共查询到20条相似文献，搜索用时 46 毫秒
1.
A.T. Baokur 《Geophysical Prospecting》1994,42(2):141-149
A new definition of apparent resistivity for the presentation of magnetotelluric sounding data is proposed. The new definition is based on the frequency-normalized impedance function. Both the existing and proposed definitions of apparent resistivity are analysed theoretically and are compared using model curves computed for a 1D earth model. Apparent resistivity curves computed using the proposed definition are a better approximation to the true resistivity values of the subsurface layers. In addition, the layers are more noticeable on the apparent resistivity curves, which is an advantage, especially for the ascending and descending type of apparent resistivity curve. 相似文献
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The magnetotelluric sounding method was used to study the active fault in Shenzhen city.Four magnetotelluric profiles with a dense station interval were laid out across the Shenzhen fault zone.The remote reference technique was used in both data observation and processing to eliminate the electromagnetic noise near the survey sites,and relatively smooth apparent resistivity curves were obtained.TM mode data and two-dimensional inversion method as NLCG were used to obtain the electrical structures underground.According to the surficial geology survey results on regional strata and distribution of magmatic bodies and faults,the electrical structures at depths less than 2000m of each profile were interpreted.Two regional faults,the Henggang-Luohu fault and the Liantang fault,and seven local faults consistent with the electrical boundaries were verified or discovered.The strata in the survey area were then related with the change of resistivity.Electrical horizontal slices of depths less than 5000m indicate that the Henggang-Luohu fault and the Liantang fault bifurcate at the shallow part but tend to merge in the deep part. 相似文献
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大地电磁资料的灵敏度研究 总被引：2，自引：0，他引：2
本文研究了大地电磁资料对地球电性结构模型参数的灵敏度,它在反演解释中具有重要意义.通过研究均匀半空间中视电导率资料的灵敏度函数与计算分析实际观测资料的Fréchet导数和斜率表明,视电导率资料的灵敏度在地表最大,从地表向下呈指数衰减,频率愈高,电导率愈大,衰减也愈快.视电阻率资料和阻抗相位资料对高频、浅层和低阻层具有更高的灵敏度,几何参数的平均灵敏度高于电性参数的平均灵敏度,通常阻抗相位资料比视电阻率资料更灵敏.同时,根据阻抗实部定义的视电阻率与Kunetz定义的时间域大地电磁响应,反映地层电性差异的灵敏度最高. 文章还指出,在实际的反演解释中,如何正确有效地应用阻抗相位资料,提高分辨能力,仍是急待解决的问题. 相似文献
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庐枞矿集区大地电磁测深强噪声的影响规律 总被引：14，自引：10，他引：4
天然大地电磁场信号微弱,极化方向随机,极易受电磁噪声污染.张量阻抗分析、远参考技术、Robust估计等对随机噪声和不相关噪声有比较好的压制效果,但对强的相关噪声目前还没有有效的压制方法.庐枞矿集区人烟稠密、工业发达,是我国著名的铁、铜、硫等矿产基地之一,区内强烈的工业、通讯、矿山、民用等电磁干扰严重污染了大地电磁测深数据.本文首先根据实测的电磁场时间域波形和卡尼亚电阻率测深曲线形态,挑选出基本未受噪声污染的测点(Y1650).然后利用数学形态学从受严重污染的电磁场时间序列中提取出类方波、三角波、阶跃、脉冲和充放电5种典型噪声的波形,并以不同的方式将这些噪声波形与Y1650的电磁场波形叠加,对比分析加噪后Y1650点电阻率和相位测深曲线的变化,进而归纳出典型强噪声对庐枞大地电磁测深资料的影响规律,为进一步压制强噪声和资料处理提供依据. 相似文献
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A new approach is presented for processing noisy magnetotelluric time series, based on a neural network algorithm. This approach structures data and removes noise and distortions from them for the subsequent correct estimation of magnetotelluric response functions. The method was tested on real data contaminated by anthropogenic noise. 相似文献
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John F. Hermance 《Physics of the Earth and Planetary Interiors》1973,7(3):349-364
The processing of magnetotelluric data involves concepts from electromagnetic theory, time series analysis and linear systems theory for reducing natural electric and magnetic field variations recorded at the earth's surface to forms suitable for studying the electrical properties of the earth's interior.The electromagnetic field relations lead to either a scalar transfer impedance which couples an electric component to an orthogonal magnetic component at the surface of a plane-layered earth, or a tensor transfer impedance which couples each electric component to both magnetic components in the vicinity of a lateral inhomogeneity.A number of time series spectral analysis methods can be used for estimating the complex spectral coefficients of the various field quantities. These in turn are used for estimating the nature of the transfer function or tensor impedance. For two dimensional situations, the tensor impedance can be rotated to determine the principal directions of the electrical structure.In general for real data, estimates of the apparent resistivity are more stable when calculated from the tensor elements rather than from simple orthogonal field ratios (Cagniard estimates), even when the fields are measured in the principal coordinates. 相似文献
