HR: 0800h
AN: GP31A-0076    [Abstracts]
TI: A numerical approach to volcanomagnetic effects associated with hydrothermal activity.
AU: * Okubo, A
EM: ayako@svo.dpri.kyoto-u.ac.jp
AF: Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University., Sakurajima-Yokoyama, Kagoshima, 891-1419 Japan
AB: We have developed a postprocessor to calculate the geomagnetic field changes due to the thermomagnetic and piezomagnetic effects caused by hydrothermal activities in the volcanic area. The main geophysical mechanisms of magnetic variations observed in hydrothermally active areas are thermomagnetic, piezomagnetic, and electrokinetic effects. In a previous work, Ishido and Pritchett (2001) numerically evaluated magnetic field changes caused by fluid production and reinjection based on a two-dimensional geothermal reservoir model. However, the piezomagnetic effect was not considered. We focus on modeling for "piezomagnetic effect" accompanied with hydrothermal activity in addition to "thermomagnetic effect" that may be the most predominant in the considered problem. The model includes the topography, which is hard to evaluate in piezomagnetism. The aim of this study is to evaluate numerically (1) how the volcanomagnetic changes are observed in the presence of hydrothermal activity and (2) how the topography affects them.
Firstly, we calculated temperature variations and hydrothermal pressurization by using a simulation code, HYDROTHERM (Ingebristen and Hayba, 1994) with various assumptions. Secondly, we evaluated geomagnetic field changes at the ground surface caused by the thermomagnetic effect of temperature changes on rock magnetization, and by piezomagnetic effect due to hydrothermal pressurization, respectively. Finally, we carried out some case studies in order to evaluate thermomagnetic and piezomagnetic effect under topographic effect. In this calculation, we expressed the topography (volcanic edifice) as a smooth spline function by using the technique of Sasai (1993) based on the circular cone model of Rikitake (1951), because the edge effect cannot be avoided when expressing the volcanic terrain with an assembly of prismatic bodies (e.g. Bhattacharyya, 1964).
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1517 Magnetic anomalies: modeling and interpretation
DE: 1545 Spatial variations: all harmonics and anomalies
DE: 1560 Time variations: secular and longer
DE: 1599 General or miscellaneous
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005