Valgma, Ingo. “MAPPING POTENTIAL AREAS OF GROUND SUBSIDENCE IN ESTONIAN UNDERGROUND OIL SHALE MINING DISTRICT”. ENVIRONMENT. TECHNOLOGY. RESOURCES. Proceedings of the International Scientific and Practical Conference 1 (June 20, 1999): 227–232. Accessed April 11, 2025. https://journals.rta.lv/index.php/ETR/article/view/1869.