Modern Methods of Airborne Electromagnetic Survey

Izvestiya, Physics of the Solid Earth - Airborne electromagnetic methods are characterized in detail. A classification of airborne electromagnetic systems is presented. Modern active...     

Exploring for Geothermal Resources with Electromagnetic Methods

Electrical conductivity of the subsurface is known to be a crucial parameter for the characterization of geothermal settings. Geothermal systems, composed by a system of faults and/or fractures filled with conducting geothermal fluids and altered rocks, are ideal targets for electromagnetic (EM) methods, which have become the industry standard for exploration of geothermal systems. This review paper presents an update of the state-of-the-art geothermal exploration using EM methods. Several examples of high-enthalpy geothermal systems as well as non-volcanic systems are presented showing the successful application of EM for geothermal exploration but at the same time highlighting the importance of the development of conceptual models in order to avoid falling into interpretation pitfalls. The integration of independent data is key in order to obtain a better understanding of the geothermal system as a whole, which is the ultimate goal of exploration.     

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.     

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.     

Applications Of Electrical And Electromagnetic Methods For Environmental And Geotechnical Investigations

Electrical and electromagnetic methods are powerful tools in environmentaland geotechnical investigations. Techniques developed for deeper applications, such as mining,geothermal and crustal studies, are scaled for shallow targets by moving to higher frequencies,earlier decay times and/or smaller array configurations. Another extremely important factor is dense stationspacing, to reduce spatial aliasing, and high quality data to resolve small features. Hence, new instrumentsare concerned with making continuous or dense measurements with high precision, and interpretationalmethods fast enough to handle large datasets quickly. Continuously measuring electrical and time-domainelectromagnetic systems have been developed for geological mapping in hydrological investigations withone-dimensional inversion routines that are rapid and robust. At a smaller scale an electrical systemis used for archaeology studies with excellent results. Working to and above the upper limits ofthe quasi-static approximation, a very early time electromagnetic system is proving successful at mapping subsurfaceinfrastructure in areas of conductive, clay cover, where ground penetrating radar is ineffective.Induced polarization (IP) and resistivity systems that employ multiplexing techniques, while not continuouslymeasuring, allow for relatively rapid production rates and dense sampling for applications ranging fromlandfill and contaminant characterization studies, to verifying the integrity of engineeredsubsurface structures and monitoring infiltration in the vadose zone.     

井间电磁波CT在昆明地铁岩溶区地质勘察中的应用

井间电磁波CT技术是利用地层间介质的物性差异,不同性质的物质对电磁波的反射和吸收能力是不同的。井间电磁波CT技术在探测岩溶裂缝等方面有较好效果,本文利用井间电磁波CT技术,对昆明地铁四号线苏家塘站岩溶专项线路工程地下岩溶发育地段进行测试,通过对工区岩石电磁波吸收系数差异的研究,依据吸收系数特征,探测到地铁线路工程地下岩溶及裂缝发育,划分出4个层次的岩溶区,经与钻孔岩心资料对比,证实探测结果的正确性。且对工程设计和施工提供了必要的基础资料。     

露天煤矿地下水电磁探测与类型判别

为准确确定露天煤矿地下水分布与类型,根据不同赋存条件电磁差异,基于磁场强度与电阻率参数进行异常划分与类型判别。以新疆某露天煤矿为例,采用高精度磁法通过磁异常强度确定烧变岩边界,并采用高密度电法和瞬变电磁法通过电阻率圈定低阻异常区,结合异常响应差异分别推断地下水分布范围与赋存类型。地面钻探验证探测成果的准确性。结果表明：露天煤矿烧变岩水与砂岩裂隙水电磁响应差异明显,基于磁场强度与电阻率的双参数综合电磁法能准确圈定其分布范围,并判别其赋存类型。     

Global Lightning Activity on the Basis of Inversions of Natural ELF Electromagnetic Data Observed at Multiple Stations around the World

The monitoring of global lightning activity and its spatial and temporal variations is known to be very essential for the study of global warming, the subject of greatest concern to human beings on planet Earth today. As a method of remote sensing for the global lightning distribution, we have proposed an inverse problem by using the data of natural electromagnetic noise in the ELF (extremely low frequency) Schumann resonance (SR) band observed simultaneously at a few stations around the world. The fundamentals of this inversion problem (or ELF tomography) to the SR data have been presented and the first attempt to deduce the global lightning distribution by means of the real SR data has been performed, which has indicated a possibility of snapshots of well-known thunderstorm centers on the globe. This ELF tomography consists of two stages. The first stage is the inversion of the ELF field power spectra to the distribution of lightning intensity by distance relative to an observation point. The obtained distance profiles of intensity of sources at a few stations are used as tomographic projections for reconstructing a spatial distribution of sources in the second stage. Maps of the global lightning distributions constructed by the result of inversions of ELF background field spectra obtained from three stations around the world show that the most active regions vary meridionally on the diurnal time scale being connected mainly with continental areas in the tropics. We do hope that this kind of inversion method to multi-stationed ELF data will be of great importance in the future.     

