Charters Towers SHARP project – Assessment of Ground Penetrating Radar for the location and risk analysis of abandoned mines shafts

Summary

Charters Towers was once one of Australia’s most productive gold fields peaking with an annual production of 319,572 ounces in 1899. The mining was predominantly underground, extending under much of the city to a depth of 2000ft. The main underground mining period ended in 1922 with many mine shafts capped with tree trunks and fill. The legacy of this period of activity is that as many as 1000 shafts exist within the Charters Towers region that have the capacity to collapse at short notice.

The Queensland Department of Mines and Energy are currently responsible for the location and capping of the shafts in the Charters Towers Shaft Repair Program (CTSHARP). In 2006 the CTSHARP Project team decided to trial Ground Penetrating Radar as a method for locating mine shafts and categorizing the risk collapse within small targeted areas. Many of these sites were located in the yards and gardens of private dwellings, horse paddocks and even currently trafficked streets. This provided a wide variety of surface and subsurface materials and site conditions in which to trial the technique.

Twenty sites were investigated and in the majority of the sites subsurface reflections were recorded that were indicative of either shaft locations or material collapsing into a shaft. The GPR data was used to both accurately plot the shaft location and give the anomaly a rating of 1 – 5 with 5 being the highest potential for collapse.

The GPR investigation results have been verified by ground truthing and an actual collapse during the heavy rains early in 2007.

Key words::

• GPR
• shaft collapse
• subsurface profiling
• risk assessment