New approaches to dealing with remanence: magnetic moment analysis using tensor invariants and remote determination of in situ magnetisation using a static tensor gradiometer

Abstract

Assuming without evidence that magnetic sources are magnetised parallel to the geomagnetic field can seriously mislead interpretation and can result in drill holes missing their targets. I present two new methods for providing information about magnetisation of anomaly sources, independent of the geometry of the causative bodies. The first method is based on analysis of magnetic gradient tensor data. Integral moments of tensor invariants locate the horizontal and vertical centres of magnetisation and estimate the magnetisation direction. The depth estimate allows correction of the integral moments for the finite range of integration, which can accordingly be restricted to the main part of the anomaly. This reduces interference from neighbouring sources. This method provides information on location, total magnetic moment (magnetisation ï?´ volume), and magnetisation direction of a compact source, without making any assumptions about its shape. The second method employs a single combined gradiometer/magnetometer, operating in base station mode within a magnetic anomaly of interest. The response to geomagnetic time variations allows the contributions of induced magnetisation and remanence to the anomaly to be separated. This method allows remote estimation, prior to drilling of (i) the total magnetisation direction of the source, which is a key to accurate modelling (ii) the remanence direction, which can provide geological information such as age of intrusion or alteration, (iii) the Koenigsberger ratio Q, which is indicative of the magnetic mineralogy of the source. If the source is compact, the method also provides a direct indication of the direction to its centroid.