UAV-based remote sensing surveys of lava flow fields: a case study from Etna’s 1974 channel-fed lava flows

International audienceDuring an eruption, time scales of topographic change are fast and involve vertical and planimetric evolution of millimeters to metersas the event progresses. Repeat production of high spatial resolution terrain models of lava flow fields over time scales of a few hours isthus a high-value capability in tracking the buildup of the deposit. Among the wide range of terrestrial and aerial methods available tocollect such topographic data, the use of an unmanned aerial vehicle (UAV) as an acquisition platform, together with structure frommotion (SfM) photogrammetry, has become especially useful. This approach allows high-frequency production of centimeter-scaleterrain models over kilometer-scale areas, including dangerous and inaccessible zones, with low cost and minimal hazard to personnel.This study presents the application of such an integrated UAV-SfM method to generate a high spatial resolution digital terrain modeland orthomosaic of Mount Etna’s January–February 1974 lava flow field. The SfM method, applied to images acquired using a UAVplatform, enabled the extraction of a very high spatial resolution (20 cm) digital elevation model and the generation of a 3-cmorthomosaic covering an area of 1.35 km2. This spatial resolution enabled us to analyze the morphology of sub-meter-scale features,such as folds, blocks, and cracks, over kilometer-scale areas. The 3-cm orthomosaic allowed us to further push the analysis tocentimeter-scale grain size distribution of the lava surface. Using these data, we define three types of crust structure and relate themto positions within a channel-fed a ā flow system. These crust structures are (i) flow parallel shear lines, (ii) raft zones, and (iii) foldedzones. Flow parallel shear lines are found at the channel edges, and are 2-m-wide and 0.25-m-deep zones running along the levee baseand inwhich cracking is intense. They result from intense shearing between themoving channel lava and the static levee lava. In zoneswhere initial levees are just beginning to form, these subtle features are the onlymarker that delimits themoving lava from the stagnantmarginal lava. Rafts generally form as the system changes from a stable to a transitional channel regime. Over this 170-m-long zone,the channel broadens from 8 to 70 m and rafts are characterized by topographically higher and poorly cracked areas, surrounded bylower, heavily cracked areas.We interpret the rafts as forming due to breakup of crust zones, previously moving in a coherent mannerin the narrow proximal channel reach. Folded zones involve arcuate, cross-flow ridges with their apexes pointing down-flow, whereridges have relatively small clasts and depressions are of coarser-grained breccia. Our folds have wavelengths of 10 m and amplitudesof 1m; are found towards the flow front, down-flow of the raft zones; and are associated with piling up of lava behind a static or slowlymoving flow front. The very high spatial resolution topographic data available from UAV-SfM allow us to resolve surfaces whereroughness has a vertical and horizontal scale of variation that is less than 1m. This is the case over pāhoehoe and a ā flow surfaces, and thus allows us to explore those new structures that are only apparent in the sub-metric data. Moreover, during future eruptions, thepossibility to acquire such information in near-real time will allow a prompt analysis of developing lava flow fields and structurestherein, such as developing lava channel systems, so as to contribute to timely hazard assessment, modeling, and projections