Geomagnetic Field Observations at Sakurajima Volcano
Masashi OGISO, Yoshitomo IKOMA, Tomoaki TAKIZAWA, Tadayuki UESUGI, Daisuke NOZAKA, Shuichi TOYODOME, Takeshi OWADA and Ikuko FUJI
Abstract
This report describes geomagnetic field observations at Sakurajima conducted to investigate the possibility of monitoring volcanicactivity at Sakurajima by observing the volcano's geomagnetic field.
Kanoya Magnetic Observatory halted geomagnetic observations at Sakurajima in 1999, since the effect of magnetized volcanic ash and the movement of the volcano's soil made geomagnetic variations arising from volcanic activity difficult to detect in the observed data. However, a new observation tunnel, the Arimura tunnel, was built on Sakurajima Island in 2006, and was available for geomagnetic field observations during the summer of that year. In August and September 2006, we conducted geomagnetic total force observations at the Arimura tunnel to test its usability and geomagneticfield variation observations at three points including the Arimura tunnel to explore the possibility of a new monitoring method.
We used two proton precession magnetometers and one Overhauser magnetometer for the geomagnetic total force observation. The observed values were small compared with those of Kanoya. This suggests that total force magnetometers are affected by magnetic bodies such as the iron frame used for the tunnel. In addition, the signal intensities of all the magnetometers were so small that it was difficult to measure the total force precisely. We conclude that it is not possible to detect geomagnetic variations in the geomagnetic total force data from the Arimura tunnel due to volcanic activity.
In the geomagnetic field variation observation, we managed to obtain good data at night while it was quiet, but during the day there was excessive man-made noise at all of the observation points. We calculated geomagnetic transfer functions at periods of 30 to 5120 seconds using a six-hour segment of the geomagnetic field variations obtained from each point at night. The transfer function shows the same characteristics for all three points, indicating the existence of a large-scale conductivity anomaly, not a small-scale anomaly caused by a volcanic dike. Standard errors for the transfer functions are at a maximum of 0.03, which is higher than the estimated value of 0.01 for monitoring volcanic activity at Sakurajima. At the Arimura tunnel, standard errors are lower than 0.01 at periods of 200 to 512 seconds. This result indicates that the monitoring of volcanic activity at Sakurajima with geomagnetic field variation observation is a possibility.
(Received 29 January 2008; received in revised form 6 March 2008; accepted 7 March2008)
