Global electromagnetic induction

Methods of analysis of long period geomagnetic variations (periods over a few hours), the available electromagnetic response function estimates, and the effect of lateral inhomogeneity within the Earth are reviewed. Recent advances in the inversion of response function data to produce conductivitydepth profiles are mentioned, and aspects of the inverse problem specific to global (spherical Earth) induction are discussed.There is a rapid rise in electrical conductivity between about 400 km and 800 km, but whether this is a gradual change or consists of one or several steps is not resolvable with the presently available data and naive inversion algorithm used here. At the greatest depths to which presently analysed variations penetrate (1000–1500 km), available data have some difficulty in resolving finer structure, but there are indications that the electrical structure of the continents becomes more laterally homogeneous as depth increases. Previously published inversions see lateral variations in electrical structure down to at least 500 km, and at shallower depths these variations are unambiguously resolved.     

电磁测深数据地形影响的快速校正

地形起伏会对电磁法的数据产生一定影响,尤其会影响浅部地层电性结果的准确性.本文通过对地形影响基本规律、经典比值校正原理的分析,认为可以用实测标准电阻率替代数值模拟中均匀半空间电阻率,提出一种新的地形影响快速校正方法,即采用小极矩直流电阻率法获得无地形影响的表层电阻率值,作为地形校正的标准电阻率,以此构造一个新的校正公式.分别对CSAMT和TEM仿真和实际测量资料进行地形校正处理,效果较好.说明新的比值校正公式,是一种快速、有效和实用的校正算法.     

Marine electromagnetic induction studies

In reviewing seafloor induction studies conducted over the last seven years, we observe a decline in single-station magnetotelluric (MT) experiments in favour of large, multinational, array experiments with a strong oceanographic component. However, better instrumentation, processing techniques and interpretational tools are improving the quality of MT experiments in spite of the physical limitations of the band limited seafloor environment, and oceanographic array deployments are allowing geomagnetic depth sounding studies to be conducted. Oceanographic objectives are met by the sensitivity of the horizontal electric field to vertically averaged motional currents, providing the same information, at much greater reliability and much lower cost, as an array of continuously operating current meter moorings.The seafloor controlled source method has now become, if not routine, at least viable. Prior to 1982, only one seafloor controlled source experiment has been conducted; now at least three groups are involved in the experimental aspects of this field. The horizontal dipole-dipole configuration is favoured, although a variant of the magnetometric resistivity method utilising a vertical electric transmitter has been developed and deployed. By exploiting the characteristics of the seafloor environment, source receiver spacings unimaginable on land can be achieved; on a recent deployment dipole spacings of 90 km were used with a clear 24 Hz signal transmitted through the seafloor. This, and prior experiments, show that the oceanic upper mantle is characteristically very resistive, 10⁵ m at least. This resistive zone is becoming apparent from other experiments as well, such as studies of the MT response in coastal areas on land.Mid-ocean ridge environments are likely to be the target of many future electromagnetic studies. By taking available laboratory data on mineral, melt and water conductivity we predict to first order the kinds of structures the EM method will help us explore.     

Direct and inverse problems in local electromagnetic induction

Various methods of solving direct and inverse problems in local electromagnetic induction are presented. In the section dealing with direct problems some improvments are suggested in the finite difference method in the case of two-dimensional modeling. Two ways of dealing with inverse problems are presented, the first continous, the other parametric. Emphasis is laid upon algebraic aspects of dealing with one-dimensional inverse problems.     

