A Sub-Audio Magnetics Case Study: Orlando Au-Cu-Bi Deposit, Tennant Creek, Australia

The first Sub-Audio Magnetics (SAM) trial survey ever conducted took place in May 1992 over the Orlando Au-Cu-Bi Deposit near Tennant Creek, Australia. The survey was conducted with prototype equipment and although the waveforms were adequately sampled, lack of synchronisation between the transmitter and magnetometer meant that data processing was extremely difficult. Furthermore, the software used to process the data was in the initial stages of development and was therefore quite primitive. Although it was always acknowledged that the main geological structures were being detected in the Orlando total field magnetometric resistivity (TFMMR) data, the quality of processing and presentation at the time was inadequate for any subtle detail in the data to be revealed. However, the survey provided the first of many invaluable SAM data sets from which more refined processing and presentation techniques have since been developed.

The Orlando TFMMR data were recently reprocessed with G-tek’s current SAM processing software in order to determine whether detail is present in the data beyond that which had been previously recognised. The reprocessed Orlando TFMMR data revealed many very subtle features, which were not evident in the original presentations. The survey was found to map the main Orlando Shear Zone and secondary shear extremely well. Offsets in stratigraphy, due to previously recognised faults, are clearly visible in the images. In addition, the correlation between the TFMMR data and the gradient-array apparent resistivity is extraordinary. However, because of its higher spatial resolution, the TFMMR data contains significantly greater detail than the conventional resistivity data.