13:45 〜 15:15
[SEM14-P15] Electrical resistivity modeling of Furano area, central Hokkaido based on broadband magnetotellurics
Mt. Tokachidake is an active volcano located in central Hokkaido, northern Japan. The volcano has been repeatedly active, including three magmatic eruptions in the last 100 years. In the 21st century, the volcano has been relatively calm, although it has shown waxing and waning in its activity. Regarding the subsurface structure of Mt. Tokachidake, The Geological Survey of Hokkaido (2016) found two low-resistivity anomalies at different depths beneath the volcano based on two-dimensional MT modeling. They interpreted that the shallower anomaly at 0~1.5km deep corresponds to a gas-condensed hydrothermal system, while the deeper one at 3~10km is indicative of magmatic hydrothermal fluid. Iwama (2021) performed an extensive MT transect across central Hokkaido and suggested the presence of an even more distinct and large-scale conductor occupying the depth range of 10-40km between Mt. Tokachidake and Furano basin, the western neighbor of the volcano. However, the detailed size and shape of the deepest conductor remained controversial because his modeling was based on a single survey line, and thus the model may not be well constrained in the lateral sides of the line. Therefore, a further survey is necessary with an increased number of measurement points on both sides of his survey line to reveal the detail of the deepest conductor and to discuss its relation to the magmatic system of Mt. Tokachidake.
To this end, we conducted a broadband magnetotelluric survey at ten sites in the Furano region in 2022. We measured the electric and magnetic field using Pb-PbCl2 electrodes and induction coils for four to six days at each site. Two components of the electric field data and three components of the magnetic field data were recorded by the MTU-5A system (Phoenix Geophysics Ltd.) at six sites (green squares), and two components of the electric field data were recorded by Elog-MT and Elog-DUAL Rev.B (NT System Design) at four sites (blue circles). We used the horizontal magnetic field data recorded at Sawauchi station by Nittetsu Mining Consultants for remote reference processing (Gamble et al. 1979) to reduce local noise. All these data were sampled at 15Hz, 150Hz, and 2400Hz.
We calculated the MT transfer functions of the data in 2022 by using SSMT2000 (Phoenix Geophysics, Ltd.) or the BIRRP code (Chave and Thomson, 2003). In the southern area of Furano (P110-190), both the sounding curves of apparent resistivity and phase angle showed a 1-D-like feature and relatively high resistivities regardless of frequency. Again, at the northern sites (P240-270), no features strongly suggestive of the deep conductor that Iwama (2021) noted have so far been found. However, at P270, the closest station to Mt. Tokachidake among them, we recognized a remarkable decrease in the apparent resistivity as well as the associating high phase angle in a period range of 0.0031-1 sec. The result indicates the presence of a conductive layer beneath the western foot of Mt. Tokachidake. It is also consistent with the previous study's result at the nearby site (Iwama, 2021). In 2023, we plan an additional MT survey in the middle part (Nakafurano town) and further north (Biei town) to obtain the data to perform a 3-D inversion modeling.
To this end, we conducted a broadband magnetotelluric survey at ten sites in the Furano region in 2022. We measured the electric and magnetic field using Pb-PbCl2 electrodes and induction coils for four to six days at each site. Two components of the electric field data and three components of the magnetic field data were recorded by the MTU-5A system (Phoenix Geophysics Ltd.) at six sites (green squares), and two components of the electric field data were recorded by Elog-MT and Elog-DUAL Rev.B (NT System Design) at four sites (blue circles). We used the horizontal magnetic field data recorded at Sawauchi station by Nittetsu Mining Consultants for remote reference processing (Gamble et al. 1979) to reduce local noise. All these data were sampled at 15Hz, 150Hz, and 2400Hz.
We calculated the MT transfer functions of the data in 2022 by using SSMT2000 (Phoenix Geophysics, Ltd.) or the BIRRP code (Chave and Thomson, 2003). In the southern area of Furano (P110-190), both the sounding curves of apparent resistivity and phase angle showed a 1-D-like feature and relatively high resistivities regardless of frequency. Again, at the northern sites (P240-270), no features strongly suggestive of the deep conductor that Iwama (2021) noted have so far been found. However, at P270, the closest station to Mt. Tokachidake among them, we recognized a remarkable decrease in the apparent resistivity as well as the associating high phase angle in a period range of 0.0031-1 sec. The result indicates the presence of a conductive layer beneath the western foot of Mt. Tokachidake. It is also consistent with the previous study's result at the nearby site (Iwama, 2021). In 2023, we plan an additional MT survey in the middle part (Nakafurano town) and further north (Biei town) to obtain the data to perform a 3-D inversion modeling.