CSEM | 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.

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.