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1.
A radio magnetotelluric study to evaluate the extents of a limestone quarry in Estonia 总被引：1，自引：0，他引：1
Electromagnetic signals from distant radio transmitters in the frequency range 15–250 kHz were measured to model an electrical resistivity structure beneath 7 profiles in the vicinity of the Karinu limestone quarry in Estonia with the aim to map the extent of the economically exploitable limestone. The resistivity models from a 2D inversion of determinant resistivity and phase values using an Occam type of regularization contained reasonably accurate information about the geometry, namely depth to the top and the bottom of the target high‐resistivity limestone. The resistivity models correlated well with existing geological evidences as well as information from closely located boreholes. However, the sharp lithological boundaries seen in the boreholes were not resolved exactly in the resistivity models. This is probably because of the smoothing regularization used in the inversion process. Combined use of borehole data together with resistivity models resulted in two major geological interpretations; a) towards the western part of the existing limestone quarry there is a NNW to NS striking fault, covered by post‐glacial sediments, b) a potential cost‐effective exploitable area containing high quality highly resistive limestone is located south of the existing quarry. This case study shows the applicability of the reasonably fast radio magnetotelluric (RMT) method for the exploration of near‐surface resources. 相似文献
2.
G. Michael Hoversten 《Geophysical Prospecting》1996,44(6):935-961
Hydrocarbon exploration in the Papuan fold belt is made extremely difficult by mountainous terrain, equatorial jungle and thick karstified Miocene limestones at the surface. The high-velocity karstified limestones at or near the surface often render the seismic technique useless for imaging the subsurface. In such areas magnetotellurics (MT) provides a valuable capability for mapping subsurface structure. The main structural interface which can be mapped with MT, due to the large electrical contrast, is the contact between the resistive Darai limestone and the underlying conductive sediments of the Ieru Formation. In some areas the base of the Darai can be mapped with reasonable accuracy by fitting 1D models to the observed MT data. However, in many cases where 2D and 3D effects are severe, 1D interpretations can yield dramatically incorrect results. Numerical and field data examples are presented which demonstrate the severity of the 1D errors and the improvements in accuracy which can be achieved using a 2D inverse solution. Two MT lines over adjacent anticlines, both with well control and seismic data, are used to demonstrate the application of 1D and 2D inversions for structural models. In both cases the seismic data provide no aid in the interpretations. The example over the Hides anticline illustrates a situation where 1D inversion of either TE or TM mode provides essentially the same depth to base of Darai as 2D inversion of both TE and TM. Both models provide base Darai depth estimates which are within 10% of that measured in the well. The example over the Angore anticline illustrates the inadequacy of 1D inversion in structurally complex geology complicated by electrical statics. The TE mode fits a 1D Darai thickness of 800 metres while the TM mode fits a 1D Darai thickness of 3500 metres, bracketing the thickness of 2450 metres observed in the well. The final 2D inversion model provides a depth estimate of 2250 metres. Four MT lines along the Angore anticline have been interpreted using 2D inversion. A high degree of correlation exists between lineaments observed on an airborne radar image and zones of low resistivity within the high-resistivity material interpreted as Darai limestone. These low-resistivity zones are interpreted as fault zones. Three-dimensional modelling has been used to simulate 3D statics in an otherwise 2D earth. These data were used to test the Groom-Bailey (GB) decomposition for possible benefits in reducing static effects and estimating geoelectric strike in the Papua New Guinea (PNG) field data. It has been found that the GB decomposition can provide improved regional 2D strike estimates in 3D contaminated data. However, in situations such as PNG, where the regional 2D strike is well established and hence can be fixed, the GB decomposition provides apparent resistivities identical to those simply rotated to strike. 相似文献
3.
