SAR Shadows for Measuring Height Changes: Application for Monitoring Pressure Changes in Volcanic Systems in the Virunga Volcanic Province

BARRIERE, JULIEN; DERAUW, DOMINIQUE; D'OREYE, NICOLAS; HALLDOR GEIRSSON

Milano

Simposio; Living Planet Symposium; 2019

European Space Agency

Several SAR sensors orbiting the Earth provide invaluable set of data since the nineties. Many applications benefit from the quasi monochromatic properties of the SAR signal to produce interferograms and compute deformation maps or Digital Elevation Models. SAR amplitude images are also used in diverse disciplines such as forestry, agriculture, archeology, geology, oceanography etc. for land cover changes detection, floods or wildfires mapping, oil spill detection, vessel tracking?In the present contribution, we use the elementary property of SAR signal propagation to travel in straight line, independently from the time of the day or night, the weather conditions. Alike the visible light in optical images, the radar illumination of the ground in SAR images is hampered by sub vertical structure, and the shadow casted by the structure is proportional to the height of the structure. There is however a fundamental difference with optical images: within SAR images, points are located in the image depending on the signal travel time between the satellite and the target on the ground, while in the optical image it depends on the illumination angle. Nevertheless, knowing the incidence angle and the range resolution of the SAR sensor, one can deduce the height of the structure from the length of the shadow casted on the ground.We apply that principle for monitoring of the lava lake level changes and the rise of the bottom of the crater at Nyiragongo volcano in in Democratic Republic of Congo. Nyiragongo is a stratovolcano hosting the largest lava lake on Earth. The city of Goma (800.000 inhabitants) is located at the foot of the volcano, in a region affected by recurrent conflicts, and where remote sensing is hence a convenient tool in assessing and monitoring volcanic hazards.We use SAR images because the quasi permanent cloud cover blanketing this equatorial region and dense gas plume emanating from the crater hamper the use of optical or multispectral imagery.From the frequent lava lake overflows, the bottom of the crater rose of more than 200 meters in a few years. The level of the lava lake is also affected by rapid level changes up to several tens of meters in a few hours or days. Despite the importance of these level changes and their evident relation with pressure changes in the magmatic system of the volcano, no permanent monitoring system was installed to measure these changes.We measure the rise of the crater floor and the lava lake level fluctuations from the module of hundreds of SAR images acquired by RADARSAT, COSMO-SkyMed, SENTIEL-1 and Envisat. The height changes are obtained from the length of the shadow casted by surrounding rims. We set an automatic method for detecting illumination/shadow transitions using a grid-search strategy among an extensive dictionary of synthetics 1-D transects. We produced the first time series of accurate and dense lava lake level measures as well as of crater?s bottom altitude changes. The measures were validated with high resolution DEMs obtain from UAV photogrammetry and field measurements.After its drainage in 2002, the crater filled up on ~400m thickness until 2008. Till 2012, the activity remained reduced (level fluctuations of various amplitude and time scales within the pit), then, after a large volume eruption at Nyamulagira volcano (located 13km away), the level progressively lowered, reaching ~70m below the crater floor. It remained at low level until the end of 2015. Early in 2016 a new vent opened within the crater, which intermittently emits important lava flows that blanket the crater. In November 2016, the lake level dropped of a hundred meters in a few hours.We compare these measures with other parameters (seismicity, SO2 degassing, ground deformation and visual observations). We infer that these lava lake level changes can be used as a proxy for the pressure changes in the magmatic system. Comparing these pressure variation measurements with ground deformation measured by the permanent geodetic KivuGnet GNSS network, we present the first evidence of interaction between these two volcanoes, Nyiragongo and Nyamulagira, located 13 km apart.The method of SAR shadow measurement presented here can of course be applied to several other type of height changes measurements such as sediment accumulation, water level variations in reservoirs etc. as long as there is a sub vertical structure casting shadow (e.g. cliff, rim, tower, dams etc.). This of course includes the monitoring of some of the other rare volcanoes hosting a lava lake, such as Kīlauea or Erta Ale.