Application of electromagnetic exploration by CSMT method to clarify the mechanism of huge landslides

2023 Fiscal Year Final Research Report

• Project/Area Number: 22K18867
• Research Category: Grant-in-Aid for Challenging Research (Exploratory)
• Allocation Type: Multi-year Fund
• Review Section: Medium-sized Section 25:Social systems engineering, safety engineering, disaster prevention engineering, and related fields
• Research Institution: Niigata University
• Principal Investigator: Watanabe Naoki 新潟大学, 災害・復興科学研究所, 准教授 (60282977)
• Project Period (FY): 2022-06-30 – 2024-03-31
• Keywords: 巨大地すべり / 高塩分濃度地下水 / CSMT法電磁探査 / 異常高圧貯留層 / 水文地質構造 / 異常間隙水圧

Outline of Final Research Achievements

The Okimi landslide, located in Joetsu City, Niigata Prefecture, Japan, is about 500 m wide and 1,500 m long, with an area of about 70 ha. The bedrock of the landslide is Miocene mudstone, and saline groundwater has been detected in observation wells around the landslide head. The CSMT survey visualized a continuous ultra-low resistivity zone (<2 Ωm) at 30 to 330 m below ground level at observation points A2 to A3 and B2 to B3, and also in the surrounding area with a thickness of about 50 m. In addition, the survey realized a saline groundwater reservoir as a saline chamber in the western part of the landslide at 100 to 300 m below ground level. We could identify that a saline water plume rising from this saline chamber toward the bottom of the landslide.

Free Research Field

水文地質学

Academic Significance and Societal Importance of the Research Achievements

新潟県の大規模～巨大地すべりでは、地球化学的に地下深部の化石海水起源とされる高濃度NaCl型地下水(以下、塩水)の存在が報告されている。これらは、石油探査で知られている地下深部の異常高圧貯留層に由来する。これまで定説のなかった大規模～巨大地すべりの発生機構および特有の水理・水文地質構造を解明するため、新潟県上越市の沖見地すべりを研究対象としてCSMT法電磁探査を行い、地下の塩水の貯留層を検出した。さらに、地すべりの底部に向かって湧昇する塩水プルームを確認した。異常高圧貯留層からの塩水の上昇は巨大地すべり発生の主因になり得るとする仮説を裏付ける証拠となった。