HR: 0800h
AN: GP41B-0878    [Abstracts]
TI: Development of Grounded Electrical Source Airborne Transient EM (GREATEM) Survey System and Its Application to Investigating a Volcano Structure
AU: * Mogi, T
EM: tmogi@eos.hokudai.ac.jp
AF: Institute of Seismology and Volcanology, Hokkaido University, N10W8, kita-ku, Sapporo, 060-0810 Japan
AU: Jomori, A
EM: neo-akira@rinku.zaq.ne.jp
AF: Neo-science Co., Hanan, Osaka, 599-0223 Japan
AU: Jomori, N
EM: neo-nob@rinku.zaq.ne.jp
AF: Neo-science Co., Hanan, Osaka, 599-0223 Japan
AU: Azuma, Y
EM: azuma@chiba.email.ne.jp
AF: Chiba Electronics Laboratory Co., Sakura, Chiba, 285-0005 Japan
AU: Fomenko, E Y
EM: fomenko@cs.msu.su
AF: Lomonosov Moscow State University, Leninsike Gor, Moscow, 142092 Russian Federation
AB: We have developed an airborne EM system that uses a grounded electrical source and an airborne magnetic field receiver to increase the depth of investigation. Using a grounded source, we can apply a large source moment with a long transmitter-receiver distance, thus we expect to obtain a greater depth of investigation. However, the area of survey is limited with this type of method, but it has some advantages such as little effect of flight level etc. We believe that a repeated survey in active area such as volcano or fault area is effective for monitoring activity in the whole area. By using the airborne survey, we will be able to make repeated survey rapidly in volcano area where it is difficult to enter by a ground survey. The initial stage of the development was reported at Mogi et al. (1998) and the survey system was named as GREATEM (Grounded electrical source airborne transient EM). Time domain data acquisition has an advantage to the deep exploration because it avoids near source field effects that occur in frequency domain measurements. To realize the method, we should overcome mainly two problems: monitoring and filtering the motion noise of the receiver and canceling the natural magnetic field variation and cultural noise without stacking in the time domain data. We prepared high accuracy fiber optic gyros to monitor the pitch and roll of the magnetic sensor in the bird to monitor the motion and also manufactured an electromagnetic receiver that can detect three components of magnetic field and a tilt meter with MI sensor which can detect up to_ 100,000 nT with a sensitivity of 0.04 mV/nT. These receiver and tilt meter are installed on a gimbal in a bird. Data acquisition system is installed in the helicopter cabin with high precision GPS synchronized with the same type GPS set in the transmitter. In this system, we designed to analyze full wave data in time-domain for investigating deep to shallow underground resistivity structure. The measured data of three-component magnetic filed, tilting, time and positioning are digitized at 25 kHz for 10 channels. We have also developed a 3D modeling scheme to investigate magnetic field responses in the air for complicated subsurface structure and topography. We carried out GREATEM surveys at Aso Volcano, southwest Japan to test the survey system and clarify the subsurface structure of the volcano. We computed the transient response of the magnetic field in the air by removing motion noise and other EM noise using special digital filters. Finally we obtained resistivity structure, assuming horizontal layer structure. The depth of investigation of present survey was estimated about 800m deep.
DE: 0925 Magnetic and electrical methods (5109)
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005