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Quantification of Ash Sedimentation Dynamics Through Depolarisation www.nature.com/scientificreports OPEN Quantifcation of ash sedimentation dynamics through depolarisation imaging with Received: 26 June 2018 Accepted: 8 October 2018 AshCam Published: xx xx xxxx Ben Esse 1, Michael Burton 1, Matthew Varnam 1, Ryunosuke Kazahaya1,2, Paul A. Wallace3, Felix Von-Aulock3, Yan Lavallée 3, Giuseppe Salerno4, Simona Scollo4 & Hugh Coe1 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 difcult 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 difuse 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, fnding good agreement with a sedimentation model based on particle size. Our new, cost-efective 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. Volcanic ash is a primary product of explosive volcanism which, while ofen beneftting the biosphere, generally poses a threat to human health and infrastructure1. Ash exposure can cause irritation to the nose, throat and eyes, as well as aggravating pre-existing health conditions such as asthma2. Heavy ash fall can also lead to building col- lapse, potentially injuring or killing those inside3. Ash is also a danger to other critical infrastructure, including electrical, water and transportation networks (especially air travel)4–7. Recent work has focussed on the key role of ash aggregation in controlling the dynamics of ash plumes8–10, and such processes are included in new mod- elling approaches11. Testing and validation of ash aggregation and sedimentation models is therefore an urgent requirement, but we have few tools capable of providing the empirical and reliable quantifcation of ash dynamics in the atmosphere. Existing ash detection methods include infrared imagery12–14, radar15,16 and LiDAR17,18, as well as combined analysis of acoustic signals and optical imagery19,20. Satellite UV instruments (such as OMI) have also been shown to be sensitive to ash21 – however little work has been done to date on ground based UV measurements. Two previous studies have used ground-based UV cameras for the observation of volcanic ash22,23, with a third meas- uring black carbon particles in ship emissions24. All three of these studies assume that absorption from the ash (or carbon particles) dominates the attenuation of the light passing through the plume and so do not explicitly detect ash. We note that for optically thick plumes refection from the surface or internal scattering within the plume would likely dominate over transmission. Ash particles are formed through explosive fragmentation of magma and are ofen very glassy in nature. Tey form sharp, irregular shapes that are hard to characterise morphologically25 and have complex interactions with radiation, leading to errors in ash mass retrievals26. Te non-sphericity of ash particles does have one advantage however: the depolarisation of scattered light. 1School of Earth and Environmental Sciences, The University of Manchester, Manchester, M13 9PL, UK. 2Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan. 3Department of Earth, Ocean and Ecological Sciences, University of Liverpool, Liverpool, L69 3GP, UK. 4Istituto Nazionale di Geofsica e Vulcanologia, Osservatorio Etneo, sezione di Catania, Piazza Roma 2, 95123, Catania, Italy. Correspondence and requests for materials should be addressed to B.E. (email: [email protected]) SCIENTIFIC REPORTS | (2018) 8:15680 | DOI:10.1038/s41598-018-34110-6 1 www.nature.com/scientificreports/ 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. Te dome complex can be seen to be growing from the collapse scar of Santa Maria. Te dotted white lines give the approximate feld of view of AshCam. Map data: Google, CNES/Airbus. Map generated with Google Earth version 7.1.8.3036 (https://www.google.com/earth/). Here, we present a new method for investigating the dynamics of ash plumes using ground-based UV-VIS imagery named “AshCam”. By measuring the intensity of light for two orthogonally polarised channels, deviations from the expected polarisation pattern of scattered sunlight can be used to infer the presence of ash. UV-VIS 27–30 cameras are widely used in volcano monitoring for measuring volcanic SO2 fuxes . Tese systems typically use two wavelength channels, one sensitive to SO2 and one not, to quantify the SO2 column amount in each pixel. Tis is usually achieved with either a single camera and a flter wheel, or two cameras with two diferent flters. By replacing the flters