Magnetotelluric measurements at Eusebio,an equatorial station

Magnetotelluric measurements in the period range 100–86400 s were conducted at a coastal station under the equatorial electrojet (Eusebio, 3.87°S, 321.58°E). The magnetotelluric data were hand-scaled and analysed to obtain a scalar apparent resistivity profile at Eusebio. The depth of the intermediate conducting layer was found to be in the range 45–75 km and the final conducting layer seems to begin at a depth of about 350 km. Possible effects of the source field equatorial electrojet are discussed and our results are compared with those of an African station under the equatorial electrojet.     

Electromagnetic measurements in the vicinity of the KTB drill site. Part II. Magnetotelluric results

Summary In addition to the magnetovariational measurements across an array in Western Bohemia, close to the KTB ultradeep borehole (Germany), discussed in part I of this paper [1], magnetotelluric results from pivot sounding point Ostrvek within the array are presented here. Good quality of long-period magnetotelluric data (period range from 30 s to about 1 hour) allowed structural dimensionality of the medium to be analysed in detail. The geoelectrical structure was identified as a slightly distorted two-dimensional regional substratum, with dominating E - W strike, overlaid by a heterogeneous subsurface layer with extremely strong and anisotropic galvanic distortion effect on the magnetotelluric data. Estimating the total static shift distortion tensor by fitting the local magnetotelluric curve to the curve of the global magnetovariational soundings (for the European continent), the static distortions were identified as of generally multidirectional origin. The resulting telluric ellipse is, however, strongly anisotropic, indicating an approximately SW - NE apparent local strike, which is in the approximate agreement with remote reference magnetovariational results. Finally, the magnetotelluric results from the station Ostrvek are compared with long-period data from the immediate neighbourhood of the KTB borehole on the German territory.     

Long-term underground temperature measurements at Syowa Station, East Antarctica

Underground temperature measurements in two shallow boreholes have been carried out by the Japanese Antarctic Research Expedition at Syowa Station, East Antarctica from April, 1981 to January, 1985. Two quartz thermometers were installed in the first borehole at depths of 2 and 5 m and three were in the second one at depths of 1, 4 and 6.8 m. The mean underground temperatures in the first borehole were −8.181 and −8.843°C at depths of 2 and 5 m, and in the second one −8.242 and −8.220°C at depths of 4 and 6.8 m. As the mean air temperature at Syowa Station was −10.8°C, the underground temperature in the 2 −6.8 m depth range is about two degrees higher than the air temperature. The thermal diffusivities of the observation area are determined by the same principle of the Ångström method, using long-term underground temperature records. The thermal diffusivity around Syowa Station is established to be about two times larger than those of ordinary igneous and metamorphic rocks measured in the laboratory. The thermal conductivity of the drilled cores and surrounding outcropping rocks are also measured by the transient method with temperature conditions at +23°C and −20°C. The thermal conductivities measured in various samples at −20°C are about 7% larger than those at +23°C. Thes tendency is consistent with the results of holocrystalline rock experiments.     

Magnetotelluric fieldwork adventures in Africa

In her RAS Presidential Address for 2005, Kathryn A Whaler discusses how mapping the electrical resistivity of the subsurface can be used to investigate continental rifting, with examples from Africa.     

Geomagnetic measurements made on the moving ICE shelf at Halley,Antarctica

The difficulties involved in making geomagnetic measurements from the moving ice shelf at Halley are considered, as are measurements giving information on this movement. These indicate that the observatory has been moving westward, accelerating from 400 m yr^–1 in 1969 to 800 m yr^–1 in 1980, and rotating at up to 30 min of arc yr^–1. The effects of both rotation and tilt on the two types of variometers installed are examined. It is concluded that the geomagnetic data obtained from Halley since 1969 are best treated as variation data.     

