STT52-P01

Room:Convention Hall

Time:May 27 18:15-19:30

Aeromagnetic constraints on the subsurface structure of the Ishikari Depression, Hokkaido
JapanOKUMA, Shigeo1∗ ; NAKATSUKA, Tadashi1
1Geological Survey of Japan, AIST

An aeromagnetic map of the Ishikari Depression and its surrounding area (total magnetic intensity, 1:200,000) (Okuma andNakatsuka, 2014) has been made as a compilation of the Digital Geoscience Map of the area (GSJ, 2014). The total magnetic
intensity anomalies within the area have been extracted from the aeromagnetic anomaly database (Nakatsuka and Okuma, 2009)
on a smoothed surface 1,500 m above terrain. The reduced to the pole anomalies have been also calculated from the total magnetic intensity anomalies on the surface and compared to the geology (Ozaki and Komatsubara, 2014) of the area.
An obvious magnetic high chain, called as the Kitakami-Ishikari magnetic belt is dominant in Ishikari Depression. By comparing with the lithology of core samples from oil exploration wells, granitic rocks as well as mafic igneous rocks can accountfor the magnetic belt but the details remain unknown.
3D imaging (Nakatsuka and Okuma, 2013) was applied to the magnetic anomalies to better understand the subsurface structureof the area. A chain of deep seated magnetic sources was imaged in the Ishikari Depression, corresponding to the KitakamiIshikari Magnetic Belt. Depths of the magnetic sources range from ˜5km (north) to ˜10km (south) below ground, whereas theirwidths range from 10km (north) to 25km (south). Their magnetization intensities (˜1 A/m) imply that they are composed mainlyof granitic rocks which were confirmed by drilling at a depth of 4,600m in the Tomakomai area.
A high-resolution aeromagnetic survey has been also conducted to better understand the subsurface structure of the Tomakomai area. Two regional magnetic highs occupy offshore Tomakomai and the eastern one is edged by a NNW-SSE trending marine
topographic step, suggesting that this step might have worked as a dam to trap sediments which contain magnetic minerals.
Keywords: aeromagnetic survey, magnetic anomaly, Kitakami-Ishikari Magnetic Belt, Ishikari Depression, Tomakomai, 3Dimaging1/1

STT52-P02

Room:Convention Hall

Time:May 27 18:15-19:30

Study of Extraction Method of the Slope tending to Cause Deep-seated landslide using
Airborne Electromagnetic method
TAKAHARA, Teruyoshi1∗ ; SETO, Shuji1 ; NOIKE, Kohei1 ; KINOSHITA, Atsuhiko1 ; SHIMIZU, Yoshikazu1 ;ISHIZUKA, Tadanori1 ; KAWATO, Katsushi2 ; OKUMURA, Minoru2 ; KAGEURA, Ryota21

Public Works Research Institute, 2 Nippon Engineering Consultants Co.,LTD.

Kind of landslide is mainly shallow landslide and deep-seated landslide, etc. Shallow landslide is a phenomenon that top
soil layer collapse. On the other hand, deep-seated landslide is the relatively large scale collapse phenomena among the slope
failures such as rock falls, slope failures, etc.; in which the slide plane arises in the deeper section than that in the case of thesurface collapse; then not only the top soil layer but also the soil at deep layer becomes the colluvial clods. Although deepseated landslide occurs less frequently than shallow landslide, sediment amount of deep-seated landslide are huge than shallow
landslide’s and it might occur serious sediment disaster. In September 2011, multiple of deep-seated landslides occurred byheavy rains associated with Typhoon Talas in Kii Penisula.
It is considered that occurrence of deep-seated landslide is linked to strength and groundwater movement of bedrock. Hence, inorder to estimate occurrence of deep-seated landslide, it is important to obtain information of bedrocks at wide area. Therefore, inthis study, we focused on airborne electromagnetic method. This method can get obtain information of geology and groundwaterat wide area. Then, we studied about extraction method of the slopes tending to cause deep-seated landslide.
Studies areas were Hayakawa River basin (Yamanashi Prefecture) and Byutanogawa River basin (Miyazaki Prefecture). Then,
we focused on slopes of the past of deep-seated landslide and the specific resistance patterns of vertical directions of bedrock
creeps in these basins.
In this study, we confirmed two-layer structure of specific resistance pattern that a shallow portion is high resistance and a
deep portion is low resistance. However, in the slope that contains a lot of muddy, we confirmed two-layer structure of specificresistance pattern that a shallow portion is low resistance and a deep portion is high resistance. Moreover, we confirmed threelayer structure of specific resistance pattern. These are future tasks.
Keywords: Airborne Electromagnetic Method, Deep-seated landslide, Specific Electrical Resistance

