SEM33-P01

Room:Convention Hall

Time:May 26 18:15-19:30

Wide-band Magnetotellurics Survey on Kuju volcanic Group, Kyushu iland
SHIOTANI, Taro1∗ ; UTSUGI, Mitsuru1 ; AIZAWA, Koki2 ; UYESHIMA, Makoto3 ; KOYAMA, Takao3 ;KANDA, Wataru4
1

Graduate School of Science, Kyoto University, 2 Institute of Seismology and Volcanology, Kyushu University., 3 Earthquake
Research Institute, University of Tokyo, 4 Volcanic Fluid Research Center, Tokyo Institute of Technology

Kuju volcano is one of the active volcano, in Kushu island, located at the prefectural border of Oita and Kumamoto. On this
volcano active fumarolic activity can be seen around Mt. Hossho which is one of the volcanic cones that construct Kuju volcano.
On Mt. Hossho, phreatic eruption was occurred in 1995, and new crater chainswere created by this activity (Imura&Kamata,1996).About Kuju volcano, many studies using seismological tomographic
method were conducted, and Yoshikawa et al.,(2004) pointed out that a magma reservoir exists beneath the southern part ofKuju volcano.
On this study, we conducted wideband MT (Magneto-Telluric) method for inferring the state of magma supply system in thismountain ranges in September and October 2014.
Five component EM fields were measured at 11site by three MTU-5A (Phoenix Geophysics Inc.) and at 23 site by seven ADU(Metoronics Inc.),and only 2component E fields at 33 site which are in the mountains, by ten ELOG. Observed time series data
are converted to wideband section by Fourier transform and calculate impedance. In our presentation, we will show the details
about our observations and also show the preliminary results of data analysis.

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SEM33-P02

Room:Convention Hall

Time:May 26 18:15-19:30

AMT resistivity soundings across the Kamogawa-teichi fault zone, Boso Peninsula
YAMAYA, Yusuke1∗ ; ITO, Shinobu1 ; MIYAKAWA, Ayumu1 ; SUMITA, Tatsuya1 ; KOMORI, Shogo1
1Geological Survey of Japan, AISTWe conducted resistivity soundings across the Kamogawa-teichi fault zone, Boso Peninsula, using the audio-frequencymagnetotelluric (AMT) method, in order to obtain a subsurface structure to discuss the existence and past activities of the fault
zone. The observed apparent resistivity and impedance phase were inverted to a resistivity section. The preliminary estimatedresistivity section is consistent with the geological age, which is categorized to three groups; Miura Group: <3 Ωm, Hota Group:∼10 Ωm and Mineoka Group: >30 Ωm. In general, an active fault is identified as a conductive zone due to saturated water into a
fractured zone. However, no significant conductor is found beneath the estimated faults and resistivity boundaries. This implies
that there are no fluids relating to fault activities in this area. We suggest possibilities that this ”fault zone” has never experienced
any fault activities or that it has spent a long time since the last active term, which cannot be resolved only by this survey.
Keywords: resitivity, active fault, Kamogawa teichi fault zone, magnetotellurics

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SEM33-P03

Room:Convention Hall

Time:May 26 18:15-19:30

Resistivity structure of the Sotobo area in Boso Peninsula, Central JapanOKUDA, Mao1∗ ; MOGI, Toru2 ; KOIZUMI, Naoki3 ; HATTORI, Katsumi3 ; HAN, Peng3 ; YMAZAKI, Tomohiro3 ;YOSHINO, Chie3
1

