Detection of traces of pyroclastic flows and lahars with satellite synthetic aperture radars

To assist volcanic hazard mitigation, detection of traces of pyroclastic flows and lahars were attempted by using satellite synthetic aperture radars (SAR). If such information can be obtained rapidly, it can help early warnings of the next pyroclastic flows and lahars because these flows often occur repeatedly at similar locations. Using three analytical approaches, two satellite SAR data, namely, the L‐band SAR of the Japanese Earth Resource Satellite (JERS‐1) and the C‐band SAR of the European Remote Sensing Satellite (ERS‐1), were tested. These approaches include subtraction of SAR backscatter coefficients, SAR coherence, and SAR interferometry (InSAR). These approaches were validated in the Unzen Volcano with digital elevation model (DEM) subtraction images created from aerial photographs. As a result, it was found that the coherence approach with JERS‐1 SAR was highly capable of detecting the traces of the pyroclastic flows and lahars. The traces appeared as either one of two characteristics on the coherence images: low coherence caused by the new traces formed in between a pair of observations or high coherence caused by the recent traces formed before a pair of observations. In contrast, we could not validate applicability of the backscatter approach or the InSAR approach.