Transient electromagnetic surveys with unimodal transverse magnetic field: ideas and results

The theory behind transient electromagnetic surveys can be well described in terms of transverse magnetic and transverse electric modes. Soundings using transverse magnetic and transverse electric modes require different source configurations. In this study, we consider an alternating transverse magnetic field excitation by a circular electric dipole. The circular electric dipole transmitter is a horizontal analogue of the vertical electric dipole. Offshore surveys using circular electric dipole might represent an alternative to the conventional marine controlled‐source electromagnetic method at shallow sea and/or for exploring relatively small targets. Field acquisition is carried out by recording either electric or magnetic responses. Electric responses bear information on the 1D structure of a layered earth and successfully resolve high‐resistivity targets in marine surveys. Land‐based circular electric dipole soundings are affected by induced polarisation. On the contrary, magnetic responses are absent on the surface of a 1D earth, and as a result, they are very sensitive to any and even very small 3D conductivity perturbations. In addition, they are sensitive to induced polarisation or some other polarisation effects in the subsurface. At present, circular electric dipole transmitters and magnetic receivers are successfully used in on‐land mineral and petroleum exploration.     

Numerical analysis of multicomponent responses of surface-hole transient electromagnetic method

We calculate the multicomponent responses of surface-hole transient electromagnetic method. The methods and models are unsuitable as geoelectric models of conductive surrounding rocks because they are based on regular local targets. We also propose a calculation and analysis scheme based on numerical simulations of the subsurface transient electromagnetic fields. In the modeling of the electromagnetic fields, the forward modeling simulations are performed by using the finite-difference time-domain method and the discrete image method, which combines the Gaver–Stehfest inverse Laplace transform with the Prony method to solve the initial electromagnetic fields. The precision in the iterative computations is ensured by using the transmission boundary conditions. For the response analysis, we customize geoelectric models consisting of near-borehole targets and conductive wall rocks and implement forward modeling simulations. The observed electric fields are converted into induced electromotive force responses using multicomponent observation devices. By comparing the transient electric fields and multicomponent responses under different conditions, we suggest that the multicomponent-induced electromotive force responses are related to the horizontal and vertical gradient variations of the transient electric field at different times. The characteristics of the response are determined by the varying the subsurface transient electromagnetic fields, i.e., diffusion, attenuation and distortion, under different conditions as well as the electromagnetic fields at the observation positions. The calculation and analysis scheme of the response consider the surrounding rocks and the anomalous field of the local targets. It therefore can account for the geological data better than conventional transient field response analysis of local targets.     

The application of spatial derivatives to non‐potential field data interpretation

Source/body edge detection is a common feature in the processing and interpretation of potential field data sets. A wide range of spatial derivatives is available to enhance the information contained in the basic data. Here the ability of these procedures to assist with the mapping interpretation of non‐potential field data is considered. The study uses airborne electromagnetic (conductivity) data but also provides a general context for other conductivity/resistivity data, provided the non‐potential field nature of active and thus spatially‐focused, measurements is acknowledged. The study discusses and demonstrates the application of a range of common spatial derivative procedures, including the analytic signal and upward continuation, to both magnetic and conductivity data. The ability of the tilt derivative to provide enhanced mapping of conductivity data is considered in detail. Tilt and its associated functions are formed by taking combinations of vertical and horizontal derivatives of the data set. Theoretical forward modelling studies are first carried out to assess the performance of the tilt derivative in relation to the detection and definition of concealed conductivity structure. The tilt derivative embodies automatic gain control that normalizes the detection and definition of both weak and strong conductivity gradients across an appropriate subsurface depth range. The use of high‐order spatial derivatives inevitably results in a degree of noise (cultural perturbation) amplification that is survey and technique specific. Both of these aspects are considered using practical case studies of jointly obtained magnetic and conductivity data at a variety of spatial scales.     

Three-Dimensional Modeling of the Casing Effect in Onshore Controlled-Source Electromagnetic Surveys

The presence of steel-cased wells and other infrastructure causes a significant change in the electromagnetic fields that has to be taken into consideration in modeling and interpretation of field data. A realistic and accurate simulation requires the borehole casing to be incorporated into the modeling scheme, which is numerically challenging. Due to the huge conductivity contrast between the casing and surrounding media, a spatial discretization that provides accurate results at different spatial scales ranging from millimeters to hundreds of meters is required. In this paper, we present a full 3D frequency-domain electromagnetic modeling based on a parallel finite-difference algorithm considering the casing effect and investigate its applicability on the borehole-to-surface configuration of the Hontomín CO₂ storage site. To guarantee a robust solution of linear systems with highly ill-conditioned matrices caused by huge conductivity contrasts and multiple spatial scales in the model, we employ direct sparse solvers. Different scenarios are simulated in order to study the influence of the source position, conductivity model, and the effect of the steel casing on the measured data. Several approximations of the real hollow casing that allow for a large reduction in the number of elements in the resulting meshes are studied. A good agreement between the modeled responses and the real field data demonstrates the feasibility of simulating casing effects in complex geological areas. The steel casing of the well greatly increases the amplitude of the surface electromagnetic fields and thus improves the signal-to-noise ratio and the sensitivity to deep targets.     