Magnetotelluric sounding of Kamchatka

The MTS data acquired in Kamchatka during the last 30 years have been analyzed and summarized. Our interpretation is based on curves oriented along and across Kamchatka. Longitudinal and transverse curves can be affected by local geoelectric inhomogeneities. These were suppressed by conformal averaging. A bimodal interpretation of average longitudinal and transverse curves yielded a deep geoelectric model, which can be adopted as a starting point to be subsequently refined by 3D numerical modeling. The model involves a crustal conductive layer extending along central Kamchatka. In the east of the peninsula this layer is connected with crustal transverse conductive zones as wide as 50 km. Those zones have extensions toward the Pacific Ocean. Major centers of present-day volcanism occur in the transverse zones. The upper mantle contains an asthenospheric conductive layer forming an uplift beneath the present-day volcanic belt of Kamchatka.     

Magnetotelluric sounding results in eastern Tibetan Plateau

The results of Zayü-Qingshuihe MT sounding profile carried out in eastern Tibetan Plateau are presented in this paper. Using 2-D RRI method, the resistivity distribution with depth is obtained along the profile. It is featured by the resistivity zones in the horizontal direction and layers in the vertical direction. The Bangong-Nujiang suture zone and Jinshajiang suture zone are both important electrical conductivity-separating zones in the plateau, and the former is a zone with relatively low resistivity while the latter is an electrical conductivity gradient zone. The highly electrical conductive bodies in the mid and lower crust of northern Qiangtang and Bayan Har Terrain might be caused by regional melting due to shear heating during the process of subduction in tectonic evolution.     

复杂三维介质的大地电磁正演模拟

存在多个三维且不规则异常体的复杂模型,与实际地质模型更为接近,用有限单元法数值模拟计算无疑具有明显优势.但由于刚度系数矩阵为大型、不定且严重病态的,因此普遍存在求解效率低的问题.在引入不完全LU分解改善其条件数后,分析、比较了几种流行的Krylov子空间方法:CGS、BICG和BICGSTAB,得出BICGSTAB具有收敛平滑、求解速度快等优点,更适合于求解大地电磁三维有限单元法正演中的线性方程组问题.通过与一维模型解析计算结果的对比以及COMMEMI-3D模型的正演计算结果,验证了算法和编制程序的有效性,为后续构建大地电磁三维反演提供基础.     

Magnetotelluric profiling of vertical conductors

The presence of lateral contrasts of electrical conductivity modifies the original pattern of electromagnetic fields radiated from remote sources. A magnetic transverse plane wave field, interacting with a vertical conductive and outcropping dike placed between two quarter-spaces of unequal electrical conductivities, creates an anomalous vertical component of the magnetic field. This anomalous field has been analysed by computation, and drafting of master curves. Two case histories are presented to illustrate the application and the effectiveness of the solution. It is concluded that: (i) the response is higher for intermediate values of the conducting body induction number; (ii) the curves can be used for the inter-pretation of magnetotelluric, AFMAG, and VLF exploration data; (iii) it is necessary to develop solutions taking into account the vertical as well as the lateral variation of conductivity.     

Magnetotelluric observational techniques on land

During the period since the Workshops in Sopron and Murnau we have witnessed some quite remarkable changes in the techniques employed in magnetotelluric (MT) observations. These changes have brought about significant improvements in the quality of MT data that can be gathered today. The new techniques are very likely to bring improvements in many areas beside MT, but are reviewed here in the light of the progress they have rendered possible for MT soundings on land. Three main subjects are covered in this review. The first one is concerned with the use of cryogenic or SQUID magnetometers. The new instruments are presented to the geomagnetist. An explanation of how these instruments work is given without, however, details of their operation and fabrication, and without showing that they usually have a much larger, i.e. better, signal-to-noise ratio than conventional magnetometers. Next, the method of the remote magnetic reference is examined. Originally the MT method assumed incident signals in the form of plane waves. While it is known today that incident wave amplitudes which vary linearly with distance are perfectly acceptable for MT soundings, there are many other higher order distortions of the incident signal which are not acceptable. Most of these high order or local perturbations are man-made and can seriously falsify MT data. All processes of MT data analysis try to minimize the errors caused by locally perturbed signals. But they are only partly successful, especially when the perturbations produce strong cross-power bias. The remote magnetic reference method has proved far superior in avoiding most of this cross-power as well as faulty autopower. The last section of this review deals with the use in the field of microcomputers or microprocessors. These have made it possible to process the data on the site already to such a degree that the sounding crew knows whether the sounding is proceeding satisfactorily, and can decide how best to continue the survey work. Of particular interest is digital filtering, which is especially easy with the microcomputer, and which may be used to avoid specific sources of perturbation.     

