SGEM 2021. 21st International Multidisciplinary Scientific GeoConference. Volume 21. Science and Technologies in Geology, Exploration and Mining : conference proceedings of selected papers. Albena, Bulgaria, 16 - 22 August, 2021. Issue 1.1 Geology, Hydrogeology, Engineering Geology and Geotechnics, Exploration and Mining, Mineral Processing, Applied and environmental Geophysics, Oil and Gas Exploration. 1. vyd. Sofia : STEF 92 Technology, 2021. 1038 s. ISBN 978-619-7603-20-0. ISSN 1314-2704.