Borehole to borehole electromagnetic tomography at Trap Gully, Callide Mine, East Central Queensland

Abstract

Borehole to borehole electromagnetic tomography, also known as the Radio Imaging Method (RIM2), has been used at Callide Coal Mine since 1996 for mapping fault structures behind high walls. The RIM2 system has been used at frequencies from 12.5kHz to 302.5kHz at borehole separations of 50 to 120m. A high level of correlation with the logged geology is obtained by using close spaced transmitter and receiver spacings and by the choice of the highest frequency at which signal penetration still produces a good dynamic range. The use of RIM2 was instrumental in improving the geologic model of the structures behind the highwalls at Trap Gully. The RIM2 system has imaged a range of structures including reverse faulting and monoclinal folding with seam offsets of less than 5m to greater than 20m. The tomographic images produced have since been confirmed by the use of other geophysical methods and geotechnical drilling.