BZ AND BZZ PEAK TO TROUGH RATIOS AND AZIMUTHS AS A RAPID MEANS TO MAP MAGNETIZATION DIRECTIONS – A METHOD BASED ON PIONEERING STUDIES BY ZIETZ AND ANDREASEN

Today we commonly investigate massive magnetic field datasets, including national and continental compilations, in which we would like to detect magnetizations rotated from the local geomagnetic field direction by remanent magnetization. The challenge of interpreting magnetic fields due to sources of unknown magnetization direction has long been recognised, and many methods, including magnetic moment analysis, magnetic field inversions, and automated search methods are now applied to selected samples of magnetic field data. Some of the methods most promising for application to large datasets require no explicit assumption about the spatial distribution of magnetization, which increases their versatility. However, we found that for discrete, compact anomalies, source magnetization direction can be recovered with greater discrimination using a dipole model assumption. We firstly select suitable anomalies mapped by the total gradient or scaled moment, taking corresponding samples from the vertical component and its vertical derivative. We then follow the observation by Zietz and Andreasen ( Remanent Magnetization and Aeromagnetic Interpretation, 1967, Mining Geophysics, vol. 2, pp. 569-590) that the peak to trough ratio is a function of inclination of magnetization, and that trough to peak orientation indicates declination of magnetization. Zietz and Andreasen applied this analysis to total field data, with the proviso that it was only valid at high geomagnetic inclinations. By using vertical component data we have extended the analysis to much lower geomagnetic inclinations. We have also derived approximate correction factors for moderately elongate anomalies. Zietz and Andreasen mostly worked with contour maps which express such relationships clearly, but in the histogram-equalised, shaded relief magnetic field imagery used now, such quantitative relationships are less obvious, and these original interpretation methods have mostly been forgotten.