The Forrestania and Nepean electromagnetic test ranges, Western Australia – a comparison of airborne systems

Summary Electromagnetic (EM) systems are often described with varying technical specifications and standards, making it difficult to directly compare and assess their application to practical field examples. Exemplar case studies provided by contractors, whilst highlighting system capabilities, do not necessarily help to refine the suitability of the system across different geological targets and environments. Test ranges provide an opportunity for direct and consistent comparison of multiple systems for objective assessment. The Forrestania and Nepean EM test ranges in Western Australia consist of readily accessible land, openly available for testing by airborne, ground and downhole EM systems. Multiple conductors at varying depths beneath 10-20 Siemens (S) conductive overburden provide challenging, real-world conductive targets. Surveying using different EM systems allows for a direct comparison of system detection and resolution capabilities in a conductive regolith environment. The conductors have been well defined by drilling and provide a large range of metrics available for measurement, varying from 60-400 m in depth, 5,000-10,000 S in conductance, and with variable lateral profiles and depth extents. Multiple airborne, ground and downhole EM systems have utilised these test ranges, and several have made their data freely available for review. These include ground methods such as moving loop EM, fixed loop EM, SAMSON, downhole EM, and helicopter systems including HeliSAM FLEM, SkyTEM, VTEM, HELITEM, HeliGEOTEM, XTEM, HoistEM and AeroTEM. The SPECTREM, and Xcite airborne systems plan to fly the test range in the near future. Comparison of the airborne results, show that most of the post-2007 systems have been adequate to good at detecting the shallow IR2 conductor at Forrestania under conductive regolith. Only the hybrid grounded loop HeliSAM system has successfully detected the deep IR4 conductor at Forrestania.