The magnetotelluric (MT) method is an electromagnetic method based on the joint analysis of elect... more The magnetotelluric (MT) method is an electromagnetic method based on the joint analysis of electric and magnetic fields through their ratio that define the MT impedance. This method has been intensively used in mineral exploration and crustal studies because of its sensitivity to conductivity contrast of rocks. The MT method is useful for delineating large-scale conductive and resistive zones within the earth. It is used for identifying structure related to juxtaposed rocks with differing resistivities, caused by differing rock types, fluid content or alteration within relatively uniform strata, and representing significant features such as partial melts, feeder dykes, structural features like faults and shear zones, magma chambers, etc. Because the method is sensitive to electrical resistivity, it offers an alternative and complementary exploration tool to seismic and gravity surveying and can be used for identifying structures that are generally undetectable by other techniques. I will talk about resistivity structure sections obtained by MT surveys in some tectonic areas such as: 1. Arc-arc collision area in Hokkaido, Northern Japan. 2. Crust strain concentration area in Central Japan. 3. Three types of tectonic areas in Indonesia. The heterogeneous electrical features in tectonic areas show a close spatial relationship with the distribution of the displacement rate and with seismicity. This suggests that the resistivity structure can be used as an index from which to evaluate the deformation characteristics of the crust.
IEEE Geoscience and Remote Sensing Letters, Aug 1, 2018
Neural network has been successfully used for fieldscale temperature estimation based on resistiv... more Neural network has been successfully used for fieldscale temperature estimation based on resistivity data. Although the methodology is able to estimate temperature distribution based on geophysical data with a limited number of temperature data, it has also been shown that the estimation accuracy can decrease as distance increases from temperature measurements. In this letter, we developed a resistivity-based neural kriging approach to improve the accuracy of estimated temperatures. The neural kriging method in this letter incorporated a variogram of temperature data, which constrained the spatial distribution of temperature. We examined the accuracy of standard neural network approaches, as well as the neural kriging approach, using a cross-validation test. The examination showed that when neural kriging was used, the estimation error and variogram error were reduced by 6%-87% and 191%-285%, respectively. The improvement was also significant at areas distant from well logs. The pattern of estimated temperatures matched the underlying geology and was in accordance with previous studies. The proposed methodology can incorporate other types of geophysical data for estimating other physical parameter distributions.
We describe a new airborne EM system which uses a grounded electrical source and an airborne magn... more We describe a new airborne EM system which 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. Although the area of survey is limited with this type of method, it has advantages over conventional AEM such as a reduced sensitivity to flight level. To fully realise the method, we have worked to overcome two main problems: monitoring the motion of the receiver in the geomagnetic field, and cancelling the natural magnetic field variation and cultural noise without stacking in the time domain. To minimise motion noise, we utilised high-accuracy fibre optic gyros to monitor the pitch and roll of the magnetic sensor in the bird. We tested the method at a geothermal field in Beppu area in southwest Japan, and obtained a clear transient response of the vertical magnetic field and its time derivative over several kilometres distance from the source at around 300 m flight height. We expect that this method will be a successful approach for deep exploration targets.
物理探査学会学術講演会講演論文集 = Proceeding of the SEGJ Conference, Nov 29, 2012
Ontake Volcano is located in central Japan, 200 km northwest of Tokyo and erupted on September 27... more Ontake Volcano is located in central Japan, 200 km northwest of Tokyo and erupted on September 27, 2014. To study the structure of Ontake Volcano and discuss the process of its phreatic eruption, which can help in future eruptions mitigation, airborne electromagnetic (AEM) surveys using the grounded electrical-source airborne transient electromagnetic (GREATEM) system were conducted over Ontake Volcano. Field measurements and data analysis were done by OYO Company under the Sabo project managed by the Ministry of Land, Infrastructure, Transport and Tourism. Processed data and 1D resistivity models were provided by this project. We performed numerical forward modeling to generate a three-dimensional (3D) resistivity structure model that fits the GREATEM data where a composite of 1D resistivity models was used as the starting model. A 3D electromagnetic forward-modeling scheme based on a staggered-grid finite-difference method was modified and used to calculate the response of the 3D resistivity model along each survey line. We verified the model by examining the fit of magnetic-transient responses between the field data and 3D forward-model computed data. The preferred 3D resistivity models show that a moderately resistive structure (30-200 Ω m) is characteristic of most of the volcano, and were able to delineate a hydrothermal zone within the volcanic edifice. This hydrothermal zone may be caused by a previous large sector collapse.
