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

Abstract

Degassing volcanic systems, expressed by fumaroles, thermal anomalies, and hydrothermal alteration and deposition at the surface provide insights into the underlying structural architecture and the magmatic system. While the fumarole sites are easily identified and investigated, areas of diffuse degassing and associated hydrothermal alteration are barely explored. Here we investigate high-resolution optical and thermal infrared (TIR) data, acquired by unoccupied aircraft systems (UAS) at the La Fossa cone (Vulcano Island) in November 2018. The data provides insights into the structural complexity of degassing sites and associated processes at the surface. Applying the Structure from Motion (SfM) approach, we generate a photomosaic database with a 0.05 m and 0.7 m pixel resolution for the optical and infrared datasets, respectively. A Principal Component Analysis (PCA) was applied to the optical data to detect, define and extract areas of hydrothermal alteration and sulfuric deposition on a pixel base, with a feature detection threshold of up to 25 cm2. By comparing optical data, PCA results and the IR data, we found a broad alteration zone dominated by diffuse degassing surrounding the main fumaroles, which with ~ 60,000 m2 is ten times larger than the area covered by fumaroles and yellowish sulfuric deposits. Spectral and thermal characteristics of this alteration zone suggest a segmentation into at least 13 distinct subregions. Hydrothermal alteration and deposition were analyzed considering their pixel density and spectral signature (RGB) and show the highest pixel density in the center of the fumarole field, accompanied by a systematic color shift. The same region is characterized by a systematic change in azimuths of thermal lineaments and sulfuric clusters from the dominating trend NW-SE by ~90 degrees to NE-SW. We conjecture this to be controlled by a permeability contrast due to a subsurface structure or crater intersection, facilitating a more direct gas ascent in the center of the fumarole field. We provide a precise and complete database for the state and extent of the La Fossa fumarole field, which can be used for comparative monitoring of spatio-temporal changes within the hydrothermal system at the surface.