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ALOS-2/PALSAR-2 Observation Result for Eruption of Mt. Agung in Indonesia

Posted: Dec. 6, 2017, 2:00 (UTC)

Overviews

  • On November 21, 2017, Mt. Agung in Bali Island, Indonesia, started the eruption for the first time in 54 years.
  • JAXA performed an emergency observation by ALOS-2 (“DAICHI-2”) on November 30, 2017 (UT),and ALOS-2 captured lava showing up in the crater.
  • Although InSAR analysis does not detect the crustal deformation around Mt. Agung, there is a low coherence area in southern side of the crater, which indicates the surface change such as the deposition of volcanic ash and ejecta.
  • The observation data were provided to local institutions via. Sentinel Asia. We can contribute to monitor volcanic activity using ALOS-2 which can observe earth surface regardless of clouds, volcanic plumes, and sun light.

On November 21, 2017 Mountain Agung in Bali Island, Indonesia, began eruption for the first time in 54 years. JAXA performed an emergency observation with the Phased Array Type L-band Synthetic Aperture Radar 2 (PALSAR-2) onboard Advanced Land Observing Satellite 2 (ALOS-2, "DAICHI-2") on around 16:38 on November 30, 2017 (UT).

Figure 1 illustrates the observation area and the overall image. The observation mode was the high-resolution 10 m mode (dual polarization). The color composite in the image represents HH polarization in red, HV polarization in green, and HH/HV in blue. In this image, black or dark blue roughly show water or bare land, green shows the vegetation covered areas, and bright green or purple show the urban areas.
Fig.1
Fig.1: Observation area of the PALSAR-2 on November 30, 2017, and the overall image by PALSAR-2.
(Click to view enlarged image)
Figure 2 shows the enlarged views of the Mt. Agung. Here, we rotated the image to make it easy to understand visually, so that the upward direction of the image is the direction of the satellite. The left of Figure 2 is the image observed on October 5, 2017 (before the eruption) for comparison, and the image of November 30, 2017 (this time) is shown in the middle of Figure 2. In this image, the state inside the crater has changed, and the lava seems to have shown up in the crater. The Synthetic Aperture Radar (SAR) image has a property that objects in higher altitude looks shifted to the satellite direction, and the lava in lower altitude at the bottom of the crater is imaged farther from the satellite (shifted in lower side in Figure 2). Based on this shift, the altitude difference from the crater edge to the surface of the lava is still 100 meters or more. On the right side of Figure 2, we show the image adding a schematic explaining how the inside of the crater is seen.
  • Fig.2
    Fig.2: Enlarged images around Mt. Agung on October 5, 2017
    (Click to view enlarged image)
  • Fig.2
    Fig.2: Enlarged images around Mt. Agung on November 30, 2017
    (Click to view enlarged image)
  • Fig.2
    Fig.2: The schematic explaining how the inside of the crater is seen
    (Click to view enlarged image)
Figure 3 shows the interferogram using PALSAR-2 data as same as those in Fig. 2. In this figure, there seems to be no signal of crustal deformation due to the eruption. The blue area in southern part of Bali Island is considered as meteorological noise on PALSAR-2 image obtained before the eruption.
Fig.3
Fig.3: Enlarged interferometric image around Mt. Agung using PALSAR-2 data acquired on October 5, 2017 and November 30, 2017 (UT).
(Click to view enlarged image)

Figure 4 shows the coherence obtained in the interferometric analysis. The coherence stands for the similarity of two images, and it decreases when the surface condition changes during two observations. In figure 4, the gray zone shows the coherence greater than 0.5, and the area whose coherence is lower than 0.5 is shown in yellow and red. Although the coherence over the entire scene is high, we find the low coherence area in southern part of the crater. This reflects the deposition of volcanic ash and ejecta, and the concentration only in the southern area of the crater is thought to be caused by wind direction. The similar area of low coherence is in northwestern Mt. Agung, but there is Lake Batur and the coherence becomes low on water surface.

These PALSAR-2 observations were requested by Sentinel Asia, and the observed data were provided to local institutions from JAXA via. Sentinel Asia.
Fig.4
Fig.4: Enlarged coherence result using the same dataset and area in Fig.4.
(Click to view enlarged image)
JAXA EORC