10:00 〜 10:15
[ACG39-05] Thaw subsidence and frost heave after 2018-20 wildfires around Batagay detected by InSAR and field observation
キーワード：InSAR、ALOS2、Sentinel-1、永久凍土、森林火災、サーモカルスト
The Arctic experienced numerous wildfires last year, and from June to August 2020, satellite data revealed record releases of carbon dioxide from wildfires. Peatland, in the Arctic, contains large amounts of organic carbon, and their discharge to the atmosphere can create positive feedbacks to further increase air temperature. Furthermore, wildfires burn the surface vegetation layer that acts as a thermal insulator, and it accelerates the thawing of permafrost over years to decades. Although the thaw depth can recover together with the recovery of surface vegetation, the massive segregated ice is not recoverable once it melted.
Around our study area of Batagay, Sakha Republic, Eastern Siberia, Sentinel-2 optical satellite image showed an increase of the burned area in 2019-20. Also, in June 2020, the highest daily maximum temperature of 38.0 degrees Celcius was recorded in Verkhoyansk, 55 km west of Batagay. To reveal the amount of fire-induced permafrost thawing, we used the remote-sensing technique called InSAR (Interferometric Synthetic Aperture Radar). InSAR can create the ground deformation map over those fire sites and detect deformation signals on a scale of several cm. Besides, we conducted a field observation in September 2019 for validations: 1) installed a soil thermometer and soil moisture meter; 2) established a reference point for leveling and first survey; 3) measured the thawing depth with a frost probe.
We analyzed two types of InSAR images, one for seasonal deformations and another for long-term deformations over a year. For seasonal ground deformations immediately after the fire, we mainly analyzed Sentinel-1 images. Sentinel-1 is the ESA's C-band SAR satellite, which has a short imaging interval of 12 days. Because of its shorter wavelength, vegetation changes can easily lose coherence and prevented us from detecting ground deformation signals in some pairs immediately after the fire. However, after the end of September, we detected displacements approaching toward the satellite line-of-sight direction at the fire sites. It indicates uplift signals due presumably to frost heave at the fire scar.
For long-term deformations over one year, we used ALOS2 images derived by JAXA's L-band SAR satellite. In the previous studies in Alaska, the ground deformation signal immediately after the fire could not be detected due to the coherence loss in the pairs derived from pre-fire and post-fire SAR images. Indeed, we could not detect deformation signals at the fire scars from the June pairs derived before and after the fire. However, the January pairs and March pairs, both of which were acquired before and after the fire, showed relatively high coherence even in the fire scar. We interpret that, because the studied Verkhoyansk Basin is very dry and has little snow cover, the microwaves could penetrate the snow layer, which allowed us to detect deformation signals even in winter. Yanagiya and Furuya (2020) validated the consistency of the winter uplift signal for the 2014 fire site. InSAR images indicated clear subsidence signals by as much as 15 cm. On the other hand, at the fire sites near Batagaika mega slump, we detected frost heave signals in the burned year and subsidence signals after the next to the burned year. To investigate the specific ground deformation at the two fire sites near Batagaika mega slump, we also analyzed the SM1 high spatial resolution data (3 m) of ALOS2 taken in 2020-21. We validated the deformation signals with the thawing depth, the time series of ground temperature, and high-resolution optical images of WorldView-2.
Around our study area of Batagay, Sakha Republic, Eastern Siberia, Sentinel-2 optical satellite image showed an increase of the burned area in 2019-20. Also, in June 2020, the highest daily maximum temperature of 38.0 degrees Celcius was recorded in Verkhoyansk, 55 km west of Batagay. To reveal the amount of fire-induced permafrost thawing, we used the remote-sensing technique called InSAR (Interferometric Synthetic Aperture Radar). InSAR can create the ground deformation map over those fire sites and detect deformation signals on a scale of several cm. Besides, we conducted a field observation in September 2019 for validations: 1) installed a soil thermometer and soil moisture meter; 2) established a reference point for leveling and first survey; 3) measured the thawing depth with a frost probe.
We analyzed two types of InSAR images, one for seasonal deformations and another for long-term deformations over a year. For seasonal ground deformations immediately after the fire, we mainly analyzed Sentinel-1 images. Sentinel-1 is the ESA's C-band SAR satellite, which has a short imaging interval of 12 days. Because of its shorter wavelength, vegetation changes can easily lose coherence and prevented us from detecting ground deformation signals in some pairs immediately after the fire. However, after the end of September, we detected displacements approaching toward the satellite line-of-sight direction at the fire sites. It indicates uplift signals due presumably to frost heave at the fire scar.
For long-term deformations over one year, we used ALOS2 images derived by JAXA's L-band SAR satellite. In the previous studies in Alaska, the ground deformation signal immediately after the fire could not be detected due to the coherence loss in the pairs derived from pre-fire and post-fire SAR images. Indeed, we could not detect deformation signals at the fire scars from the June pairs derived before and after the fire. However, the January pairs and March pairs, both of which were acquired before and after the fire, showed relatively high coherence even in the fire scar. We interpret that, because the studied Verkhoyansk Basin is very dry and has little snow cover, the microwaves could penetrate the snow layer, which allowed us to detect deformation signals even in winter. Yanagiya and Furuya (2020) validated the consistency of the winter uplift signal for the 2014 fire site. InSAR images indicated clear subsidence signals by as much as 15 cm. On the other hand, at the fire sites near Batagaika mega slump, we detected frost heave signals in the burned year and subsidence signals after the next to the burned year. To investigate the specific ground deformation at the two fire sites near Batagaika mega slump, we also analyzed the SM1 high spatial resolution data (3 m) of ALOS2 taken in 2020-21. We validated the deformation signals with the thawing depth, the time series of ground temperature, and high-resolution optical images of WorldView-2.