Risk assessment from historical coal undermining using helicopter TDEM and ground geophysics: The Ermelo Ring Road study, South Africa
J. Legault, K. Kwan, G. Plastow, R. Wilson, P. Roux and S. Fourie
Event name: SAGEEP 2017 - 30th Annual Symposium on the Application of Geophysics to Engineering and Environmental Problems
Session: Infrastructure II
Publication date: 19 March 2017
Info: Extended abstract, PDF ( 1.15Mb )
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. Coal mines were operational in the mid 1900’s, the majority of which have been abandoned. Little information was readily available about the undermining, and a decision was taken to evaluate the planned route across the undermined area for old mine voids (tunnels and cavities). This would assist in determining whether the planned route was safe. A methodology was applied that combined airborne geophysics, principally Time Domain Electromagnetics, Ground Geophysics, Drilling and Borehole Geophysics to investigate the subsurface under the planned route. The airborne geophysics was employed to survey the large planned area swiftly with a helicopter, equipped with the necessary equipment. This was aimed at identifying target areas inside the survey area to be investigated in more detail, while eliminating areas that are considered stable. Experience with old coal undermining areas indicated that voids could potentially be filled with Acid Mine Water (AMW). The AMW has a low PH and is considered very conductive, which lends these waters to be an excellent target for TDEM. This was the main reason why an airborne TDEM survey was recommended. A combined airborne magnetic survey was also flown along with TDEM in order to delineate possible geological structures in the area, such as dykes and faults. The VTEM helicopter time-domain EM (TDEM) system was selected for the Ermelo study, with ground follow-up using TDEM, Electrical Resistivity Tomography (ERT) and gravity. The results were used to guide drilling to verify undermining targets, which was successful.