Feasibility study on monitoring the volcanic activity at Sakurajima by geomagnetic field observation
Ikuko Fujii
Abstract
We report on simulated results of volcanomagnetic effects at the Sakurajima volcano, Japan.
We modeled the demagnetization orinduction effecton the geomagnetic field in three cases here; (1) a recent long-term magma supply, (2) an eruption at the Syowa crater in June 2006, and (3) an intrusion of a small scale dike.
The demagnetization effects in the cases (1) and (2) at areas more than 2 km away from the Minamidake crater are estimated to be on the order of 0.01 nT, which is the lower limit of observation by a proton-precession magnetometer. If this level of amplitude is required, an underground observation should be considered to avoid the influence of volcanicash, and this would result in a very limited number of observation sites.
On the other hand,avariationofthegeomagnetictransferfunctiongeneratedinthecase (3) could be on the order of 0.01 at periods from 0.1 to 10 seconds, which is an observable level by a fluxgate magnetometer. Disturbances due to volcanic ash would be negligible for the case of the fluxgate magnetometer because the sensor is usually buried.
Noises of DC trainsare expected to be smalleratthe requested period band than at periods from 10 to 1000 seconds.
It is indicated that a geomagnetic observation by a proton-precession magnetometer could detect only an abruptchange generated by a huge eruption, while thatby a fluxgate magnetometer could monitor an apparent resistivity change in the shallow area as well as an abrupt change.
Received 1 March 2007; Accepted 5 March 2007
