Nyiragongo crater collapses measured by multi-sensor SAR amplitude time series

Abstract

Crater morphology at active volcanoes can change rapidly. Quantifying changes during the course of a volcanic unrest episode may help assess the level of volcanic activity. However, limitations such as crater accessibility, cloud cover or intra-crater eruptive activity may hamper regular optical or on-site crater monitoring. Here we use multi-sensor satellite Synthetic Aperture Radar (SAR) imagery to produce dense time series of quantitative indicators of crater morphological changes. High temporal resolution is achieved by combining images from a variety of sensors and acquisition modes, though the diversity of acquisition geometries (incidence angle, viewing direction, resolution…) prevents direct comparison between the different images. Using basic trigonometry assumptions, we develop PickCraterSAR, an open-access tool written in Python to measure crater radius and depth from SAR amplitude images in radar geometry. We apply our methodology to study the crater collapse associated with the May 2021 and January 2002 eruptions of Nyiragongo volcano. After the 2021 collapse, we estimate the maximum depth of the crater to be 850 m below the rim and the total volume to be 84$\pm$10 Mm$^3$ (270 m deeper but only 15-20 \% more voluminous than the post-2002 eruption crater). We also show that the 2021 crater collapse occurred progressively while a dike intrusion was migrating toward the south.

Local Infrasound Monitoring of Lava Eruptions at Nyiragongo Volcano (D.R. Congo) Using Urban and Near‐Source Stations

“…Using colocated seismo-acoustic records at KBTI over ∼2 years (September 2015-November 2017), Barrière et al (2018) identified long-period (<2 Hz) repetitive seismic events and continuous tremor (i.e., 2023) for more details). PickCraterSAR data are smoothed with a ±1 day moving average window (Smittarello et al, 2023) or ±7 days for updated results based on COSMOSkyMed images only. (c) Space-based daily solutions for SO 2 emissions obtained from TROPOMI and thermal hotspots associated to Nyiragongo (S2 stands for Sentinel-2, L8/9 for Landsat-8/9).…”

“…The absence of observable lava-related thermal anomaly was noticed one day later on 1 June 2021 (Figure S7 in Supporting Information S1). This date corresponds to the deepest crater bottom after successive deepening and enlarging of the pit crater (∼580 m deeper than before the eruption), after which the crater refilled the crater bottom from collapsing walls (Smittarello et al, 2023). Both renewal of intra-crater activity on 28 September and 25 November 2021 are clearly identified in the infrasound records, further validated by thermal imagery on 29 September despite very cloudy conditions (Figure S8 in Supporting Information S1) or by visual inspection on 1 October (Figure 2a).…”

“…Smittarello et al. (2023) developed the method called PickCraterSAR, which extend the applicability of SAR amplitude when the layover effect limits the maximum depth reachable by the SAsha method (see both studies for more details). In the present study, we provide the merged data set between SAsha and PickCraterSAR up to July 2021 and extend the time series with newly processed COSMOSkyMed images up to April 2022.…”

“…Because a large lava lake and a spatter cone disappeared, the changing source properties (Rosenblatt et al., 2022) and modified acoustic response of the crater due to evolving geometry (Watson et al., 2019) can largely influence the frequency and amplitude content of this new infrasound signature. During the May 2021 eruption lasting only ∼6 hr (Smittarello et al., 2023), the draining of the lava lake and the lava fountaining on the flank can also be tracked using records from KBTI and NYI (Barrière et al., 2023). These aspects, which are beyond the scope of this paper, will be tackled in a dedicated study.…”

“…The analysis of radar amplitude images acquired by Synthetic-Aperture Radar (SAR) satellite sensors offers the opportunity to detect morphology changes of volcanic craters as evidenced at African lava-lake volcanoes Nyiragongo, D.R. Congo (Barrière et al, 2018(Barrière et al, , 2022Smittarello et al, 2023), its neighbor Nyamulagira (Burgi et al, 2021) and Erta' Ale, Ethiopia (Moore et al, 2019). Sulfur dioxide is a key constituent of volcanic plumes which can be measured from space-based spectrometers owing to its distinct absorption spectral signature in the ultraviolet (e.g., Carn et al, 2016;Theys et al, 2019).…”