Deep structure of Western Yakutia from magnetotelluric soundings

Analysis of magnetotelluric (MT) sounding studies in western Yakutia showed that amplitude curves are strongly affected by the regional distribution of electrical conductivity heterogeneities of the sedimentary cover, while phase curves are distorted to a lesser extent. The distribution of the longitudinal conductance in the sedimentary cover is constructed, and MT fields and resistivity and phase curves are modeled. Based on combined analysis of experimental and model data and results of inversion of corrected curves, an interpretation of main anomalies of MT fields is given and a regional crustal anomaly of the electrical conductivity is discovered.     

A new approach to the electrical conductivity anomalies in the Drauzug-Bakony geological unit

A major electrical conductivity anomaly has been detected in Transdanubia (West Hungary) in the eastern part of the Drauzug-Bakony geological unit (DBU), a collision zone of the Alpine orogeny. Assuming the source of the conductivity anomaly to be a characteristic formation of the whole DBU, long period magnetotelluric (MT) measurements were made in two regions of the western part of the DBU, in the Gail valley and in the Karawanken. The general features of this formation were studied also in order to elucidate the cause of the Transdanubian conductivity anomaly. The conductivity anomaly detected by long period MT in the Western DBU lies much deeper (12–17 km) than in Transdanubia (5–9 km). As the anomaly in both regions is associated with a well-known tectonic zone, its origin was initially explained by ionic conduction, arising from the presence of pore fluids. The very low resistivity values, the great anisotropy, the effect of charges on the H-polarized MT curves, and the anomalous magnetic field variations also indicated an increase of electronic conduction, possibly caused by graphitic rocks or ores nearer to the surface.

In the Gail valley, audiomagnetotelluric measurements made in 1986 have led to the discovery of these standing blocks of graphitic shales lifted up gradually to the surface by tectonic forces from below the dolomites of the Carnic Alps. It is suggested that the same plate collision arranged the Paleozoic graphitic shales to narrow dikes in the Gail valley as well as in Transdanubia along the Balaton line and zones parallel to it.     

The North American Central Plains conductivity anomaly and its correlation with gravity, magnetic, seismic, and heat flow data in Saskatchewan, Canada

The North American Central Plains conductivity anomaly (NACP) lies, virtually in its entirety, within the Trans-Hudson Orogen. Accordingly, should these two features prove to be contemporaneous, then the geometrical relationship between these two is of foremost importance to any evolutionary tectonic model proposed to explain the collision of the Superior and Churchill Provinces in the Hudsonian, and a model that does not include a mechanism for the generation of this anomaly is obviously untenable.

In order to map better the trend of the NACP in the Province of Saskatchewan, Canada, two magnetotelluric (MT) profiles were conducted over the NACP previously defined by magnetometer arrays and profiles. Data from these two profiles, along with an earlier MT profile just north of the U.S./Canadian border, suggest that the NACP is not a continuous feature, but rather that it exhibits a definite break at latitude 51° N. Other geophysical evidence examined herein is concordant with this characteristic. If a second MT anomaly mapped to the northwest of this break is, in fact, a manifestation of the same geological structure, then one possible interpretation is of a major NW-SE trending sinistral fault in the deep crust, previously undetected, with a movement of some 100–150 km along strike.

The MT data from the southernmost profile, after correction for static shift, are modelled in a 2D manner, and it is shown that the NACP is consistent with an arcuate structure in vertical section of high conductivity (> 2 S m⁻¹) beginning at a depth of 10 km centred on 103° W dipping down to the west reaching possibly the base of the crust. It is also shown that such an arcuate shape in section can explain an observed gravity high of 40 mgal. Such high conductivities cannot be explained in terms of connected fluids, as it would require implausibly high porosities (12–20%). A comprehensive interpretation of all the geophysical data would have to account for the observed characteristics of the anomalous region; which are, high electrical conductivity, positive density contrast, no magnetization, and high heat flow in the basement, in terms of a single causative body. One possible explanation is presented in terms of a zone of lithospheric weakness along which was emplaced low-density differentia from a mantle-derived body. An alternative explanation could be the phase transformation of obducted material into eclogite or serpentinite. However, difficulties exist with both of these explanations and further data are required.     

