Electrical Structure in Marine Tectonic Settings     

Kiyoshi Baba 《Surveys in Geophysics》2005,26(6):701-731

This review paper presents recent research on electrical conductivity structure in various marine tectonic settings. In at least three areas, marine electromagnetic studies for structural exploration have increasingly progressed: (1) data accumulations, (2) technical advances both for hardware and software, and (3) interpretations based on multidisciplinary approaches. The mid-ocean ridge system is the best-studied tectonic setting. Recent works have revealed evidence of conductive zones of hydrothermal circulation and axial magma chambers in the crust and partial melt zones of the mid-ocean ridge basalt source in the mantle. The role of water or dissolved hydrogen and its redistribution at mid-ocean ridges is emphasized for the conductivity pattern of the oceanic lithosphere and asthenosphere. Regions of mantle upwelling (hotspot or plume) and downwelling (subducting slab) are attracting attention. Evidence of heterogeneity exists not only in the crust and the upper mantle, but also in the mantle transition zone. Electrical conductive zones frequently overlap seismic low-velocity zones, but discrepancies are also apparent. Some studies have compared conductivity models with the results of seismic and other studies to investigate the physical properties or processes. A new laboratory-based conductivity model for matured oceanic lithosphere and asthenosphere is proposed. It takes account of both the water distribution in the mantle as well as the thermal structure. It explains observed conductivity patterns in the depth range of 60–200 km.  相似文献   

Effects of galvanic distortion on magnetotelluric data over a three-dimensional regional structure   总被引：2，自引：0，他引：2  

Juanjo Ledo  Pilar Queralt  & Jaume Pous 《Geophysical Journal International》1998,132(2):295-301

Although the galvanic distortion due to local, near-surface inhomogeneities is frequency-independent, its effect on the magnetotelluric data becomes, in a 3-D structure, frequency-dependent. Therefore, both the apparent resistivity and the phase responses are disturbed, and a correction should be carried out prior to the 3-D interpretation in order to retrieve the 3-D regional impedance tensor. In many cases, the structure is 2-D for depths corresponding to a first range of periods and 3-D for longer periods (called 2-D/3-D). For these cases, a simple method which allows us to retrieve the 3-D regional impedance tensor (except the static shift) is presented. The method proposed uses the Groom & Bailey decomposition of the distortion matrix for the short periods. Three examples are presented: two using synthetic data and one employing real data. These examples show the effect of the galvanic distortion over a regional 2-D/3-D model and the retrieval of the regional transfer functions from the distorted ones.  相似文献   

Sensitivity studies applied to a two-dimensional resistivity model from the Central Andes     

Katrin Schwalenberg  Volker Rath  Volker Haak 《Geophysical Journal International》2002,150(3):673-686

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The statistical distribution of magnetotelluric apparent resistivity and phase   总被引：1，自引：0，他引：1  

Alan D. Chave  Pamela Lezaeta 《Geophysical Journal International》2007,171(1):127-132

The marginal distributions for the magnetotelluric (MT) magnitude squared response function (and hence apparent resistivity) and phase are derived from the bivariate complex normal distribution that describes the distribution of response function estimates when the Gauss–Markov theorem is satisfied and the regression random errors are normally distributed. The distribution of the magnitude squared response function is shown to be non-central chi-squared with 2 degrees of freedom, with the non-centrality parameter given by the squared magnitude of the true MT response. The standard estimate for the magnitude squared response function is biased, with the bias proportional to the variance and hence important when the uncertainty is large. The distribution reduces to the exponential when the expected value of the MT response function is zero. The distribution for the phase is also obtained in closed form. It reduces to the uniform distribution when the squared magnitude of the true MT response function is zero or its variance is very large. The phase distribution is symmetric and becomes increasingly concentrated as the variance decreases, although it is shorter-tailed than the Gaussian. The standard estimate for phase is unbiased. Confidence limits are derived from the distributions for magnitude squared response function and phase. Using a data set taken from the 2003 Kaapvaal transect, it is shown that the bias in the apparent resistivity is small and that confidence intervals obtained using the non-parametric delta method are very close to the true values obtained from the distributions. Thus, it appears that the computationally simple delta approximation provides accurate estimates for the confidence intervals, provided that the MT response function is obtained using an estimator that bounds the influence of extreme data.  相似文献   

Deep electrical anomaly in the M7.5 Qiongzhou earthquake region and its relation with future seismicity     

胡久常白登海  王薇华林镇向小娟  王立风 《地震学报(英文版)》2007,20(3):273-279

From the magnetotelluric detection in the epicentral region and the adjacent areas of the 1605 M7.5 Qiongzhou earthquake, we have discovered there is a low resistive body in the deep crust of the epicentral region. The low resistive body extends straightly from the depth of about 13 km to the upper mantle, which is supposed as an up-rising mantle pole. We therefore consider it is just the existing mantle pole and its upwelling thermal material that result in the faulting and stick-slipping activities of the upper crust, which is an important factor for the M7.5 Qiongzhou great earthquake occurrence. The postseismic faulting activity is characterized by creep, which shows that the risk is greatly decreased for the occurrence of a great earthquake with similar intensity in the future.  相似文献   

Interpretation of magnetotelluric sounding data for the 2-D inhomogeneous model of the Earth     

L. N. Porokhova  L. A. Morozov  I. L. Vardanyants 《Geophysical Journal International》1991,106(1):77-84

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The relationship between the spectral function and the underlying conductivity structure in 1-D magnetotellurics     

Peter Weidelt 《Geophysical Journal International》2005,161(3):566-590

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Geoelectric crustal structures off the SW border of the São Francisco craton, central Brazil, as inferred from a magnetotelluric survey     

Maurício S. Bologna  Antonio L. Padilha  Ícaro Vitorello 《Geophysical Journal International》2005,162(2):357-370

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Comment on 'Electrical conductivity and crustal structure beneath the central Hellenides around the Gulf of Corinth (Greece) and their relationship with the seismotectonics' by Pham et al.     

I. I. Rokityansky  P. Varotsos  N. Sarlis 《Geophysical Journal International》2005,162(2):332-336

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高压密封舱与磁场传感器的综合频响变异及其实验验证     

邓明  魏文博  金胜  叶高峰  张启升  景建恩 《地球科学》2007,32(4):453-460

大地电磁测深在陆地上的方法技术成熟, 但将其移植到海洋中却遇到了较多的技术问题.其中之一是如何将磁场传感器运抵海底实施测量.为抵御海水的侵蚀和抗衡海下的环境压力, 需要研发装载磁场传感器的密封舱.该舱体需满足测磁的技术要求, 即制造材料的非磁性, 且兼顾海洋作业设备轻便的特点.LC4超硬铝合金在可选的材料中性价比居高, 然而用它制成的高压密封舱并非十分完美.因它的弱磁性, 使得磁场传感器装入舱体后, 整体频率响应发生了部分变异.这种变异虽不影响磁场传感器的使用, 但要对其规律特征予以测定, 方能剔除由此产生的对磁场测量的不良影响.在所构思的实验装置中, 通过人为产生扫频磁场, 对密封和非密封条件下的磁场传感器进行激励.在响应输出端, 测取幅频特性和相频特性, 获得归一化的对比资料和频响异常曲线, 从而对密封前后磁场传感器的性能变化有较清楚的认识.实验结果揭示, 由高压密封舱引起的频响变异发生在中高频区段.实验数据可作为标定资料, 对海底大地电磁探测的实测信号进行校正后, 即可还原出真实的海底场源信息.   相似文献