岷山隆起带与西秦岭构造带中段位于青藏高原物质东向流动的必经之处,又是南北地震带的组成部分和GPS速度场非连续性衰减和转换的关键部位,其地壳结构及地壳变形机制受到国内外地质地球物理学家的广泛关注,了解研究区深部细结构及主要边界断裂空间展布特征,对青藏高原隆升机制及中强地震孕震构造的研究有重要意义.本文依托分别横跨岷山隆起带及西秦岭构造带中段的两条大地电磁剖面(SG-WQL-L1与SG-WQL-L2)小点距观测数据,采用大地电磁相位张量分解技术对两条剖面上各个测点的电性走向、二维偏离度进行计算分析,根据分析结果对原始数据进行主轴方位角校正处理,进一步采用NLCG(非线性共轭梯度)二维反演方法开展TE与TM模式的相位和电阻率联合反演,获取沿剖面方向30 km以浅的电阻率结构模型,并完成了地质地球物理综合解释.两条大地电磁剖面勘探成果揭示出,马尔康地块中上地壳发育的壳内低阻层与峨山隆起上地壳低阻体在深部交汇,岷江断裂带与虎牙断裂带受控于马尔康地块与岷山隆起带上地壳底部的滑脱面,滑脱面呈现往东角度逐渐变陡峭的趋势且在岷江附近出现"断坡"构造,历史强震震源深度显示虎牙断裂为岷山隆起带新生代强震的发震断裂;西秦岭构造带中段中上地壳沿剖面方向表现为横向分块、纵向分层的电性结构特征,中地壳12~25 km左右发育厚度不等的壳内低阻层,壳内低阻层多与研究区次级地块的边界断裂在深部交汇,次级地块以及区分次级地块的活动断裂带可能是GPS速度场在研究区呈现非连续性的递减并伴随方向转换的构造成因;青藏高原内部的软流圈物质向NE和SSE流动,驱动巴颜喀拉地块东缘上地壳沿中上地壳低阻层东向运移,受到摩天岭高阻地块的阻挡作用,软弱的岷山隆起带发生地壳褶皱变形并向东逆冲推覆从而形成高耸的岷山山脉,岷江断裂与虎牙断裂的左旋运动加速了岷山的隆起. 相似文献
4.
A. Gabàs A. Macau B. Benjumea P. Queralt J. Ledo S. Figueras A. Marcuello 《Surveys in Geophysics》2016,37(5):897-921
The structure of Cerdanya Basin (north-east of Iberian Peninsula) is partly known from geological cross sections, geological maps and vintage geophysical data. However, these data do not have the necessary resolution to characterize some parts of Cerdanya Basin such as the thickness of soft soil, geometry of bedrock or geometry of geological units and associated faults. For all these reasons, the main objective of this work is to improve this deficiency carrying out a detailed study in this Neogene basin applying jointly the combination of passive seismic methods (H/V spectral ratio and seismic array) and electromagnetic methods (audio-magnetotelluric and magnetotelluric method). The passive seismic techniques provide valuable information of geometry of basement along the profile. The maximum depth is located near Alp village with a bedrock depth of 500 m. The bedrock is located in surface at both sites of profile. The Neogene sediments present a shear-wave velocity between 400 and 1000 m/s, and the bedrock basement presents a shear-wave velocity values between 1700 and 2200 m/s. These results are used as a priori information to create a 2D resistivity initial model which constraints the inversion process of electromagnetic data. We have obtained a 2D resistivity model which is characterized by (1) a heterogeneous conductivity zone (<40 Ohm m) that corresponds to shallow part of the model up to 500 m depth in the centre of the profile. These values have been associated with Quaternary and Neogene sediments formed by silts, clays, conglomerates, sandstones and gravels, and (2) a deeper resistive zone (1000–3000 Ohm m) interpreted as Palaeozoic basement (sandstones, limestones and slates at NW and conglomerates and microconglomerates at SE). The resistive zone is truncated by a discontinuity at the south-east of the profile which is interpreted as the Alp-La Tet Fault. This discontinuity is represented by a more conductive zone (600 Ohm m approx.) and is explained as a combination of fractured rock and a fluid network. The result highlights that the support between different geophysical methods is essential in producing geophysical meaningful models. 相似文献
5.
