[SVC35-P04] Research on observation method of volcanic ash-fall accumulation using spaceborne SAR
Keywords:ash-fall distribution, SAR, coherence
In this development, we use InSAR coherence analysis. In InSAR results, clear fringes are seen if the coherence is high. On the other hand, noise is dominant if the coherence is low, and fringe becomes unclear. Generally, loss of coherence (decorrelation) is caused by difference of radar scatter at the pixel for InSAR pair. In a result from an interferometric pair including the eruption occurrence, decorrelation due to radar scatter change caused by volcanic ash accumulation is generally seen around the crater. For example, we analyzed PALSAR-2 InSAR pairs acquired before and after the Kuchinoerabujima eruption on January 17, 2019 and revealed that decorrelation area due to volcanic ash accumulation was seen within the range of about 1 km from the Shin-dake crater. Furthermore, decorrelation area extended to a range of about 1.5 km along the southwest and northwest valleys, which is consistent with the direction of pyroclastic flow.
Additionally, we analyzed InSAR pairs for December 18, 2018 and January 29, 2019 eruptions, and revealed the similar decorrelation around the crater. However, they were obviously small relative to that for January 17, 2019 eruption. Especially, that for January 17, 2019 eruption was limited to the vicinity of the crater. Thus, we could clearly see the difference in the volcanic ash accumulation area from InSAR coherence analysis.
If the relation between decorrelation and the amount of volcanic ash accumulation becomes clear quantitatively, the amount of volcanic ash accumulation will be obtained from InSAR coherence analysis. Then, in this development, we estimate decorrelation only due to volcanic ash accumulation, using a temporal coherence change model without volcanic ash effect constructed from InSAR coherence time-series analysis. Furthermore, we estimate coefficient converting coherence to the amount of volcanic ash accumulation from case studies. However, coherence basically saturates easily and thin accumulation of volcanic ash is difficult to detect, and therefore, it is difficult to detect the spatial distribution of volcanic ash accumulation. So, we attempt to estimate spatial distribution of volcanic ash accumulation by combining InSAR coherence analysis and ash-fall simulation in this development.