Electromagnetic sounding of geothermal zones

Electromagnetic (EM) data provide a substantial contribution to the geophysical mapping and monitoring of geothermal reservoirs. This paper presents an up-to-date picture of the achievements of EM methods for geothermal exploration as they have emerged over the last few years. It has been proved that EM sounding of geothermal zones and distant monitoring macro-parameters of the reservoirs, fluid-filled faults and other elements of the geothermal system are possible provided that modern 3-D inversion is used along with techniques that reduce the effects of industrial and geological noise. In addition, geological and petrophysical data also need to be included in the analysis.     

地热地球物理勘探新进展

地热作为一种清洁能源具有巨大的开发潜力,将在我国的经济发展中起到巨大的作用.在传统的热水型地热开发的基础上,国际上非常重视热干岩(Enhanced Geothermal)型地热的勘探开发.在这两类地热勘查和开发中,地球物理方法具有非常重要的作用.本文从地热系统的目标体岩石的地球物理性质出发,分析岩石的地球物理性质与温度、压力和含水量等影响因素的关系.例如随着温度的升高,岩石会出现去磁、电阻率降低、密度降低、弹性波速度也现明显降低等现象.进而分析地球物理方法应用到具体的地热勘查地质-地球物理异常模型.结合国际上21世纪以后的新方法技术,分析了重磁、电、地震方法在利用由于岩石温度的升高而出现的特殊地球物理现象,并应用于地热勘探.通过国内外实例介绍了各种地球物理勘探方法在地热勘查中成功应用,为进一步提高我国地热勘查水平,提供一些参考.     

Future Directions of Electromagnetic Methods for Hydrocarbon Applications

For hydrocarbon applications, seismic exploration is the workhorse of the industry. Only in the borehole, electromagnetic (EM) methods play a dominant role, as they are mostly used to determine oil reserves and to distinguish water from oil-bearing zones. Throughout the past 60 years, we had several periods with an increased interest in EM. This increased with the success of the marine EM industry and now electromagnetics in general is considered for many new applications. The classic electromagnetic methods are borehole, onshore and offshore, and airborne EM methods. Airborne is covered elsewhere (see Smith, this issue). Marine EM material is readily available from the service company Web sites, and here I will only mention some future technical directions that are visible. The marine EM success is being carried back to the onshore market, fueled by geothermal and unconventional hydrocarbon applications. Oil companies are listening to pro-EM arguments, but still are hesitant to go through the learning exercises as early adopters. In particular, the huge business drivers of shale hydrocarbons and reservoir monitoring will bring markets many times bigger than the entire marine EM market. Additional applications include support for seismic operations, sub-salt, and sub-basalt, all areas where seismic exploration is costly and inefficient. Integration with EM will allow novel seismic methods to be applied. In the borehole, anisotropy measurements, now possible, form the missing link between surface measurements and ground truth. Three-dimensional (3D) induction measurements are readily available from several logging contractors. The trend to logging-while-drilling measurements will continue with many more EM technologies, and the effort of controlling the drill bit while drilling including look-ahead-and-around the drill bit is going on. Overall, the market for electromagnetics is increasing, and a demand for EM capable professionals will continue. The emphasis will be more on application and data integration (bottom-line value increase) and less on EM technology and modeling exercises.     

A Review Of Environmental Applications Of Quasi-Stationary Electromagnetic Techniques

Electromagnetic (EM) techniques are the most commonly used geophysical methods in mineral exploration. However, the use of EM measurements for environmental and engineering applications like the detection of contaminant plumes or the exploration of waste sites is relatively new.The reason for the success of the application of EM methods to environmental problems lies in the variation of conductivity caused by different geometry of pore fluids and clay contents in rocks, and by the presence of organic and inorganic contaminants.Many EM methods/instruments used for mapping near surface geology exist and nowadays they play a central role in environmental geophysics. In general, these methods can be classified in two blocks: EM methods using a plane wave source of excitation and EM methods using a controlled source like a magnetic dipole or a loop source. The Very Low Frequency (VLF, VLF-R) and Radiomagnetotelluric (RMT) methods are chosen as representative methods for plane wave techniques, while horizontal loop EM methods operating in low induction numbers (EM31, EM34) and Transient Electromagnetic methods (TEM) are chosen as representatives of magnetic dipole or loop source techniques. Basic principles, advantages and disadvantages of each technique as well as their connection to specific environmental problems will be discussed.Different successful applications of these methods are reported in the literature. However, this review will focus on three major subjects: waste site exploration, detection of contaminated earth layers, and groundwater exploration. Case histories are presented illustrating the suitability of EM methods for solving such problems.     

