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A multi-faceted approach to estimate total grain size distribution of pyroclastic density current deposits using unoccupied aircraft systems, wet sieving, and high-resolution digital microscopy

Franco Garin , Sylvain J. Charbonnier

Abstract

The study of pyroclastic density currents (PDC) dynamics, as well as related erosional and depositional processes still primarily rely on depositional evidence. Capturing data during an active PDC event is limited by hazardous conditions, as well as difficult logistical and technological challenges. Hence, acquiring geological samples that are analogous to individual PDC deposits is of the utmost importance, particularly for the quantification of the total grain size distribution (TGSD). Justifiably, volcanologists struggle to do so, as the grain sizes can range from a few microns to several meters, resulting in the commonality of truncated GSD data. Although different approaches have been proposed and used to estimate TGSD, improvements can be made to address the approximation of the buried portions of coarse clasts as well as enhancing the fidelity of the fine range partitioning. Thus, we developed a dependable and standardizable method aimed towards capturing the full range of grain sizes and reducing associated analytical uncertainties by melding different technologies. Complementarily, we used i) unoccupied aircraft systems (UAS) to capture imagery in-situ and then processed it to produce millimeter-scale Digital Outcrop Models (DOMs) for coarse-to-intermediate clasts (meter-to-centimeter scale), ii) traditional wet-sieving techniques for intermediate-to-fine clasts (centimeter-to-micrometer scale), and iii) a high-resolution digital microscope for the finest clasts (millimeter-to-micrometer scale). This approach aims to standardize the collection of PDC field data, contributes towards the adoption of discrete element methods to advance our understanding of PDC dynamics, and could even potentially improve volcanic hazard assessment efforts.