We have combined tensor radio magnetotelluric- (RMT, 15–250 kHz) and controlled source tensor magnetotelluric (CSTMT, 1–12 kHz) data for the mapping of aquifers in gravel formations lying in between crystalline bedrock and clay rich sediments in the Heby area some 40 km west of Uppsala in Sweden. The estimated transfer functions, the impedance tensor and the tipper vector generally satisfy 1D or 2D necessary conditions except for the lowest CSTMT frequencies where near field effects become more dominant.The data measured from 8 profiles were inverted with the Rebocc code of Siripunvaraporn and Egbert (2000) assuming plane wave conditions. This meant that only 12 frequencies in the range of 4–180 kHz could be used. The four lowest frequencies of CSTMT in the range of 1–2.8 kHz were excluded because of source effects. Data from all profiles were inverted with a starting model of 100 Ω-m and a relative error floor of 0.02 on apparent resistivity, corresponding to less than 1° on phase. Tipper vectors are generally small except when source effects become dominant in the lowest frequencies of CSTMT and were therefore not used for inversion. Comparing with models derived from vertical electrical soundings, refraction and reflection seismic data as well as ground truth from exploration wells assessed the reliability of the deep part of the models. Furthermore we carried out a non-linear resolution analysis to better quantify the depth extent of the aquifers.The inverted models from the Heby area show well the thickness variations of glacial deposits overlying crystalline bedrock. Generally, the upper 20 m of the models have resistivities below 40 Ω-m, taken to represent clay rich formations. Below the clay layer resistivities increase to about 40–400 Ω-m, interpreted to represent sand/gravel formations with a maximum thickness of about 40 m and a width of several hundred metres. This is a potential aquifer that extends in approximately N–S direction for some kilometres. 相似文献
6.
Joint inversion of long‐offset and central‐loop transient electromagnetic data: Application to a mud volcano exploration in Perekishkul,Azerbaijan 
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下载免费PDF全文 J. Adrian R. Bergers M. Gurk B. Tezkan A. L. Mammadov A. G. Novruzov 《Geophysical Prospecting》2015,63(2):478-494
Mud volcanism is commonly observed in Azerbaijan and the surrounding South Caspian Basin. This natural phenomenon is very similar to magmatic volcanoes but differs in one considerable aspect: Magmatic volcanoes are generally the result of ascending molten rock within the Earth's crust, whereas mud volcanoes are characterised by expelling mixtures of water, mud, and gas. The majority of mud volcanoes have been observed on ocean floors or in deep sedimentary basins, such as those found in Azerbaijan. Furthermore, their occurrences in Azerbaijan are generally closely associated with hydrocarbon reservoirs and are therefore of immense economic and geological interest. The broadside long‐offset transient electromagnetic method and the central‐loop transient electromagnetic method were applied to study the inner structure of such mud volcanoes and to determine the depth of a resistive geological formation that is predicted to contain the majority of the hydrocarbon reservoirs in the survey area. One‐dimensional joint inversion of central‐loop and long‐offset transient electromagnetic data was performed using the inversion schemes of Occam and Marquardt. By using the joint inversion models, a subsurface resistivity structure ranging from the surface to a depth of approximately 7 km was determined. Along a profile running perpendicular to the assumed strike direction, lateral resistivity variations could only be determined in the shallow depth range using the transient electromagnetic data. An attempt to resolve further two‐dimensional/three‐dimensional resistivity structures, representing possible mud migration paths at large depths using the long‐offset transient electromagnetic data, failed. Moreover, the joint inversion models led to ambiguous results regarding the depth and resistivity of the hydrocarbon target formation due to poor resolution at great depths (>5 km). Thus, 1D/2D modelling studies were subsequently performed to investigate the influence of the resistive terminating half‐space on the measured long‐offset transient electromagnetic data. The 1D joint inversion models were utilised as starting models for both the 1D and 2D modelling studies. The results tend to show that a resistive terminating half‐space, implying the presence of the target formation, is the favourable geological setting. Furthermore, the 2D modelling study aimed to fit all measured long‐offset transient electromagnetic Ex transients along the profile simultaneously. Consequently, 3125 2D forward calculations were necessary to determine the best‐fit resistivity model. The results are consistent with the 1D inversion, indicating that the data are best described by a resistive terminating half‐space, although the resistivity and depth cannot be determined clearly. 相似文献
7.
