EM Monitoring of Crustal Processes Including the Use of the Network-MT Observations
EM Monitoring of Crustal Processes Including the Use of the Network-MT Observations
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DOI：
10.1007/s10712-007-9023-x
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发表时间：
2007-09
影响因子：
4.6
通讯作者：
M. Uyeshima
中科院分区：
文献类型：
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作者：
M. Uyeshima
有几种耦合机制可以将地震或火山活动产生的机械能、化学能或热能转换为电磁能。由于实验室和理论研究的集中努力，电磁源强度与机械、化学和热状态变化之间的基本关系正在建立。随着电磁场模拟技术的发展，现场敏感性评价成为可能。基于这一进展，也由于测量技术的广泛改进，已经进行了许多现场实验，以阐明地震和火山爆发的准备阶段，发病和随后的愈合阶段的地下地球物理过程。在火山研究中，许多研究报告了电磁信号的测量，这些信号被成功地解释为各种驱动机制。虽然在地震研究中已经有大量的关于异常电磁信号存在的报道，但只有少数可以成功地用所提出的机制来解释，而同震现象往往与这些机制相一致，包括没有可检测到的信号。在许多情况下，需要一个或两个更高的灵敏度，特别是对于寄生信号。通常，电磁法对近地表现象更敏感。由于这些近地表源的电磁信号往往与地壳活动无关，因此有必要对它们进行识别。建议进一步努力通过改进观测技术和数据处理技术来提高现场灵敏度。与此同时，对电磁现象的有效性进行多学科的确认也是必然的。为确定大尺度深部电导率结构，发展了网络化大地电磁观测技术。在该方法中，利用电话线网络或专用的长基线电缆来测量具有长电极间隔的电压差。由于电场的平均效应，可以减轻由于小尺度、近表面横向不均匀性引起的静态偏移问题。几个现场实验揭示了区域尺度的深部电导率结构板片俯冲或其停滞，这使我们能够阐明板片运动所造成的基本物理过程。该技术还可用于监测与地壳活动有关的电位场。法国阿尔卑斯山的势场和电导率的年变化被解释为是由湖水位的年变化引起的。该方法还用于监测地震和火山爆发前和爆发时SP场和电导率的区域尺度时空变化。
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.