Offshore-onshore resistivity imaging of freshwater using a controlled-source electromagnetic method: A feasibility study

Abstract

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 freshwater distribution is necessary. Freshwater distribution in a coastal area can extend across the shoreline and into the offshore region. Offshore-onshore 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 have considered a controlled-source electromagnetic (CSEM) method for offshore-onshore resistivity imaging of freshwater at a depth range of 0–500 m below the seafloor. Our CSEM method is novel in considering an array of onshore-offshore electromagnetic receivers with onshore electric dipole transmitters. We have conducted a feasibility study to investigate the ability of the CSEM method for offshore-onshore resistivity imaging of freshwater in a coastal area. The test results indicate that the method could image the resistivity distribution of freshwater located at a depth of 500 m below the seafloor. Our model study also indicates that the offshore-onshore CSEM method can detect offshore aquifers up to 5 km from the shoreline. These numerical test results imply that our CSEM method is a promising technique for offshore-onshore resistivity imaging of freshwater in coastal areas.