with two linear polarising ones these cameras can be modifed to measure ash, allowing AshCam to be easily integrated into existing monitoring networks. Te high frame rate (~1 Hz) of the cameras allows for the investigation of ash dynamics, for example the ash settling velocity, and individual flaments of ash can be tracked between frames, providing detailed quantifcation of ash velocities. We report on observations of mild explosive activity at Santiaguito Volcano, Guatemala. We show for the frst time that it is possible to measure the depolarising efect of ash through passive measurements of sunlight and demonstrate how these measurements can be used to investigate the dynamics of ash plumes. Results Field site. Santiaguito (14.7500° N, 91.5667° W, 2520 m) is an andesitic-dacitic lava dome complex in Guatemala (Fig. 1a) which formed afer the 1902 eruption and subsequent collapse of Santa Maria31,32. Santiaguito exhibits regular explosions approximately once every 26 minutes to two hours from the currently active vent feeding “Caliente” dome33,34 (Fig. 1b). Tis makes it an excellent natural laboratory for testing ash remote sens- ing techniques20,35. Te observations presented here were made on 18th January 2018 from an observation site approximately 4 km north-west from the dome (Fig. 1c). During the observation period two explosions occurred at 09:00 and 11:10 (local time), lasting approximately 1 and 10 minutes respectively. Here, we focus on the second explosion as a test case for AshCam due to its longer duration. Depolarisation images. Figure 2 shows examples of depolarisation images taken before, during and afer the explosion, as well as graphs showing the cross-section of the plume (blue line on the images). Te cross-sections are averaged vertically across ten pixels. Te explosion started with a smaller impulsive event, followed by a more prolonged ash emission that formed the main body of the explosion – a common feature at Santiaguito20,34,35. A full video of the depolarisation images for the explosion can be found in Supplementary Video S1. Images were acquired with a frequency of 0.2 Hz – although the cameras are capable of higher frame rates (5–15 Hz depending on acquisition quality settings), 0.2 Hz was chosen in the feld due to data storage prac- ticalities and to increase the image quality. Te main, optically-thick aerosol plume rising from the dome has a strong depolarisation ratio (~1.25) – however this is likely due to refections from the plume surface and multiple internal scattering within the optically thick plume itself. Tis means the dynamics of the ash cannot be separated from the other aerosols in the plume. We note that this result suggests direct observation of aerosol-rich plumes with SO2 cameras may be strongly afected by refected sunlight. Te ash settling out from the main plume could instead be readily observed as optically thin depolarisation features on the lef side of the images. We highlight that the fne settling ash was not clearly visible to the naked eye or with normal video recording equipment (see Supplementary Fig. S1). Te depolarisation signal (~1.1) is signifcantly above the background noise in the image (~1.01). Te background sky shows little change between the frames, remaining close to 1 throughout the explosion. A false signal can be seen in the top lef corner of frame (c) due to refections from low meteorological clouds. Additionally, an enhancement of the signal can be seen on the edge of the main plume and on the dome due to a slight spatial misalignment of the horizontally and vertically polarised images. SCIENTIFIC REPORTS | (2018) 8:15680 | DOI:10.1038/s41598-018-34110-6 2 www.nature.com/scientificreports/ Figure 2. Example depolarisation images before (a), during (b) and afer (c) an explosion at Santiaguito. Te timings are relative to the onset of the explosion. Te graphs depict the cross-sections indicated by the blue lines on the images. Te cross-sections are averaged across 10 pixels vertically. Main Plume Downwind Figure 3. Example fow map of the output from the optical fow 275 s afer the onset of the explosion. Te length of the arrows is proportional to the fow speed. Te white boxes show the main plume and downwind areas chosen for further analysis. Plume dynamics. Te plume dynamics were investigated using an optical fow algorithm (see methods section for details). Tis was achieved using the Farnebäck algorithm from the python OpenCV library.
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