Electromagnetic Induction Methods in Mining Geophysics from 2008 to 2012

In the period from 2008 to 2012, the topic of electromagnetic (EM) induction methods applied to mineral exploration has been the subject of more than 50 papers in journals and more than 300 extended abstracts presented at conferences (about 100 of which contain developments worthy of mentioning). Most of the work at the universities has been on modelling, inversion and data processing, and most of this material is published in the refereed literature. However, academia has also undertaken work on system geometry changes, system calibration and sensor design. There have been papers describing new systems developed for mineral exploration and case histories describing the use of EM methods to directly discover mineral deposits or to map the geology. Most of this work is by the service companies and mining companies and reported in the unrefereed literature. Since 2008, the pace of development of helicopter time-domain systems has slowed and more effort has been directed to developing natural source magnetic systems and to modelling and inverting this data. A number of studies comparing the results from natural source methods with the results from artificial source methods conclude that the natural source methods can see large-scale geological structures usually when there is a weak conductivity contrast with the surrounding material, but the natural source methods are unable to see small features that have a very large conductivity contrast with the country rock. Hence, they are not a good detector of mineral deposits unless one is looking for a large porphyry system.     

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.     

Kaipohan: An apparently nonthermal manifestation of hydrothermal systems in the Philippines

Hydrothermal systems in regions of high relief may not have obvious thermal features indicative of their central upflow zone at depth. Investigation of such areas for geothermal energy production in the Philippines has, however, encountered relatively large ( > 10,000 m²) areas of volumetrically significant but diffuse, cold-gas emission and associated intense argillic alteration. These features are likely to be best developed above, or close to, hydrothermal upflow zones. The identification of such features is important for interpreting the hydrology of active geothermal systems in the course of exploration for power development, or when interpreting the paleohydrology of fossil geothermal systems which host epithermal mineral deposits. Such zones of argillic alteration are likely to be barren of precious metals or other elements (except mercury) which are commonly used as pathfinders for precious-metal deposits, but are indicative of subsurface processes significant in ore genesis.It is proposed that the term “kaipohan” be used for such features. Their existence depends on the presence of either a low-permeability geologic formation or where a suitable relationship exists between the hydrology of the deep geothermal reservoir and that of the shallow groundwater system. Such a situation occurs in steep terrain, where much of the area supplying meteoric recharge to the geothermal system is at a lower elevation than the area overlying the centre of the system, and where there is a high annual rainfall. These conditions are typical of andesitic stratovolcanoes in tropical or subtropical island arcs.     

Analysis and interpretation of gravity data from the Aluto-Langano geothermal field of Ethiopia

The Aluto-Langano geothermal field is located in the central southern portion of Ethiopia within the Ethiopian Rift Valley. The gravity of the area was surveyed in an attempt to delineate the subsurface structure and to better understand the relationship between the geothermal systems and the subsurface structure. The gravity data were analyzed using integrated gradient interpretation techniques, such as the Horizontal Gradient (HG), Source Edge Detection (SED), and Euler Deconvolution (ED) methods. These techniques detected many faults that were compared with the mapped faults in the surface geology. The results of the present study will lead to an improved understanding of the geothermal system in the study area and aid the future geothermal exploration of the area.     

电磁波层析成像100例的统计分析

本文给出了最近十二年间１００个电磁波层析成像实例的统计资料，分析了电磁波层析的勘查精度与像元尺寸的关系，以及电波仪的频率设置，野外测量时的覆盖次数、方程组的欠定性等问题，并得出结论：为了提高电磁波层析的实用水平，需要研制频带宽、功率大和天线短的新一代电波仪；也需要研究欠定方程组模式下的成像方法和重建算法。与此同时，叙述了重建算法研究了模型设计与实际情况脱节的不足之处。     

Efficient simulation of geothermal processes in heterogeneous porous media based on the exponential Rosenbrock–Euler and Rosenbrock-type methods

Simulation of geothermal systems is challenging due to coupled physical processes in highly heterogeneous media. Combining the exponential Rosenbrock–Euler method and Rosenbrock-type methods with control-volume (two-point flux approximation) space discretizations leads to efficient numerical techniques for simulating geothermal systems. In terms of efficiency and accuracy, the exponential Rosenbrock–Euler time integrator has advantages over standard time-discretization schemes, which suffer from time-step restrictions or excessive numerical diffusion when advection processes are dominating. Based on linearization of the equation at each time step, we make use of matrix exponentials of the Jacobian from the spatial discretization, which provide the exact solution in time for the linearized equations. This is at the expense of computing the matrix exponentials of the stiff Jacobian matrix, together with propagating a linearized system. However, using a Krylov subspace or Léja points techniques make these computations efficient.The Rosenbrock-type methods use the appropriate rational functions of the Jacobian of the ODEs resulting from the spatial discretization. The parameters in these schemes are found in consistency with the required order of convergence in time. As a result, these schemes are A-stable and only a few linear systems are solved at each time step. The efficiency of the methods compared to standard time-discretization techniques are demonstrated in numerical examples.     