大地电磁局部畸变异常特征三维数值模拟研究

局部畸变问题曾经困扰大地电磁资料反演解释几十年,大地电磁三维数值模拟技术的发展为剖析局部畸变特点和得到可靠的反演成像结果提供了技术基础。本文采用三维数值模拟成像方法对典型三维局部畸变模型进行模拟分析。三维数值模拟结果显示:电场分量垂直电性分界面的极化模式视电阻率曲线(对应二维情况下TM模式)在穿越低阻异常体界面时,曲线会先上移后下移,而在穿越高阻异常体界面时,曲线会先下移后上移,这与电性分界面处积累面电荷产生的二次电场有关。三维模型中XY模式、YX模式视电阻率和相位在三维异常体附近的水平变化是呈现近似垂向对称的,该现象与电场垂直跨越电性界面时视电阻率的变化规律是吻合的,当测线分别沿X方向和Y方向展布时,三维情况下的XY和YX模式分别对应二维情况下的TM模式。低阻小异常体对区域构造响应的畸变影响比高阻小异常体要严重。低阻小异常体对二维区域响应的两种极化模式视电阻率和相位都有非常明显的畸变影响,相比较而言对TE模式的畸变要大于TM模式,因此我们在做二维反演解释时,可优先考虑拟合TM模式数据。位于小异常体中心上方测点的三维畸变响应虽然与对应真实二维区域响应的差异比较大,但可以等效于某种二维模型响应,这种由局部畸变造成的假二维响应在实际野外数据的解释中是需要注意的。      

2D modelling of resistivity and magnetotelluric data from the Belvedere Spinello salt mine,Italy1

The Belvedere Spinello salt mine is located in the Catanzaro Province of Calabria in Southern Italy. An extensive mining program has caused the development of Underground cavities filled with brine and the migration of this brine has been of great environmental concern to the mine owners. This paper presents the results of a multidimensional interpretation of a two-phase resistivity and magnetotelluric (MT) survey that was performed in an attempt to determine the complex conductivity structure of the mine area and to gain information on brine development and migration pathways. Key resistivity soundings were interpreted using a 2.5D algorithm based on the Polozhii decomposition method. The MT data were interpreted using a 2D finite-element code. A conductivity model was developed, integrating available geological and drill-hole information. The interpretation of the MT data, collected five years after the acquisition of the resistivity data, shows a conductive feature of depth that is not resolved in the resistivity interpretation. This feature has been interpreted as a thick brine zone that has developed as a result of mining during the interval between the resistivity and the MT measurements.     

华北地区上地幔及过渡带电性结构研究

采用远参考道和Robust技术,处理了华北地区14个地磁台站资料,得到了相干度超过0.8的地磁测深响应函数.并将其转换为大地电磁测深的响应函数,获取了105~107 s周期范围内的视电阻率和相位.应用ρ+理论对数据进行了一致性检验和反演,结果表明417km,850km深度附近可能存在电性间断面.同时采用基于一维最光滑模型的Occam反演方法得到了300~1000km范围的地幔电性结构,并与前人在其他地区的研究结果进行了对比.发现华北地区地幔过渡带的电导率在大兴安岭—太行山重力梯度带东西两侧表现不同,重力梯度带附近及西侧台站下方过渡带深度的电导率和北美的Tucson地区相当,而华北地区东部的电导率在地幔过渡带范围高出西侧约2~5倍,这很可能和太平洋板块的俯冲有关.     

