Using marine controlled-source electromagnetics to study potential offshore groundwater : Canterbury Basin, New Zealand

Abstract

Continental shelves are the submarine edges of the continental crust and in some areas have been shown to contain freshwater aquifers beneath the seafloor. During the Quaternary, continental shelves experienced maximum exposure to meteoric water during the Last Glacial Maximum. Offshore groundwater systems are slowly adjusting toward a new equilibrium in present-day conditions as a result of rising sea-level. Information regarding the location, characteristics and dynamics of offshore groundwater systems and whether they can be used sustainably as a potential source of drinking water may be obtained by applying a range of geophysical techniques including marine controlled-source electromagnetics (CSEM) and multichannel seismics. However, the methods applied to map the extent, volume and connectivity of offshore aquifers are still in the experimental phase. One promising geophysical tool to study offshore groundwater is the CSEM method. CSEM is capable of mapping the spatial variations of saline and fresh water zones in the sub-seafloor. Currently, there are only a few studies that have successfully applied marine CSEM to quantify offshore groundwater aquifers. This study constitutes a new attempt in using CSEM to detect the offshore groundwater at large spatial scales (around 100 km).