Graduate School of science, Hokkaido University, 2 Institute of Seismology and Volcanology, Faculty of Science, Hokkaido
University, 3 Department of Earth Sciences, Graduate School of Science, Chiba University
Boso peninsula is located in a tectonically complicated area where the Pacific plate, the Philippine Sea plate and the NorthAmerican plate converge. The Philippine Sea plate is subducting along the Sagami trough, where megathrust earthquakes haverepeatedly occurred. Near the megathrust earthquake source region, slow slop events (SSEs) have occurred at least five times
within 20 years and the recurrence interval has decreased [Ozawa et al., 2014]. The studies of SSEs have been advanced actively.
For example, it was indicated that SSE is a major driving process for earthquake swarms [Hirose et al., 2014]. On the other hand,
Obara [2007] argued that SSE may be related to fluid liberated from down-going slab by dehydration process. But the details of
the subsurface structure where it occurred have not still understood well.
A magnetotelluric (MT) survey was carried out between November and December in 2014 to investigate the structure relating
to SSE and subduction of the Philippine Sea plate under the continental plate. Time series of the MT data were recorded using theMTU-5 system manufactured by Phoenix Geophysics Co. Large artificial electromagnetic noise was observed in this area during
operation time of DC trains. Reduction of the serious noise will be essential to investigate deeper depth. In this preliminary stage,
we analyzed only data for two hours per a day that was obtained from 2 to 4 a.m. to avoid the DC train noise. However the lower
frequency bands (0.1 - 0.001 Hz) data still have not well identified due to local noise. We used the data at the frequency bandof 320 to 1 Hz and applied the remote reference method referring the other site observed in same time. We acquired apparent
resistivity and phase on each site based on the above process, and we also computed the phase tensor and induction vectors.
The phase tensor was nearly circle at each frequency, and the induction vectors have not point to the specific direction in thefrequency range. Finally we inverted the apparent resistivity and phase data and drew out a MT model.
MT images have showed basically three layer structure. Top layer, having 10 Ω-m, is extending to the several hundred meter
depth and underlying the very low resistivity layer (>1 ohm-m). According to the drilling data, these two layers are interpreted
as Shimosa and Kazusa Group respectively. The bottom of Kazusa group in the west area seems to be deeper than that of theeast area of the survey area and has showed basin like structure. The resistive layer is distributed at several kilo-meters depths
in the northeast area. This layer was interpreted as pre-Tertiary bed rocks. As shown here, we imaged subsurface structure of
the Sotobo area at depths of several kilo-meters in this stage using noise free frequency band data. However we would like todelineate image the deeper area than the result of this survey to elucidate the structure of SSE or the relation of the two plates, so
we need to remove noises from obtained data and observe wider area in the next stage.
Keywords: Magnetotelluric, Resistivity structure, Boso Peninsula, Kazusa Group

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SEM33-P04

Room:Convention Hall

Time:May 26 18:15-19:30

Broad-band telluric and magnetotelluric measurements in Oita prefecture, Kyushu, JapanAIZAWA, Koki1∗ ; UYESHIMA, Makoto2 ; TAKAMURA, Naoya3 ; UTSUGI, Mitsuru4 ; INOUE, Hiroyuki4 ;SHIOTANI, Taro4 ; UCHIDA, Kazunari1 ; TSUKASHIMA, Yuko1 ; WAKABAYASHI, Asuma1 ; FUJITA, Shiori1 ;MATSUSHIMA, Takeshi1 ; KOYAMA, Takao2 ; KANDA, Wataru5 ; YOSHIMURA, Ryokei6 ; YAMAZAKI, Ken’ichi6 ;KOMATSU, Shintaro6 ; SHITO, Azusa7
1

