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The Transfer Function Method to Reduce the Noise of the Geomagnetic Observation at Sakurajima Volcano and Aso Volcano
In order to make the fundamental research for prediction of volcanic explosions，geomagnetic observation start at Sakurajima volcano in 1978, at Aso volcano in 1979. Sakurajima volcano is apart about 50 km from Kanoya geomagnetic observatory, reference station for the geomagnetic observation at Sakurajima volcano and Aso volcano, Aso volcano about 350km. The geomagnetic total field is continuously measered at two stations. At sakurajima volcano, one of these stations, SHI (Shirahama) is situated at a distance of about 4 km at the north of Minamidake crater, the other, MCH (Mochiki) at a distance of about 5 km to the south-south-by-west of the crater. At Aso volcano, one of continuous observation stations, AHK( Aso Sanroku) is situated at a distance of about 5 km to the north of Nakadake crater and the other, ASJ (Aso Sanjijo) at a distance of about 0.8km to the north-west-by-west of the crater. The simple difference method in which data at a remote reference station are simply subtracted from those at observation site are appear to include the noise due to the local difference of time-varying field of external origin. For the purpose of reducing the noise the correction using transfer function method was made. As the results, the noise due to the external field was reduced to about one-third of the value before the correction applied.
The slight anomaly variations (about 0.5nT) in mutual difference between ASJ and AHK was observed before the explosion occurred Sep. 6 1979 at Aso volcano. For the purpose to detect such a slight volcano-magnetic effect in the mutual difference between Sakurajima, Aso and Kanoya this method is not sufficient, but the capability to detect volcano-magnetic effect is improved by this method.
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