The magnetotelluric (MT) method is an electromagnetic method based on the joint analysis of elect... more The magnetotelluric (MT) method is an electromagnetic method based on the joint analysis of electric and magnetic fields through their ratio that define the MT impedance. This method has been intensively used in mineral exploration and crustal studies because of its sensitivity to conductivity contrast of rocks. The MT method is useful for delineating large-scale conductive and resistive zones within the earth. It is used for identifying structure related to juxtaposed rocks with differing resistivities, caused by differing rock types, fluid content or alteration within relatively uniform strata, and representing significant features such as partial melts, feeder dykes, structural features like faults and shear zones, magma chambers, etc. Because the method is sensitive to electrical resistivity, it offers an alternative and complementary exploration tool to seismic and gravity surveying and can be used for identifying structures that are generally undetectable by other techniques. I will talk about resistivity structure sections obtained by MT surveys in some tectonic areas such as: 1. Arc-arc collision area in Hokkaido, Northern Japan. 2. Crust strain concentration area in Central Japan. 3. Three types of tectonic areas in Indonesia. The heterogeneous electrical features in tectonic areas show a close spatial relationship with the distribution of the displacement rate and with seismicity. This suggests that the resistivity structure can be used as an index from which to evaluate the deformation characteristics of the crust.
IEEE Geoscience and Remote Sensing Letters, Aug 1, 2018
Neural network has been successfully used for fieldscale temperature estimation based on resistiv... more Neural network has been successfully used for fieldscale temperature estimation based on resistivity data. Although the methodology is able to estimate temperature distribution based on geophysical data with a limited number of temperature data, it has also been shown that the estimation accuracy can decrease as distance increases from temperature measurements. In this letter, we developed a resistivity-based neural kriging approach to improve the accuracy of estimated temperatures. The neural kriging method in this letter incorporated a variogram of temperature data, which constrained the spatial distribution of temperature. We examined the accuracy of standard neural network approaches, as well as the neural kriging approach, using a cross-validation test. The examination showed that when neural kriging was used, the estimation error and variogram error were reduced by 6%-87% and 191%-285%, respectively. The improvement was also significant at areas distant from well logs. The pattern of estimated temperatures matched the underlying geology and was in accordance with previous studies. The proposed methodology can incorporate other types of geophysical data for estimating other physical parameter distributions.
We describe a new airborne EM system which uses a grounded electrical source and an airborne magn... more We describe a new airborne EM system which 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. Although the area of survey is limited with this type of method, it has advantages over conventional AEM such as a reduced sensitivity to flight level. To fully realise the method, we have worked to overcome two main problems: monitoring the motion of the receiver in the geomagnetic field, and cancelling the natural magnetic field variation and cultural noise without stacking in the time domain. To minimise motion noise, we utilised high-accuracy fibre optic gyros to monitor the pitch and roll of the magnetic sensor in the bird. We tested the method at a geothermal field in Beppu area in southwest Japan, and obtained a clear transient response of the vertical magnetic field and its time derivative over several kilometres distance from the source at around 300 m flight height. We expect that this method will be a successful approach for deep exploration targets.
物理探査学会学術講演会講演論文集 = Proceeding of the SEGJ Conference, Nov 29, 2012
Ontake Volcano is located in central Japan, 200 km northwest of Tokyo and erupted on September 27... more Ontake Volcano is located in central Japan, 200 km northwest of Tokyo and erupted on September 27, 2014. To study the structure of Ontake Volcano and discuss the process of its phreatic eruption, which can help in future eruptions mitigation, airborne electromagnetic (AEM) surveys using the grounded electrical-source airborne transient electromagnetic (GREATEM) system were conducted over Ontake Volcano. Field measurements and data analysis were done by OYO Company under the Sabo project managed by the Ministry of Land, Infrastructure, Transport and Tourism. Processed data and 1D resistivity models were provided by this project. We performed numerical forward modeling to generate a three-dimensional (3D) resistivity structure model that fits the GREATEM data where a composite of 1D resistivity models was used as the starting model. A 3D electromagnetic forward-modeling scheme based on a staggered-grid finite-difference method was modified and used to calculate the response of the 3D resistivity model along each survey line. We verified the model by examining the fit of magnetic-transient responses between the field data and 3D forward-model computed data. The preferred 3D resistivity models show that a moderately resistive structure (30-200 Ω m) is characteristic of most of the volcano, and were able to delineate a hydrothermal zone within the volcanic edifice. This hydrothermal zone may be caused by a previous large sector collapse.
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