In this study, we construct a 3-D density structure model with high-accuracy based on gravity data in northern Chugoku Region, Japan. It is necessary to set constraint conditions appropriately in order to perform 3-D model analysis. We applied the improved Moving-Window Poisson (MWP) method using gravity and magnetic data in the target area to detect density structure boundaries in horizontal direction. From the analysis, horizontal density structure boundaries were detected. We applied the result for structure analysis. Finally, 3-D density structure models of wide area with higher accuracy than a homogeneous two layers structure model. We expect that the result is used as worthful data for numerical modeling of subsurface structure in earthquake disaster prevention.

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  This report describes the results of accuracy verification test of attitude measurement system using L1 GPS (Global Positioning System) and MEMS (Micro Electronic Mechanical Systems) IMU (Inertial Measurement Unit). Airborne gravity survey is conducted for estimating spatial fluctuation of gravity over a wide area. In an airborne gravity survey, a gravimeter is generally installed into a carrier like an airplane. If the carrier is able to fly with a lower altitude and a slower speed, it might be possible to estimate the spatial change of gravity with higher spatial resolution and accuracy. Therefore, the research using a RC (Remote Controlled) helicopter as the carrier is investigated by some researchers. In airborne gravity survey, it is very important to keep the attitude of gravimeter stable. Therefore, a gravimeter is generally supported by a gimbal system. However, the gimbal system is too heavy to hang from a RC helicopter due to the limitation of its payload. So, a gravimeter with wide dynamic range and a small and light attitude measurement system are required. In this research, we develop a prototype of an attitude measurement system using MEMS IMU and L1 GPS receiver. This system measures raw GPS data, acceleration and angular velocity, and saves them on its flash memory. The measured data is analyzed in a post-processing manner. Attitude estimation program is also developed based on the loose coupling algorithm which is one of the INS/GPS integration algorithms. Field experiments are conducted using the developed system to check whether or not the system can be attached on a RC helicopter and analyzable data can be measured in the field flight test. The results of analyzing the raw data by means of the developed attitude estimation program show that the position and attitude are successfully determined. However, the accuracy of attitude cannot be evaluated because the true angle is never known in such a field experiment. Therefore, we conduct an experiment under perfectly known condition and verify the accuracy of estimated attitude by comparing the estimated result with the known tilt angle. As a result, it is found that the attitude measurement system can estimate the tilt angle with accuracy of 0.1 degree under the experimental condition.

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  The Nankai Trough is a convergent plate boundary which lies under the Pacific Ocean, off southwest Japan. Borehole drilling surveys by IODP (Integrated Ocean Drilling Program) and a 3-D multi-channel seismic reflection survey by JAMSTEC (Japan Agency for Marine-Earth Science and Technology) were examined around Kashinosaki Knoll which located on the Nankai Trough. We implement seismic inversion analysis for the seabed sediment layers with these data: computing acoustic impedance depth profiles for the drilling sites from P-wave velocity and density data, constructing seismic convolution models with the acoustic impedance profiles and a source wavelet which is extracted from the seismic data, and adjusting the seismic models to observed seismic traces with inversion method. A 3-D acoustic impedance data is obtained from the 3-D seismic data with seismic inversion analysis, and the data is also converted to a 3-D porosity data. This porosity data shows that porosity values are generally decreasing together with TWT in the 3-D porosity data, though in some areas porosity does not subject this trend; we focus such porosity anomaly areas and describe them in relation with the horizons. We confirm such layers as the porosity anomaly area: having high and low porosity alterations, or continuous and clear low porosity stratum. All these porosity anomaly areas become obvious toward the landward part of the region. We conclude that the continuous low porosity layer represents ancient channel sediments, and show the prospects of planar and confined distribution of channel sediments around Kashinosaki Knoll with comparison on the rock stratigraphy between preceding boreholes.

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  Electromagnetic (EM) methods have been often used for geothermal exploration because resistivity is a physical property sensitive to temperature and the state of fluids, and because EM methods such as MT, CSAMT, and TEM are applicable to geothermal investigations over depths of several kilometers. In geothermal exploration, it is required to estimate the regional structure of a geothermal system including the geothermal reservoirs, heat sources, and geothermal fluids. The resistivity structure calculated from EM data shows only the distribution of resistivity, and therefore, it is necessary to interpret this as a geological structure. This paper describes a method for the estimation of a regional geothermal system based on EM exploration. It focuses on how the regional resistivity structure of a geothermal system can be interpreted and introduces two case studies of EM surveys in the Ogiri geothermal area of Kagoshima and the Toyoha geothermal area of Hokkaido in Japan.

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[in Japanese]

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