9:15 AM - 9:30 AM
[STT34-02] The topographic effect in magnetic structure analysis
Keywords:magnetic inversion analysis, topographic effect
Magnetic anomalies observed above ground include magnetic anomalies due to the contrast between atmospheric and surface magnetization, i.e. the topographic effect. Therefore, it is very important to separate magnetic anomalies caused by subsurface structures from the topographic effect when carrying out magnetic structure analysis, and many researchers have attempted to separate the topographic effect from observed magnetic anomaly data by using certain filtering and modelling schemes (Grauch, 1987). The most commonly used of these is the idea that the magnetic field produced by a uniformly magnetized model domain is equivalent to the topographic effect (Grauch, 1987; Makino and Kaneko, 1988). In this case, the uniform magnetization within the model domain that produces the topographic effect can be regarded as the average magnetization of the rock in the area of interest, referred to as the average magnetization in this study.
In previous studies, the estimation of the average magnetization producing the topographic effect and the estimation of the magnetic structure model have been carried out individually and in two steps (e.g. Grauch, 1987; Tada et al., 2021). However, it has been found that when this two-step estimation is performed, biases may occur in the estimated model. To overcome this problem, in this study the average magnetization and the magnetization structure are modelled. The scheme was developed to simultaneously optimize the average magnetization, which creates the topographic effect, and the magnetic structure model. As a result, it was found that the average magnetization and the magnetic structure model can be accurately estimated even for problems with poor conditions where conventional two-step estimation does not work well. In addition to the results of synthetic tests demonstrating the effectiveness of the scheme in this study, we will present the results of its application to magnetic anomaly data acquired over the Himekami pluton in Iwate Prefecture. The data used in the analysis of Himekami pluton are from the Aeromagnetic Database of Japan (Nakatsuka and Okuma, 2005).
In previous studies, the estimation of the average magnetization producing the topographic effect and the estimation of the magnetic structure model have been carried out individually and in two steps (e.g. Grauch, 1987; Tada et al., 2021). However, it has been found that when this two-step estimation is performed, biases may occur in the estimated model. To overcome this problem, in this study the average magnetization and the magnetization structure are modelled. The scheme was developed to simultaneously optimize the average magnetization, which creates the topographic effect, and the magnetic structure model. As a result, it was found that the average magnetization and the magnetic structure model can be accurately estimated even for problems with poor conditions where conventional two-step estimation does not work well. In addition to the results of synthetic tests demonstrating the effectiveness of the scheme in this study, we will present the results of its application to magnetic anomaly data acquired over the Himekami pluton in Iwate Prefecture. The data used in the analysis of Himekami pluton are from the Aeromagnetic Database of Japan (Nakatsuka and Okuma, 2005).