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.     

Seismic imaging of the geothermal field at Krafla,Iceland using shear-wave splitting

Shear-wave splitting is emerging as a useful exploration method for geothermal reservoirs as it can detect the geometry of the fracture system, the intensity of cracking and possibly, changes in fluid pressure within the reservoir. The method is based on the analyses of polarizations and time delays of shear-waves that have been distorted by the anisotropy of the medium through which the seismic waves have propagated. Observations of shear-wave splitting within the Krafla–Leirhnúkur geothermal field, Iceland, using a 20-station 3-component portable seismic array have provided evidence for at least two major crack systems of microfractures, oriented approximately N–S and E–W. Located microearthquakes align roughly along the E–W direction of the geothermal field, with shallow focal depths mostly around the injection well, probably related to the ongoing injection. This unexpected direction is however consistent with results from a simultaneous MT (magnetotelluric) survey.     

Geophysical greenfield exploration in the permo‐carboniferous Saar–Nahe basin—The Wiesbaden Geothermal Project,Germany

Structurally bound upwelling and outflowing hot fluids in the city of Wiesbaden suggest an interesting geothermal potential at the northern margin of the permo‐carboniferous Saar–Nahe basin, Germany. The Saar–Nahe basin in the Wiesbaden area was previously not a target for intensive geophysical prospecting of the deep underground. We present an exploration concept to investigate potential geothermal reservoirs near Wiesbaden for heat and power production. The acquired geophysical exploration data give new insights into the local geometry and internal structure of the Saar–Nahe basin. The exploration combines a wide range of methods, including the analysis of data, a dense grid gravity survey, as well as two‐dimensional and three‐dimensional seismic surveys. The resulting data, which for the first time reveal the deep structure of the Mainz and Saar–Nahe basins near Wiesbaden down to a depth of c. 4,000 m, show that the Saar–Nahe basin is strongly segmented and contains deep‐reaching active faults. Combining these information with numerical models of the stresses in the fault system and of the temperature distribution in the exploration area, a promising drilling target has been identified, which promise high permeability at a low risk of induced or triggered seismicity.     

中低温对流型地热系统中氢同位素的变化

中低温对流型地热系统在阐明地区构造活动性、地热资源形成与分布、水-岩相互作用以及有用元素富集成矿等方面具有十分重要的意义。而水作为地热系统的载体,研究其同位素变化机制是分析此系统中水-岩相互作用和地热水来源的必要前提。文中统计了中国大陆前人发表的167个温泉水与同一地区冷水的氢同位素(δD值)资料,并讨论了这些温泉水与区域活动构造的空间分布关系。结果表明,温泉水δD值均低于同一地区冷水,且91.11%的温泉发育于活动构造周边。文中还对造成温泉水δD值低于冷水的原因进行了分析: 主要是水由地表向地下渗透的过程中,水分子H-D同位素产生分馏作用(深循环作用),且分馏速率与温度相关,温度越高H-D分馏作用越明显; 其次,对于围岩中含高岭土矿物的地区,水与高岭土的H-D同位素也可能发生交换反应,从而降低温泉水中D同位素的含量。     

电磁勘探中各向异性研究现状和展望

电各向异性广泛存在于自然界中,已成为电磁勘探资料解释中不可忽视的因素.特别是在一些沉积岩地区,由于层理发育导致地下介质电阻率随电流方向发生变化,表现出很强的各向异性导电性.此时,利用各向同性模型进行电磁数据解释将引起很大的误差.本文回顾了电磁勘探中电各向异性研究的历史,简要介绍电各向异性成因、数值模拟中的数学描述及电各向异性介质中电磁场正反演模拟方法等研究现状,总结电各向异性在大地电磁、可控源电磁、航空电磁和感应测井等方面的最新研究进展及未来的挑战.随着多维电磁数据采集及正反演模拟技术的进步和计算能力的提高,给电各向异性信息提取和应用带来了新的契机.电各向异性研究将在矿产资源勘查、油气存储与运移特征分析、地下水和地热、环境工程地质调查、大地构造及地质灾害预测等方面发挥积极作用.     

