Muography. Группа авторов. Читать онлайн. Newlib. NEWLIB.NET

Автор: Группа авторов
Издательство: John Wiley & Sons Limited
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Жанр произведения: Физика
Год издания: 0
isbn: 9781119723066
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and persistent degassing also occurs in silicic volcanoes during quiescent periods between eruptions (Delgado‐Granados et al., 2001; Edmonds et al., 2003; Kazahaya et al., 1994).

Schematic illustration of conduit magmatic convection model of Satsuma-Iwojima volcano.

      The magma supply rate will be underestimated if only the geodetic modeling of the expansion rate of the volcanic body is applied. If the magmatic convection process is assumed as a model of magma degassing, the rate of the magma supply to the shallow part of the volcano will be estimated as more than one order of magnitude larger than the surficial expansion rate of the volcano (Kazahaya & Mori, 2016).

      1.3.3 Phreatic Explosions and Magmatic Eruptions

      Additionally, the 2006 measurement imaged a low‐density region that indicated a vacant magma pathway underneath the volcanic plug, as shown in Fig. 1.6a. The statistical error for the bulk density of this region was 0.2 g/cm3. This low‐density region was interpreted as a porous magma pathway that was plugged by magma deposited on the crater floor, which was created by the following process. After the eruption process was completed in 2004, the magma deposit on the crater floor cooled and solidified, and the magma in the pathway drained away, leading us to speculate that it left a vacant or highly porous pathway. The muographic image captured a structure consistent with this interpretation in the anticipated region below the crater floor. If this space is over‐pressured by future volcanic activities, the plug may explode, rapidly releasing fragments of this magma deposit.

Schematic illustration of muographic image of Asama volcano, Japan (a) and images before and after 2009 eruption (b).

      It is difficult to directly observe phreatic explosions with muography because the gas flow doesn’t generate significant density variations inside a volcano. However, like an example shown above, indirect evidence could be captured as the structural modification as a consequence of phreatic explosion. In contrast with phreatic explosions, magmatic eruptions could be more directly captured with muography. Dense magmatic materials ascend through the highly porous pathway during magmatic eruptions. On June 4, 2013, the eruption alert level had risen from level 1 (signs of volcano unrest) to level 2 (minor eruptive activity) at Satsuma‐Iwojima volcano, Japan. Time sequential muographic images showed the ascent and descent of the magma head, which synchronized to the visual observation timings of volcanic glows during this eruption episode (Tanaka et al., 2014). Currently, the automatic analysis and data visualization system is available for volcano muography (Tanaka et al., 2020c). The resultant muographic images were similar to medical radiographic images. In the field of medical imaging, by taking advantage of recent deep learning techniques, for example, convolutional neural network (CNN), image processing techniques have been highly developed to realize automated medical image analysis and evaluation. A deep learning technique similar to what is used now for medical image processing has been found to be applicable to analyzing time‐sequential muographic images (Nomura et al., 2020).

      1.3.4 Plate Tectonics and Volcanism

      Muography has been also applied to studying the tectonic history of the Earth by revealing the subterranean bulk density distribution averaged over the hectokilometric‐scale area. By installing the detectors inside the multiple tunnels, which are randomly but uniformly distributed over this area, the density distribution above these tunnels can be measured. Tanaka (2015) conducted tunnel muography to the southern part of Izu peninsula, the tectonically active peninsula that was made by collision with Honshu Island, Japan, about one million years ago. The formation history of Izu peninsula will be summarized below.