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可控源音频大地电磁法(CSAMT)一直沿用大地电磁法(MT)的办法,通过计算电场分量与磁场分量的比值,求取卡尼亚视电阻率.而CSAMT场源已知,电场分量和磁场分量都与地下电阻率存在一定的关系,可以单独采用CSAMT电场分量或者磁场分量提取地下介质的视电阻率.本文通过分析电场分量与磁场分量的数据特性,提出利用CSAMT电场单分量数据进行视电阻率的计算,用改进的广义逆矩阵反演方法,使初始模型中的地电层数等于频道个数,克服了以往反演计算中层数较少的问题;实现全场区电场分量视电阻率曲线的拟合反演.同时对单分量视相位计算方法进行分析,结合山西大同地区积水采空区探测及数据解释结果,论证本文提出的单分量解释方法的有效性. 相似文献
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The Millennium uranium deposit is located within the Athabasca Basin of northern Saskatchewan. The basement rocks, comprised primarily of paleo‐Proterozoic gneisses, are electrically resistive. However, the deposit is associated with highly conductive graphitic metasediments that are intercalated with the gneisses. An unconformity separates the basement rocks from the overlying, horizontally stratified, Proterozoic sandstones of the Athabasca Group (which are also highly resistive). The strike extents of the graphitic metasedimenary packages are extensive and therefore electromagnetic (EM) survey techniques are successful at identifying these zones but do not identify the specific locations where they are enriched in uranium. Through drilling it has been noted that hydrothermal processes associated with mineralization has altered the rocks in the vicinity of the deposits, which should in theory result in a resistivity low. A significant resistivity low has been mapped coincident with the Millennium deposit using ground resistivity survey techniques. However, a comparison of the airborne EM and ground resistivity results reveals that the two data sets have imaged different features. The resistive‐limit (on‐time) windows of the MEGATEM data show conductive features corresponding to lakes located to the west and south of the deposit. The late‐time windows show a feature to the east of the deposit, interpreted as being associated with the east‐dipping graphitic basement conductors (similar to that observed in historical ground EM data collected in this area). The early‐time TEMPEST windows (delay times less than 0.2 ms) show a broad resistivity low located at approximately the same location as where the alteration has been identified through drilling. Modelling the data is not easy but a response that decays prior to 0.3 ms is consistent with 500 Ωm material in the sandstone, a resistivity value close to the lower limit with respect to the hydrothermally altered Athabasca group sediments in this area. The MEGATEM system does not see a conductive zone over the alteration as clearly but the high signal‐to‐noise ratio in the late‐time MEGATEM data means that the conductive material at a greater depth is more coherently imaged. 相似文献
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Qibin Xiao Xinping Cai Xingwang Xu Guanghe Liang Baolin Zhang 《Geophysical Prospecting》2010,58(6):1177-1192
The WSINV3DMT code makes the implementation of 3D inversion of magnetotelluric data feasible using a single PC. Audio‐magnetotelluric data were collected along two profiles in a Cu‐Ni mining area in Xinjiang, China, where the apparent resistivity and phase curves, the phase tensors and the magnetic induction vectors indicate a complex 3D conductivity structure. 3D inversions were carried out to reveal the electrical structure of the area. The final 3D model is selected from the inversion results using different initial Lagrange values and steps. The relatively low root‐mean‐square (rms) misfit and model norm indicate a reliable electrical model. The final model includes four types of low resistivity areas, the first ones coincide with the known location of an orebody and further forward modelling indicates that they are not in full connectivity to form a low resistivity zone. The second ones are not controlled by magnetotelluric sites and embody little information of the observed data, they are considered as tedious structures. The third one is near to the regional Kangguer fault and should be treated carefully considering the effect of the fault. The last ones are isolated and existing at a limited level as the first ones, they should be paid more attention to. 相似文献
