Sweet-spots for estimation of source magnetization direction

Meaningful estimates of magnetization direction can only be recovered from analysis or inversion of magnetic field data over concentrations of magnetization that give rise to distinct total gradient (TG) or normalised source strength (NSS) peaks. We term these `sweet-spots for estimation of magnetization direction'. For well sampled fields the NSS peaks better locate the horizontal centre of magnetization than do the TG peaks, but for many measured fields both peaks are almost co-centred at the sharpest field gradients where individual flight-line segments are closest to magnetization. Magnetization direction cannot be reliably determined away from sweet-spots, and individual sweet-spots support estimation of only a single magnetization direction with little or no power to resolve any internal inhomogeneity. We investigate estimation of magnetization from inversion of an anomaly near Ebagoola in eastern Queensland. The NSS and TG peaks of measured TMI are similarly located, marking short-wavelength features on individual flightlines. For TMI computed from the same TMI inversion model across a constant elevation surface the NSS and TG peaks diverge, consistent with the pattern observed in synthetic modelling of a dipole source of the same magnetization direction. Regardless of the deviation between their peak locations, the NSS and TG anomalies are predominantly coincident and define a sweet-spot from which we recover consistent estimates of magnetization direction from inversion of the TMI data using a range of source model geometries.