Kyushu University, 2 ERI, University of Tokyo, 3 NT system design, 4 Aso Volcanological Laboratory, Kyoto University, 5 KSVO,
TITECH, 6 DPRI, Kyoto University, 7 Institute for Geothermal sciences, Kyoto University
Recent development of 3-D magnetotelluric (MT) inversion codes (e.g., Siripunvaraporn et al., 2009; Kelbert et al., 2014)
allows us to deduce the 3-D resistivity structure. However, it is still hard to obtain MT data at many sites. In particular,
measurements of broad-band MT data is not easy to be conducted; because the apparatus is expensive, heavy, and has highelectric power consumption (12˜18W). These factors sometimes hamper to obtain large MT dataset, though a sufficient numberof sites are crucial for a reliable 3-D resistivity structure.
For MT impedance calculations, it is not necessary to record the magnetic field in all sites. Alternatively, the MT impedance
can also be calculated by using the magnetic field of other sites. Indeed, such kind of impedances has been used to deduce thehigh-resolution resistivity structure (e.g., Unsworth et al., 1997). In an extreme case, it is possible to deduce the 3-D resistivity
structure with single magnetic field site (e.g., Hata et al., 2015). Considering that omitting of magnetic field measurements leads
to reduce the effort in the field, we developed a new telluric measurement apparatus with low power consumption (1.8W). In thepresentation, we will show the result of the field test that is performed by using the new telluric apparatus and the MT apparatus.
The broad-band (200˜0.001 Hz) MT impedances obtained in the Oita prefecture, Kyushu, Japan, will also be shown.

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SEM33-P05

Room:Convention Hall

Time:May 26 18:15-19:30

Modification of resistivity structure beneath the Biwako fault, the southeastern part of the
Yamasaki fault system
ITO, Shuhei1∗ ; YAMAGUCHI, Satoru2 ; ODA, Yusuke2 ; MURAKAMI, Hideki3 ; KATOH, Shigehiro4 ;UYESHIMA, Makoto5 ; KATAKAMI, Satoshi1
1Department of Geosciences, Osaka City Univ., 2 Department of Geosciences, Graduate school of Science, Osaka City Univ.,
Natural Sciences Cluster-Science Unit, Kochi Univ., 4 Division of Natural History, Hyogo Museum of Nature and Human Activities, 5 Earthquake Research Institute, The University of Tokyo
3

Introduction
The Yamasaki fault zone (YFZ) consists of the Nagisen Fault, the main part of the YFZ, and the Kusadani Fault. The mainpart of the YFZ is further divided into a northwestern (NW) group (the Ohara, Hijima, Yasutomi, and Kuresakatouge Faults)
and a southeastern (SE) group (the Biwako and Miki Faults) based on their latest faulting events and mean left-lateral slip rates;
AD 868 and 1.0 m/kyr for the NW group vs. AD400 - 600 and 0.8 m/kyr for the SE group (Okada, 1987; Earthquake ResearchCommittee, 2013).
The audio-frequency magnetotelluric (AMT) method is a powerful tool for investigating the structure of active faults in theupper few km of the Earth crust. In particular, this method is more sensitive to the structure of a strike-slip fault, where verticalto high-angle fault planes and fracture zones are expected, than seismic reflection or refraction surveys.
Observation
An AMT surveys were made at 81 stations along 7 survey lines across the faults of the NW group, while the survey was madeat only 8 stations along 1 survey line across the faults of the SE group. It is important to make clear the subsurface structure of
the SE group to know the whole nature of the YFZ and difference between the NE and SE groups of the main part of the YFZ.
We focused on the Biwako faults of the SE group in this study. The Biwako fault is a vertical dipping, left-lateral strike-slip
fault system along a general strike of N50W.
An AMT survey was undertaken in June 2014 at nine stations along the transect across the Biwako fault. This transect passed
the site of trench excavation survey (Earthquake Research Committee, 2013). To apply remote reference method, the remote station of the magnetic field was set ˜15km north from the northeastern end of the transect. Two horizontal components of electricalfield and three components of magnetic field were measured.
Analysis
After calculated MT response functions based on the remote reference method (Gamble et al ., 1978), we adopted the phasetensor analysis (Caldwell et al ., 2004) to estimate dimensionality of the resistivity structure beneath the study area and to determine the direction of the regional strike, if the structure is two-dimensional. The two-dimensional resistivity model wasconstructed using the code of Ogawa and Uchida (1996) from the MT responses of both TE and TM modes.
Interpretation
The optimum model obtained is characterized by the clear resistivity boundaries correspond to the surface traces of west andeast segments of the Biwako fault and by low resistivity zone dominated on the north side of the fault. We interpreted that thedominant low resistive zone on the north side of the fault is caused by aqueous fluid in wide damaged zone which is developedin the tensile region between the western and eastern segments. The clear resistivity boundary near the fault is caused by a fault
core which can be a barrier to water flow across a fault plane. It is still unclear how the conductive region at the south side of thissurvey line generated.

