Topographic change of the summit crater of Asama Volcano during the 2004 eruption derived from Airborne Synthetic Aperture Radar (SAR) measurements

An active volcano, Mt. Asama, located in the central part of the Honshu island of Japan, erupted on September 1, 2004. Since then, thick volcanic fumes have prevented monitoring of the topography on the summit crater floor by standard optical methods. To detect geomorphic changes of the volcano, GSI repeatedly carried out Airborne Synthetic Aperture Radar (AirSAR) measurements including interferometry between September 2004 and March 2005. The comparison of AirSAR results with a digital elevation model (DEM) by Airborne Laser Scanning measurements in October 2003 revealed that a pancake shape lava mound had formed on the crater floor, and the volume of the lava mound amounted to 2.1×10 m. From this, AirSAR measurement is recognized as an effective method for monitoring the topographic change of active volcano craters, and for foreseeing crises. 1. Topographic Observation using AirSAR AirSAR is a high resolution imaging radar attached to an aircraft which acquires terrain imagery by transmitting microwaves and receiving their reflection from the earth's surface. The transmission is made in the range direction downwards from the aircraft. GSI's AirSAR system (GSI-SAR) has two receiver antennas attached to the frame such that each antenna receives the same reflection signal at a slightly different position from each other (Photo 1). Operating interferometric measurements using both receivers' observations, it generates a digital elevation model (DEM) of the area. Concerning terrain observation from an aircraft, the introduction of SAR technology has the following advantages and disadvantages. 1) Observation through clouds, mists, or volcanic smoke is available. SAR uses microwave radiation for observation. Microwaves can penetrate through clouds and mist, which ordinary optical devices such as a camera cannot. Also, as it is an active sensor that transmits the radiation by itself, it can acquire information about terrain with or without sunlight (Fig. 1). This wide ability of observation provides a big advantage to SAR devices in cases like the monitoring of active volcanoes when observation by ordinary photography or laser scanning is impossible. 2) Observation conditions can be determined flexibly Compared to Satellite SAR systems, AirSAR has an advantage in the determination of observation conditions such as flight altitude, flight direction, and timing. These are very important abilities for efficient observation of volcano activities (Sato et al, 2002). 3) Distortion and radar shadow The output image of SAR is generated using the results of distance measurement between the radar and the terrain surface. So the image carries distinctive characteristics of the radar measurement such as distortions and shadows caused by steep mountains (Fig. 1). Reflected microwave signals are processed to be allocated on an image from the near side according to the trip time between the antenna and the terrain. Thus, the tops of mountains are shifted in the range direction to the near side of the image from their proper position. No distortions exist in the azimuth direction of the aircraft. The transmitted microwaves cannot reach the opposite side of mountains, so they cause a shadow area on the image where no signals come back from the terrain. Because of the use of an aircraft as the platform, AirSAR observation can be made from various directions toward the target, so the shadow area is reduced to the minimum even at obstructed targets like volcanic craters. Topographic Change of the Summit Crater of Asama Volcano during the 2004 Eruption Derived from Airborne Synthetic Aperture Radar (SAR) Measurements Bulletin of the Geogrephical Survey Institute, Vol. 53 March, 2006 2 2. Observation of the Summit Crater of Asama Volcano using AirSAR Since the major eruption of Asama volcano on September 1, 2004, GSI has repeatedly carried out AirSAR measurements around the summit crater, and observed its topographic change. The measurements were carried out considering the AirSAR characteristics described above. The results of the measurements were analyzed by interpretation from the output images, and comparison of the DEM generated using interferometric processing of SAR data and the DEM derived from a laser scanner measurement taken before the eruption by Tonegawa River Basin Sabo Office, Kanto Regional Development Bureau. From the DEM comparison, the volume of lava eruptions was also calculated. 2.1 Outline of measurement The measurement was set to have the most suitable altitude and off-nadir angle for observing inside the crater, and the flight courses were planned to circulate the summit so the measurements would cover all directions. Based on this plan, four sets of Ph oto 1 Assembled airborne SAR equipment used for the repeated measurements over the Asama volcano on September 16, October 22 and December 15, 2004 (left). The aircraft (Cessna 208) with onboard SAR instruments mounted (right). Fi g. 1 A schematic illustration of AirSAR measurements over a volcano. SAR is advantageous in measurements during night or through cloud, mist and volcanic fumes. Photo 1 Assembled airborne SAR equipment used for the repeated measurements over the Asama volcano on September 16,