Network-magnetotelluric method and its first results in central and eastern Hokkaido, NE Japan

Abstract

Summary A new field observation technique based on the magnetotelluric (MT) method has been developed to determine deep and large‐scale 3‐D electrical conductivity distributions in the Earth. The method is named ‘Network‐MT’, and employs a commercial telephone network, to measure voltage differences with long dipole lengths ranging from 10 to several tens of kilometres. This observation configuration enables us to obtain the telluric field distribution with nearly continuous coverage over a target region. Response functions are estimated between the respective voltage differences and the horizontal magnetic fields at a reference point. Owing to the long electrode spacing, the observed responses are relatively free from the effects of small‐scale near‐surface heterogeneity with a scalelength shorter than the typical electrode spacing. Therefore, physically meaningful direct comparison between the observations and model responses is feasible even if the fine‐scale features of near‐surface heterogeneity are ignored. This extensively reduces the difficulty, especially in 3‐D MT interpretation. The first Network‐MT experiment was performed in central and eastern Hokkaido, NE Japan, in 1989. It took about five months to complete all of the measurements, and used 209 dipoles to cover the target area of 200(EW) × 200(NS) km2. The long electrode spacing enabled us to obtain the voltage differences with a high signal‐to‐noise ratio. For 175 dipoles, the squared multiple coherency between the voltage difference and the horizontal magnetic field at Memambetsu Geomagnetic Observatory was determined to be more than 0.9 in the period from 102 to 104 s. 193 MT impedances were computed in tensor form by linear combination of the response functions. The estimated impedances generally possessed smooth period dependence throughout the period range. No drastic spatial change was observed in the characteristics of the tensors for neighbouring sites, and some regional trend could be detected in the spatial distribution. Thus, we confirmed the merit of the Network‐MT method, that its responses are little affected by small‐scale near‐surface structures. The regional feature of the response implied a significant influence of the coast effect, and was well correlated with the regional geological setting in Hokkaido. Conventional Groom–Bailey tensor decomposition analysis revealed that the target region is not regionally one‐ or two‐dimensional. Therefore, we developed a 3‐D forward modelling scheme specially designed for the Network‐MT experiment, and tried to reproduce the Network‐MT responses directly. In the 3‐D model, a realistic land–sea distribution was considered. The resistivity of sea water was fixed to be 0.25 Ω m and, as a first trial of 3‐D modelling, the resistivity of the land was assumed to be uniform and its value was determined to be 200 Ω m by a simple one‐parameter inversion. Overall agreements between the observations and the best‐fit model responses indicated the importance of the 3‐D coast effect in the target region....

Estimation of geomagnetically induced currents based on the measurement data of a transformer in a Japanese power network and geoelectric field observations

“…This suggests existence of conductive layer in east of Memanbetsu. Uyeshima et al (2001) suggested a significant coast effect in eastern Hokkaido based on the observation by the networkmagnetotelluric (network-MT) method. Consideration of underground conductivity is necessary to understand the measured GIC data as a future work.…”

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

“…The NMT method was developed and first applied in central and eastern Hokkaido, northeastern Japan (NEJ), by Uyeshima (1990) and Uyeshima et al (2001). Yamaguchi et al (1999) and Satoh et al (2001) applied the method in eastern Shikoku, SWJ, and eastern Hokkaido, NEJ, respectively.…”

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.…”

“…We here describe the electrical resistivity structure beneath the Kyushu subduction zone in the Southwest Japan arc (Figure 1), which was determined using measurements recorded by the network-magnetotelluric (network-MT) sounding method [Uyeshima et al, 2001]. Various geophysical methods can be applied in the observation of subsurface structures, depending on the target of interest (e.g., fluids triggering partial melt below volcanoes or earthquake-triggering fluids).…”

“…In this study, the period range of the network-MT data is 480-40,960 s; thus, the penetration depth reaches the upper mantle. The network-MT method was first applied in central and eastern Hokkaido (Japan) in 1989 [Uyeshima et al, 2001;Uyeshima, 2007] to obtain the large-scale electrical resistivity structure. After this pioneering work, several network-MT observations were conducted in Japan, including on Kyushu Island [Uyeshima et al, 2002].…”