Keywords: Biwako fault, Yamasaki fault system, active fault, resistivity structure, Magnetotelluric method

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SEM33-P06

Room:Convention Hall

Time:May 26 18:15-19:30

A report on the fundamental investigations of an electrical resitivity structure beneath
Chugoku and Shikoku regions
SHIOZAKI, Ichiro1∗ ; YAMAMOTO, Takenao1 ; HAMADA, Akiyoshi2 ; MURAKAMI, Hideki3 ; YOSHIMURA, Ryokei4 ;OSHIMAN, Naoto4 ; IIO, Yoshihisa4 ; UYESHIMA, Makoto5
1

Graduate School of Engineering, Tottori University, 2 Faculty of Engineering, Tottori University, 3 Faculty of Science,Kochi
University, 4 Disaster Prevention Research Institute, Kyoto University, 5 Earthquake Research Institute, The University of Tokyo
In order to contribute to a reduction in damage caused by earthquakes and volcanic eruptions, heterogeneities of crustal andupper mantle structure should be clarified based on fundamental investigations of electrical resistivity sructure in Chugoku andShikoku regions, southwestern Japan arc. In this presentation, a preliminary report on fundamental surveys, using data acquiredin 2014 incorporated in the existing data, will be shown.
Our research group has shown that there is a clear relationship between resistivity and seismicity in the Sanin and Shikoku
regions. In the eastern part of San-in region, it was found that a conductive area exists in the deep crust part under the seismic
region, which is a resistive area, along with the seismic activity area stretching nearly in the east and west direction. However,recent observation result conflicts with the model advocated by the group including the author that has studied electrical resistivity
in Sanin region (ex, Ozaki et al., 2013). That is, there is a possibility that the deep low resistivity area beneath the Sanin region
does not exist in series. Assuming that inland earthquakes occur because of local stress concentration caused by heterogeneity
beneath a seismic activity band (Iio, 2009), the heterogeneity in this area should be clarified hereafter. Wideband MT obsevations
were carried out at 5 sites in the western extension area of Shikano and Yoshioka faults, from late Sep to early Oct 2014. The area
is located between the 1943 Tottori earthquake (M7.2) and 1983 Misasa earthquake (M6.2) source regions. Natural geomagnetic
and electric field variations were measured using Phoenix MTU5 systems. Apparent resistivity and phase data were used formodel analysis assuming an EW strike direction. A preliminary two-dimensional model shows almost a similar resistivity
structure, as a whole, to those obtained by the existing studies in this region. However, the location of the deep crustal conductor
is slightly shifted to the south.
On the other hand, in the Shikoku region, investigations were carried out mainly in the outer zone, and the result suggested thata remarkable conductive area should exist in the upper crust and that the conductive area in the central and western part shouldhave a clear relation with the non-seismic area. These studies suggest that high conductivity(low resitivity) is possibly causedby the existence of deep crustal fluids, which probably play an important role in the inland earthquake occurrence mechanism ofthese regions. However, the existence of the plate is not thoroughly identified in the geological inner zone of the southwesternJapan Arc. Therefore, in order to grasp a whole tectonic setting, from the fore to the back arc side in the southwestern Arc,
quantitative discussions based on the wideband MT survey covering whole these regions should be required (Shiozaki et al.,2014).
In order to elucidate the regional characteristics of the large scale resistivity structure, fundamental wideband MT observations
have been conducted at 5 sites in the observations gap area in the east and central region of Shikoku from late Nov to mid-Dec
2014. Preliminary results show that the derived apparent resistivity and the phase curves have a common feature for all 5 sites.
We would like to express sincere gratitude for the Nittetsu Mining Consultants Co. Ltd. kindly let us use their continuous
geomagnetic records as remote references. This study was supported by the Ministry of Education, Culture, Sports, Scienceand Technology (MEXT) of Japan, under its Observation and Research Program for Prediction of Earthquakes and Volcanic
Eruptions, and also supported by a Tottori Prefecture Environmental Academic Research Promotion Project issued in Fiscal year
2014. Last of all, we would like to express our thanks to T.Uto, H.Hataoka, S.Yamamoto, Y.Ikezoe, M.Fukunari, A.Hamada,
Y.Tsuyoshi and S.Yoshida of Tottori University for their help during data acquisition.
Keywords: electrical resitivity, fundamental investigation, Chugoku and Shikoku region

