Monitoring underwater volcanic degassing using Exail (iXblue) SeapiX volumetric sonar

Volcanic gases are a main trigger of explosive eruptions, but the largest amounts are emitted through passive, non-eruptive, degassing during quiescence. It is thus necessary to accurately map bubble clouds, and to monitor their dynamics, to reduce volcanic risks.

Contrary to atmosphere, gases are easily detected in water column, particularly using hydro-acoustic methods (Vandemeulebrouck et al., 2000). Two pioneering studies have monitored gas venting into Kelud Crater Lake (Indonesia) from a hydroacoustic station shortly before a Plinian eruption in 1990 [1] and, nearly two decades later, by empirically quantifying CO2 fluxes using acoustic measurements in the same lake just before a non-explosive eruption [2]. However, despite hydroacoustic detection capabilities, fundamental advances are limited by technology performances. Overall acoustic detection of a bubble field is easy, while its quantification remains complex due to the 3D structure of clouds and the acoustic interactions between bubbles.

We present results from near-surface geophysics of sedimentary deposits and water column gas seepage at the Laacher See (Eifel, Germany), using Exail Seapix 3D multibeam echosounder & Echoes high-resolution sub-bottom profiler. Backscatter profiles of water column elements distinguish macrophytes, gas bubbles and fishes and highlight several bubble plumes. Target Strength (TS) of bubbles is centered around -70 dB, suggesting they are of very small size (35 μm), much smaller than observed elsewhere using single beam echosounders. This would explain why, in the same spot, we did not observe any gas bubbling using camera mounted on ROV. Recent measurements at the nadir of a gas flare, in static positioning, using the steerable mills cross multibeam capability of the SeapiX, offered a 4D observation of the gas bubbling. It also provided an equivalent TS of the bubbling we observed two years earlier. We will also present CO2 flow rates that were also extracted from backscatter of gas bubbling in 4D. These calculations are currently being constrained using different backscatter models and represent the last technical aspect before developing an efficient early warning system. Meanwhile, Echoes 10 000 provides high-resolution paleoenvironmental reconstruction using 3D modeling of remobilized materials, and gas diffusion through the sediment. Fusion of all geophysical data using Delph Roadmap allows 3D modeling of gas flare dynamic from 40m in sediment to water-atmosphere interface. Our scientific approach contributes to improve forecasting of volcanic and limnic eruptions and participates to improve early warning systems by constant collaborations with academic research.

[1] Vandemeulebrouck et al (2000) J. Volcanol. Geotherm. Res 97, 1-4: 443-456

[2] Caudron et al (2012) JGR: Solid Earth 117, B5