Ermelo Ring Road Risk Assessment for Coal Undermining using Airborne TDEM and Ground Geophysics

The South African National Roads Agency Ltd (SANRAL) appointed AECOM SA (PTY) Ltd (AECOM) in August 2014 to investigate the viability of constructing the new proposed ring road around the town of Ermelo, in the Mpumalanga Province, South Africa. A portion of the new proposed route was affected by previously undermined areas around the north-eastern part of the town. A decision was taken to evaluate the planned route across the undermined area for old mine voids (tunnels and cavities) using airborne and ground geophysics, followed by drilling. Experience with old coal undermining areas indicated that voids can potentially be filled with Acid Mine Water (AMW). AMW’s low pH makes it conductive. The air-layer above it is resistive. VTEM helicopter TDEM and aeromagnetics were employed to survey the large planned area swiftly. This was followed by ground TDEM, electrical resistivity tomography (ERT) and gravity. The results were used to guide drilling to verify undermining targets, which was successful.