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

Abstract

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, nearsurface lateral heterogeneities can be alleviated. Several field experiments revealed regional scale deep electrical conductivity structures related to slab subduction or its M. Uyeshima (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.

Modification of the Network-MT method and its first application in imaging the deep conductivity structure beneath the Kii Peninsula, southwestern Japan

“…To overcome this problem, we developed the modified NMT method, which uses the local metallic lines that run between telephone stations and end-users within the region covered by each station (the region is called the local service area) as these metallic lines are still widely deployed in Japan. This method is based on similar research undertaken in small, specific areas with the aim of monitoring temporal variations in voltage differences arising from crustal activity (Uyeshima, 2007). Figure 2 shows a sketch of the experimental design of the modified NMT system.…”

Three‐dimensional electromagnetic imaging of upwelling fluids in the Kyushu subduction zone, Japan

“…Theoretical details describing the procedure of the network-MT analysis are given in Uyeshima et al [2001] and Uyeshima [2007]. This section explains the main data-processing steps applied in the present study.…”

Variations of geoelectric potential differences associated with an anomalous volumetric strain change in the region of expected Tokai Earthquake, Japan

“…Metal lines of telecommunication facility connected the electrodes. The long-dipole telluric observation system like this is often used in earlier works Uyeshima et al, 1989) and the Network-MT survey (Uyeshima, 2007). For data acquisition, we installed SES96 data logger (ADOSYSTEMS Co. Ltd.) at site WDS, equipped with a 2-s low-pass filter and a 20-bit A/D converter.…”