1. School of Engineering, Newcastle University , Newcastle Upon Tyne, UK
2. Istituto Nazionale di Geofisica e Vulcanologia , Sezione di Catania – Osservatorio Etneo. Piazza Roma, 295125 Catania , Italy
3. Geospatial Research Ltd, Department of Earth Sciences, Durham University , UK
4. Department of Earth Sciences, Durham University , UK
5. Department of Earth and Planetary Sciences , Birkbeck College, Malet Street, London, WC1E 7HX , UK