Global electromagnetic induction in the Moon and planets

A summary of experiments and analyses concerning electromagnetic induction in the Moon and other extraterrestrial bodies is presented. Magnetic step-transient measurements made on the lunar dark side show the eddy current response to be the dominant induction mode of the Moon. Analysis of the poloidal field decay of the eddy currents has yielded a range of monotonic conductivity profiles for the lunar interior: the conductivity rises from 3·10^?4 mho/m at a depth of 170 km to 10^?2 mho/m at 1000 km depth. The static magnetization field induction has been measured and the whole-Moon relative magnetic permeability has been calculated to be $\text{μ}\text{μ}{}_0\text{= 1.01 ± 0.06}$. The remanent magnetic fields, measured at Apollo landing sites, range from 3 to 327 γ. Simultaneous magnetometer and solar wind spectrometer measurements show that the 38-γ remanent field at the Apollo 12 site is compressed to 54 γ by a solar wind pressure increase of 7·10^?8 dyn/cm². The solar wind confines the induced lunar poloidal field; the field is compressed to the surface on the lunar subsolar side and extends out into a cylindrical cavity on the lunar antisolar side. This solar wind confinement is modeled in the laboratory by a magnetic dipole enclosed in a superconducting lead cylinder; results show that the induced poloidal field geometry is modified in a manner similar to that measured on the Moon. Induction concepts developed for the Moon are extended to estimate the electromagnetic response of other bodies in the solar system.     

Inversion problem in regional electromagnetic induction studies

For completeness of the theory proposed by Oni (1972), the inversion problem is examined. It is shown that parameters which constrain distinct solutions of the inverse problem can be determined in the application of the theory. The fundamental basis of the inversion and the strategy to be adopted are discussed.     

An introduction to electromagnetic induction in the ocean

The governing equations for the induction of electromagnetic fields in the ocean by ionospheric and oceanic sources are presented. A uniformly conducting layered model and a nonuniformly conducting thin-sheet model are discussed with reference to the interpretation of fields observed in the ocean. A procedure for the separation of the electric field continuum into parts of ionospheric and oceanic origin is presented.     

Theoretical models for electromagnetic induction in the oceans

The different methods and techniques employed in the theory of electromagnetic induction in thin sheets are reviewed and the methods for approximation to the solution are indicated. These depend on whether the sheet is closed or finite and on whether the integrated conductivity and/or the frequency of variations is high or low.Results for induction in finite sheets which are suitable for ocean modelling are given. These include sheets of perfect conductivity and sheets of finite conductivity which is either discontinuous or continuous at the boundary. The dependence of the “coastline effect” for a global ocean on the location of the edge of the continental shelf, the period of variation of the external field and the conductivity of the underlying earth is explained.     

Near-surface models in Saudi Arabia

A single‐layer model of the near surface throughout the Kingdom of Saudi Arabia is available. While this simple model suffices for most areas and large subsurface structures, it fails in situations where the surface topography is complex, the base of weathering is below the datum, or where the time structural closure is less than the uncertainty in the static correction. In such cases, multiple‐layered models that incorporate velocities derived from analysis of first arrivals picked from seismic shot records have proved to be successful in defining the lateral heterogeneity of the near surface. The additional velocity information obtained from this first‐arrival analysis (direct as well as refracted arrivals) vastly improves the velocity–depth model of the near surface, regardless of the topography. Static corrections computed from these detailed near‐surface velocity models have significantly enhanced subsurface image focusing, thereby reducing the uncertainty in the closure of target structures. Other non‐seismic methods have been used either to confirm qualitatively or to enhance the layer models previously mentioned. Gravity data may be particularly useful in sandy areas to confirm general structure, while geostatistical modelling of vibrator base‐plate attributes has yielded information that enhances the velocity field. In the global context, exploration targets of the oil and gas industry are seeking smaller and lower relief‐time structures. Thus, near‐surface models will need to enhance and integrate these methods, particularly in areas where the assumption of flat‐lying near‐surface layers cannot be met.     

A numerical study of the topographic effect on electromagnetic fields in a three-dimensional conductivity model

Summary The topographic effect on electromagnetic fields in a three-dimensional conductivity model is investigated using a finite-difference numerical method. The model is that of a conducting block buried in the host Earth beneath a conducting hill. The altitude effect as well as the effect of conductivity of the hill has been studied by comparing the results for the field components at the surface of the hill with those at the surface of a flat half-space. The results indicate that both altitude and the hill conductivity influence the behaviour of the electromagnetic fields. For certain traverses beyond the edge of the block, it is seen that the variations in some field components are mainly due to the topographic effect and that the contribution from the block itself is negligible.     

