Recovery of resultant magnetisation vectors from magnetic anomalies

Abstract

A magnetic body has a resultant magnetisation that is the vector sum of its induced and remanent components. Ignoring the role of remanence may lead to erroneous resultant magnetisation direction and hence inaccurate geophysical models. Determination of this resultant magnetisation direction may be obtained from total magnetic intensity data for a well separated magnetic anomaly. We present a new method to recover the resultant magnetisation direction that proceeds by iteratively calculating an approximate source layer, and cross correlating trial magnetisation directions. This method is tested with a number of remanent component directions for compact and elongate sources. For a compact source, the accuracy to which the correct resultant magnetisation direction can be recovered is generally found to be less than 5°. The method retains accuracy for low inclination resultant magnetisation directions, however, begins to lose sensitivity as the direction approaches the pole. Application of the method to the case study of the Black Hill Norite recovers resultant magnetisation directions in agreement with paleomagnetic results. Here the resultant directions from the two methods are found to be consistent, with minor differences possibly due to limited paleomagnetic sampling of magnetisation directions that changed as the intrusion cooled. A higher bulk Q value, than found from the limited sampling, is suggested for one of the three anomalies studied, as supported by the results of other authors.