[S-EM12] Electric, magnetic, and electromagnetic survey technologies and the scientific achievements

[SEM12-04] Offshore–onshore seamless resistivity imaging of freshwater using a controlled source electromagnetic method

*Keiichi Ishizu¹, Yasuo Ogawa¹ (1.Tokyo Institute of Technology)

Keywords:Resistivity, Electromagnetic survey, Coastal areas

Coastal freshwater provides a water source for more than one billion people living in coastal regions. For sustainable groundwater management in coastal areas, an understanding of the freshwater distribution is necessary. The freshwater distribution in a coastal area can be across the landward and seaward sides. Offshore–onshore seamless mapping of freshwater helps us to gain a comprehensive understanding of the freshwater distribution in coastal areas. Resistivity imaging using electromagnetic methods has been used to reveal the freshwater distribution in coastal areas because resistivity provides information on porosity and porewater salinity. We used a controlled source electromagnetic (CSEM) method for offshore–onshore seamless resistivity imaging of freshwater at a depth range of 0–500 m below the seafloor. Our CSEM method consists of onshore transmitters and onshore–offshore receivers. We conducted a feasibility study to investigate the ability of the CSEM method for offshore–onshore seamless resistivity imaging of freshwater in a coastal area. We found that the method could image the resistivity distribution of freshwater located at a depth of 500 m below the seafloor. The test results also showed that the detectability of freshwater was high enough at all sea depths between 10 m and 100 m, which is a typical range in coastal areas. These numerical test results imply that the proposed CSEM method is a promising technique for offshore–onshore seamless resistivity imaging of freshwater in coastal areas.