Research Highlight | Keiichi Ishizu

Controlled-Source Electromagnetic Survey in Volcanic Areas (Ishizu et al., 2025, GJI)

Wed, 01 Jan 2025 00:00:00 +0000

Ishizu, K. et al. (2025). Controlled-source electromagnetic survey in a volcanic area: relationship between stacking time and signal-to-noise ratio and comparison with magnetotelluric data. Geophysical Journal International, 240(2), 1107-1121.

Point 1: Significant improvement in S/N ratio achieved using EM-ACROSS

Point 2: Detection of a vapor layer undetected by conventional MT surveys

Magnetotelluric (MT) methods, which use natural electromagnetic signals, have primarily been used for structural investigation and monitoring of volcanic bodies. However, their application was limited by unstable signal sources and artificial noise, which reduced the signal-to-noise (S/N) ratio.

In this paper, we demonstrate that a significant improvement in the S/N ratio is possible by using EM-ACROSS, a high-precision controlled-source method, combined with long-duration measurements. This result suggests that high S/N ratio electromagnetic exploration data can be obtained even in areas where MT investigation was previously difficult due to noise levels.

Furthermore, MT methods often struggle to detect thin high-resistivity layers because horizontal electric fields dominate. By using a controlled source, we can excite vertical currents, enabling the detection of vapor layers (thin high-resistivity bodies) that can cause phreatic eruptions. This research introduces a new observation method to the field and represents a significant breakthrough in electromagnetic exploration.

Inversion Algorithm Determining Sharp Boundaries (Ishizu et al., 2025, Geophysics)

Wed, 01 Jan 2025 00:00:00 +0000

Ishizu, K. et al. (2025). Inversion algorithm determining sharp boundaries in electrical resistivity tomography. Geophysics, 90(3), 1-46.

Point 1: Data-driven inversion algorithm capable of reproducing sharp boundaries without excessive reliance on prior information

We have developed a new inversion algorithm that can accurately depict sharp resistivity boundaries. This data-driven approach represents a significant advancement by minimizing reliance on prior assumptions about boundary locations.

Accurately depicting sharp resistivity boundaries is crucial for improving the precision of resource exploration, particularly for metallic deposits. This method can be implemented by incorporating an ABIC search into existing inversion codes, and we hope it will be adopted across many platforms in the future.

Estimating Gas Hydrate Distribution Using Electrical and Seismic Data (Ishizu et al., 2024, Scientific Reports)

Mon, 01 Jan 2024 00:00:00 +0000

Ishizu, K. et al. (2024). Electrical resistivity tomography combined with seismic data estimates heterogeneous distribution of near-seafloor concentrated gas hydrates within gas chimneys. Scientific Reports, 14(1), 15045.

Point 1: Estimating gas hydrate distribution using electrical and seismic data

Point 2: Discovery of heterogeneous gas hydrate distribution within gas chimneys

By combining electrical resistivity tomography and seismic data, this study successfully estimated the distribution of shallow gas hydrates in the Sea of Japan. Shallow gas hydrates are found within gas chimneys, where gas is supplied from the deep subsurface. Our research revealed that gas hydrates are not uniformly present within these chimneys but exist in a spatially heterogeneous manner. This suggests that such heterogeneous distributions must be considered when evaluating future resource development and environmental protection.

Successful 3D Subsurface Mapping of Embedded Massive Sulfides (Ishizu et al., 2024, Geophysics)

Mon, 01 Jan 2024 00:00:00 +0000

Ishizu, K. Kasaya, T., Goto, T. N., Koike, K., Siripunvaraporn, W., Iwamoto, H., … & Ishibashi, J. I. (2024). A marine controlled-source electromagnetic application using towed and seafloor-based receivers capable of mapping seafloor and embedded massive sulfides. Geophysics, 89(3), E87-E99.

Point 1: Successful 3D subsurface mapping of embedded massive sulfides

Conventional marine electromagnetic methods for deep-sea metallic deposit exploration involved towing cables with current transmitters and receivers. While this allowed for mapping massive sulfides on the seafloor (mound-type seafloor massive sulfides), it was difficult to detect anomalies buried beneath the seafloor (embedded seafloor massive sulfides).

In this paper, we developed a system that combines the aforementioned towed system with multiple seafloor-based electric and magnetic field receivers. We demonstrated its effectiveness for simultaneously exploring embedded anomalies through numerical simulations and real-world data. Since embedded seafloor massive sulfides are highly promising as resources but lacked established exploration methods, the development of this technique represents a breakthrough in deep-sea metallic deposit exploration. Applying this method to various hydrothermal fields in the future may lead to more accurate resource assessments.

Successful 3D Subsurface Mapping of Seafloor Massive Sulfides (Ishizu et al., 2022, Geophysics)

Sat, 01 Jan 2022 00:00:00 +0000

Ishizu, K. Siripunvaraporn, W., Goto, T. N., Koike, K., Kasaya, T., & Iwamoto, H. (2022). A cost-effective three-dimensional marine controlled-source electromagnetic survey: exploring seafloor massive sulfides. Geophysics, 87(4), E219–E241.