电性各向异性地层频率域电磁响应模拟

在现有计算机内存和运算速度条件下,电性各向异性地层中频率域电磁响应的数值模拟是一个难点. 为探索各向异性介质模型电磁响应的有效算法,同时为进一步研究各向异性地层测量响应的校正方法奠定基础,从Maxwell电磁响应方程出发,使用由三个彼此垂直的发射线圈和三个彼此垂直的接收线圈组成的线圈结构模型,应用交错网格有限差分法推导了三维频率域电磁响应的差分计算格式,采用预条件双共轭梯度法对所形成的复系数矩阵进行了迭代求解,从而得到电性各向异性地层模型下的线圈测量响应. 应用这些方法进行了多种条件下电磁响应的计算,计算结果表明,所形成的计算方法是准确可靠的,预条件双共轭梯度法的运算效率也较高;电性各向异性地层的电磁响应与地层倾角（或井斜角）、仪器测量方位、地层的水平电阻率和垂直电阻率有关;根据水平方向磁场强度分量可以确定地层界面;在电磁响应计算过程中,背景电导率的取值要尽量接近整个计算区域的平均电导率.     

Evaluation of the feasibility of recovering the magma chamber’s parameters by 3D Bayesian statistical inversion of synthetic MT data

Feasibility of recovering the magma chamber’s parameters by 3D Bayesian statistical inversion of magnetotelluric data is estimated for the simplified conductivity model of the Vesuvios volcano. The results indicate that in the lack of prior information and data, the most efficient approach may consist in successive estimation of the geometry and the depth of the anomaly followed by estimation of the electric conductivity distribution in it. The horizontal boundaries of the target could be outlined by the high gradients of the impedance determinant phase pseudosections determined by the upward analytical continuation of the anomalous electromagnetic fields from the relief surface to the artificial reference plane located above the summit of the volcano. The vertical boundaries and the target extension as well as the electric conductivity could be estimated successively by means of 3D Bayesian statistical inversion of the collected magnetotelluric data carried out in the domain delimited by the estimated horizontal boundaries.     

一维电阻率各向异性对海洋可控源电磁响应的影响研究

地下介质的电阻率常常表现为各向异性, 海底裂隙地层和层状沉积序列可能形成宏观电阻率各向异性.在解释海洋电磁资料时, 电阻率各向异性的影响不应该被忽略, 否则可能会得到错误的海底地电模型.作者编写了电阻率任意各向异性一维层状介质海洋可控源电磁场计算程序, 计算了电阻率各向异性层状模型的海洋可控源电磁响应, 讨论了覆盖层和高阻储层分别具有电阻率各向异性时的电磁场响应特征.     

THE EFFECT OF A SUPERPARAMAGNETIC LAYER ON THE TRANSIENT ELECTROMAGNETIC RESPONSE OF A GROUND*

A thin superparamagnetic layer on the earth's surface greatly affects the transient electromagnetic response of a conducting ground. The effect of the layer is most evident for singleloop transient electromagnetic data where transient voltages decay as 1/t. Even when a separate transmitter and receiver are used, the effect of the superparamagnetic layer is still pronounced. In this case the effect of the 1/t term in the equation is much less. More dominant now is a 1/t² term. The effect of the superparamagnetism can readily be seen in the analytical expressions for the apparent resistivities. If the presence of the superparamagnetic layer is not recognized, then the apparent resistivities decrease with time rather than approach the true value of the host rock.     

层状介质中水平磁偶极子电磁场空间分布特征

针对岩层中介电常数和磁导率随深度变化的情况,给出水平磁偶极子源电磁场的水平或垂直空间分布特征及相应的变化规律。采用Kong给出的汉克尔(Hankel)J₀变换线性滤波器(241点)和汉克尔J₁变换线性滤波器(241点)算法,选用均匀大地和两层地电模型,讨论介质的介电常数和磁导率变化时的水平磁偶极子电磁场空间分布特征。计算结果表明,磁场的H_x分量幅值与磁导率反相关;磁场H_z和电场E_y分量随着磁导率的增大而增大;低频电磁测深中介电常数变化时对电磁场基本没有影响。     

我国深地资源电磁探测新技术研究进展

诸多研究表明我国深部资源潜力巨大,但目前的开发开采深度普遍停留在500 m以浅,开展"攻深探盲"是构建国家资源安全体系的有效途径.应用最先进的科学技术手段,提取深部地质信息,已成为我国当前地球物理科学研究的发展方向.作为地球物理学的重要分支,电磁法是矿产资源探查的主体手段之一.在分析我国现阶段航空、地面及海洋电磁探测技术进展的基础上,本文重点说明了极低频电磁法（简称WEM法）,多通道瞬变电磁法（简称MTEM）和电性源短偏移瞬变电磁法（简称SOTEM）等电磁探测新技术.WEM法建立一套包括岩石层、大气层和电离层在内的全空间电磁传播理论,通过新研制的观测系统,获取地下10 km的地电信息;MTEM方法是地下埋深4 km目标体精细勘查的有效手段;SOTEM实现地下1.5 km深度范围内目标体的精细探测.通过多种电磁探测技术组合,可实现地下10 km深度范围内多尺度探测,达到"望远镜+放大镜+显微镜"探测效果.同时,本文指出进一步研发与新方法配套的装备、资料处理技术和大数据人工智能识别等将是我国电磁法未来的发展方向.     

我国海底大地电磁探测技术研究的进展

几年以前 ,我国尚未独立开展过海洋电磁探测 ,没有获取海洋岩石层电性资料的技术手段。然而 ,地下岩层的电性参数较之其他物性参数能更好地反映岩石的性质以及岩石所处的物理状态 ,因此迫切需要发展海底大地电磁探测技术。海底大地电磁探测是把仪器布置在海底 ,采集海底大地电磁场数据 ,从而研究海底以下不同深度上介质导电性的分布规律 ,达到了解地下不同深度地质情况的目的。经过两年的努力 ,我们已基本实现了海底大地电磁探测技术的研究目标     

水平层状大地中任意方向偶极源激发的电磁场

目前已存在多种物理模型用于解释地震前兆电磁异常,不同机制下产生电磁波场的源可等效为不同类型偶极源（磁型/电型）的组合.针对不同偶极源激发的电磁响应特征的研究对地震孕育过程中观测的电磁信号的提取与解释具有重要意义.基于此,本文推导了一套正演算法,可用于研究任意方向（方位角和倾角）、大小和位置的一个或多个偶极源在地壳或岩石圈-大气圈-电离层（Lithosphere-Atmosphere-Ionosphere,LAI）模型中产生的电磁波场的传播特征.基于多源组合,该算法可进一步拓展到线电流源和面电流源的电磁响应的计算.本文给出了水平、垂直和倾斜电偶极源响应计算的实例,并针对不同类型的偶极源激发下的地中、地面以及空间电磁场响应特征进行了初步分析.模拟结果显示,相比于单纯水平或垂直的电偶极源,地中倾斜电偶极源激发的电磁响应在地表的各场量辐射花样虽然仍可反映偶极子的方位角和空间位置,但不再具有对称性,其模式更复杂.本文提出的正演算法可用于地中低频电磁辐射源的正演模拟和反演定位.

     

Large-scale ionospheric gradients over Europe observed in October 2003

It is well known that ionospheric perturbations are characterised by strong horizontal gradients and rapid changes of the ionisation. Thus, space weather induced severe ionosphere perturbations can cause serious technological problems in Global Navigation Satellite Systems (GNSS) such as GPS. During the severe ionosphere storm period of 29–31 October 2003, reported were several significant malfunctions due to the adverse effects of the ionosphere perturbations such as interruption of the WAAS service and degradation of mid-latitudes GPS reference services. To properly warn service users of such effects, a quick evaluation of the current signal propagation conditions expressed in a suitable ionospheric perturbation index would be of great benefit. Preliminary results of a comparative study of ionospheric gradients including vertical sounding and Total Electron Content (TEC) data are presented. Strong enhancements of latitudinal gradients and temporal changes of the ionisation are observed over Europe during the 29–30 October storm period. The potential use of spatial gradients and rate of change of foF2 and TEC characterising the actual perturbation degree of the ionosphere is discussed. It has been found that perturbation induced spatial gradients of TEC and foF2 strongly enhance during the ionospheric storm on 29 October over the Central European region in particular in North–South direction exceeding the gradients in East–West direction by a factor of 2.     

