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Summary An integrated airborne survey was conducted around Aso volcano in Kyushu, Japan. In this survey, four geophysically different methods; the electromagnetics, the magnetics, the infrared images and the gamma-ray spectrometry, were applied in almost same survey lines by using a same helicopter. Electric resistivity structure down to 800m, vertical magnetic gradient map, temperature distribution on the ground surface and total gamma-ray intensity map were estimated and produced by the survey. These results were confirmed by comparing with the geology and the ground survey results in the survey area.
Introduction In Japan, we have experienced many natural disasters for example, large earthquakes, large landslides and volcanic eruptions. Geophysical explorations are thought to be able to contribute for predicting and monitoring the underground structure. But it is hard to investigate by the ground survey in such disaster areas, because of topography, access and cost. Airborne, particularly helicopter survey is useful to collect two-dimensional data in such dangerous areas as volcanoes and rugged terrains in a short time. We have been developing an integrated airborne survey as a three year project since 2003 (kaieda et. al., 2004). In this survey, four geophysically different measurements are applied along same survey lines at almost same time with a helicopter. We think that this integrated measurement and analysis will be very effective for estimating the underground structure more precisely. As shown in Figure 1, we designed an integrated airborne survey system in which four geophysically different methods; the electromagnetics, the magnetics, the infrared images and the gamma-ray spectrometry are applied along same survey lines in the same time using a same helicopter. The electromagnetic measurement with a grounded electric source is used to investigate deep underground resistivity structure in three-dimension. This system is based on GREATEM (Mogi et al., 1998). The magnetic measurement is to investigate geology and thermal condition near the ground surface. The infrared image is to observe temperature distribution on the ground surface. The gamma-ray spectrometry is to investigate geological structures. Most equipments of this system have been designed and constructed by 2004 and some field measurements were conducted to confirm practical use of the developed system in 2004. In this paper, we describe the field measurement results obtained around Aso volcano in Kyushu, Japan October in 2004.
Measurement around Aso volcano Aso volcano is famous for an active volcano. Geology on the ground surface in this area is well studied. Some measurement lines covered around the volcano in an about 6 km * 6 km area to investigate the underground geological structure. The survey area is divided into three areas (A, B, C) as shown in Figure 2. There are some peaks and a crater around Aso volcano. CR, NK, TK, EB, KJ, and OJ in Figure 2 represent Naka-dake creator, Naka-dake, Takadake, Ebisu-dake, Kijima-dake and Ohjyo-dake, respectively.
Electromagnetic survey results The electromagnetic survey was conducted with an electric power line of about 3 km length at north edge of measurement area. A direct current of 20 A was charged to the line for 0.4 second and cut off.
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