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SEM33-P07

Room:Convention Hall

Time:May 26 18:15-19:30

Dense AMT observations across the Japan Median Tectonic Line Izumi fault zone
OUCHI, Yuhei1∗ ; YOSHIMURA, Ryokei2 ; TANAKA, Daisuke2 ; YONEDA, Itaru2 ; YAMAZAKI, Tomoya2 ;OSHIMAN, Naoto2
1

Graduate School of Science, Kyoto University, 2 Disaster Prevention Research Institute, Kyoto University

The Japan Median Tectonic Line Fault Zone (MTL) is a fault system that extends for about 360km and locates along the geological boundary between the Ryoke and the Sanbagawa belts. Izumi segment fault zone of the MTL consists of Gojodani and
Shobudani faults, etc. Wideband Magnetotelluric (MT) soundings were carried out across these faults (Yoshimura et al ., 2014).
The obtained resistivity model was characterized by a contrast around the MTL. However, the shallow resistivity structure was
not so clear.
In order to delineate fine subsurface structure around the fault, we carried out audio-frequency magnetotellurics (AMT) measurements at 38 sites along a 5km profile perpendicular to the Gojodani and Shobudani faults in November, 2014. MT responses
of a frequency band of 10,400-0.35 Hz were obtained using remote reference processing (Gamble et al ., 1979). As a result,relatively good quality MT responses were obtained at most sites.
In this presentation, we will show the final MT response functions at each site and report a preliminarily results of twodimensional inversions.

Keywords: the Japan Median Tectonic Line Fault Zone, magnetotelluric, damage zone, shallow resistivity structure

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SEM33-P08

Room:Convention Hall

Time:May 26 18:15-19:30

The 3-D magnetic imaging using the L-1 norm regularization, Part II.
UTSUGI, Mitsuru1∗
1

Kyoto Univ.

Recently some new methods for 3-D magnetic imaging were proposed (Li and Oldenburg,1996, Portniaguine and Zhdanov,2001,
Pilkington,2013). Some of them have a goal to obtain the “sparse” model i.e. the simplest model that reproduce the observed
data. This is because, most of the traditional way of inversion for the potential data provides distorted or unfocused images ofreal magnetic structures. In this study, we propose a new method introducing an L-1 norm penalized least square procedure andtried to obtain a simple, high-resolution and focused model.
Lasso (Tibshirani, 1996) is a linear regression and variable selection procedure based on the L1 penalized least square. L1
penalty has an effect of shrinkage the value of model parameters which have weak contributions to be zero. So, the Lasso does
both continuous shrinkage and automatic variable selection simultaneously. On the other hand, Lasso has some drawbacks. Oneof them is, at most Lasso algorithm can select nonzero variables of same number of observed data. So, in the case of p <<nproblem, i.e. when the number of unknown parameters (n) is larger than the number of observations (p), this algorithm cannot
be adopted or overly shrinkage model will be obtained.
To overcome this limitation, Zou and Hastie (2005) proposed a new L-1 penalized method named ’elastic net’, and Hebiri andvan de Geer (2011) proposed ’S-Lasso’. These methods are the compromise of the L-1 and L-2 or some quadratic regularization
method. Using these methods, we can treat p <<n problems in the framework of L-1 penalized method.
In This study, we propose a new 3-D magnetic inversion method based on the Lasso-type regularization (i.e. generalized elasticnet) and show the results of applying our method to the synthesized and real magnetic data.

