1. Department of Drilling, Oil and Gas Production & Transport, Faculty of Geology & Exploration, University of Mining and Geology St. Ivan Rilski, 1700 Sofia, Bulgaria
2. Department of Mathematics, Faculty of Sciences, University of Oviedo, 33007 Oviedo, Spain
3. Instituto Universitario de Ciencias y Tecnologías Espaciales de Asturias (ICTEA), University of Oviedo, 33004 Oviedo, Spain