b'National differential RTP gridTechnical noteThe Australian national differential RTP gridClive Foss1, Des Fitzgerald2 and Mike Barlow3 inclination from -65 to -45 reduces the peak to trough ratio of the anomaly and shifts the anomaly peak further north 1CSIRO Mineral Resources, 2Intrepid Geophysics, 3Geoscience Australia away from the centre of magnetisation. North-south profiles des@intrepid-geophysics.com through these anomalies and for a -90 inclination are plotted in Figure 3. For the anomaly in a vertical inclination field the Over the last 50 years total magnetic intensity (TMI) has beenTMI peak is directly above the dipole centre, the anomaly of the the most measured expression of the magnetic field, not forsymmetric source is itself symmetric and the peak to trough interpretational advantage but for practical and cost-effectiveratio is extremely high, giving clear advantages for mapping acquisition. Since 2011, Geoscience Australia has producedmagnetisation. A reduced-to-pole (RTP) phase transform of the a series of national TMI compilations from a minimum of 795field and induced magnetisation from the local geomagnetic individual survey grids. TMI measurements are (except indirection to the vertical can be used to extend this advantage regions of intense local fields) the scalar magnitude of theto other latitudes (Baranov 1957, Blakely 1995) but the vector component parallel to the local geomagnetic field.conventional transform from a single TMI direction to vertical There is significant variation in direction of the magneticcannot span the whole of Australia.field across Australia, and therefore the nature of TMI also varies considerably. The shallowest geomagnetic inclinationArkani-Hamed (1988) and Swain (2000) developed differential of -40 at the north coast of Queensland and the NorthernRTP to transform TMI of continuously variable direction, and Territory (Figure 1) is steeper than would normally be expectedCooper and Cowan (2005) developed an iterative algorithm at those latitudes, and is associated with a substantialto perform the task using a spatial domain Taylor series northward departure of the geomagnetic equator (the line ofapproximation. Geoscience Australia applied this method to zero inclination) through the South China Sea. The steepestthe Edition 4 TMI compilation (Nakamura and Milligan, 2015) inclination of -70 is at the south coast and in Tasmaniausing the Intrepid software product and have since updated (Figure1). the grid using the 2019 TMI compilation (Poudjom Djomani and Minty, 2019). The regular variation of the geomagnetic Figure 2 shows anomalies due to an induced dipole at 1 km depth in fields of -65 and -45 inclination. Shallowing of Figure 1.Inclination and declination changes of the geomagnetic fieldFigure 2.Dipole TMI anomalies in (top) a steep -65 inclination towards the across Australia, 2021. Red square defines the case study location. south coast and (bottom) a lower -45 inclination towards the north coast.55 PREVIEW DECEMBER 2021'