Integrated interpretation of the magnetotelluric and magnetic data from Mahallat geothermal field,Iran

Magnetotelluric (MT) and ground magnetic surveys were conducted on the Mahallat geothermal field situated in Markazi province, central Iran, as a primary part of the explorations and developments of a geothermal energy investigation program in the region. Mahallat region has the greatest geothermal fields in Iran. MT survey was performed in November 2011 on an 8 km profile crossing the hot springs with a total of 17 stations. The 2D inversion of the determinant MT data was performed using a 2D inversion routine based on the Occam approach. The 2D resistivity model obtained from the determinant data shows a low resistivity zone at 800-2000 m depth and a higher resistivity zone above the low resistivity zone, interpreted as geothermal reservoir and cap rock, respectively. It also revealed two major concealed faults which are acting as preferential paths for the circulation of hydrothermal fluids. To obtain more geophysical evidence, a ground magnetic survey with 5000 stations was also performed over an area of 200 km² around the MT profile. Magnetic measurements show a main positive anomaly of about +1000 nT over the study area, which could be interpreted as an intrusive body with the high magnetic susceptibility (i.e. mafic and ultramafic rocks) into the sedimentary host rocks. We interpret the body as the heat source of the geothermal system. Structural index and depth estimation of the anomaly indicate that the intrusive body is similar to a cylinder extending from about one kilometer depth down to greater depths. The results of MT and magnetic investigations indicate a geothermal reservoir which proves the preliminary geological observations to a great extent.     

Application of Schlumberger transverse profiling method to detecting a strike fault

Because it is difficult to detect a strike fault, its physical properties are discussed in this paper. Using physical simulation, numerical modeling and the in situ data, the differences between the apparent resistivity of low resistivity model obtained by transverse profiling method （TPM） whose electrode array is vertical to the profile and those by longitudinal profiling method （LPM） whose electrode array is parallel to the profile are analyzed, respectively. Our results show that the former has much marked amplitudes of anomaly. Therefore, TPM can be used to detect a strike fault more effectively and locate it more precisely, and is expected to be a new approach for detecting a sliding fault.     

利用对称四极横向剖面法探测走滑断层的应用

从走滑断层难以探测的实际出发,讨论了其物性特征;通过对低阻板模型的物理模拟、数值模拟以及对实地观测资料的分析,研究了对称四极纵、横向剖面法视电阻率曲线的变化特征和差异. 结果表明,横向剖面法相对纵向剖面法异常幅度明显增加,可对地震活动断层进行更为有效的探测和定位. 这一研究为走滑断层探测提供了新的思路. 同时,利用对称四极横向剖面法可以解决走滑断层难以探测这一技术难题.      

MAGNETO-TELLURIC MEASUREMENTS IN THE NORTH GERMAN BASIN*

Magneto-telluric measurements were carried out at a group of eight sites, from Braunschweig to Lübeck, to determine the resistivity values associated with the North German conductivity anomaly. The data were analyzed for scalar and tensor apparent resistivities over the period range 5–2000 seconds. The results in general indicate very conductive (?I Ω-m) sediments overlying a resistive basement which is strongly lineated. Interpreted basement depth (?6 km) is compatible with published seismic refraction data. Principal conductivity axes in the basement appear to be directly related to trends of salt domes and major troughs. A major change in axis direction occurs near the center of the North German anomaly.     

华北大地电磁测深阵列观测实验与岩石圈导电性快速成像模型

关于中国大陆岩石圈导电性结构研究越来越引起人们的重视,而目前研究大陆岩石圈导电性结构的主要方法是大地电磁测深（Magnetotelluric sounding）.为此,在国家项目"深部探测技术与实验研究（SinoProbe）"专项里开展了"大陆电磁参数标准网实验（SinoProbe-01）"研究,完成了华北1°×1°地理坐标网度的大地电磁测深"标准点"阵列（Array）观测.本文详细论述了华北SinoProbe-01项目1°×1°MT"标准点"阵列观测实验的概况,以及通过精细的MT数据处理和一维Niblett-Bostick变换快速成像,所获取的华北地区岩石圈导电性三维成像模型.在分析华北岩石圈导电性结构特征的基础上,从电性结构角度把华北与邻区岩石圈划分为胶辽、燕山、鲁西、太行—吕梁等低导电性（高电阻率）块体,内蒙古、阿拉善和祁连中等导电性块体和黄淮、鄂尔多斯、秦岭良导电性（低电阻率）块体,进一步从导电性的角度证实了华北克拉通是由多个块体集合而成的观点.     

