f Detection of disturbed zones over old mine workings using electrical tomography

Coal exploitation in the open cast mine of Kolubara, Serbia, progressed up to a zone of old mine workings abandoned since 1971, which has been determined as a high risk zone due to the presence of cavities. It was thus necessary to investigate the presence of empty spaces or significantly altered zones particularly to the depth of about 15 m. which was considered to be the zone of highest risk. Since the problem couldn't be solved only by drilling, among several geophysical methods, electrical tomography has been applied (D.H. Griffiths, J.S. Turnbull, 1985). Twenty to thirty five electrode arrays were used with a ten meters unit spacing. Depending on the number of electrodes, the measurements were carried out on six to ten levels (n), corresponding to depths of investigation ranging from 30-50 m .