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STT52-P03

Room:Convention HallTime:May 27 18:15-19:30

Semi-automatic interpretation methods using gravity gradient tensor data obtained by airborne gravity gradient survey
KUSUMOTO, Shigekazu1∗
1

Graduate School of Science and Engineering for Research (Science), University of Toyama

The importance and usefulness of gravity gradient tensor for the estimation of subsurface structures, especially to find structural boundaries, have been pointed out since the latter half of the 1930’s. Presently, not only the classical methods, such as
horizontal first derivative, vertical first derivative and vertical second derivative, are being used, but more advanced and complex
techniques designed by combining the classical methods are being discussed and analyzed. These techniques reveal structural
boundaries through a differential in the space domain of gravity anomalies caused by variations in subsurface structures; they area kind of high-pass filters that emphasize the short wave length signals of gravity anomalies.
In general, the techniques that estimate subsurface structures from gravity anomaly without geological and geophysical constraint conditions are called semi-automatic interpretation methods. These methods not only employ high-pass filtering but also
estimate causative sources by using eigenvalues and/or eigenvectors of gravity gradient tensor.
In this presentation, we will first review the filtering and semi-automatic interpretation methods that use gravity gradient tensor.
Then, we will report the results obtained by applying these methods to the gravity gradient data acquired for the Kuju geothermal area of Kyushu district through FALCON (R) AGG. We will also discuss the characteristic features of these methods andimprovements in their practical application.
The results presented here were obtained from the project ‘Investigations on applications of airborne gravity surveys to geothermal resources surveys’ by JOGMEC. We are most grateful to JOGMEC for making the data available to us.

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STT52-P04

Room:Convention Hall

Time:May 27 18:15-19:30

Three-dimensional resistivity modelling of GREATEM survey data from the Nojima Fault,Awaji Island, south-east Japan.
ABD ALLAH, Sabry1∗ ; MOGI, Toru1 ; ITO, Hisatoshi2 ; JOMORI, Akira3 ; YUUKI, Youichi4 ;FOMENKO, Elena5 ; KIHO, Kenzo2 ; KAIEDA, Hideshi2 ; SUZUKI, Koichi2 ; TSUKUDA, Kazuhiro21

Institute of Seismology and Volcanology, Hokkaido University, 2 Civil Engineering Research Laboratory, Central Research Institute of Electrical Power Industry, 3 NeoScience Co., 5-11-22 Osato, Sennan, Osaka, 590-0526, Japan, 4 Geotechnical Center, Oyo
Co., 2-61-5 Toro, Saitama, 331-8688, Japan., 5 Faculty of Computational Mathematics and Cybernetics, Lomonosov Moscow
State University, Moscow
An airborne electromagnetic (AEM) survey using the grounded electrical-source airborne transient electromagnetic (GREATEM)
system was conducted over the Nojima Fault on Awaji Island, south-east Japan, to assess GREATEM survey applicability forstudying coastal areas with complex topographic features. To obtain high-quality data with an optimised signal-to-noise ratio, aseries of data processing techniques was used to acquire the final transient response curves from the field survey data.
The 1D inversion results were feasible in that the horizontal resistivity contrast was not much higher than the true contrast, butthey were not reasonable in that the horizontal resistivity values were greatly changed. To circumvent this problem, we performed
numerical forward modelling using a finite-difference staggered-grid method (Fomenko and Mogi, 2002) adding a finite-length
electrical dipole source routine to generate a three-dimensional (3D) resistivity structure model from GREATEM survey data of
the Nojima Fault area. The 3D model was based on an initial model consisting of two adjacent onshore and offshore layers ofdifferent conductivity such that, a highly conductive sea of depth (10?40 m) is placed on top of a uniform half-space, assuming
the presence of topographic features on the inland side. We examined the fit of the magnetic transient responses between fielddata and 3D forward-model computed data, the latter were convolved with the measured system response of the correspondingdataset. The inverted 3D resistivity structures showed that the GREATEM system has the capability to map underground resistivity structures as deep as 500 m onshore and offshore. The GREATEM survey delineated how seawater intrudes on the landside of the fault and indicated that the fault is a barrier to seawater invasion.

Keywords: 3D EM forward modeling, GREATEM, Numerical approximations, Airborne Electromagnetic, Fault zone survey

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