Point 1: Successful 3D subsurface mapping of seafloor massive sulfides

Point 2 (Most Important): Proposal of a new 3D marine electromagnetic exploration technology that reduces receivers and costs while maintaining performance equivalent to conventional methods

Marine electromagnetic exploration is effective for estimating the 3D subsurface distribution of seafloor massive sulfides. However, existing methods require a large number of receivers to obtain 3D information, leading to high survey costs. In this paper, we proposed a new 3D marine electromagnetic technology that reduces the number of receivers and survey costs while maintaining performance equivalent to conventional methods. The proposed method is very simple, involving the placement of a single line of receivers in the center of the survey track. We first demonstrated the effectiveness of this method using a synthetic model and data. We then applied the proposed method to explore the Ieyama hydrothermal field in the Okinawa Trough and estimated the 3D subsurface distribution of seafloor massive sulfides, successfully identifying areas likely to be deposit bodies.

Successful Visualization of a Supercritical Geothermal Reservoir (Ishizu et al., 2022, JGR)

Sat, 01 Jan 2022 00:00:00 +0000

Ishizu, K. Ogawa, Y., Nunohara, K., Tsuchiya, N., Ichiki, M., Hase, H., et al. (2022). Estimation of spatial distribution and fluid fraction of a potential supercritical geothermal reservoir by magnetotelluric data: A case study from Yuzawa geothermal field, NE Japan. Journal of Geophysical Research: Solid Earth, 127, e2021JB022911.

Point 1: Successful visualization of a supercritical geothermal reservoir

Point 2 (Most Important): Discovery of a silica cap layer above the supercritical geothermal reservoir and its role in reservoir formation

This research relates to new technologies that can contribute to reducing carbon dioxide emissions. Using supercritical geothermal reservoirs for power generation is expected to enable higher power output compared to conventional hydrothermal reservoirs (below 350°C). However, the development mechanisms and spatial distribution of these reservoirs were previously unknown. In this study, we used electromagnetic exploration to visualize the spatial distribution of a supercritical geothermal reservoir and elucidated its development mechanism by incorporating temperature information from drilling. The mechanism suggests that as magma below the reservoir solidifies, it releases fluids, providing supercritical geothermal fluids to the area above. These fluids are restricted from further upward migration by a silica cap layer, resulting in the accumulation of supercritical fluids and the development of the reservoir below the cap. We believe this is the first study to quantitatively clarify the relationship between the silica cap layer and the supercritical geothermal reservoir suggested by geophysical exploration using silica solubility calculations.

Press release articles can be read here:

Successful Visualization of the Two-Story Distribution of Seafloor Massive Sulfides (Ishizu et al., 2019, GRL)

Tue, 01 Jan 2019 00:00:00 +0000

Ishizu, K. Goto, T., Ohta, Y., Kasaya, T., Iwamoto, H., Vachiratienchai, C., Siripunvaraporn, W., Tsuji, T., Kumagai H. and Koike K. (2019). Internal structure of a seafloor massive sulfide deposit by electrical resistivity tomography, Okinawa Trough. Geophysical Research Letters, 46(20), 11025-11034.

Point 1: Successful visualization of the “two-story” distribution (upper and lower deposits) of seafloor massive sulfides

Point 2 (Most Important): Elucidation of a new development mechanism for lower deposits

Seafloor massive sulfides (SMS) are next-generation metallic resources containing rare metals and precious metals. These deposits are confirmed to exist in areas with hydrothermal circulation on the seafloor. In Japan, SMS deposits have been discovered in the Okinawa Trough and the Izu-Ogasawara regions. However, their subsurface distribution and development mechanisms were previously unknown. This paper details the visualization of SMS distribution in the Iheya hydrothermal field of the Okinawa Trough using a marine electrical resistivity tomography system where both the transmitter and receivers are towed on a cable. Marine electrical exploration works like a dowsing machine by injecting electricity into the seafloor and estimating areas with high electrical conductivity (i.e., where metallic deposits exist). Put simply, it’s like a treasure-hunting machine that allows us to obtain information below the seafloor without drilling.

The marine electrical exploration device used in this study utilized 10 receivers on the cable to significantly increase the amount of data collected (conventional devices typically use 5 or fewer). This increased data volume allowed for detailed subsurface imaging down to about 60m. From this detailed map, we revealed that SMS deposits exist not only on the seafloor (upper deposit) but also at a depth of about 30m (lower deposit), a highly novel finding. By integrating this detailed subsurface map with drilling data, we proposed a new development mechanism for the lower deposit: hydrothermal fluids are cooled below a cap layer, causing metallic components to precipitate and form the deposit beneath the cap. This is the most significant novel point of this paper.

Press release articles can be read here: