Effects of Magnetic Anisotropy on Total Magnetic Field Anomalies

Abstract

Modeling of magnetic anomaly data is a powerful technique to gain information on the shape of subsurface rock bodies. Most models are based on the assumption that the magnetization in the source body is parallel to the direction of the Earth's magnetic field. It has long been recognized that remanent magnetization affects the magnetization direction, intensity and shape of the anomaly, and therefore the interpreted structure. The effects of anisotropy, however, have only received little attention so far. This study uses synthetic models and a case study to investigate how anisotropy affects magnetization and anomalies over a thick dipping sheet and determines expected errors in interpreted magnetic properties and geometry of the source body for various anisotropy degrees and field inclinations. Anisotropy affects both the shape and amplitude of anomalies. For an oblate uniaxial fabric with the minimum susceptibility normal to the sheet and P = 1.5, errors in interpreted dip are up to 12°, depending on the field inclination, dip, and profile orientation, and errors in estimated mean susceptibility are up to −30%/+20%, if anisotropy is not taken into account during modeling. These effects are larger for higher degrees of anisotropy. A case study over the megacyclic unit IVe′ layer in the Bjerkreim Sokndal layered intrusion, Norway, investigates the contributions of (an)isotropic induced and remanent magnetizations to the total field anomalies. There, the influence of anisotropy is mainly related to remanence deflection. The results shown here will help to further improve interpretation of magnetic potential field data.