b'National differential RTP gridTechnical noteFigure 5.Outline of inversion source bodies on TMI for anomalies inFigure 7.Almost coincident induced (black) and free magnetisation (blue) northeast Western Australia (for location see Figure 1). inversion body outlines with differential (black) and standard (red) RTP contours.susceptibilities of 12700 and 13000 metres and 0.43 and 0.49 to Geoscience Australias national differential RTP - the onlySI for the western and eastern sources (outlines as plotted in differences will arise from differences in the TMI compilationsFigure 5). The apparent susceptibilities (equivalent to over that they are applied to. GSWA use over 2500 input grids to10% magnetite by volume) are high for such large volumes create their state super grid which locally introduce additionaland may suggest contribution from remanent magnetisation. detail. Spatial details of the sources at these depths are not reliably recovered but nevertheless the inversion bodies and differential To illustrate and calibrate the role of RTP we have selectedRTP peaks are coincident as shown in Figure 6. This does not a pair of adjacent anomalies in northeast Western Australiaconclusively prove that both methods are correctly achieving (location shown in Figure 1). The magnetic field inclination andtheir objectives of locating the magnetisation because they declination at the area are -51.3 and +3.0 respectively. Theboth share the common assumption that magnetisation TMI anomalies are shown in Figure 5. Interpretive separationis parallel to the local geomagnetic field. To establish the of the TMI anomalies from the background field gives peakappropriate magnetisation direction, we repeated the inversion to trough amplitudes of 830 and 880 nT and peak to troughwith a free magnetisation direction. This results in a slightly ratios of 2. 9 and 2.3 for the western and eastern anomaliesshallower inclination of magnetisation but the difference of respectively. The differential RTP anomalies are shown inless than 5 degrees is not clearly significant and location of the Figure6 with peaks 6300 and 8200 metres south of the TMImagnetisation does not shift substantially. Figure 7 also shows peaks. A simultaneous inversion of the two TMI anomaliesthe very close match between the differential RTP contours and assuming induced magnetisation and using horizontal-topcontours of a local standard RTP. Figure 8 shows the anomalies elliptic-section bodies gives depths and apparent magnetic Figure 8.Anomaly sources in the Australian Remanent Anomalies Database Figure 6.Outline of inversion source bodies on the differential RTP. presented in the AUSGIN portal.57 PREVIEW DECEMBER 2021'