A feasibility study of offshoreonshore resistivity imaging of freshwater using a controlled source electromagnetic method

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 extend across the shoreline and into the offshore region. Offshoreonshore 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 electrical resistivity in these settings is primarily controlled by porosity and porewater salinity. We consider a controlled source electromagnetic (CSEM) method for offshoreonshore resistivity imaging of freshwater at a depth range of 0500 m below the seafloor. Our CSEM method is novel in considering an array of onshoreoffshore electromagnetic receivers with onshore electric dipole transmitters. We conducted a feasibility study to investigate the ability of the CSEM method for offshoreonshore resistivity imaging of freshwater in a coastal area. The test results showed that the method could image the resistivity distribution of freshwater located at a depth of 500 m below the seafloor. Our model study also showed the offshoreonshore CSEM method can detect offshore aquifers up to 5 km from the shoreline. These numerical test results imply that the proposed CSEM method is a promising technique for offshoreonshore resistivity imaging of freshwater in coastal areas.