井间电磁波CT技术在长江大堤岩溶探测中的应用

地下井间电磁波法层析技术(CT)是一门新技术,它将医学CT的原理成功地运用于地学中,依照电磁波在地下有耗介质中传播规律及一定的物理和数学关系,反演透视剖面上的物理参数的分布,最后以图像的形式表现出来。长江大堤武汉段附近的碳酸盐岩地区,由于地势较低和长江水位动态变化,使地下水长期作用于灰岩裂隙,往往使岩溶或破碎带更加发育,形成地下溶洞,给长江大堤和附近的工农业生产、居民生活带来极大的隐患和危险。通过井间电磁波法层析技术共90个电磁波CT剖面测试,发现有28个CT剖面出现异常。此次的探测,为长江大堤周边安全的防治提供可靠精确的地下地质资料。     

城市地震活断层探测的地球物理方法

通过国内外有关城市地震活断层探测的实例，对城市地震活断层探测的主要地球物理方法进行了介绍。由于城市地区特殊的干扰环境，城市地震活断层探测需要对传统的技术方法进行必要的改进和调整。以提高探测方法的抗干扰能力，城市地球地震活断层探测应充分利用高分辨浅层地震勘探和人工地震测深精度高的优势，根据不同城市的地球物理特征，配合电法勘探、磁法勘探、重力勘探和天然地震观测等其他地球物理方法，通过对资料的综合分析和解释，提高探测结果的分辨率和可靠性。     

瞬变电磁法在煤矿采空区及富水性探测中的应用

瞬变电磁法作为地球物理勘查的重要手段之一,利用对电阻的异常灵敏反应,将采集到的数据分析处理、绘制出视电阻率测深剖面、视电阻率解释顺层切片,可直观地对煤系地层的采空区展布、含水层及其富水性、断层的富、导水性进行评价,可有效避免矿井开采过程中带来的地质危害及资源浪费.     

The Physics of Lightning

An overview of the physics of cloud-to-ground lightning is given, including its initiation, propagation, and attachment to ground. Discharges artificially initiated (triggered) from natural thunderclouds using the rocket-and-wire technique are discussed with a view toward studying properties of natural lightning. Both conventional and runaway breakdown mechanisms of lightning initiation in thunderclouds are reviewed, as is the role of the lower positive charge region in facilitating different types of lightning. New observations of negative-leader stepping and its attachment to ground are compared to similar processes in long laboratory sparks. The mechanism and parameters of compact intracloud lightning discharges that are thought to be the most intense natural producers of HF-VHF (3–300 MHz) radiation on Earth are reviewed. The M-component mode of charge transfer to ground and its difference from the leader/return-stroke mode are discussed. Lightning interaction with the ionosphere and the production of energetic radiation (X-rays and gamma radiation) by cloud-to-ground leaders are considered.     

Detection of Hazardous Cavities Below a Road Using Combined Geophysical Methods

Assessment of the risk arising from near-surface natural hazard is a crucial step in safeguarding the security of the roads in karst areas. It helps authorities and other related parties to apply suitable procedures for ground treatment, mitigate potential natural hazards and minimize human and economic losses. Karstic terrains in the Salento Peninsula (Apulia region—South Italy) is a major challenge to engineering constructions and roads due to extensive occurrence of cavities and/or sinkholes that cause ground subsidence and both roads and building collapse. Cavities are air/sediment-filled underground voids, commonly developed in calcarenite sedimentary rocks by the infiltration of rainwater into the ground, opening up, over a long period of time, holes and tunnels. Mitigation of natural hazards can best be achieved through careful geoscientific studies. Traditionally, engineers use destructive probing techniques for the detection of cavities across regular grids or random distances. Such probing is insufficient on its own to provide confidence that cavities will not be encountered. Frequency of probing and depth of investigation may become more expensive. Besides, probing is intrusive, non-continuous, slow, expensive and cannot provide a complete lateral picture of the subsurface geology. Near-surface cavities usually can be easily detected by surface geophysical methods. Traditional and recently developed measuring techniques in seismic, geoelectrics and georadar are suitable for economical investigation of hazardous, potentially collapsing cavities. The presented research focused on an integrated geophysical survey that was carried out in a near-coast road located at Porto Cesareo, a small village a few kilometers south west of Lecce (south Italy). The roads in this area are intensively affected by dangerous surface cracks that cause structural instability. The survey aimed to image the shallow subsurface structures, including karstic features, and evaluate their extent, as they may cause rock instability and lead to cracking of the road. Seismic refraction tomography and ground-penetrating radar surveys were carried out along several parallel traverses extending about 100 m on the cracked road. The acquired data were processed and interpreted integrally to elucidate the shallow structural setting of the site. Integrated interpretation led to the delineation of hazard zones rich with karstic features in the area. Most of these karstic features are associated with vertical and subvertical linear features and cavities. These features are the main reason of the rock instability that resulted in potentially dangerous cracking of road.     

