Special issue “Studies on electromagnetic induction in the earth: recent advances”
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Limitations such as highly resistive terrains, coils that are too long to fit in a narrow tunnel space, and the absence of a GPS signal make it difficult to use conventional surface EM receivers to measure the signal inside tunnels. In their work “Electromagnetic receiver with capacitive electrodes and triaxial induction coil for tunnel exploration,” Kai et al. (2017) introduced capacitive electrodes and triaxial induction coils that help avoid these limitations. They successfully developed their new design, and their tests in a mine showed that the new equipment can be used to measure EM signals inside tunnels with typical noise characteristics.
As the electrical resistivity values of rocks of the crust and upper mantle are quite sensitive to the presence aqueous fluids and partial melts, MT plays a major role in these scenarios. In their research paper “Regional electrical structure of the Andean subduction zone in central Chile (35°–36°S) using magnetotellurics,” Reyes-Wagner et al. (2017) used broadband MT data acquired in the Southern Volcanic Zone of the Andes to map the subduction zone and establish its relation with the volcanic arc at 35°–36°S latitude. Their final resistivity model brought out the resistive nature of the forearc structure and also a wide region of high conductivity extending from the volcanic front to the east indicating the highly active magmatism of the region.
The geodynamics of the Cambay Rift Zone, India, is quite complex. Danda et al. (2017) conducted a broadband and long-period MT profile with a length of 200 km in the area and processed the result to yield a 2-D resistivity model in their work “Geoelectric structure of northern Cambay rift basin from magnetotelluric data.” They interpreted the high-conductivity zones as fluid emplacement in the west and the presence of fluids and/or interconnected sulfides in the east. In addition, a highly resistive body outside of the rift zone was interpreted as an igneous granitic intrusive complex.
Tracing the Indian crustal front beneath Tibet remains a controversial issue. In their paper “Varying Indian crustal front in the southern Tibetan Plateau as revealed by magnetotelluric data,” Xie et al. (2017) used previous MT data to generate a 3-D geoelectrical model in southern Tibet to cope with the Indian crustal front. They found conductive layers beneath the mid- to lower crust, suggesting that the Indian crustal front varies irregularly from west to east. This observation was also supported by seismic results.
Marine EM studies
In contrast to the MT data, which show no evidence of electrical anisotropy, recent seismic data from the NoMelt experiment presented a strong anisotropy in the upper mantle. Matsuno and Evans (2017) revisited the MT data to answer the question of whether electrical anisotropy really existed in the data in their express letter “Constraints on lithospheric mantle and crustal anisotropy in the NoMelt area from an analysis of long-period seafloor magnetotelluric data.” Interestingly, they found that electrical anisotropy is possible at lower lithosphere depths. However, this is not related to the anisotropy found in the seismic data at upper and mid-lithosphere depths. Seismic anisotropy at this depth is not expected to generate measurable MT data.
To study the nature of old oceanic upper mantle, Baba et al. (2017) analyzed many seafloor MT data in the northwestern Pacific in their full research paper “Electrical conductivity of old oceanic mantle in the northwestern Pacific I: 1-D profiles suggesting differences in thermal structure not predictable from a plate cooling model.” Significant differences in resistive layer thickness were observed between the four zones of investigation, and these could not be explained by only a plate cooling model. The authors suggested that these differences may have been caused by the influence of the plume associated with the formation of the Shatsky Rise or by spatially non-uniform and small-scale convection in the asthenosphere.
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- Baba K, Tada N, Matsuno T, Liang P, Li R, Zhang L, Shimizu H, Abe N, Hirano N, Ichiki M, Utada H (2017) Electrical conductivity of old oceanic mantle in the northwestern Pacific I: 1-D profiles suggesting differences in thermal structure not predictable from a plate cooling model. Earth Planets Space. https://doi.org/10.1186/s40623-017-0697-0 Google Scholar
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