of each method was estimated by a forward modeling simulation. For both methods, it is clear that the spatial resolution increases with the number of observation directions. If the internal density structure is rather simple, it is possible to detect density contrasts as a blurred distribution, even if the number of observation directions is only four. However, the density structure inside a volcano can be made clearer when other observational measurements are available (e.g., gravity data). Further technological developments, reconstruction methods for three‐dimensional imaging, and multiple observational measurement techniques will provide new insights into volcanic structures.
ACKNOWLEDGEMENTS
The authors would like to acknowledge Hideaki Aoki of Ike-kankou for collaborating on our study. We also thank Masato Koyama of Shizuoka University and Yusuke Suzuki for their discussions regarding Omuroyama volcano. This work was supported by JSPS KAKENHI Grant Numbers 19H01988, Izu Peninsula Geopark Academic Research Grant in 2018, the joint research program of the Institute of Materials and Systems for Sustainability at Nagoya University in 2017-2020, and JSPS Fellowship (DC2, 19J13805).
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