地面瞬变电磁法（TEM）研究进展

较详细地介绍了近年来国内外地面瞬变电磁法（TEM）的基本原理、特点、影响因素、测量仪器、数据处理、数据解译（数据的正演和反演）方面的研究状况和所取得的进展,简要介绍了其应用前景,由于具有探测深度大、空间分辨率高等优势,TEM在地质灾害、建筑工程、水文地质工程特别是在深部找矿等方面得以广泛应用。     

VLF RESISTIVITY MAPPING AND VERTICALIZATION OF THE ELECTRIC FIELD1

For over 20 years, powerful VLF transmitters have been used as electromagnetic sources for subsurface investigations in mining exploration. Measurements initially concerned the vertical component of the magnetic field or the inclination of the field and were later extended to measurement of the horizontal electric field in the direction of the transmitter, to determine the resistivity of the terrain. Measurement of the electric field is usually performed with electric lines, grounded or not, with lengths of at least 5 m. This paper presents the concept of a VLF resistivity meter with a very short electric sensor (1 m) and the results obtained with it. This technique improves the measurement of the electric field, which is in principle a point value. It also permits a higher spatial sampling rate and, by closely linking the electric sensor with the magnetic sensor on a lightweight mount, makes it possible for the instrument to be used by a single operator. In addition, transformation of the electric field data, analogous to reduction to the pole in magnetism, is proposed to correct the horizontal deformation of the anomalies created by polarization of the primary field. Comparison with direct current electrical measurements shows highly satisfactory correlations. This transformation, validated for VLF, can be extended to any electrical or electromagnetic method using a uniform primary field, i.e. gradient array in direct current or low-frequency magnetotellurics. We call this verticalization of the electric field. Resistivity measurements and mapping using the VLF frequency range can be applied not only to mining but also to a wide range of shallow geophysical studies (hydrology, civil engineering, etc.) and are not limited to problems concerning the location of conductive targets     

The observation and interpretation of geomagnetic variations in a small area of Hainan Province,China

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Development and evaluation of airborne microwave refractometer for studies on atmospheric tropical boundary layer radiorefractive index

Observations of high-resolution data on radio refractivity were obtained by the airborne microwave refractometer over the Indian sub-continent (a tropical country) from 1971 to 1988. Detailed vertical and horizontal distributions of radio refractivity on a near-real-time basis in the atmospheric boundary layer were determined. Radiosonde observations cannot detect the thin refractivity gradients which characterize the propagation environment in this low-altitude region. This knowledge is required to design reliable and efficient communication systems for strategic, tactical and operational needs. However, the results demonstrate the layer structures and the variability of the boundary layer in time and space. The radio refractive effects on electromagnetic propagation and the future direction of radio refractivity fine-structure measurements are discussed.     

Petroleum Electromagnetic Prospecting Advances and Case Studies in China

Remarkable progress has been made in electromagnetic (EM) techniques as applied to the petroleum industry in instruments, data acquisition, and processing and interpretation in China. Included here is equipment, such as high-power Controlled Source EM (CSEM) acquisition systems, acquisition methods, such as the three dimensional small-bin Continuous Electromagnetic Array acquisition method, Time and Frequency Domain Controlled Source Electromagnetic, Borehole-to-surface Electromagnetic technique and marine magnetotelluric method. Data processing methods, such as fast three dimensional inversion using nonlinear conjugate gradients, and data interpretation methods, like Induced Polarization and Resistivity anomalies for hydrocarbon detection, are also included. These new techniques have been applied in petroleum survey and many cases are in complicated areas. They have successfully served the investigation of deep igneous rock reservoirs, and prediction of potential hydrocarbon targets. The cases indicate that electromagnetic techniques can help seismic survey to effectively detect hydrocarbon reservoir and remarkably improve drilling successes.     

跨孔电磁波层析成像在岩溶三维空间分布上的应用

灰岩地区岩溶发育对城市建设有着极大的威胁,一般岩溶探测主要采用钻探勘探,很难充分反映地下岩溶发育规模与特征,而跨孔电磁波层析成像方法,能查明工区地下岩溶发育及延伸情况.对钻孔间地层进行电磁波层析成像正演,表明电磁波层析成像对岩溶勘察的准确性与可行性.结合地铁轨道交通岩溶勘察实例,应用电磁波层析成像技术反演电磁波吸收系数...     

A mathematical structure for a theory of electromagnetic induction in the Earth in low latitude

Summary In low latitude the spatial distribution functions of the source field over the surface and the dimensions of the source, are important in any theory of electromagnetic induction developed for studying the conductivity structure of the Earth. The author has built up a mathematical structure for a theory of electromagnetic induction in anyn-layered earth model in low latitude. No simple solution is assumed for the horizontal distribution function of the source field and no assumption is made about the horizontal gradients of the source. The mathematical structure involves the concept of downward continuity of the field equations inside then-layered earth model. The resulting mathematical functions derived for anyn-layered earth model are complex. Hence a new matrix algebra of complex numbers is introduced by the author and this is built into the theory. From the upward continuity of the field equations, an inequality equation is derived in order to determine the heighth ₀ at which the induction field of the earth becomes negligible compared with the source field. The comparison of such heights at two or more stations under the same influence of the source field can be used for the resolution of the lateral distribution of the earch conductivity structure at these stations. The application of the theory will follow in a subsequent paper.