Keywords: L1 regularization, magnetc inversion, aeromagnetic survey

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SEM33-P09

Room:Convention Hall

Time:May 26 18:15-19:30

Geomagnetic three-component secular changes in eastern Hokkaido, NE JapanMISHIMA, Wataru1∗ ; HASHIMOTO, Takeshi2 ; MOGI, Toru2
1

Graduate School of Science, Hokkaido University, 2 Institute of Seismology and Volcanology, Faculty of Science, Hokkaido
University
Based on the total field records, regional anomaly in the rate of geomagnetic secular changes have been observed in eastern
Hokkaido, NE Japan (Oshima et al., 1994; Hashimoto et al., 2012). This area is under a compressive regime due to subductionof Pacific plate from east-southeast toward west-northwest (Ishikawa et al., 2001). Positive geomagnetic anomalies are also
distributed in the southern coastal area, suggesting the existence of highly-magnetized rocks (Sugisaki et al., 2001). For thesereasons, Nishida et al. (2004) suspected the piezomagnetic effect as the predominant cause of the rate of geomagnetic secularchanges. We started three-component absolute measurements at eight continuous magnetic stations in 2009, aiming at revealing
the mechanisms of the anomaly. The measurements have been performed twice or three times at each station by 2014. We reporta brief overview of the absolute measurements up to 2014.
In the total field, the values at each station have been subtracted from values at the reference station Memambetsu Magnetic
Observatory (MMB) of Japan Meteorological Agency to remove the variations of extra-terrestrial origins. We applied the sameprocedure also to declination, horizontal and vertical components. We then recognized that the rate of secular change in individualcomponent also differs from site to site. Now, it is clear that the anomalous secular changes in the total field in this area were notbrought by the so-called orientation effect. Trends of horizontal and vertical components were steeper near the southern coast as
in the case of the total field. Declination seemed more complicated, presumably showing clockwise rotation in the northern area,
while anti-clockwise in the south.
Current results are still preliminary because the number of absolute measurements is small yet. It is necessary to add further
data in order to make the discussion more accurate. In addition, numerical modeling of piezomagnetic effect in three-component
will contribute to constrain the mechanism of the anomaly.

Keywords: geomagnetic three-component absolute measurements, geomagnetic secular change, eastern Hokkaido

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SEM33-P10

Room:Convention Hall

Time:May 26 18:15-19:30

Distribution of electromagnetic fields induced in JapanTOMINAGA, Hiroki1∗ ; ENDO, Arata1 ; FUJITA, Shigeru2 ; FUJII, Ikuko2
1

Japan Meteorological Agency, 2 Meteorological College, Japan Meteorological Agency