《Journal of Applied Geophysics》2006,58(4):227-241
We compare the resolution power of single frequency very low frequency (VLF) electromagnetic data (real and imaginary parts of the tipper) and multi-frequency RadioMagnetoTelluric (RMT) data in delineating conductive structures typical for the sedimentary cover over crystalline basement in Scandinavia. Using VLF field data from five parallel profiles reveals that the estimated models have responses that fit the observed data well, and the models show an overall agreement with more detailed models derived from broadband RMT data. It is suggested that VLF data be used as a fast mapping tool to fill in the gaps between profiles along which more detailed RMT measurements are made. A generic model with conductive clay lenses and sandy formations over crystalline basement is used to generate synthetic data for the two cases. Using regularized inversion the corresponding estimated models clearly shows the strength and the weakness of both methods. Being inductive methods, they both have difficulties in clearly resolving the depth transition from conductive to resistive units. Especially the single frequency VLF data can be interpreted with very smooth models at depth. However, both methods resolve very well the lateral boundaries of the clay lenses and the RMT data also constrain the thickness of the clays quite well compared with the VLF models, which are less distinct at depth. Single frequency scalar VLF data emphasize those conductive structures that have dominant strikes in the direction of the transmitter. Multi-frequency VLF (tensor VLF) measurements provide the tipper vector which depends upon the underlying conductivity structure only. Real conductivity structures have significant 3D components which can be delineated easily by tensor VLF measurements. We propose that new VLF instrumentation be developed with this in mind. 相似文献
8.
在横跨鄂尔多斯盆地西缘构造带北段的查甘池—银川—五湖洞约200 km长的东西向剖面上,进行了67个测点的大地电磁探测.使用“远参考道”和Robust技术处理数据.分析了各测点视电阻率、阻抗相位、二维偏离度、电性主轴方位角、磁实感应矢量等参数,采用NLCG二维反演方法对TE和TM两种模式的数据进行了二维反演.得到的二维电性结构表明,沿剖面查汗断裂带、贺兰山东缘断裂带和黄河断裂带是明显较大型电性边界,为超壳断裂带,而三关口断裂带深部延深不大.沿剖面阿拉善地块、贺兰山褶皱带、银川断陷盆地和鄂尔多斯地块具有明显不同的深部电性结构特征.阿拉善地块内部除浅表电阻率较低外,以下到深度约50 km都表现为高电阻特性.贺兰山褶皱带电性结构复杂,电阻率高低相间.银川盆地具有上宽下窄最深达约8 km低阻层,具有断陷盆地特征.鄂尔多斯地块具有低-高-低的深部电性结构特征,成层性较明显. 相似文献
9.
A. Gabàs A. Macau B. Benjumea F. Bellmunt S. Figueras M. Vilà 《Surveys in Geophysics》2014,35(4):983-1002
10.
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. 相似文献
11.
射频大地电磁法(RMT)是以潜艇天线发射的射线源等作为场源的一种地球物理勘探方法,目前被广泛应用于近地表工程和环境地球物理勘探.RMT数据解释常采用基于静态假设的大地电磁法(MT)程序,往往会反演出不真实的浅层目标体.为解决这一长期困扰RMT资料解释的问题,本文实现了考虑位移电流的RMT有限元数值模拟.为了处理任意复杂模型,如起伏地形,非结构的三角形网格被用于离散RMT模型.首先通过算例对比,验证了程序的正确性和可靠性.通过Dike模型讨论了空气层厚度对RMT数值解的影响,结果表明当空气层厚度大于1/4波长即可满足精度要求.以山脊模型为例计算了位移电流对RMT响应的影响,表明位移电流的影响会随着山脊高程的增加而增大.最后通过舒家店实际模型进一步验证了位移电流在RMT中的重要性. 相似文献
12.
