1. a Department of Natural Resources and Environmental Engineering, Escuela Técnica Superior de Ingenieros de Minas , University of Vigo , Vigo , Spain
2. b Centro Universitario de la Defensa, Academia General Militar , Zaragoza , Spain
3. c Department of Statistics, Escuela Técnica Superior de Ingenieros de Minas , University of Vigo , Vigo , Spain
4. d Serumano S.L., C/Enrique Mariñas , A Coruña , Spain