兰州市西部地热资源大地电磁探测

甘肃与青海边界的民和盆地为中新生代盆地,其中含有油气和煤炭资源,随着开采资源枯竭,适合就近利用的地热资源的勘探评价具有重要的实际价值.通过4条大地电磁剖面(D5,D9,D13,D17)对民和盆地局部地壳电性结构、区域构造进行了探测,对本区地热资源进行初步评价.采用德国Geometrix公司GMS07e电磁系统,按64 ...     

音频大地电磁法在黄土区填图中的应用

通过在工作区内布设音频大地电磁测线,并结合当地已有的地质与测井资料对测深数据进行约束反演,结果表明:①音频大地电磁测深法对工作区的地层岩性分层具有较好的效果,并对深部的基岩面与上部土层的分界具有较好的反映;②工作区内的地层产状比较稳定;③测点点距对结果精准度有一定的影响。进一步总结了音频大地电磁法在黄土区填图中的适用性,对指导音频大地电磁法在黄土区填图中应用具有重要意义。     

Earthquakes in continental Antarctica

No earthquake had been definitely located in the interior of the Antarctic continent before that in Dronning Maud Land, which occurred on 1982 November 4, and was found by the “search” procedure of the International Seismological Centre. A further earthquake of similar magnitude (Mb 4.4) has now been located about 200 km inland from the coast of Wilkes Land, near 69°S, 123°E. The second event occurred on 1983 September 20, and was also found by the ISC search procedure, from readings at four Antarctic seismograph stations, and six farther afield. Reflected phases suggest that the focus of this earthquake is in the upper few kilometres of rock beneath the ice cap. Two later earthquakes occurred on 1984 May 19 about 400 km west of the event of 1983 September 20. They had magnitudes (Mb) of 5.0 and 4.6, and were large enough to be located by the US National Earthquake Information Service.     

Magnetotelluric data inversion with seismic data constraint

Introduction The research on the structure and physical property of ancient hidden hill, igneous rocks and basement is relatively difficult by using seismic data only. If we combine seismic data, magneto-telluric (MT) data and geophysical data together, better results can be obtained for the above problem. A number of geophysicists at home and abroad, such as CHEN and WANG (1990), Siri-punvarapor and Egbert (2000) have tried many methods to solve the problem by the inversion of seismic da…     

大地电磁地震前兆初步分析系统

介绍了大地电磁(MT)地震前兆初步分析系统的建立及主要功能,运用该系统对2003年度祁连山地震带8个大地电磁复测点进行了MT多参量综合对比、变化幅度的计算及异常前兆判定,取得较好效果。     

Magnetotelluric survey across the active spreading zone in Southwest Iceland

The results of 10 magnetotelluric soundings, performed along a 110-km-long profile crossing the constructive plate boundary in southwest Iceland, are presented. Apparent resistivities are interpreted by a horizontally stratified earth model to yield a pseudo cross-section along the profile. The crust-mantle interface contains a well conductive layer. The depth to the good conductor increases with age of the crust and the distance from the axial zone. This layer is interpreted as partially molten basalt, at a temperature about 1100°C and a volume fraction of the melt phase in the range 10–20%.The high-conductivity layer probably disappears west of the Borgarnes anticlinal axis, which separates the older (to the west) and younger (to the east) flood basalts in western Iceland, indicating that the temperature below the oldest part of the profile lies below the solidus curve of basalt. Recent seismic crustal investigations in the same area indicate a state of partial melting or a magma chamber, which agrees with the results of the magnetotelluric soundings.