inner core is considered. The 3D magnetic field and velocity field are resolved in the physical space for r- and -coordinates, whereas the sin- and cos-decomposition is applied to the -coordinate. The additional boundary conditions for the case of non-zero velocity field on the boundaries of the liquid spherical shell and for different magnetic diffusivities of the inner and outer core are applied. The computer code was tested by free decay mode solutions and comparisons were made also with results reported by other authors. This work is a part of a project to study 3D inviscid geodynamo models.     

Großkreisdifferenzgleichen

Summary Echo soundings of the U.S. Cruiser Milwaukee in the Puerto Rico Trough in 1939 are briefly discussed, and two depths of 30246 feet or 9219 m, found at 19° 36 N, 68° 20.5 W and at 19° 35N, 68° 8.75W, are stated to be the greatest depths which are known so far in the Atlantic Ocean.     

The interaction of elastic waves with a solid-liquid interface,with applications to the core-mantle boundary

The paper discusses basic ideas and principles underlying methods, which have proved useful in the interpretation of diffraction and scattering phenomena by a smooth or slightly rough solid-liquid interface. Generally, the wave interaction may be formulated as an excitation problem; it involves (1) finding an equivalent dislocation or source distribution on the interface, and (2) evaluating the excited wave field. These steps are taken through perturbation theory and/or adopting the appropriate generalization of ray theory. In this context an explicit form of Green's function is also given. The methods have been applied to the core-mantle boundary, with a view toward the interpretation of recent data: (a) diffracted P and S waves around the core (in particular, their attenuation); (b) scattered short-period core phases (in particular, precursors to PKP and PKKP). Other types of wave interaction, and implications for models of the core-mantle boundary structure, are briefly mentioned.Paper presented at the EGS/ESC workshop on Generation and propagation of seismic waves in Neustadt, Federal Republic of Germany, August 1978.     

Two-dimensional, regularised inversion of VLF data

Very low frequency electromagnetic (EM) methods using VLF transmitters have found many applications in subsurface geophysical investigations. Surface measurements involving both the vertical component of the magnetic field (VLF-EM or VLF-Z) and of the apparent resistivity (VLF-R) are increasingly common. Although extensive VLF data sets have been successfully used for mapping purposes, modelling and interpretation techniques which asess the third (i.e. depth) dimension appear limited.Given a profile of VLF-R measurements the main purpose of the present study is to demonstrate an automatic method for the construction of a resistivity cross-section. The technique used is one of a new generation of regularised inversion methods. These techniques attempt to overcome the problem of equivalence/non-uniqueness in EM sounding data by constructing the resistivity distribution with the minimum amount of structure that fits the data.VLF data represent a special case of plane-wave EM sounding in that they conform, in practice, to a single-frequency technique. This fact imposes a limitation in the amount of vertical resolution that we can expect using such data. In the case of two-dimensional modelling and inversion, resolution through the cross-section is a resultant attribute from both vertical and lateral resistivity gradients within the subsurface. In order to provide insight into the practical application of regularised inversion techniques to VLF data, both synthetic and field examples are considered. Both sets of examples are primarily concerned with VLF data applied to near-surface fault mapping where the main aim is to assess the location, dip and depth extent of conductive subsurface features.     

Pitfalls of 1D inversion of small-loop electromagnetic data for detecting man-made objects

In order to interpret field data from small-loop electromagnetic (EM) instruments with fixed source–receiver separation, 1D inversion method is commonly used due to its efficiency with regard to computation costs. This application of 1D inversion is based on the assumption that small-offset broadband EM signals are insensitive to lateral resistivity variation. However, this assumption can be false when isolated conductive bodies such as man-made objects are embedded in the earth. Thus, we need to clarify the applicability of the 1D inversion method for small-loop EM data. In order to systematically analyze this conventional inversion approach, we developed a 2D EM inversion algorithm and verified this algorithm with a synthetic EM data set. 1D and 2D inversions were applied to synthetic and field EM data sets. The comparison of these inversion results shows that the resistivity distribution of the subsurface constructed by the 1D inversion approach can be distorted when the earth contains man-made objects, because they induce drastic variation of the resistivity distribution. By analyzing the integrated sensitivity of the small-loop EM method, we found that this pitfall of 1D inversion may be caused by the considerable sensitivity of the small-loop EM responses to lateral resistivity variation. However, the application of our 2D inversion algorithm to synthetic and field EM data sets demonstrate that the pitfall of 1D inversion due to man-made objects can be successfully alleviated. Thus, 2D EM inversion is strongly recommended for detecting conductive isolated bodies, such as man-made objects, whereas this approach may not always be essential for interpreting the EM field data.     

