2.5D magnetization vector inversion of vector magnetic data

Abstract

The remanent magnetization vector records the earth’s magnetic field at the time of the formation process of magnetized geologic units. Recovering the magnetization vector from magnetic data can provide extra information regarding the source properties to differentiate geologic units and reveal thermal evolution or tectonic history. To provide this information, the magnetization vector inversion (MVI) method has been used to invert for the magnetization vector. The MVI usually works with the total-field magnetic anomaly magnitude ([Formula: see text]). The magnitude is an approximation of the projection of the magnetic anomaly vector onto the normal geomagnetic field. For highly magnetic sources, the approximation error of [Formula: see text] cannot be ignored. However, this approximation error can be avoided by using measured vector magnetic data. To reduce the severe ambiguity of 3D MVI, specific constraints based on extra prior information are usually used. For 2D or 2.5D cases, MVI only inverts for magnitude and inclination of the magnetization, which encounters less ambiguity than that of the 3D case. We have developed a 2.5D MVI approach using vector magnetic data. To reduce the divergent and smooth trends in magnetization vectors recovered by MVI in the Cartesian framework, we make use of a focusing constraint method to improve the imaging. The inversion results of synthetic data indicate that the method is able to recover magnetization magnitude and inclinations close to the true values and is fairly robust to inappropriate choice of the profile location and heading. Finally, the method is applied to the measured airborne vector magnetic data from the Qixin area of the East Tianshan Mountains in China. The distributions and directions of magnetization of the mafic-ultramafic rocks and linear structures are revealed, which provides the information for distinguishing geologic units and geologic differentiation.