高度変化に富む空中磁気探査データの"交点"コントロール処理とその磁気異常変化抽出への応用

We have been developing a software system for helicopter-borne magnetic survey using an equivalent source technique. As one of the functions of this system, we are investigating the procedure of generalized mis-tie control. In a helicopter survey of mountainous area, even at the intersection of horizontal trajectories of traverse and tie lines, the altitudes of both lines differ considerably in practice. This means that the conventional crossover analysis cannot be applied. As the position fix is accurate by the use of GPS, only magnetic level shifts between lines are considered. An algorithm of estimating the magnetic level shifts was developed as an extension of the data reduction procedure (underdetermined inverse problem) of using an equivalent source analysis of survey line data. In solving for unknown parameters of different kinds of quantity, adequate scaling for parameters is essential for the underdetermined problem. In the case of allowing variable level shifts along survey lines, improper scaling may remove some components of actual magnetic anomalies. The method with parameter scaling was applied to actual survey data, which revealed a capability of removing the erroneous magnetic level shifts. There is another application of this technique in detecting the magnetic anomaly change by repeated surveys. Repeated aeromagnetic surveys over an active volcano are one of the promising ways of examining volcanic activity. Since the track lines of different surveys cannot be the same, the inspection for the repeatability of a magnetic anomaly pattern is quite important to acquire genuine information of the activity. It was proven that the use of our method of generalized mis-tie control could eliminate the spatial alias effect, even in cases of insufficient survey line spacing.