三维频率域航空电磁反演研究

航空电磁数据的三维解释由于数据量大需要有高效的反演算法作为支撑.本文利用两种目前主流的数值优化技术（非线性共轭梯度和有限内存的BFGS法）实现了三维频率域航空电磁反演,并进一步比较了两种方法的有效性和运算效率.在反演过程中,为了更好地反演异常体的空间位置,模型方差矩阵中的光滑系数在反演起始阶段取值较大;当数据拟合差下降趋于平缓时,再利用较小的光滑因子约束反演过程来实现聚焦和获得精确的反演结果.理论数据反演表明这两种优化策略具有相似的内存需求,但是有限内存的BFGS技术比非线性共轭梯度法在计算时间和模型反演分辨率上具有一定的优越性,因此有限内存BFGS法更适合于求解大规模三维反演问题. 模型试验进一步表明目前主流的迭代法求解技术不适合大规模航空电磁数据反演,未来移动平台多源电磁数据快速正反演可通过引入矩阵分解技术来实现.     

Model studies on electro-telluric interpretation problems of some geological inhomogeneities

Summary The rotating nature of telluric field is simulated in an electrolytic model tank. Variations of this rotating field due to the presence of a few two and three dimensional geological models, simulated in the tank, are studied. The telluric ellipses, recorded on a C.R.O. Screen, are found to be useful for qualitative and quantitative interpretation of the non-linearly polarised telluric field data. Relative usefulnesses of the various parameters, e.g., (i)M=ratio of the semimajor and semiminor axis, (ii)K = square root of normalized surface areas, (iii) = linear eccentricity ((a ²/b ²-1) and (iv)e = conventional eccentricity ((1-b ²/a ²)), of a telluric ellipse are tested. Linear eccentricity appears to be the most sensitive parameter. Methods for determining depth and horizontal extent of a structure from a telluric map are suggested. Near surface inhomogeneities, as revealed from the experiment are unlikely to affect the telluric map due to a basement structure.     

Three-dimensional em modeling

Three-dimensional (3D) interpretation of electromagnetic (EM) dsta is still in its infancy, due to a lack of practical numerical solutions for the forward problem. However, a number of algorithms for simulating the responses of simple 3D models have been developed over the last ten years, and they have provided important new insight. Integral equation methods have been more successful than differential equation methods, because they require calculating the electric field only in small anomalous regions, rather than throughout the earth. Utilizing a vector-scalar potential approach and incorporating symmetry through group theory improves the general 3D integral equation solution. Thin-sheet integral equation formulations have been particularly useful. Much recent research has focused on hybrid methods, which are finite element differential equation solutions within a mesh of limited extent, with boundary values determined by integrating over the interior fields. An elegant eigencurrent technique has been developed for calculating the transient response of a thin 3D sheet in free space, but general 3D time domain responses have only been calculated by Fourier transforming frequency domain results. Direct time domain calculations have been carried out only for 2D bodies.     

Tensional crack development in physical models with inhomogeneities under load

Summary The paper deals with a study of the mean stress field (_M) and its gradient (_M/x) in uniaxially loaded two-dimensional models of small thickness with respect to other dimensions, in which two inhomogeneities, e.g., a circular opening and a linear slit have been made. Particular attention was paid to the stress field development in the treated models durin gthe change of the mutual geometric configuration of both the inhomogeneities in question. The problem was analysed theoretically by means of Muskhelishvili's potentials of the two-dimensional theory of elasticity. To compare the results of calculations, the schlieren method was used to visualize and record the studied gradients of stress fields in physical models. The results of the calculations were calibrated by laboratory tests. In this way the individual model situations were classified from the point of view of their resistance to the applied load.     

