EM Monitoring of Crustal Processes Including the Use of the Network-MT Observations

There are several kinds of coupling mechanisms which can convert mechanical, chemical or thermal energies due to seismic or volcanic activities into electromagnetic energies. As a result of concentrated efforts in laboratory and theoretical research, the basic relationship between the intensity of electromagnetic sources and changes in mechanical, chemical and thermal state is becoming established. Also with the progress of the electromagnetic simulation techniques, it has been possible to evaluate in situ sensitivity. Based on this progress and also due to extensive improvement in measuring techniques, many field experiments have been performed to elucidate subsurface geophysical processes underlying the preparation stage, onset, and subsequent healing stage of earthquakes and volcanic eruptions. In volcanic studies, many studies have reported the measurement of electromagnetic signals which were successfully interpreted in terms of various driving mechanisms. Although there have been numerous reports about the existence of precursory electromagnetic signals in seismic studies, only a few of them could be successfully explained by the proposed mechanisms, whereas coseismic phenomena are often consistent with those mechanisms including the absence of detectable signals. In many cases, one or two orders of higher sensitivity were required, especially for precursory signals. Generally, electromagnetic methods are more sensitive to near-surface phenomena. It will be necessary to discriminate electromagnetic signals due to these near-surface sources, which often possess no relationship with the crustal activities. Further efforts to enhance in situ sensitivity through improvements in observation techniques and in data processing techniques are recommended. At the same time, multi-disciplinary confirmation against the validity of electromagnetic phenomena will inevitably be necessary. A Network-MT observation technique has been developed to determine large-scale deep electrical conductivity structure. In the method, a telephone line network or purpose-built long baseline cables are utilized to measure voltage differences with long electrode separations. Because of the averaging effect of the electric fields, static shift problems due to small-scale, near-surface lateral heterogeneities can be alleviated. Several field experiments revealed regional scale deep electrical conductivity structures related to slab subduction or its stagnation, which enable us to elucidate underlying physical processes caused by the slab motion. The technique can also be applied to monitor the electric potential field related to crustal activities. The annual variation of the potential field and electrical conductivity in the French Alps were interpreted to be caused by the annual variation of lake water level. The method was also used to monitor the regional scale spatio-temporal variation of the SP field and electrical conductivity before and at the onset of earthquakes and volcanic eruptions.     

用大地电磁网法在长春等地探测上地幔电导率结构

使用长周期大地电磁测深网 (Network -MT)等技术 ,在长春和三岔河等地对太平洋板块弧后的上地幔电导率结构进行了探测。用陆地电话线或电缆接收长基线的电场 ,由设置在长春地磁台的三分量磁通门磁力仪记录磁场变化 ,得到Network -MT等观测资料的最长周期可达 10 5～10 6 s,反演结果的深度可达约 10 0 0km。 1)在两个区的 10 0～ 2 0 0 ,30 0 ,50 0～ 6 0 0km深度附近存在电性间断面 ;2 )长春测区的 850km深度存在电性间断面 ;3)平滑模型的电导率随深度增大而单调增加 ;4 )三岔河测区 6 0 0km深度、长春测区 850km深度的电导率达到 1S/m。上述结果表明 ,地幔内存在不连续界面和不均匀性 ,有些结果可与岩石学的研究结果对比     

网式大地电磁阻抗张量及其影响因素分析

网式大地电磁（Network-MT,N-MT）法采用长数公里至数十公里的电话线为电极线测量电场,很难形成两条笔直且相互垂直的电极线,因此阻抗张量的计算不如大地电磁法中直接.本文依据阻抗张量的旋转规则提出了一种计算N-MT阻抗张量的简便算法.依据该算法计算了中国东北地区5个N-MT测站的阻抗张量,获得了基于阻抗张量的视电阻率、相位曲线和最佳主轴方位角分布图像,为我国东北N-MT资料的进一步处理和解释提供了基础性数据.此外,本文对比分析了朝阳测站中6条N-MT观测电极线上阻抗张量旋转值与观测值之间的差异,重点讨论了产生这种系统性偏差的各种因素,提出其主要因素可能来自“电场等效各向异性”效应,即测站附近的地壳内部存在与观测电极线尺度相比拟的横向非均匀构造,而测站各电极线沿不同方向跨越不均匀构造,此时各电极线上的电场分量不遵循同一电场矢量的分解准则,导致地表观测三角形内阻抗分量不满足统一的阻抗张量旋转规则.