In the Kanto region of Japan, a large quantity of natural gas is dissolved in brine. The large-scale production of gas and iodine in the region has caused large-scale land subsidence in the past. Therefore, continuous and accurate monitoring for subsidence using satellite remote sensing is essential to prevent extreme subsidence and ensure the safety of residences. This study focused on the small baseline subset (SBAS) method to assess ground deformation trends around the Kanto region. Data for the SBAS method was acquired by the Advanced Land Observing Satellite (ALOS)-2 Phased Array type L-band Synthetic Aperture Radar (PALSAR)-2 from 2015 to 2019. A comparison of our results with reference levelling data shows that the SBAS method underestimates displacement. We corrected our results using linear regression and determined the maximum displacement around the Kujyukuri area to be approximately 20 mm/year; the mean displacement rate for 2015–2019 was −7.9 ± 2.9 mm/year. These values exceed those obtained using past PALSAR observations owing to the horizontal displacement after the Great East Japan Earthquake of 2011. Moreover, fewer points were acquired, and the root mean-squared error of each time-series displacement value was larger in our results. Further analysis is needed to address these bias errors.

High-resolution synthetic aperture radar (SAR) data are widely used for disaster monitoring. To extract damaged areas automatically, it is essential to understand the relationships among the sensor specifications, acquisition conditions, and land cover. Our previous studies developed a method for estimating the phase noise of interferograms using several pairs of TerraSAR-X series (TerraSAR-X and TanDEM-X) datasets. Atmospheric disturbance data are also necessary to interpret the interferograms; therefore, the purpose of this study is to estimate the atmospheric effects by focusing on the difference in digital elevation model (DEM) errors between repeat-pass (two interferometric SAR images acquired at different times) and single-pass (two interferometric SAR images acquired simultaneously) interferometry. Single-pass DEM errors are reduced due to the lack of temporal decorrelation and atmospheric disturbances. At a study site in the city of Tsukuba, a quantitative analysis of DEM errors at fixed ground objects shows that the atmospheric effects are estimated to contribute 75% to 80% of the total phase noise in interferograms.

In a previous study, we developed a method for assessing building damage at the district level in markedly damaged areas using satellite-derived synthetic aperture radar (SAR) data and conducted a coherence analysis of urban areas. We previously evaluated the relationship between the average coherence within a 200-m mesh and degree of building damage in Mashiki, Kumamoto Prefecture, Japan, using L-band ALOS-2 PALSAR-2 data obtained before and after the Kumamoto earthquake. Our findings indicated a significant negative correlation between these parameters. Currently, C-band Sentinel-1 provides global coverage at a high observation frequency. However, previous studies have not sufficiently evaluated the effects of differences in acquisition conditions. Therefore, we aimed to investigate how acquisition conditions affect the relationship between coherence and the building damage rate using images captured by Sentinel-1 C-SAR during the Kumamoto earthquake. The results showed that coherence tended to decrease as the damage rate increased but the correlation was not significant. The difference in coherence per 10% of the building damage rate was less pronounced compared with the results obtained from ALOS-2. Our findings also indicated that the effects of the pre-earthquake acquisition dates and the baseline were less than those of the orbit direction and incidence angle.

これまで著者らは，衛星SARデータを用いて，被害が大きい地域において地区レベルで建物の被害状況の把握を目標とし，予め都市域を切り出したコヒーレンス画像を用いた手法の開発を行ってきた．本研究では，熊本地震を取り上げ，LバンドのALOS-2 PALSAR-2による複数の撮影条件（異なる軌道，撮影方向）で取得した画像を用いて，コヒーレンスと建物の被害度の関係を明らかにすることを目的とした．熊本県益城町を解析対象とし，益城町内の200メートルメッシュ内のコヒーレンスの平均値と建物の被害度の関係を評価した．その結果，撮影条件によらず，これらに有意な相関があることとともに，被害度が小さいほど撮影条件によるコヒーレンスのばらつきが大きい傾向があることを示した．さらに本研究による土地被覆を考慮したコヒーレンスと被害度のモデルの構築には，撮影日，撮影間隔，基線長100m程度の違いよりも，軌道や撮影方向による違いの影響が大きいことを明らかにした．