Computation of the scattering functions of the haze from skylight measurements,considering the multiple scattering

Summary The primary scattering of atmospheric haze is computed by modification of an approximative method evolved byde Bary, basing on skylight measurements carried out on the Musalla, a mountain of 3000 m height in Bulgaria, at various altitudes of the sun and in different horizontal circles at a wavelength of 450 nm. This measured total radiation is reduced by the radiation induced by the scattering at molecules as well as by secondary or multiple scattering at turbidity aerosol. The single scattering of the atmospheric haze thus computed is discussed for various assumed Linke turbidity factors.The concluding deductions relate to a comparison of haze scattering functions with theoretical scattering functions, the results attained agreeing closely with superimposed logarithmic Gaussian distributions and only partly with the scattering functions computed byBullrich. These deductions prove optically the increase of larger particles (r ₀0.64 m) during forenoon. The analysis of the scattering functions conductes to a shortage of particles withr ₀=0.2 m in agreement with other investigations.

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Zusammenfassung In Abwandlung einer vonde Bary angegebenen Approximationsmethode wird die Primärstreuung des atmosphärischen Dunstes berechnet. Zugrundegelegt werden Himmelsllichtmessungen, die auf dem etwa 3000 m hohen Mus Alla in Bulgarien bei verschiedenen Sonnenhöhen und in verschiedenen Horizontalkreisen bei einer Wellenlänge von 450 nm durchgeführt wurden. Von dieser gemessenen Gesamtstrahlung werden die Anteile substrahiert, die durch Streuung an Molekülen sowie durch zwei-oder mehrfache Streuung an Trübungsaerosol hervorgerufen werden. Die so berechnete Einfachstreuung des atmosphärischen Dunstes wird für verschiednee angenommene Linke'sche Trübungsfaktoren diskutiert.Abschließend werden die Dunststreufunktionen mit theoretischen Streufunktionen verglichen, wobei eine gute Übereinstimmung mit überlagerten logarithmischen Gauß-Verteilungen und nur teilweise mit vonBullrich berechneten Streufunktionen erzielt wurde. Es kann dabei optisch die Zunahme großer Teilchen (r ₀0,64 m) im Laufe des Vormittags gezeigt werden. Die Analyse der Streufunktionen führt zu einem Fehlen von Teilchen mitr ₀=0,2 m in Übereinstimmung mit anderen Untersuchungen.

     

地球电磁感应研究的新进展

地球电磁感应方法的应用研究在陆地上不断深化的同时,正在向海洋区和大陆边缘扩展,各种电磁方法的观测、资料处理和解释技术正在提高到新的水平,模型和反演技术的研究正在开创一些新的思路,畸变效应及消除问题更加引起人们的注意.     

基于高斯牛顿法的频率域可控源电磁三维反演研究

三维反演解释是电磁法勘探发展的重要趋势,而如何提高三维反演的可靠性、稳定性和计算效率是算法开发者们目前的研究重点.本文实现了一种频率域可控源电磁(CSEM)三维反演算法.其中正演基于拟态有限体积法离散化,利用直接矩阵分解技术来求解大型线性系统方程,不仅准确、稳定,而且特别有利于含有大量发射场源位置的CSEM勘探情况;对目标函数的最优化采用高斯牛顿法(GN),具有近似二次的收敛性;使用预条件共轭梯度法(PCG)求解每次GN迭代所得到的法方程,避免了显式求解和存储灵敏度矩阵,减小了计算量.以上这些方法的结合应用,使得本文的三维反演算法准确、稳定且高效.通过陆地和海洋CSEM勘探场景中的典型理论模型的反演测试,验证了本文算法的有效性.     

Active and laboratory experiments in space plasma physics

This report focuses on active experiments in space as well as laboratory experiments which are directly related to space phenomena. The active experiments involve particle releases in the CRESS and AMPTE missions. Unexpected results such as the motion of the plasma triggered several interesting computer simulations and a laboratory experiment which will be reviewed. Critical Ionization Phenomena will be discussed in this context. A recent release over Arecibo was used to create a plasma lens to focus radiation from the HF heater beam. The higher power triggered several nonlinear processes. Advances in plasma sources and diagnostics have made it possible to scale many laboratory experiments. Discussed here are experiments on Alfvén waves generated by a localized source, 3D magnetic field line reconnection, three dimensional magnetized double layers and simulations of a tethered satellite.     

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.