Discussion: On 'Magnetization structure of Aogashima Island using vector magnetic anomalies obtained by a helicopter-borne magnetometer' (Isezaki, N., and J. Matsuo, 2009, Exploration Geophysics, 40,17-26; Butsuri-Tansa, 62,17-26; Mulli-Tamsa, 12, 17-26).

摘要

In 'Magnetization structure of Aogashima Island using vector magnetic anomalies obtained by a helicopter-borne magnetometer', Isezaki and Matsuo discussed a source of inaccuracy in the inversion of total intensity magnetic anomalies, and presented an example of a helicopter three-component magnetic survey and its inversion. In their paper, I consider there to be some problems and points to be discussed. Here I refer to the figures in their paper by the original figure numbers, and use the label D-l for the new figure in this discussion. Isezaki and Matsuo described the algebra of the total intensity anomaly (TIA), and discussed the error introduced by the assumption that TIA is approximated by the projected total intensity anomaly (PTA). Their figures 3 and 4 are correct although the description of the horizontal axis of figure 4 could be clearer. It is obvious that the relative error ε_T/TIA ( = (TIA- PTA)/ TIA) becomes infinite where 7X4 = 0. However, the situation is not as severe as their assertion implies, because such a large error occurs only in a quite limited region near PTA = 0, but the result of analyses of magnetic anomalies is commonly more dependent on the locations and values of the high and low anomaly peaks, where the approximation above is fully valid. To illustrate this, I show a simple synthetic model.