Review of Acquisition and Signal Processing Methods for Electromagnetic Noise Reduction and Retrieval of Surface Nuclear Magnetic Resonance Parameters

Surveys in Geophysics - Surface nuclear magnetic resonance (sNMR) is an electromagnetic hydrogeophysical method directly sensitive to liquid phase water in the upper $$\approx $$ 100 m of the...     

基于DMIS的断层三维表面形态自动检测方法研究

针对断层的测量需求,利用计算机控制技术对空间曲面自动测量程序进行优化,以DMIS(Dimensional Measuring Interface Specification)为开发平台分别形成四边形键入坐标式自动检测程序和任意多边形自动检测程序,且通过所述程序完成塔里木盆地塔中26井区某断层模型表面形态的仿真,给出两种针对断层三维表面形态检测的建系方法:模型建系法和机床建系法。并将空间曲面的自动测量技术应用到断层的三维表面形态测试中,形成一套针对断层复杂表面形态的自动检测方法,使得断层的三维表面形态可以通过上述程序自动测量,且可以使用多个测头角度连续进行一次测量。该方法可以克服在传统的断层检测过程中边界不能完全衔接的问题,提高断层测量的效率和自动化程度,降低测量过程中的人为因素和后期的数据处理难度。     

Lightning activity and precipitation structure of hailstorms

By using the cloud-to-ground (CG) lightning location data from the lightning detection network of He- nan Province, surface Doppler radar data and standard orbit data of PR, TMI and LIS on TRMM satellite, the spatial and temporal characteristics of CG lightning flashes in 10 severe hailstorms are analyzed. The results show that the percentage of CG lightning in these hailstorms is high with an average value of 45.5%. There is a distinct increase in CG flash rate during the rapid development stage of hailstorms. The hailstone falling corresponds to an active positive flash period, and the increase of CG flash rate is generally accompanied with a decrease of –CG flash rate. The flash rate declines rapidly during the dissipating stage of hailstorms. The precipitation structure and lightning activity in two typical hail- storms are studied in detail. It is found that strong convective cells with reflectivity greater than 30dBZ mainly are situated in the front region of hailstorms, whereas the trailing stratiform region is in the rear part of the hailstorms. The maximum heights of echo top are higher than 14 km. Convective rain con- tributes much more rainfall to the total than stratiform rain, and the convective rain takes about 85% and 97% of the total in the two cases, respectively. Total lightning in the hailstorms is very active with the flash rate up to 183 fl/min and 55 fl/min, respectively. The results also indicate that most lightning flashes occurred in the echo region greater than 30 dBZ and its immediate periphery. The probability of lightning occurrence is 20 times higher in the convective region than in the stratiform region. The result suggests that the lightning information is helpful to the identification of convective rain region. The linear relationship between flash rate and ice water content is disclosed primarily.     

Solar Activity,Lightning and Climate

The physics of solar forcing of the climate and long term climate change is summarized, and the role of energetic charged particles (including cosmic rays) on cloud formation and their effect on climate is examined. It is considered that the cosmic ray-cloud cover hypothesis is not supported by presently available data and further investigations (during Forbush decreases and at other times) should be analyzed to further examine the hypothesis. Another player in climate is lightning through the production of NO_x; this greenhouse gas, water vapour in the troposphere (and stratosphere) and carbon dioxide influence the global temperature through different processes. The enhancement of aerosol concentrations and their distribution in the troposphere also affect the climate and may result in enhanced lightning activity. Finally, the roles of atmospheric conductivity on the electrical activity of thunderstorms and lightning discharges in relation to climate are discussed.