Gas detection and quantification using iXblue Echoes high-resolution sub-bottom profiler and Seapix 3D multibeam echosounder from the Laacher See (Eifel, Germany)

Volcanic gases constitute the main fuel of explosive eruptions and volcanoes mostly emit massive amounts of gas during quiescence. They can also trigger limnic eruptions such as the 1986 eruption at Nyos volcano (Cameroon) that killed more than 1500 people. Contrary to atmosphere, gases are easily detected in water column, particularly using hydro-acoustic methods [1]. 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 by 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. It is thus necessary to accurately map the different bubble clouds, to monitor their evolution and to dissociate different gas origins to reduce the volcanic risk, which is major in aqueous environments. Here, we present preliminary results from near-surface geophysics of sedimentary deposits and water column gas distributions and quantification from an Eifel crater lake (Germany), using iXblue Echoes high-resolution sub-bottom profiler and Seapix 3D multibeam echosounder. Derived from Seapix analyses, backscatter profiles of elements in the water column allowed to distinguish fish and gas bubbles, which demonstrates a potential for the development of an automatic gas detection module using the Seapix software. Ongoing research on the Target Strengh (TS) of bubbles suggest they are of very small size (35 $\mu$m), much smaller than observed elsewhere using single beam echosounders, which might also explain why, in the same spot, we did not observe gas bubbles using camera mounted on ROV. This also raises new perspectives to improve CO2 estimates from volcanic bubbles release. Meanwhile, the Echoes 10 000 provided high-resolution images of the architecture of the lake deposits and visualized in real time using Delph Software. More than 30 m of penetration with a theoretical 8 cm-resolution highlight paleoenvironmental and paleoclimatic reconstruction perspectives, 3D modeling of remobilized materials and tephra deposits from volcanic activity, as well as differentiation between shallow and deep gas diffusion derived from organic matter decomposition and volcanic activity respectively. Next step is to use Delph Seismic Interpretation to construct DTM of all main reflectors, thus providing 3D modeling of the sedimentary infill and to allow interpretation in terms of sedimentary dynamics. Echoes/Delph & SeapiX complement each other to follow gas flares in underwater environments. This complementarity allows to verify that the vertical acoustic signal attributed to gas flares is not an artefact potentially related to macrophytes or other plants. Indeed, fusion of all geophysical data using Delph Roadmap allow to verify if each gas plume in water column coincides with gas diffusion within the sediment. Our geophysical scientific approach contributes to a wider scientific effort to increase forecasting of volcanic and limnic eruption and improve early warning systems through constant collaborations with academic research.