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为研究AMT数据的"死频带"畸变特征及其校正方法,在长江中下游地区,利用凤凰MTU-5A仪器,系统观测了音频电磁场的"死频带"数据.对不同季节、不同时段的AMT"死频带"数据进行了分析.提出AMT"死频带"频率域畸变数据的Rhoplus校正方法,给出了该方法的适用条件、关键技术与评价方案,并提供了大量实测数据论证其应用效果.观测实验表明:在"死频带"内天然音频电磁场信号强度极低,正交电磁场分量相干度极低;造成了阻抗视电阻率、相位及相位张量等数据的畸变;畸变范围可达10k~100Hz,以5~1kHz频段最为明显;并且数据畸变程度与观测时段密切相关,在长江中下游地区为秋冬季畸变强夏季弱,日间畸变强夜间弱.处理实例表明:Rhoplus校正方法可以有效地处理AMT"死频带"内畸变的视电阻率、相位数据,相干度阈值与人机交互相结合的处理策略快速客观,所得结果曲线光滑连续,数据与实测未畸变数据基本一致. 相似文献
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Field test of sub‐basalt hydrocarbon exploration with marine controlled source electromagnetic and magnetotelluric data 
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下载免费PDF全文 David Myer David Alumbaugh Oliver Hermann Randall Hobbet 《Geophysical Prospecting》2015,63(5):1284-1310
The recent use of marine electromagnetic technology for exploration geophysics has primarily focused on applying the controlled source electromagnetic method for hydrocarbon mapping. However, this technology also has potential for structural mapping applications, particularly when the relative higher frequency controlled source electromagnetic data are combined with the lower frequencies of naturally occurring magnetotelluric data. This paper reports on an extensive test using data from 84 marine controlled source electromagnetic and magnetotelluric stations for imaging volcanic sections and underlying sediments on a 128‐km‐long profile. The profile extends across the trough between the Faroe and Shetland Islands in the North Sea. Here, we focus on how 2.5D inversion can best recover the volcanic and sedimentary sections. A synthetic test carried out with 3D anisotropic model responses shows that vertically transverse isotropy 2.5D inversion using controlled source electromagnetic and magnetotelluric data provides the most accurate prediction of the resistivity in both volcanic and sedimentary sections. We find the 2.5D inversion works well despite moderate 3D structure in the synthetic model. Triaxial inversion using the combination of controlled source electromagnetic and magnetotelluric data provided a constant resistivity contour that most closely matched the true base of the volcanic flows. For the field survey data, triaxial inversion of controlled source electromagnetic and magnetotelluric data provides the best overall tie to well logs with vertically transverse isotropy inversion of controlled source electromagnetic and magnetotelluric data a close second. Vertical transverse isotropy inversion of controlled source electromagnetic and magnetotelluric data provided the best interpreted base of the volcanic horizon when compared with our best seismic interpretation. The structural boundaries estimated by the 20‐Ω·m contour of the vertical resistivity obtained by vertical transverse isotropy inversion of controlled source electromagnetic and magnetotelluric data gives a maximum geometric location error of 11% with a mean error of 1.2% compared with the interpreted base of the volcanic horizon. Both the model study and field data interpretation indicate that marine electromagnetic technology has the potential to discriminate between low‐resistivity prospective siliciclastic sediments and higher resistivity non‐prospective volcaniclastic sediments beneath the volcanic section. 相似文献
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DEEP TECTONIC STRESS CHANGE BASED ON LONG-TERM APPARENT RESISTIVITY TIME SERIES: A CASE STUDY FROM NORTH SECTION OF YISHU FAULT ZONE 下载免费PDF全文
