Magnetization structure of Aogashima Island using vector magnetic anomalies obtained by a helicopter-borne magnetometer
Nobuhiro Isezaski 1 2 Jun Matsuo 1 21 Graduate School of Science and Technology, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan.
2 Corresponding authors. Email: nisezaki@earth.s.chiba-u.ac.jp, mastuo@oyointer.com
Exploration Geophysics 40(1) 17-26 https://doi.org/10.1071/EG08112
Submitted: 6 September 2008 Published: 27 February 2009
Abstract
On Aogashima Island, a volcanic island located in the southernmost part of the Izu Seven Islands Chain, vector magnetic anomalies were obtained in a helicopter-borne magnetic survey. The purpose of this study was to understand the volcanic structure of Aogashima Island in order to mitigate future disasters.
Commonly, to obtain the magnetic structure of a volcanic island, total intensity anomalies (TIA) have been used, even though they have intrinsic errors that have not been evaluated correctly.
Because the total intensity magnetic anomaly (TIA) is not a physical value, it does not satisfy Maxwell’s Equations, Laplace’s Equation, etc., and so TIA is not suitable for any physical analyses. In addition, it has been conventionally assumed that TIA is the same as the projected total intensity anomaly vector (PTA) for analyses of TIA. However, the effect of the intrinsic error (ॉT = TIA–PTA) on the analysis results has not been taken into account. To avoid such an effect, vector magnetic anomalies were measured so that a reliable analysis of Aogashima Island magnetization could be carried out.
In this study, we evaluated the error in TIA and used vector anomalies to avoid this erroneous effect, in the process obtaining reliable analysis results for 3D, vector magnetization distributions. An area of less than 1 A/m magnetization was found in the south-west part of Aogashima Island at the depth of 1–2 km. Taking the location of fumarolic activity into consideration, the lower-magnetization area was expected to be the source of that fumarolic activity of Aogashima Island.
Key words: Aogashima Island, helicopter-borne vector magnetometer, vector magnetic anomalies, vector magnetization.
Acknowledgments
Mr Kikuchi, chief of the Aogashima village office, and Mr Chiba and Ms Okuyama, who belong to the Disaster-Prevention Department of Aogashima village office, have given us much support for our study. Dr Harada and Dr Kato, formerly of Chiba University, helped us set up the measuring system for the vector geomagnetic survey. Dr Tsukui gave us information about the volcanic and geological structure of Aogashima Island.
Baranov, V., 1957, A New Method for interpretation of aeromagnetic maps: PSEUDO-GRAVIMENTRIC Anomalies: Geophysics 22, 359–383.
| Crossref | GoogleScholarGoogle Scholar |
Hughes, D. S., and Pondrom, W. L., 1947, Computation of vertical magnetic anomalies from total magnetic field measurements: Transactions – American Geophysical Union 28, 193–197.
Yamamoto, M., Seama, N., and Isezaki, N., 2005, Geomagnetic paleointensity over 1.2 Ma from deep-tow vector magnetic data across the East Pacific Rise: Earth, Planets, and Space 57, 465–470.