郯庐断裂带中段(沂沭断裂带)电性结构研究与孕震环境

穿过郯庐断裂带中段（沂沭断裂带,36°N）所做的大地电磁测深（MT）剖面长约150km.使用Robust技术和远参考道大地电磁方法处理观测数据.通过分析视电阻率、阻抗相位、Swift二维偏离度和区域走向,定性确定测区的电性结构.二维反演解释中选择非线性共轭梯度（NLCG）方法,使用TE、TM两种模式资料联合反演,沿剖面的二维电性结构显示：自西向东,鲁西隆起、郯庐断裂带、胶莱坳陷及鲁东隆起4个电性区块分别对应,鲁东和鲁西隆起区为高阻,郯庐断裂带电性结构复杂,高、低阻相间,胶莱坳陷为低阻（高导）区.沿MT剖面附近曾发生3个地震,其震源区处在电性变化剧烈部位,并在震源区附近存在高导体.     

河北石家庄地区深部结构大地电磁探测

石家庄位于太行山隆起带和华北平原冀中坳陷盆地接触区,西邻太行山山前断裂带,1966年在其东南曾发生邢台7.2级强震.为研究该区的深部构造背景,并为分析地震活动性趋势提供基础资料,2010年10月采用宽频带大地电磁法对该区的深部结构进行探测研究.大地电磁剖面穿过石家庄南侧区域,长约167km,获得了64个测点数据.采用远...     

On the Coupling of Geodynamic and Resistivity Models: A Progress Report and the Way Forward

Magnetotelluric (MT) studies represent the structure of crust and mantle in terms of conductivity anomalies, while geodynamic modelling predicts the deformation and evolution of crust and mantle subject to plate tectonic processes. Here, we review the first attempts to link MT models with geodynamic models. An integration of MT with geodynamic modelling requires the use of relationships between conductivity and rheological parameters such as viscosity and melt fraction, which are provided by laboratory measurements of rock properties. Owing to present limitations in our understanding of these relationships, and in interpreting the trade-off between scale and magnitude of conductivity anomalies from MT inversions, most studies linking MT and geodynamic models are qualitative rather than providing hard constraints. Some recent examples attempt a more quantitative comparison, such as a study from the Himalayan continental collision zone, where rheological parameters have been calculated from a resistivity model and compared to predictions from geodynamic modelling. We conclude by demonstrating the potential in combining MT results and geodynamic modelling with examples that directly use MT results as constraints within geodynamic models of ore bodies and studies of an active volcano-tectonic rift.     

激发极化法探测油气田——效果及异常模式探讨

本文是在20多个油气藏上作激发极化法找油气的试验结果。在华北的第三系油田,四川的中生界油气田,新疆的中生、古生界大逆掩断层带油田,以及华东的第三系油田上试验,普遍得到好的效果。各地区的充电率、电阻率及金属因数等参数的反应特点有差异,异常明显程度受盖层影响很大。该方法可以探测到埋深不小于3000m的油气藏,并在许多油气藏发现了具有共同特征的典型异常模式:它在浅部中心部位表现为高阻,高极化;两侧为低阻,低极化;深部表现为低阻,高极化;深浅两带之间为过渡带,异常无固定的特征。此模式可用以分辨油气藏异常,选择合适工作极距,以及研究引起异常的机制等。     

Electric field variations measured continuously in free air over a conductive thin zone in the tilted Lias-epsilon black shales near Osnabrück,Northwest Germany

