An inverse method proposed by Thomas Moriarty (1988) to determine the direction of remanent magnetisation associated with magnetic anomalies is evaluated. Moriarty's method utilises the properties of the phase spectrum of the observed magnetic field, and its relationship to the Green's function for magnetic sources. His technique was developed assuming any observed magnetic anomaly was purely due to remnant magnetism. Moriarty claimed that this assumption was appropriate for the seamounts he was studying. However, in general the magnetisation vector of an anomalous region has a significant component due to induced magnetisation.
Fortran code is developed and used together with PEST software to trial Moriarty's inversion technique for the more general case where the magnetisation vector of the source has a significant component due to the induced field. Two essential assumptions must be met for the inversion process to work. Firstly, the centre of symmetry of the body must be at the origin of the coordinate system. Secondly, the anomaly must be unaffected by surrounding sources.
The technique is accurate for synthetic data when the interpreted centre of symmetry is close to the true centre of symmetry. For real data the technique is less robust. Moriarty's technique requires that the anomaly be the result of one body with uniform susceptibility and remanence, and that the anomaly be unaffected by surrounding sources. Geologically this is highly unlikely, making Moriarty's technique impractical for exploration work.
Moriarty, T.D., 1988, Estimation of the Direction of Remanent Magnetisation: An Inverse Method Using the Phase Spectrum of a Magnetic Anomaly: Master's Thesis, Texas A&M University.