The electric field induced by a geomagnetic field variation can be enhanced in Japan by a large conductivity contrast between
seawater and rocks (the coast line effect). We should know plausible geomagnetically induced currents there when unprecedentedly large geomagnetic disturbances occur. Distributions of the electromagnetic fields induced at the surface of a realistic earth
are estimated as the first step.
The electromagnetic fields induced at the surface of a 3D earth under an inducing magnetic field in the magnetosphere arecomputed in the frequency domain by the finite difference code developed by Uyeshima and Schultz (2000). The periods usedare 200, 800 and 3600 seconds. The grid size is 12.5km x 12.5km at latitudes between 25-50o N and longitudes between 125150o E and is gradually increased at the outside the region.
A 3D distribution of the electrical conductivity inside the Earth is obtained down to 12.5 km deep by using the ETOPO1
bathymetry data and the sediment thickness data by Laske and Masters (1997). A radially symmetric conductivity distribution isused at depths lower than 12.5km. The conductivities of seawater and sediment are fixed as 3 and 0.1 S/m, respectively, while
the rock conductivity is used either of 0.01 or 0.0001 S/m to demonstrate the effect of a conductive or resistive lithosphere.
The inducing dipole magnetic field is given at 10 Earth radii as a boundary condition. If it is an axial dipole, the source
magnetic field variation is polarized mainly in the north-south direction in Japan. We rotate the dipole every 30 degrees against
the equator plane to realize other polarizations.
We pick up the maximum amplitude of the induced electric field among all source field patterns at every surface grid point toconstruct the distribution map of the maximum possible induced electric field in Japan. The map shows that the electric field isenhanced along the coast lines and the electric field amplitude is larger for the resistive lithosphere than that for the conductive
one. The amplitude reaches twice compared to that from a homogeneous earth model. In most surface grids, the electric field
amplitude at a shorter period is larger than that at a longer period. The electric field is enhanced especially at Tsugaru-Toshimapeninsulas, Toyama bay, Izu peninsula, and east coasts of Kyusyu Island where offshore sea depths quickly become deeper.
The geomagnetic field at the surface also shows heterogeneity because of induction. When the source magnetic field is anaxial dipole, the east-west component is generally weak and the vertical components are can be large at coastal areas. The northsouth component at Tohoku and Kyusyu is similar to that of a 1D homogeneous earth, while it is weaker at areas from Kanto to
Chugoku.
Distributions of the impedance tensor are also obtained. The impedance tensor substantially varies over Japan suggesting thatthe influence of sea on the regional induction is immense. For instance, Hokkaido could be divided into several areas following
the spatial pattern of the impedance tensor, which show some similarities to the impedance tensor at Doto obtained by Uyeshima
et al (2001). Similarly, several districts over Japan are recognized from the viewpoint of the spatial pattern of the impedance
tensor depending on the direction of the source magnetic field.
The comparison between the computed and observed impedance tensors at Kakioka, Kanoya and Memambetsu indicates thatthe computed tensors qualitatively reproduce the observed ones, but the amplitudes and frequency dependencies show discrepancies. A more realistic conductivity model is necessary, if a more precise estimate of the electromagnetic field induced at thesurface.
Keywords: electromagnetic induction, geomagnetically induced currents, impedance tensor

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SEM33-P11

Room:Convention Hall

Time:May 26 18:15-19:30

Estimating the distribution of electrical conductivity in the earth with the maximumlikelihood method
IWASHITA, Kodai1∗ ; TOH, Hiroaki2
1Graduate School of Science, Kyoto University, 2 Data Analysis Center for Geomagnetism and Space Magnetism, Kyoto University
We estimated radial electrical conductivity distribution in the Earth by solving an inversion problem with an objective functionthat allows errors in the model. In general, geophysical inverse problems are solved by the least squares method that minimizes
the objective function consisting of linear combination of the data misfit and the regularization term. The predicted data is obtained by a forward calculation for the set of optimized model parameters. However, this method implicitly premises that themodel has no error.
We tried to solve an inversion problem without the premise. Solutions by the maximum-likelihood method are not necessarilyequal to those by the least square method. Specifically, We applied this method to the vector geomagnetic observatory data all
around the globe in November, 2003 and estimated the radial electrical conductivity profile. A heterogeneously conducting shell
was placed at the top of the radially symmetric sphere. We worked with the data per minute at 69 observatories during the large
geomagnetic storm and subtracted the vector average for 5 quietest days. We then separated the corrected time-series into internaland external origin. The separated internal magnetic field can be reproduced by our maximum-likelihood method using forward
response of the radially symmetric conducting sphere to the separated external magnetic field.
It was revealed that the maximum-likelihood method led to the different solution from that by the least squares method andthat the maximum-likelihood method gave us appropriate information on model choice. However, it was necessary to apply apriori information on the model parameters as much as possible (e.g., all electrical conductivities must be positive) and to add a
regularization term to the objective function. We will also discuss the pertinent way of adding errors to the synthetic data and theeffect of initial models on the final model.

