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Sennan, Japan

Ito H.,Japan Central Research Institute of Electric Power Industry | Kaieda H.,Japan Central Research Institute of Electric Power Industry | Mogi T.,Hokkaido University | Jomori A.,NeoScience Co. | Yuuki Y.,Oyo Corporation
Exploration Geophysics | Year: 2014

Grounded electrical-source airborne transient electromagnetics (GREATEM), a type of semi-airborne electromagnetics, was used to examine Aso Volcano in south-west Japan, to verify its applicability to surveying deep subsurface resistivity structures. Comparison of the GREATEM resistivity values with those of ground-based transient electromagnetics (TEM) data, repeated GREATEM survey results at the same and different flight heights, and lithologic descriptions indicated that GREATEM can successfully identify underground structures as deep as ∼800m in rugged mountainous areas. An active volcanic region (Naka-Dake crater) was mapped as a low-resistivity zone from the surface to a depth of 100m. This low-resistivity zone extended to the west-north-west, implying future volcanic activity in this area. Therefore, the GREATEM method is useful for surveying deep structures in large, inaccessible areas, such as volcanic provinces, in a quick, cost-effective way. © 2014 ASEG. Source


Okazaki K.,Japan Civil Engineering Research Institute for Cold Region | Mogi T.,Hokkaido University | Utsugi M.,Kyoto University | Ito Y.,Japan Civil Engineering Research Institute for Cold Region | And 10 more authors.
Physics and Chemistry of the Earth | Year: 2011

Airborne geophysical surveys enable us to clarify three-dimensional subsurface structures in large and inaccessible areas. Therefore, they have been used recently to investigate large-scale landslides and volcanoes, as well as in mineral and hydrocarbon explorations. This paper describes a geological assessment for tunnel design and construction using helicopter-borne geophysical surveys. Airborne electromagnetic surveys using a grounded electric dipole source and magnetic surveys were conducted to delineate resistivity and magnetization structures in deeper parts of tunnel construction sites in the Otoineppu area of Hokkaido, northern Japan. The survey area is mainly composed of Cretaceous sedimentary rocks, with serpentinite dykes intruded into the sedimentary rocks. The surveys covered the tunnel site and its surroundings to estimate the distribution of sediment rocks and serpentinite. The resistivity structure of deep sections and the distribution of magnetization anomalies delineated the serpentinite types and their distribution, which is useful in understanding potential geotechnical issues when excavating a tunnel. © 2011 Elsevier Ltd. Source


Abd Allah S.,Hokkaido University | Mogi T.,Hokkaido University | Ito H.,Japan Central Research Institute of Electric Power Industry | Jomori A.,NeoScience Co. | And 6 more authors.
Journal of Applied Geophysics | Year: 2013

An airborne electromagnetic (AEM) survey using the Grounded Electrical-Source Airborne Transient Electromagnetic (GREATEM) system was conducted over the Kujukuri coastal plain in southeast Japan to assess the system's ability to accurately describe the geological structure beneath shallow seawater. To obtain high-quality data with an optimized signal-to-noise ratio, a series of data processing techniques were used to obtain the final transient response curves from the field survey data. These steps included movement correction, coordinate transformation, the removal of local noise, data stacking, and signal portion extraction. We performed numerical forward modeling to generate a three-dimensional (3D) resistivity structure model from the GREATEM data. This model was developed from an initial one-dimensional (1D) resistivity structure that was also inverted from the GREATEM field survey data. We modified a 3D electromagnetic forward-modeling scheme based on a finite-difference staggered-grid method and used it to calculate the response of the 3D resistivity model along each survey line. We verified the model by examining the fit of the magnetic-transient responses between field data and the 3D forward-model computed data, the latter of which were convolved with the measured system responses of the corresponding data set. The inverted 3D resistivity structures showed that the GREATEM system has the capability to map resistivity structures as far as 800. m offshore and as deep as 300-350. m underground in coastal areas of relatively shallow seawater depth (5-10. m). © 2013 Elsevier B.V. Source


Allah S.A.,Hokkaido University | Mogi T.,Hokkaido University | Ito H.,Japan Central Research Institute of Electric Power Industry | Jymori A.,NeoScience Co. | And 6 more authors.
Exploration Geophysics | Year: 2014

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 for studying coastal areas with complex topographic features. To obtain high-quality data with an optimised signal-to-noise ratio, a series 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, but they 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 of different conductivity such that, a highly conductive sea of depth (10-40m) 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 field data and 3D forward-model computed data, the latter were convolved with the measured system response of the corresponding dataset. The inverted 3D resistivity structures showed that the GREATEM system has the capability to map underground resistivity structures as deep as 500m 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. © 2014 ASEG. Source

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