Trace-element distribution in an active hydrothermal system, Roosevelt hot springs thermal area, Utah

Chemical interaction of thermal fluids with reservoir rock in the Roosevelt Hot Springs thermal area, Utah, has resulted in the development of characteristic trace-element dispersion patterns. Multielement analyses of surface rock samples, soil samples and drill cuttings from deep exploration wells provide a three-dimensional perspective of chemical redistribution within this structurally-controlled hot-water geothermal system.Five distinctive elemental suites of chemical enrichment are recognized, each characteristic of a particular combination of physical and chemical conditions within the geothermal system. These are: (1) concentrations of As, Sb, Be, and Hg associated with siliceous material at locations of liquid discharge, fluid mixing or boiling; (2) concentrations of Mn, Ba, W, Be, Cu, Co, As, Sb and Hg in manganese and iron oxide deposits; (3) high concentrations of Hg in argillized rock near fumaroles and lower concentrations in a broad diffuse halo surrounding the thermal center; (4) concentrations of As in sulfides and Li in silicate alteration minerals immediately surrounding high-temperature fluid flow-controlling fractures; (5) deposits of CaCO₃ at depth where flashing of brine to steam has occurred due to pressure release. The geochemical enrichments are not, in general, widespread, pervasively developed zones of regular form and dimension as are typical in many ore-forming hydrothermal systems.As the geothermal system develops, changes and eventually declines through time, the chemical deposits are developed, remobilized or superimposed upon each other, thus preserving within the rocks a record of the history of the geothermal system. Recognition of trace-element distribution patterns during the exploration of a geothermal system may aid definition of the present geometry and interpretation of the history of the system.     

3D forward modeling and response analysis for marine CSEMs towed by two ships

A dual-ship-towed marine electromagnetic (EM) system is a new marine exploration technology recently being developed in China. Compared with traditional marine EM systems, the new system tows the transmitters and receivers using two ships, rendering it unnecessary to position EM receivers at the seafloor in advance. This makes the system more flexible, allowing for different configurations (e.g., in-line, broadside, and azimuthal and concentric scanning) that can produce more detailed underwater structural information. We develop a three-dimensional goal-oriented adaptive forward modeling method for the new marine EM system and analyze the responses for four survey configurations. Oceanbottom topography has a strong effect on the marine EM responses; thus, we develop a forward modeling algorithm based on the finite-element method and unstructured grids. To satisfy the requirements for modeling the moving transmitters of a dual-ship-towed EM system, we use a single mesh for each of the transmitter locations. This mitigates the mesh complexity by refining the grids near the transmitters and minimizes the computational cost. To generate a rational mesh while maintaining the accuracy for single transmitter, we develop a goal-oriented adaptive method with separate mesh refinements for areas around the transmitting source and those far away. To test the modeling algorithm and accuracy, we compare the EM responses calculated by the proposed algorithm and semi-analytical results and from published sources. Furthermore, by analyzing the EM responses for four survey configurations, we are confirm that compared with traditional marine EM systems with only in-line array, a dual-ship-towed marine system can collect more data.     

电离层影响下不同类型源激发的电磁场特征

大地电磁法(MT)和可控源音频大地电磁法(CSAMT)虽然在多种勘探领域均得到了广泛的应用,但是也存在着一些问题.于是结合了这两种勘探方法的优点,一种采用固定的大功率源进行电磁波发射,在全国范围内进行电磁信号接收的人工源电磁法得到了发展.此方法中收发距可达上千公里,在此大尺度范围下如何保证电磁信号的强度成为一个关键问题,而其中发射源的类型是决定着信号强度的重要因素.当收发距很大时,电离层的存在将影响到电磁信号的传播,为了探讨适合于大功率固定源方法的发射源类型,本文将大功率固定源方法模型抽象为地-电离层模型,研究电离层影响下的三维积分方程法,其中地-电离层模式背景模型的格林函数用波数域中的层矩阵法获得.利用此正演方法模拟对比了发射源分别为水平长线源、环状源和L型源时电离层影响下的电磁场传播特征,并初步探讨了L型发射源对三维异常体的分辨能力.综合分析认为L型源是较优的发射源,有利于在大功率固定源方法中进行实际应用.     

地热资源探测方法研究进展

地热资源将成为未来一种重要的生态型资源，具有巨大的市场潜力，地热资源的研究与利用历史比较悠久，其探测方法与技术也处于不断的革新过程中，本文总结了地热资源探测的方法与技术及其进展情况，并探讨了利用3S技术和计算机技术的融合在地热资源探测中的应用情况及发展前景。     

Study of Shallow Low-Enthalpy Geothermal Resources Using Integrated Geophysical Methods

The paper is focused on low enthalpy geothermal exploration performed in south Italy and provides an integrated presentation of geological, hydrogeological, and geophysical surveys carried out in the area of municipality of Lecce. Geological and hydrogeological models were performed using the stratigraphical data from 51 wells. A ground-water flow (direction and velocity) model was obtained. Using the same wells data, the ground-water annual temperature was modeled. Furthermore, the ground surface temperature records from ten meteorological stations were studied. This allowed us to obtain a model related to the variations of the temperature at different depths in the subsoil. Integrated geophysical surveys were carried out in order to explore the low-enthalpy geothermal fluids and to evaluate the results of the model. Electrical resistivity tomography (ERT) and self-potential (SP) methods were used. The results obtained upon integrating the geophysical data with the models show a low-enthalpy geothermal resource constituted by a shallow ground-water system.