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  • Observation Results of Mt. Ontake Volcano eruption by ALOS-2/PALSAR-2 and Pi-SAR-L2

Observation Results of Mt. Ontake Volcano eruption by ALOS-2/PALSAR-2 and Pi-SAR-L2

The Japan Aerospace Exploration Agency (JAXA) observed Mt. Ontake volcano, which was suddenly erupted on September 27th, 2014, 11:53 (JST), with the Phased Array Type L-band Synthetic Aperture Radar 2 (PALSAR-2) aboard "DAICHI-2" (ALOS-2) satellite and the airborne L-band synthetic aperture radar (Pi-SAR-L2).

Table 1 lists the data acquisitions by PALSAR-2 over Mt. Ontake before (No. 1) and after (No. 2-6) the eruption. JAXA has provided those observed data to the agencies concerning disaster response.
Table 1: List of PALSAR-2 observation of Mt. Ontake
Obs. date and time (JST) Obs. mode Obs. direction Incidence angle(degrees) Orbit direction
2014/08/18 23:59 Stripmap [3m] right 46.4 Ascending
2014/09/27 23:17 Stripmap [3m] left 18.0 Ascending
2014/09/28 12:45 Stripmap [3m] left 46.4 Descending
2014/09/28 23:37 Stripmap [3m] right 21.9 Ascending
2014/09/29 23:59 Stripmap [3m] right 46.4 Ascending
2014/10/02 12:30 Stripmap [3m] left 32.4 Descending

(1) Change in the shape of the volcano

Figure 1 shows Mt. Ontake volcano observed in Stripmap (3m) mode on September 29 and 28 (after eruption), and August 18 (before eruption). In the circled area near the crater, a new hole appeared after the eruption. Figure 2 is an interactive comparison of images on Sep. 29 and Aug. 18 (after and before eruption).
Fig.1
Fig.1: Mt. Ontake volcano observed on Sep. 29 and 28 (after eruption), and Aug. 18 (before eruption).
(Click to view enlarged image)
Fig.2 Fig.2
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Fig.2: Interactive comparison of Mt. Ontake volcano observed on Sep. 29 and Aug. 18 (after and before eruption)

(Drag a slider over the image or click on the arbitrary position of the image to compare the images)*

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(2) Distribution of volcanic ash accumulation

More than 50cm thickness of volcanic ash accumulation is reported in regard to this eruption. Figure 3 shows a color composite, which assigned the images on Aug. 18 (before eruption) and Sep. 29 (after eruption) to red and cyan, respectively. Since accumulated ash absorbs the microwave used in observation, distribution of ash accumulation is considered to be seen as red area in this image.
Fig. 3
Fig. 3: Color composite of Aug. 18 (before eruption, assigned to red) and Sep. 29 (after eruption assigned to cyan).
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(3) Deformation detected by differential SAR interferometry

The observation data of Sep. 29 and Aug. 18 were acquired from the same orbital path. From these two datasets, differential SAR interferometry (DInSAR) processing can be used to detect the small deformation of the volcano.

Figure 4 shows the result of DInSAR over the area of 50km square including Mt. Ontake. A close-up image over 6km square around the volcanic crater is shown in figure 5. This interferogram depicts deformation of land surface (change of the distance between the satellite and the ground) in color, where green is no-change area.

From this result, deformation up to 9cm toward to the satellite is detected in the south-west side of the mountain. This change is considered to be a land motion in west direction associated with the volcanic activity. Note that this interferogram contains atmospheric error especially in high altitude area.

Figure 5 shows an interactive 3D view of the interferogram.
Fig. 4
Fig. 4: DInSAR result over Mt. Ontake.
(Click to view enlarged image)
Fig. 5
Fig. 5: Close-up image of the DInSAR result around the volcanic crater.
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Fig. 6
Fig. 6: Interactive 3D view of DInSAR result around the volcanic crater.
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(4) Observation results by Pi-SAR-L2

JAXA observed Mt. Ontake by the airborne L-band synthetic aperture radar (Pi-SAR-L2) on September 30th, 2014. Observations from multiple directions and 13,000km altitude were conducted to monitor the volcanic activity such as new craters.

Table 2 lists the data acquisitions by Pi-SAR-L2 over Mt. Ontake with multiple flight directions and incidence angles.
Table 2: List of Pi-SAR-L2 observation of Mt. Ontake
# Flight direction Incidence angle(degrees)
a east → west 45
b north → south 55
c west → east 45
d south → north 55
e east → west 55
f west → east 55
Table 2: List of Pi-SAR-L2 observation of Mt. Ontake
# Flight direction Incidence angle(degrees)
a east → west 45
b north → south 55
c west → east 45
d south → north 55
e east → west 55
f west → east 55

Figure 7 shows a bird's-eye view of the Pi-SAR-L2 image and an aerial photo captured from approximately-same direction. The craters can be seen in the circled area.

Images in figure 8 show the Pi-SAR-L2 images from multiple direction. In these polarimetric color composite (R:G:B = HH:HV:VV), green area roughly represents vegetation area and red area corresponds to bare soil.
Fig.7
Fig.7: Bird's-eye view of the Pi-SAR-L2 image (left) and aerial photo from same direction (right).
(Click to view enlarged image)
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Fig.8: Pi-SAR-L2 images from multiple direction. Symbol (a)-(f) correspond to the symbol in Table 2.
(Click to view enlarged image)

About the processing

Processing software: Sigma-SAR 2014

DEM: GSI 10m-mesh DSM (Geospatial Information Authority of Japan)

Atmospheric correction for interferogram: Topographic correction of dry atmospheric delay
Bird's-eye view of Mt. Ontake observed by Pi-SAR-L2 (YouTube movie)
JAXA EORC