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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS007 Oral Presentation 1955
High Density Helicopter-Borne Aeromagnetic Survey in Aso Volcano
Dr. Mitsuru Utsugi
Aso Volcanology Laboratory Kyoto Univ. IAGA
Yoshikazu Tanaka
Recently, geomagnetic field observation is successfully applied to many active volcanos to detect the
volcano-magnetic changes, i.e., geomagnetic field changes associated with the volcanic activities. These
observations are usually based on the continuous or repeated observation stations setting on the
ground near the active area. From these observations, we can obtain high accurate information about
the temporal geomagnetic field changes. But we can obtain only limited information about the special
distribution of geomagnetic changes because of it is quite difficult to maintain many observation
stations under the extremely bad environment around the active area of volcano. To interpret the
geomagnetic field changes to underground heat transfer, we have to know the special distribution of
the geomagnetic changes. To obtain the detailed information about the spatial distribution, we tried to
use the aeromagnetic survey. Based on the aeromagnetic survey, we can get many data of spatial
distribution of the field easily. Our goal is to detect the volcano-magnetic changes from the repeated
aeromagnetic survey. The main problem of aeromagnetic repeated observation is the difficulty of the
observation point control. In the two flights, it will be impossible to flight exactly same position. So that,
it is very difficult to separate observed field changes to temporal variation due to the volcanic activities
and the spatial variation due to the difference of the observation points. If the detailed distribution of
geomagnetic field is obtained on quiet period of the volcano, and the field intensity on the arbitrary
point around the active area is estimated interpolating the observed data, we can correct the spatial
variation of the repeated aeromagnetic survey data caused by the difference of flight position, and it
may be possible to detect the field changes associated with the volcanic activities. For this purpose, we
made very high density and low altitude helicopter-borne aeromagnetic survey on Aso Volcano, central
Kyushu Island of Japan, in July 2002. The survey area was NS1200 x EW1200 x 300m region above the
Nakadake crater which is the most active area on Aso volcano. In this survey, we used a high-resolution
portable cesium magnetometer. The sampling time of measurements was 0.1 second. The flight was
made in 8 heights (1370, 1400, 1450, 1500, 1540, 1570, 1600 and 1640 m from the sea level). The
total numbers of measurements were about 8200. Using these data, we tried to estimate the special
distributions of the geomagnetic field around the Nakadake crater. Observed data likely contain some
noise or miss-observed data. To extract these inadequate data, we applied the technique of equivalent
anomaly method. This method is usually used to calculate the upward continued aeromagnetic data.
Based on this method, inharmonic component of observed data is carried away and the harmonic field
distribution can be estimated in the upper region of the flight surface. In this study, we used this
method to extract harmonic component from the observed data. The accuracy of the obtained
distribution over the Nakadake crater is about 10nT, and if comparatively larger scale eruption is
occurred, we can detect the volcano-magnetic changes using our estimated distribution of the
geomagnetic field.
Keywords: aeromagneticsurvey, volcanomagneticeffect