Common studies on the static electric field distribution over a conductivity anomaly use the self-potential method. However, this method is time consuming and requires nonpolarizable electrodes to be placed in the ground. Moreover, the information gained by this method is restricted to the horizontal variations of the electric field. To overcome the limitation in the self-potential technique, we conducted a field experiment using a non conventional technique to assess the static electric field over a conductivity anomaly. We use two metallic potential probes arranged on an insulated boom with a separation of 126 cm. When placed into the electric field of the free air, a surface charge will be induced on each probe trying to equalize with the potential of the surrounding atmosphere. The use of a plasma source at both probes facilitated continuous and quicker measurement of the electric field in the air. The present study shows first experimental measurements with a modified potential probe technique (MPP) along a 600-meter-long transect to demonstrate the general feasibility of this method for studying the static electric field distribution over shallow conductivity anomalies.Field measurements were carried out on a test site on top of the Bramsche Massif near Osnabrück (Northwest Germany) to benefit from a variety of available near surface data over an almost vertical conductivity anomaly. High resolution self-potential data served in a numerical analysis to estimate the expected individual components of the electric field vector.During the experiment we found more anomalies in the vertical and horizontal components of the electric field than self-potential anomalies. These contrasting findings are successfully cross-validated with conventional near surface geophysical methods. Among these methods, we used self-potential, radiomagnetotelluric, electric resistivity tomography and induced polarization data to derive 2D conductivity models of the subsurface in order to infer the geometrical properties and the origin of the conductivity anomaly in the survey area. The presented study demonstrates the feasibility of electric field measurements in free air to detect and study near surface conductivity anomalies. Variations in Ez correlate well with the conductivity distribution obtained from resistivity methods. Compared to the self-potential technique, continuously free air measurements of the electric field are more rapid and of better lateral resolution combined with the unique ability to analyze vertical components of the electric field which are of particular importance to detect lateral conductivity contrasts. Mapping Ez in free air is a good tool to precisely map lateral changes of the electric field distribution in areas where SP generation fails. MPP offers interesting application in other geophysical techniques e.g. in time domain electromagnetics, DC and IP.With this method we were able to reveal a ca. 150 m broad zone of enhanced electric field strength.     

全球地磁感应测深数据三维反演

全球地磁感应测深能获得地幔转换带及下地幔上部的导电结构.但目前稀疏的地磁台站分布及部分台站的观测数据稳定性较差,影响了三维反演对地下电性结构的分辨力和反演可靠性.为此,区别于传统的L2-范数反演方法,本文提出并实现了基于L1-范数的地磁测深响应三维反演技术.在反演中,利用L1-范数度量数据预测误差,降低"飞点"数据的影响,将相关系数较小的C-响应估计也纳入反演数据中.三维正演模拟采用球坐标系下的交错网格有限差分法,反演采用有限内存拟牛顿法.文中利用指数概率密度分布函数构造非高斯噪声的合成数据,并采用棋盘模型对反演方法的可靠性进行了验证.之后,我们将本文提出的三维反演方法用于全球129个地磁观测台站的C-响应数据反演,结果表明在地幔转换带深部,中国东北地区为高导电异常,南欧和北非则均为高阻;夏威夷在900km以下为高导;菲律宾海及以东地区在转换带表现为明显的高阻,这些结果与前人研究结果一致.由于采用了更多的台站数据,我们的反演结果还发现一些新的异常:南美洲南端,转换带表现为明显的高导;澳大利亚东南部,地幔转换带深部,也存在一个明显的高导异常,这些异常分布和地震层析成像的低速区一致.因此,L1-范数三维反演能够充分利用全球C-响应数据信息,提高地磁测深对地球深部电性结构的分辨能力,更好的研究全球地幔电性结构.     

Volcanic structure of the southern sector of Mt. Etna after the 2001 and 2002 eruptions defined by magnetotelluric measurements

Two magnetotelluric (MT) surveys were carried out on the Mt. Etna volcano after two of the most intense eruptions of the last 30 years which took place in summer 2001 and winter 2002–2003. Surveying was pursued for two main reasons. First, we sought to contribute to the definition of the first-order structure and physico-chemical state (temperature, fluids, melts) of a volcano that has been extensively explored and monitored by means of various geophysical methods, but where only few electrical and electromagnetic surveys have been performed. Secondly, we acquired MT data in the same sites in the two different surveys with the aim of monitoring the possible changes of the first-order structure, since conditions are expected to vary on an active volcano such as Etna, and are supposed to be linked to the eruptive events. Soundings have been acquired in an E-W 10 km-long profile across the southern flank of Mt. Etna, at a distance of almost 6 km south from the Central Crater. The first survey was carried out three months after the 2001 eruption. Inverse models define a pronounced (4 km thickness) low resistivity section at a depth of about 1 km b.s.l. to the west. To the east, a low resistivity section is still present, but appears deeper, thinner and more resistive, and a shallow low resistivity anomaly also exists. The shallow anomaly to the east is tentatively correlated with altered and clayey volcanic units and/or temporary groundwater storage. The deep anomalies are interpreted as being due to melt storage at shallow depths which was not exhausted during the eruption. This would be confirmed by the abundance of lava erupted within one year from the end of the survey. The few good sites retrieved in the second survey, carried out a few weeks after the eruption of 2002–2003, confirm the picture defined in the first survey, and provide a better definition of the bottom of the deep anomaly located in the sedimentary basement.     