下载免费PDF全文 Almost all the destructive earthquakes, so called tectonic earthquake, are produced by tectonic movement. The tectonic movement will cause resistivity changes in the earth. It's valuable for strong earthquake forecast to study the relationship between the resistivity change and tectonic movement. According to magnetotelluric sounding method, we establish a robust estimate method to calculate the long-term apparent resistivity from seismic station monitoring data. We first use 1D layered earth models to test the sensitivity of this method. Then we process the continually observed long term electromagnetic data in 4 years (from 2011 to 2014) from Anqiu seismic station which is very close to the north section of Yishu fault zone. The apparent resistivity curves and the apparent resistivity time series for different frequencies are obtained. The time series show the resistivity increase in the selected 4 years, which is in good agreement with the GPS baseline data. This also satisfies the results from rock mechanics experiments on deep rocks (upper mantle) that the resistivity will increase with improved stress in a high temperature and pressure environment. The method is valuable for studying electromagnetic precursor of strong earthquakes and strong earthquake prediction. 相似文献
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为了避免形态滤波方法在大地电磁强干扰分离中的"过处理"、进一步保留大地电磁低频段的有用信息,提出基于信噪辨识的矿集区大地电磁噪声压制方法.首先,从信号处理的角度剖析矿集区典型强干扰与天然大地电磁微弱信号之间的定量辨识关系,利用形态分形维数和形态膨胀谱熵对大地电磁信号与强干扰进行信噪辨识.然后,结合形态滤波技术和阈值法,仅对辨识出明显不是天然大地电磁信号的异常波形进行噪声压制.最后,重构大地电磁有用信号,并对算法进行性能评价.仿真结果表明,形态分形维数和形态膨胀谱熵能较好地定量辨识大地电磁信号与强干扰,大地电磁信号中一些缓变化的低频信息得到了更为精细的保留;与形态滤波整体处理相比,本文所提方法获得的卡尼亚电阻率曲线更为光滑、连续,视电阻率值相对稳定,其结果更为真实地反映了测点本身所固有的大地电磁深部构造信息. 相似文献
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3D joint inversion of magnetotelluric and airborne tipper data: a case study from the Morrison porphyry Cu–Au–Mo deposit,British Columbia,Canada 下载免费PDF全文
下载免费PDF全文 Benjamin M. Lee Martyn J. Unsworth Juliane Hübert Jeremy P. Richards Jean M. Legault 《Geophysical Prospecting》2018,66(2):397-421
Z‐axis tipper electromagnetic and broadband magnetotelluric data were used to determine three‐dimensional electrical resistivity models of the Morrison porphyry Cu–Au–Mo deposit in British Columbia. Z‐axis tipper electromagnetic data are collected with a helicopter, thus allowing rapid surveys with uniform spatial sampling. Ground‐based magnetotelluric surveys can achieve a greater exploration depth than Z‐axis tipper electromagnetic surveys, but data collection is slower and can be limited by difficult terrain. The airborne Z‐axis tipper electromagnetic tipper data and the ground magnetotelluric tipper data show good agreement at the Morrison deposit despite differences in the data collection method, spatial sampling, and collection date. Resistivity models derived from individual inversions of the Z‐axis tipper electromagnetic tipper data and magnetotelluric impedance data contain some similar features, but the Z‐axis tipper electromagnetic model appears to lack resolution below a depth of 1 km, and the magnetotelluric model suffers from non‐uniform and relatively sparse spatial sampling. The joint Z‐axis tipper electromagnetic inversion solves these issues by combining the dense spatial sampling of the airborne Z‐axis tipper electromagnetic technique and the deeper penetration of the lower frequency magnetotelluric data. The resulting joint resistivity model correlates well with the known geology and distribution of alteration at the Morrison deposit. Higher resistivity is associated with the potassic alteration zone and volcanic country rocks, whereas areas of lower resistivity agree with known faults and sedimentary units. The pyrite halo and ≥0.3% Cu zone have the moderate resistivity that is expected of disseminated sulphides. The joint Z‐axis tipper electromagnetic inversion provides an improved resistivity model by enhancing the lateral and depth resolution of resistivity features compared with the individual Z‐axis tipper electromagnetic and magnetotelluric inversions. This case study shows that a joint Z‐axis tipper electromagnetic–magnetotelluric approach effectively images the interpreted mineralised zone at the Morrison deposit and could be beneficial in exploration for disseminated sulphides at other porphyry deposits. 相似文献
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针对匹配追踪计算量大、大地电磁数据处理效率低的问题,提出基于匹配追踪和遗传算法的大地电磁噪声压制方法.首先,利用Gabor原子构建过完备原子库,并对过完备原子库集合进行划分.然后,借助遗传算法的自适应性,快速搜寻最优匹配原子及所在位置.最后,运用最优匹配原子对待处理信号进行稀疏分解,重构有用信号.通过对计算机模拟的典型强干扰和矿集区实测大地电磁数据进行分析处理,实验结果表明,相对于匹配追踪和正交匹配追踪,文中所提方法能从过完备原子库中快速、自适应地选取最优匹配原子与不同噪声干扰类型高精度的匹配,极大地提升了计算效率;大地电磁时间域序列中的大尺度强干扰被有效剔除,视电阻率曲线更为光滑、连续,低频段的数据质量得到明显改善. 相似文献