Anicet Beauvais Michel Ritz Jean‐Claude Parisot Christian Bantsimba 《地球表面变化过程与地形》2003,28(10):1071-1080
An Erratum has been published for this article in Earth Surface Processes and Landforms 28(13) 2003, 1491. Granite domes, boulders and knobs buried within saprolite have been detected beneath lateritic weathering landsurfaces using 2D electrical resistivity tomography (ERT). This technique provides a valuable means of mapping the bedrock topography and the regolith structures underneath landsurfaces, as it is intrinsically very sensitive to the electrical properties of superimposed pedological, hydrological and geological layers, allowing the determination of their relative geometry and spatial relationships. For instance, 2D inverse electrical resistivity models including topographic data permit the de?nition of lithostratigraphic cross‐sections. It shows that resistive layers, such as the more or less hardened ferruginous horizons and/or the bedrock, are generally well differentiated from poorly resistive layers, such as saprolite, including water‐saturated lenses, as has been corroborated by past and actual borehole observations. The analysis of the 2D geometrical relations between the weathering front, i.e. the bedrock topography, and the erosion surface, i.e. the landsurface topography, documents the weathering and erosion processes governing the development of the landforms and the underlying structures, thus allowing the etching hypothesis to be tested. The in?ltration waters are diverted by bedrock protrusions, which behave as structural thresholds compartmentalizing the saprolite domain, and also the regolith water table, into distinct perched saturated subdomains. The diverted waters are thus accumulated in bedrock troughs, which behave like underground channels where the saprolite production rate may be enhanced, provided that the water drainage is ef?cient. If the landsurface topography controls the runoff dynamics, the actual bedrock topography as depicted by ERT imaging in?uences the hydrodynamics beneath the landsurface. In some way, this may control the actual weathering rate and the shaping of bedrock protrusions as granite domes and knobs within thick saprolite, before their eventual future exposure. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
13.
Application of 2D magnetotelluric methods in a geological complex area, Xinjiang, China 总被引：2，自引：0，他引：2
Qibin Xiao Xinping CaiGuanghe Liang Xingwang XuBaolin Zhang 《Journal of Applied Geophysics》2011,75(1):19-30
Magnetotelluric (MT) data along two profiles were collected in Tulargen Cu-Ni mining area, Xinjiang, China. The MT profiles, crossing the known ore-bearing magmatic body, are interpreted to have further understanding of the ore-bearing magmatic bodies' stretch and shape in depth, and to evaluate the prospect in the mining area. The values of phase sensitivity skewness indicate a contaminated two-dimensional (2D) structure (there exist local three-dimensional (3D) structures at shallow level) over 3D structures. G-B decomposition of the impedance tensors indicates the dominant geoelectrical strike in survey area is in East-West direction. To obtain reliable electrical structures, four different 2D inversion codes as REBOCC, NLCG, OCCAM, and ABIC are adopted, inversions of a synthetic 3D model using these codes indicate TM data are more suitable to invert the electrical structures along the profiles in Tulargen Cu-Ni mining area, while TE and TE + TM inversions can be used to verify the transverse variation of electrical structures at shallow level. The 2D electrical structures show high-resistivity basement covered by relatively low-resistivity strata in the mining area, the conductive anomalies are imbedded in the upper covers. The known ore-bearing magmatic body is coincided with the positions of two conductive anomalies in both profiles at shallow level; its extension at depth is deduced according to the shapes of the anomalies. Other conductive anomalies, isolated and existing only at limited depth as the known ore-bearing magmatic body, are presumed of good prospect. 相似文献
14.
A combination of magnetotelluric (MT) measurements on the surface and in boreholes (without metal casing) can be expected to enhance resolution and reduce the ambiguity in models of electrical resistivity derived from MT surface measurements alone. In order to quantify potential improvement in inversion models and to aid design of electromagnetic (EM) borehole sensors, we considered two synthetic 2D models containing ore bodies down to 3000 m depth (the first with two dipping conductors in resistive crystalline host rock and the second with three mineralisation zones in a sedimentary succession exhibiting only moderate resistivity contrasts). We computed 2D inversion models from the forward responses based on combinations of surface impedance measurements and borehole measurements such as (1) skin-effect transfer functions relating horizontal magnetic fields at depth to those on the surface, (2) vertical magnetic transfer functions relating vertical magnetic fields at depth to horizontal magnetic fields on the surface and (3) vertical electric transfer functions relating vertical electric fields at depth to horizontal magnetic fields on the surface. Whereas skin-effect transfer functions are sensitive to the resistivity of the background medium and 2D anomalies, the vertical magnetic and electric field transfer functions have the disadvantage that they are comparatively insensitive to the resistivity of the layered background medium. This insensitivity introduces convergence problems in the inversion of data from structures with strong 2D resistivity contrasts. Hence, we adjusted the inversion approach to a three-step procedure, where (1) an initial inversion model is computed from surface impedance measurements, (2) this inversion model from surface impedances is used as the initial model for a joint inversion of surface impedances and skin-effect transfer functions and (3) the joint inversion model derived from the surface impedances and skin-effect transfer functions is used as the initial model for the inversion of the surface impedances, skin-effect transfer functions and vertical magnetic and electric transfer functions. For both synthetic examples, the inversion models resulting from surface and borehole measurements have higher similarity to the true models than models computed exclusively from surface measurements. However, the most prominent improvements were obtained for the first example, in which a deep small-sized ore body is more easily distinguished from a shallow main ore body penetrated by a borehole and the extent of the shadow zone (a conductive artefact) underneath the main conductor is strongly reduced. Formal model error and resolution analysis demonstrated that predominantly the skin-effect transfer functions improve model resolution at depth below the sensors and at distance of \(\sim \) 300–1000 m laterally off a borehole, whereas the vertical electric and magnetic transfer functions improve resolution along the borehole and in its immediate vicinity. Furthermore, we studied the signal levels at depth and provided specifications of borehole magnetic and electric field sensors to be developed in a future project. Our results suggest that three-component SQUID and fluxgate magnetometers should be developed to facilitate borehole MT measurements at signal frequencies above and below 1 Hz, respectively. 相似文献
15.