A regional strategy for geothermal exploration with emphasis on gravity and magnetotellurics

As part of the resource evaluation and exploration program conducted by Los Alamos Scientific Laboratory for the national Hot Dry Rock (HDR) Geothermal Program, a regional magnetotelluric (MT) survey of New Mexico and Arizona is being performed. The MT lines are being located in areas where the results of analysis of residual gravity anomaly maps of Arizona and New Mexico, integrated with other geologic and geophysical studies indicate the greatest potential for HDR resources.The residual gravity anomalies are derived by applying the concept of predicting gravity anomalies from topography. This can be accomplished by employing reductions similar to those used in some isostatic investigations, in which a regional topographic surface is used as the Bouguer reduction datum. The datum is derived by comparison of various harmonics of Bouguer anomalies and elevations of stations. Topography can be used to predict Bouguer anomalies because of isostatic compensation; the resultant anomalies can be considered high frequency residual anomalies or isostatic anomalies corrected for regional compensation. Such maps have been produced for Arizona, New Mexico, west Texas, and Chihuahua, Mexico.The main objective of the MT project is to produce a regional geoelectric contour map of the pervasive deep electrical conductor within the crust and/or upper mantle beneath the Colorado Plateau and the adjacent Basin and Range Province and Rio Grande Rift. The MT survey consists of 200 sites along several long profiles with site spacing of 15–20 km. Pre-existing available MT data are being integrated with the new data. After the data are processed, a one-dimensional inversion is applied to the sounding curve and used as a starting point for 2-D modeling. Such a project and ultimate map will be of major value in studying the regional geophysics and tectonics of the southwest United States as they now apply to HDR resources in particular and geothermal resources in general.Electrical conductivity anomalies of large areal extent are of particular interest in geothermal exploration. Correlation analysis of large conductive anomalies with other geophysical, geological, and geotectonic data is being performed. Preliminary analysis of the data has suggested several major regions of anomalously shallow high electrical conductivity. Among these is the Aquarius area of northwest Arizona which is the site of a longwavelength residual anomaly low, which when modeled and correlated with other geophysical data can be shown to be possibly related to low density and high temperature in the crust at depths of 20 km or less. Preliminary analysis of MT data indicates the possible existence of a mid-crustal high electrical conductivity anomaly in this same region.     

地电阻率观测装置布设与金属管线类干扰的关系研究——以平凉台为例

以平凉井下地电观测装置为例,采用有限元数值模拟的方法分析地电阻率观测装置布设与金属管线类干扰的关系。结果表明：(1)垂直于测线方向的金属管线对地电阻率观测的影响明显小于平行于测线方向的影响,最有效的抑制方法是增大金属管线的避让距离。(2)金属管线位于布极中心区域时,加深电极埋深并不一定能有效抑制干扰;对于金属管线垂直于测线方向的情况效果是显著的,但对于平行于测线方向的情况效果并不理想。(3)金属管线对井下地电阻率观测的影响是相对复杂的,它与装置系统(观测极距、电极埋深)、分层介质电性结构、金属管线自身的属性(电导率、横截面积、长度、距离)等因素有关。本研究方法可以快速判定资料异常变化的性质,对异常跟踪工作具有一定的参考意义,还可以为新建井下地电观测装置提供理论依据,从而提高台站的观测效能。