Keywords: electrical conductivity, magnetic storm, inversion

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SEM33-P12

Room:Convention Hall

Time:May 26 18:15-19:30

Direct simulation of resistivity on porous model obtained from high-resolution X-ray CT
SAOMOTO, Hidetaka1∗ ; KATAGIRI, Jun1 ; TAKADA, Naoki1 ; TAKAHASHI, Manabu1
1

National Institute of Advanced Industrial Science and Technology

Resistivity imaging is widely used to estimate both the class ofgeo-materials (e.g., rock, sand, etc.) and the state variables ofgeo-materials (e.g., porosity, degree of saturation, etc.) on a cross-sectional plane (survey area) placed in the underground. For
engineering use, the resistivity imaging of underground is adopted to observe the behavior of groundwater flow and to comprehend the changes of ground state caused by ground improvement. In order to increase the accuracy in the estimation of geologicalproperties derived from resistivity imaging, we need to reveal the relationship between the resistivity and several physical parameters such as porosity, degree of saturation, and electrical conductivity of pore fluid. In other words, the resistivity has thedependency with those physical parameters.
Here, we have conducted a series of finite element simulations with above physical parameters to discuss the relationshipbetween the resistivity of porous media and those physical parameters. The three-dimensional porous models are created by aseries of sectional images obtained from the high-resolution X-ray CT. Although this method requires a precise micro-structureof porous media before the consideration, we can quantitatively estimate the resistivity of the porous model. This method alsoenables us to discuss the anisotropic properties on the resistivity by changing the direction of energization along the x, y, andz-axis.
Subsequently, the simulation results are compared with those obtained from experiments and with those derived from theempirical law, i.e., Archie’s equation. According to the comparison, the simulation results are in good agreements with experimental results and indicate similar function form that proposed in Archie’s equation being available for unsaturated state. Also,
we have tried to extract the micro-scopic physical parameter such as the tortuosity. The tortuosity is one of the key parameters tocharacterize the transport properties of porous media, but it is difficult to measure the tortuosity through experiments.
Finally, we discuss the limitation of Archie’s equation and infer that Archie’s equation is applicable to geo-materials having a
degree of saturation exceeding 40%.
Keywords: resistivity, porous media, finite element method, X-ray CT, Tortuosity

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SEM33-P13

Room:Convention Hall

Time:May 26 18:15-19:30

Development of resistivity modeling code designed to high-density electrical prospecting
for cylindrical rock samples
SUZUKI, Takeshi1∗ ; YOSHIMURA, Ryokei2 ; OSHIMAN, Naoto2
1

Graduate School of Science, Kyoto University, 2 DPRI, Kyoto University

Crustal electrical resistivity structures are obtained through geoelectromagnetic observations, and are used to interpret characteristics of subsurface crustal activities such as earthquake occurrence and volcanic activities (e.g., Ogawa et al., 2001; Yoshimura
et al., 2009). Because electrical properties are very sensitive to the existence of fluid, geoelectromagnetic methods are used todetect crustal fluid.
Resistivity images are interpreted in relation between the porosity of rock and its connectivity with several mixing lows (e.g.,
Archie, 1942; Hashin and Shtrikman, 1962; Glover et al., 2000). In order to verify the applicability and scalability of suchinterpretations, we plan to carry out high-density electrical prospecting for rock samples whose other geophysical characteristics
are well known.
As the first step of laboratory experiments, we have developed a numerical simulation code of DC method for cylindricallyshaped samples as a tool for feasibility studies. In our code, three dimensional finite-difference methods described in Dey andMorrison (1979) is modified for the cylindrical coordinate system.
For performance evaluation, the results of numerical experiment were compared with results of electrical prospecting to simulation sample (conductive plastic). As a result, we confirmed a cylindrical-shaped outer boundary is represented consistently
by developed code. Also, we detected thin structure whose thickness is 1mm or less and estimated its resistivity by forward
modeling.
In this presentation, we will report the results of comparison between numerical simulations and electrical prospecting to simulation sample.
Keywords: rock experiments, electrical resistivity, numerical simulations

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