2.5维起伏地表条件下频率域地空电磁正演模拟

频率域地空电磁探测方法是指在地面布设人工场源,在空中测量电磁场的一种高效的地球物理勘探技术.该方法具有大范围、高通过性、快速测量的优势,尤其适合崎岖山地、沙漠、沼泽、海陆交互带等复杂地貌区域的资源勘查.但是这些地区的地形起伏通常较大,因此分析地形对地空电磁响应的影响具有重要意义.本文利用有限元法对频率域地空电磁响应进行了正演计算,分析了起伏地表条件下的频率域地空电磁响应特征.首先利用傅里叶变换将2.5维问题转化成二维问题,利用伽辽金加权余量法推导了相应的离散有限元方程组.采用任意四边形单元对区域进行不均匀网格剖分,源和异常体附近网格加密处理,保证计算精度,远离目标区域网格逐渐稀疏,模拟无穷远边界,降低对计算资源的要求.在单元内进行插值,将有限元方程组变换为线性方程组,采用总场算法,利用具有一定面积的伪δ函数表达源电流分布,源项近似为分布在以电偶极源为中心的25个节点上.通过求解线性方程组得到波数域电磁响应,再对波数域电磁场响应进行反傅里叶变换从而获得空间域2.5维频率域电磁场值.通过对比2.5维正演结果与均匀半空间解析解,验证了本文算法的精度,同时本文还对地空电磁场与地面电磁场的响应特...     

On the inversion of global electromagnetic induction data

The initial phase of any inversion of geophysical data must examine the question of the existence of globally distinct solutions. Previous inversion st point of view. A basic inversion strategy for geophysical data is considered. It is concluded that future progress depends on the use of synthetic data to resolve questions about the potential constraining power of GEMI data.     

Unexploded ordnance discrimination using time-domain electromagnetic induction and self-organizing maps

Self-organizing maps (SOM) are implemented for discrimination of geologic noise, buried metal objects and unexploded ordnance using the geophysical method of time-domain electromagnetic induction. The learning and misfit measures are based on a Euclidean metric. The U*-matrix method is shown to be a reliable tool for determining data clusters and cluster boundaries. The performance of SOM for data-type discrimination was tested using three synthetic, idealized geophysical datasets consisting of exponential, multi-exponential and stretched-exponential decaying transients. In addition, experimental data were acquired using a modified Geonics EM63 instrument. Results from the synthetic examples show that SOM clusters the data based on their functional origin, when represented using U*-matrices. The percentage of correct classification is 100%. Unsupervised learning using the field dataset obtained with the Geonics EM63 succeeded in producing a multi-clustered map in which the background transients cluster themselves and are separated from clusters associated with metal clutter objects and UXO. Even though in some cases the SOM did not produce a single cluster for each type of causative body, it was able to separate clutter data from target data by producing several small clusters. The results are encouraging in view of the heterogeneity and sparsity of the training dataset.     

Spectral theory of electromagnetic induction in a radially and laterally inhomogeneous earth

Summary The partial differential equations of electromagnetic induction in a 3-D Earth of inhomogeneous conductivity are reduced to a system of ordinary differential equations of the 2nd order for the spectral coefficients of the field.

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Terminology and symbols for use in studies of electromagnetic induction in the Earth

Induction studies currently suffer from a bewildering number of terms describing the same phenomenon, by definitions which are incompatible and by diverse and confusing presentation methods. IAGA Working Group I-2 (Electromagnetic Induction in the Earth) established an ad hoc Semantics Committee charged with proposing a set of standard terminologies and presentation methods for use in geomagnetic induction studies. The main text of this paper details the proposed standards following much involvement with the induction community. Background information detailing this involvement is given in a series of appendices.     

Generalised relative and cumulative response functions for electromagnetic induction conductivity meters operating at low induction numbers

Relative and cumulative analytical response functions have been widely used as a powerful tool for forward modelling and interpretation of measurements obtained by electromagnetic induction conductivity meters operating at low induction numbers for one‐dimensional layered earth models. These well‐known functions were derived and should be used for the instruments laid on the surface of the earth. In this paper, we extended the response functions and obtained new generalised analytical expressions, which can be used for instruments carried at any height from the surface. The proposed new equations were compared with numerically constructed functions, obtained using the full solution of Maxwell's equations, and proved to be in very good agreement at low induction numbers. Quantitative analyses of the behaviour of the relative response and the depth of investigation of electromagnetic induction instruments, when raised from the ground, could also be done using the generalised functions.     

