Implementation Of A Method For Determination Of Magnetization Direction

Abstract

The knowledge of the total magnetization direction has always been a drawback in among geological and geophysical exploration when using magnetic data. Remanent magnetization, present in almost all magnetic bodies, can significantly alter this direction. Usually the reduced to pole transform is applied to magnetic data but, without the knowledge of the total magnetization direction the data will not be reduced to pole correctly. Consequently, the interpretation will be erroneous. The need of a more precise and accurate interpretation always leads us to new methods and techniques. In this paper a method to determine the magnetization direction was tested and analyzed. The methodology is based on the cross-correlation of the vertical gradient and the total gradient of the reduced to pole anomaly. The method is only applicable to isolated anomalies and showed consistence and efficiency when applied to synthetic and real data, resulting in a more symmetrical and centered anomaly. Introduction A variety of interpretation techniques of magnetic data requires the knowledge of the total magnetization direction (TMD). The presence of remanent magnetization can influence this direction and consequently affect the interpretation and modeling of magnetic bodies. The total magnetization direction of a magnetic body is the vector sum of the induced magnetization and the remanent magnetization. The induced magnetization is aligned with the magnetic field of the earth, and in most cases it is the dominant magnetization and the only magnetization assumed when treating the magnetic anomaly. However, the remanent magnetization can be strong and alter significantly the direction of the total magnetization leading to erroneous interpretation. This paper is a reproduction and analyses of a technique used to determine the total magnetization direction of magnetic bodies. The proposed method, described by Dannemiller and Li, 2006, is based upon correlation between two quantities used in magnetic data interpretation: the vertical gradient and the total gradient of the reduced-to-pole (RTP) field. The method was tested on synthetic data and on real magnetic anomalies. Method The process of reducing to the pole transforms a total field anomaly, which is asymmetrical out of the pole, into a symmetrical anomaly generated as it would occur in the pole. The reduction to pole can only be correctly applied if the total magnetization is known. Numerically, this transformation (RTP) can be done using an estimate of the magnetization direction, but the result will not be an anomaly reduced to pole. The anomaly will still be asymmetrical when compared to that reduced to pole. So, the problem consists in finding the magnetization direction that, when applied to the reduction to pole, will generate the most symmetrical anomaly possible for our magnetic causative body. The methodology is based on the cross-correlation of two well known quantities used in interpretation of magnetic data: the vertical gradient and the total gradient of the reduced to pole anomaly. Once calculated the vertical and total gradient of all possible direction of magnetization, defined by the pair magnetic inclination and declination (IM, DM), the correct reduced to pole transformation will be the one that reaches the highest correlation between the two quantities. The cross-correlation can be computed using the following formula: