Surveying fumarole sites and hydrothermal alteration by unoccupied aircraft systems (UAS) at the La Fossa cone, Vulcano Island (Italy)

Hidden mechanical weaknesses within lava domes provided by buried high-porosity hydrothermal alteration zones

“…Drone imagery has also been used for image band combination, Structure-from-Motion (SfM), and image classification 38 . While many hydrothermal alteration effects are visible in the drone photos, the Principal Component Analysis (PCA) is an additional efficient statistical tool that can reveal information that is not accessible to the naked eye 39 – 41 . In a first step we use the drone image SfM approach for the identification of the geomorphology and dimensions of the evolving Merapi lava dome, revealing two main domes, a younger one covering an older one, both located inside the Merapi crater open to the South.…”

“…For instance, the dimensionality reduction from 3 optical channels to single principal components, PC1, PC2 or PC3, allows us to separate and identify areas associated with brightness estimates (PC1) or alteration-typical greenish-yellowish colorization (PC2). Using this approach, we can indirectly identify characteristic pixel colorization through the PC2 that is likely associated with sulfuric deposition and acid sulfate alteration 41 . We carefully compared regions located outside the area of interest (the lava dome) and found that the number of pixels detected in PC2 in many parts was constant in 2015 and 2017, whereas a strong increase of the number of pixels from 800 m 2 to over 2200 m 2 was recorded along the eastern segment of a horseshoe -shaped fracture system and on the southern sector of the active lava dome (Figs.…”

“…Drone based optical photogrammetry data containing pixel-wise information in RGB space can therefore not only provide the topographic dataset showing a nested dome structure (Fig. 6 ), but also provide a qualitative indication for both alteration and sulfuric deposition effects 41 . Repeat drone data at Merapi volcano from 2014 to 2017 suggests an intensification of alteration processes in the sulfuric-yellowish areas (Fig.…”

“…3 ). Applying PCA to these data will eventually allow us to quantify the intensity of alteration in such alteration zones 41 , as the progressively increasing degree of dome rock alteration was clearly recognizable. These rocks were eventually covered by new dome lava effusions in December 2018 and thereafter.…”

Alteration‐Induced Volcano Instability at La Soufrière de Guadeloupe (Eastern Caribbean)

“…Our modeling and 3D strength map therefore highlight the importance of monitoring the extent and evolution of hydrothermal alteration, and the formation of low-strength altered layers, at La Soufrière and at other active volcanoes worldwide. Such monitoring can be achieved, for example, using geophysical methods such as electrical tomography (Ahmed et al, 2018;Byrdina et al, 2017;Ghorbani et al, 2018;Rosas-Carbajal et al, 2016) and muon tomography (Lesparre et al, 2012;Rosas-Carbajal et al, 2017), near-surface seismic imaging (Amoroso et al, 2018), thermal and gas monitoring (de Moor et al, 2019;Edmonds et al, 2003;Jessop et al, 2021;Moretti et al, 2020;Tamburello et al, 2019), geological mapping (van Wyk de Vries et al, 2000), deformation monitoring (Moretti et al, 2020), magnetic methods (Finn et al, 2007), and remote and/or ground-based optical and spectroscopic methods (Crowley & Zimbelman, 1997;Darmawan et al, 2018;John et al, 2008;Kereszturi et al, 2020;Mueller et al, 2021). Further, although we document a reduction in strength as a function of alteration (Figure 5c), alteration can also increase strength and potentially promote volcanic instability by creating zones of high pore fluid pressure (Heap, Baumann, et al, 2021;Reid, 2004).…”