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Near-surface inhomogeneities can distort magnetotelluric (MT) data, thereby limiting interpretational accuracy. In the specific case of near-surface galvanic distortion the resulting frequency independent shift in apparent resistivity sounding curves can be most readily identified in data which are predominantly one-dimensional (1 D). We here consider data from a detailed grid survey which are thoroughly contaminated by parallel offsets. The data were collected over a limited portion of the basalt-covered Paraná basin in Brazil. The data appear largely 1 D over a three decade bandwidth down to a low frequency limit of about 0.1 Hz. A realistic conceptual model consists of a variable thin-sheet over a basin in which the layer parameters vary sufficiently slowly (laterally) that a 1 D interpretation of the static corrected data is valid down to some maximum depth (in this case basement). We consider the performance of three methods which attempt to remove static offsets and provide assessments of the configuration of the geoelectric units of the basin. The three methods consist of curve shifting, statistical/spatial averaging, and the application of parameter constraints. The unambiguous removal of static offsets undoubtedly requires one or more independent constraints at each measurement location. In the absence of such control the three methods are necessarily statistical and each must supply a constraint that has least conflict with the data characteristics. Simple spatial averaging of our data proves effective but the degree of success is generally not known. Parametric constraints, applied as uniform layer resistivities, allow phase-only (non-distorted) data inversion. Inverse models obtained from this approach are found to provide accurate estimates of the depth to the main resistivity interface, the base of the basalt cover, in the vicinity of a control well. 相似文献
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时间域航空电磁数据往往在测量过程中受到天然和人文噪声的干扰.如果不能很好滤除这些电磁噪声,那么将会降低资料质量、影响反演的精度,甚至获得错误的解释结果.本文提出了一种基于核主成分分析的去噪方法,通过核主成分分析提取叠加后衰减曲线的主成分,然后使用能量占比方法分离反映地下介质的有效信号和噪声,最后使用反映地下介质的特定成分进行重构.本文所推荐的去噪方法不仅能剔除天然噪声,例如天电产生的尖脉冲或者振荡,而且能有效地抑制人文噪声.分别使用基于核主成分分析的去噪方法,以及AeroTEM软件的处理方法对同样的吊舱式时间域直升机航空电磁勘查系统实测数据进行处理,并比较其结果.处理结果表明：所推荐的去噪方法要优于AeroTEM软件. 相似文献
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We report on strong coast effect distortions observed for broadband marine magnetotelluric (MT) data collected on the forearc offshore northeastern Japan. Eight days of horizontal electric and magnetic fields recorded at eight seafloor stations and the horizontal magnetic fields from a land remote station were processed with a robust multiple-station algorithm, yielding good MT responses and inter-station transfer functions at periods of 7–10,000 s. Transverse electric (TE) mode responses have cusps in apparent resistivity and negative phases at periods around 1000 s, while the transverse magnetic (TM) mode responses are galvanically depressed below the TE responses. An analysis of inter-station transfer functions confirms that the apparent resistivity cusps are a magnetic field, rather than electric field, phenomenon, consisting of an amplitude minimum and rapid phase change around a characteristic frequency. Poynting vectors for a TE coast effect model study illustrate that the anomalous phases are associated with energy diffusing back up to the seafloor from below, after being turned around from its usual downward propagating trajectory by inductive coupling between the conductive ocean and the resistive seafloor along the continental margin. We show that the characteristic frequency and position of the TE mode apparent resistivity cusps are determined by a relatively simple combination of the electrical resistivity of the seafloor, the depth of the ocean, and the distance from the coastline. By including coastlines and bathymetry in 2D inversion, we recover the seafloor conductivity structure along the forearc, demonstrating that broadband data can constrain the thickness of conductive forearc sediments and the underlying high resistivity associated with the mantle wedge and subducting oceanic lithosphere. 相似文献