横跨大兴安岭与海拉尔盆地和松辽盆地结合地带的大地电磁测深剖面揭示了盆山构造的深部电性结构.剖面西起海拉尔盆地东缘,向东延伸穿过大兴安岭中部,一直到达松辽盆地西缘.本文对剖面测点的二维偏离度、构造走向等进行了计算和分析,采用非线性共轭梯度(NLCG)二维反演方法对TM模式的数据进行了反演,获得了该剖面的地壳、上地幔电性结构模型,划分出三个典型构造单元:海拉尔盆地、大兴安岭和松辽盆地.研究结果表明,海拉尔盆地东缘和松辽盆地西缘浅部都呈低阻特征,但松辽盆地西缘深部电性结构比较复杂,而大兴安岭整体呈高阻特征.海拉尔盆地东缘可能属于兴安块体,松辽盆地西缘与大兴安岭接触关系复杂.海拉尔盆地东缘岩石圈厚度约为110km,大兴安岭岩石圈厚度约为110~150km.大兴安岭上地壳基本呈高阻特征,可能为多次叠置的岩浆岩,代表大兴安岭经历了多期次岩浆作用;中下地壳横向存在较大范围低阻体,可能反映了大兴安岭地壳内部非刚性的特点;残存在岩石圈地幔的高阻异常,说明其下地壳可能发生过拆沉作用.大兴安岭与松辽盆地结合带存在一个岩石圈尺度的西倾低阻带,向下延伸到岩石圈底部,可能是早期松嫩地块向兴安地块俯冲并以软碰撞形式拼合的构造遗迹. 相似文献
16.
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。 相似文献
17.
针对大地电磁二维反演中TE、TM极化模式的选择问题,设计了多个二维、三维模型进行了正演计算,从数据对比和反演结果对比两方面入手,分析研究了三维模型和二维模型响应数据的差异、反演中极化模式的选择、以及三维模型数据的二维反演近似等问题.研究结果表明:在三维模型条件下,利用二维模型进行反演时,TE模式对模型的二维的近似程度要求远高于TM模式;当三维结构影响较明显时,利用TM模式数据进行二维反演比利用TE模式或利用TE+TM模式联合反演都更合理,反演结果中的虚假结构明显减少;对于TM模式,相位受三维畸变影响较小,视电阻率较大,所以二维反演中可适当加大相位的权;对于实测数据的二维反演,应优先考虑采用TM模式数据进行二维反演,其次是TM+TE模式,一般不要单独采用TE模式. 相似文献
18.