Analyzing electromagnetic induction data: Suggestions from laboratory measurements

Recent inversions of electrical profiles of the upper mantle beneath the oceans permit a variety of conductivity-depth profiles ranging from models with monotonically increasing conductivity to layered models having decreases of conductivity with depth. Laboratory data on possible mantle materials can physically explain high mantle conductivities in terms of a fluid phase (partial melt, hydrous fluid) or a good solid conductor (amorphous or graphitic carbon) and favor a profile having a high conductivity layer (HCL) underlain by a more resistive layer.     

On some effects connected with electromagnetic induction in a rotating earth

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An analogue model study of electromagnetic induction in the Tasmania region

Laboratory analogue model magnetic measurements are carried out for a model of the region including Tasmania, Bass Strait with its highly conductive deep sedimentary basins, and the south coast of mainland Australia. The model source frequencies used simulate naturally occurring geomagnetic variations of periods 5–120 min. In-phase and quadrature magnetic H_x, H_y and H_z field measurements for the modelled region are presented for an approximately uniform overhead horizontal source field for E-polarization (electric field of the source in the N-S direction) and for H-polarization (electric field of the source in the E-W direction). Large anomalous in-phase and quadrature model magnetic fields are observed over Bass Strait and the coastal regions at short periods for both E- and H-polarization, but with increasing period, the field anomalies decrease more rapidly for E-polarization, than for H-polarization. The difference in response with polarization for the Bass Strait region is attributed to current induced in the deep ocean, for all periods, being channelled through Bass Strait for H-polarization but not for E-polarization. The persistent large coastal field anomalies elsewhere, for H-polarization, can be accounted for by the coastal current concentrations due to currents induced in the deep ocean for all periods deflected to the south and to the north by the shelving sea-floor and channelled through Bass Strait and around the southern coast of Tasmania. The phenomena of current deflection and channelling for H-polarization for the geometry of the southern Australia coastline and associated ocean bathymetry is particularly effective in producing field anomalies for a large period range.The coastal horizontal H_x and H_y field anomalies, present for E-polarization at short periods and for H-polarization at all periods, do not extend far inland, and thus, for inland station sites somewhat removed from the coast, should not present serious problems for magnetic soundings in field work. The sharp vertical field (H_z) gradient over Tasmania at short periods, which is predominantly in the E-W direction for E-polarization and the N-S direction for H-polarization, is strongly frequency dependent, becoming almost undetectable at 60 min. The behaviour of the H_z field gradients, however, are very similar from traverse to traverse over inland Tasmania, and thus, the effects of the ocean should not present too serious a problem in the interpretation of field station studies. The discrepancies between model and field station results should be useful in mapping geological boundaries in the region.     

Techniques and instrumentation for study of natural electromagnetic induction at sea

Electromagnetic fluctuations in the ocean have external sources above (ionospheric) and below (secular variation of the earth's magnetic field), and internal, purely oceanic sources associated with interaction between water velocity fields and the earth's field. Energy diagrams indicative of the electromagnetic activity in the sea are presented. From the latter, estimates of the resolution required in electromagnetic research at sea can be made. Absolute minima of 1 γ and 0.05 μV/m are necessary for magnetic and electric fields, respectively. Because the ocean shields overhead sources at frequencies above a few hundred c/h and because motional fields have weak signatures, a resolution at least 10 times higher would considerably enhance the scope of such research.The response of electric field instruments to motionally induced fields depends upon whether they are fixed or drifting, but both types respond similarly to fields of external origin.The most stringent limitation to electric field sampling in the sea is the difficulty in achieving low-noise electrical continuity between measuring circuits and sea water. Even the best matched silver—silver chloride electrodes introduce variable electrochemical signals hard to maintain below a millivolt. These mask very low frequency signals unless sophisticated techniques such as electrode switching are used.