Quantification of ash sedimentation dynamics through depolarisation imaging with AshCam

Abstract

Even modest ash-rich volcanic eruptions can severely impact a range of human activities, especially air travel. The dispersal of ash in these eruptions depends critically on aggregation and sedimentation processes - however these are difficult to quantify in volcanic plumes. Here, we image ash dynamics from mild explosive activity at Santiaguito Volcano, Guatemala, by measuring the depolarisation of scattered sunlight by non-spherical ash particles, allowing the dynamics of diffuse ash plumes to be investigated with high temporal resolution (>1 Hz). We measure the ash settling velocity downwind from the main plume, and compare it directly with ground sampled ash particles, finding good agreement with a sedimentation model based on particle size. Our new, cost-effective technique leverages existing technology, opening a new frontier of integrated ash visualisation and ground collection studies which could test models of ash coagulation and sedimentation, leading to improved ash dispersion forecasts. This will provide risk managers with improved data quality on ash location, reducing the economic and societal impacts of future ash-rich eruptions.

Figure 1

Geography of Santiaguito. (a) Sketch map showing the location of Santiaguito in Central America (red circle). (b) Image of Caliente dome taken from the measurement location at the onset of an explosion (image taken by Ben Esse). (c) Satellite image of the area surrounding Santiaguito with the measurement and sample locations marked. The dome complex can be seen to be growing from the collapse scar of Santa Maria. The dotted white lines give the approximate field of view of AshCam. Map data: Google, CNES/Airbus. Map generated with Google Earth version 7.1.8.3036 (https://www.google.com/earth/).

Figure 2

Example depolarisation images before (a), during (b) and after (c) an explosion at Santiaguito. The timings are relative to the onset of the explosion. The graphs depict the cross-sections indicated by the blue lines on the images. The cross-sections are averaged across 10 pixels vertically.

Figure 3

Example flow map of the output from the optical flow 275 s after the onset of the explosion. The length of the arrows is proportional to the flow speed. The white boxes show the main plume and downwind areas chosen for further analysis.

Figure 4

Average vertical flow speeds for the main plume (orange circles) and downwind (blue crosses) regions. The x-axis is the time with respect to the onset of the explosion, determined from the imagery. Positive velocities correspond to upwards motion. The solid lines represent the 5 point moving average.

Figure 5

Sample ash collected from Santiaguito. (a) BSE SEM image of the ash sample. (b) Particle size distribution of the ash collected from Santiaguito on 20^th January 2018. The size fractions were sorted by dry-sieving the sample.

Figure 6

Setup of AshCam. The cameras are fitted with a 380 nm band filter (FWHM = 10 nm) and a polarising filter mounted to the front of each lens. The cameras are mounted on a standard tripod and powered using a pair of Lithium Polymer batteries.

Figure 7

Diagram of the geometry used in the Rayleigh Sky Model. The vector r represents the viewing direction, γ_s is the solar azimuth angle and α is the scattering angle. The observer is at the origin.