西秦岭造山带与南北地震构造带接触区是中国大陆最重要的南北向和东西向构造转化的接合部位之一.本文介绍了分别位于该区106°E东、西两侧的LMS-L3和DBS-L1两条大地电磁剖面的探测结果,两条剖面分别跨过了龙门山构造带东北段的青川段和宁强段.采用大地电磁相位张量分解技术对两条剖面上各测点的电性走向、二维偏离度等进行了计算和分析,采用NLCG二维反演方法对TE+TM模式的视电阻率和阻抗相位数据进行了二维联合反演.反演得到二维电性结构,在经度106°西侧LMS-L3剖面的深部电性结构自北向南揭示出,西秦岭北缘、成县盆地北缘、康县（即勉略构造带）和平武-青川断裂带都表现为明显的电性梯度带,深部延伸可达几十公里;西秦岭造山带、碧口地块与龙门山构造带东北段3个构造单元整体表现为高电阻体、呈现往南叠合且角度逐渐变陡的趋势.在106°E西侧西秦岭造山带区域的深部存在壳内低阻层,而东侧区域表现为高电阻体,深部电性结构在106°E东、西两侧的差异与该区深部速度结构特征一致,东、西两侧深部结构差异可能是该区中强地震分布差异的深层原因.LMS-L3和DBS-L1两条剖面南段的深部电性结构图像揭示出龙门山构造带东北部的青川段和宁强段内的平武-青川断裂带具有明显不同的深部结构特征,平武-青川断裂带在青川段为明显的电性梯度带,在宁强段不再表现为电性梯度带,而是完整的高电阻块体.汶川强余震向东北发展止于青川青木川附近,与平武-青川断裂带延伸深度和向北东方向的延伸长度密切相关,同时高电阻块体的宁强段对汶川强余震东北发展起到了阻挡作用. 相似文献
19.
In many coastal areas of North America and Scandinavia, post-glacial clay sediments have emerged above sea level due to iso-static uplift. These clays are often destabilised by fresh water leaching and transformed to so-called quick clays as at the investigated area at Smørgrav, Norway. Slight mechanical disturbances of these materials may trigger landslides. Since the leaching increases the electrical resistivity of quick clay as compared to normal marine clay, the application of electromagnetic (EM) methods is of particular interest in the study of quick clay structures.For the first time, single and joint inversions of direct-current resistivity (DCR), radiomagnetotelluric (RMT) and controlled-source audiomagnetotelluric (CSAMT) data were applied to delineate a zone of quick clay. The resulting 2-D models of electrical resistivity correlate excellently with previously published data from a ground conductivity metre and resistivity logs from two resistivity cone penetration tests (RCPT) into marine clay and quick clay. The RCPT log into the central part of the quick clay identifies the electrical resistivity of the quick clay structure to lie between 10 and 80 Ω m. In combination with the 2-D inversion models, it becomes possible to delineate the vertical and horizontal extent of the quick clay zone. As compared to the inversions of single data sets, the joint inversion model exhibits sharper resistivity contrasts and its resistivity values are more characteristic of the expected geology. In our preferred joint inversion model, there is a clear demarcation between dry soil, marine clay, quick clay and bedrock, which consists of alum shale and limestone. 相似文献
20.
Three‐dimensional modelling of magnetotelluric data to image Sehqanat hydrocarbon reservoir in southwestern Iran 下载免费PDF全文
下载免费PDF全文 A detailed magnetotelluric survey was conducted in 2013 in the Sehqanat oil field, southwestern Iran to map the geoelectrical structures of the sedimentary Zagros zone, particularly the boundary between the Gachsaran Formation acting as cap rock and the Asmari Formation as the reservoir. According to the electrical well logs, a large resistivity contrast exists between the two formations. The Gachsaran Formation is formed by tens to hundreds of metres of evaporites and it is highly conductive (ca. 1 Ωm–10 Ωm), and the Asmari Formation consists of dense carbonates, which are considerably more resistive (more than 100 Ωm). Broadband magnetotelluric data were collected along five southwest–northeast directed parallel lines with more than 600 stations crossing the main geological trend. Although dimensionality and strike analysis of the magnetotelluric transfer functions showed that overall they satisfied local 2D conditions, there were also strong 3D conditions found in some of the sites. Therefore, in order to obtain a more reliable image of the resistivity distribution in the Sehqanat oil field, in addition to standard 2D inversion, we investigated to what extent 3D inversion of the data was feasible and what improvements in the resistivity image could be obtained. The 2D inversion models using the determinant average of the impedance tensor depict the main resistivity structures well, whereas the estimated 3D model shows significantly more details although problems were encountered in fitting the data with the latter. Both approaches resolved the Gachsaran–Asmari transition from high conductivity to moderate conductivity. The well‐known Sehqanat anticline could also be delineated throughout the 2D and 3D resistivity models as a resistive dome‐shaped body in the middle parts of the magnetotelluric profiles. 相似文献