Testing the potential of satellite based Synthetic Aperture Radar for the detection and mapping of volcanic ash
Dr Susanna Ebmeier
Reconstructions of total ashfall thickness provide valuable information for estimating volcanic eruption volumes and for understanding plume dynamics and particle transport. Isopach maps of ash thickness after an explosive volcanic eruption are traditionally constructed from (sometimes sparse) field measurements and eyewitness accounts, occasionally supplemented by aerial or optical satellite images. Eruptive deposits are rapidly removed by water and by clean up operations, so satellite imagery acquired immediately after an eruption has the potential to provide better constraints on the spatial extent of ash thickness.
Fresh ash generates signals in both the amplitude and phase of Synthetic Aperture Radar (SAR) data, but it has not yet been widely used for mapping ash deposits. The deposition of volcanic ash causes changes to the scattering properties of the Earth’s surface and therefore to the amplitude of SAR images, allowing it to be identified by using ‘change detection’ methods after radiometric correction. Recent ash layers may also shift the phase of back-scattered radar, in a similar manner to snow. Where a coating of ash is even and uniform (and conditions are dry), this may result in a uniform gradient in interferometric phase. Where ash changes scattering properties, this will result in phase incoherence, in a similar manner to fresh lava or rockfall.
This project aims to catalogue and characterise the impact of volcanic ash on SAR images sapnning recent explosive eruptions, with the aim of testing their usefulness for reconstructing total ashfall thickness. Initial targets will include the eruption of Calbuco, Chile (April, 2015) and Kelud, Indonesia (February, 2014). In both cases ash isopach maps constructed from field observations and optical imagery will be available for comparison to SAR data.
The summer student will have the opportunity to:
- Conduct a search through published articles and satellite data catalogues to identify recent explosive eruptions, with significant ash fall, for which SAR data were acquired.
- Analyse SAR interferograms and amplitude imagery from eruptions at Calbuco and Kelud volcanoes using GIS tools/Matlab/IDL to map the extent of evidence for ash fall.
- Compare evidence for ash fall in SAR imagery to isopach maps constructed from field data and test simple quantitative relationships between SAR observations and ash thickness.
Development of the project, depending on the student’s interests and skills may include the construction of an algorithm for the automatic detection of ash in SAR data. This would have to be calibrated against a bigger dataset and would require processing data from recent explosive eruptions chosen by the student. The student will have the chance to develop skills in processing SAR data as well as an understanding of explosive volcanic deposits and their impacts.