09:00 〜 09:15
[SEM14-01] Mapping submarine groundwater using electromagnetic methods
★Invited Papers
キーワード：groundwater, electromagnetic, salinity, offshore, submarine, magnetotelluric
Submarine freshened groundwater contained in continental shelves is thought to be a significant global phenomenon and could serve as a valuable resource in water-stressed regions. However, in most places, the distribution of offshore groundwater is poorly known due to limited sampling by boreholes and the insensitivity of seismic imaging data to pore water salinity. Electromagnetic (EM) geophysical methods present the opportunity to improve our knowledge of offshore groundwater by remotely mapping its spatial extent and salinity. The conductivity contrast between sediments containing resistive fresh or brackish groundwater and those containing conductive seawater and brines can present a significant anomaly for marine EM techniques to image. Here we look at data collected on the US Atlantic margin as part of a pilot study of using large-scale marine EM methods for mapping offshore groundwater. In this region, boreholes have revealed low salinity groundwater far offshore but are unable to characterize the aquifer’s lateral extent and the deeper groundwater system. Previous analysis of the seafloor magnetotelluric (MT) and short-offset surface-towed controlled source electromagnetic (CSEM) data revealed a low resistivity anomaly consistent with a brackish aquifer system extending up to 90 km offshore and which is underlain by a deeper conductive brine. Here we augment the analysis to include long-offset seafloor CSEM data collected by the same seafloor receivers used for the seafloor MT data. We use a combination of regularized inversion and trans-dimensional Bayesian inversion to compare and contrast the different resolving capabilities of each data type. We also present an analysis of the relative merits of each data type by considering the combination of spatial and depth resolution in conjunction with the efficiency of data collection. Our results can be used to guide and inform future surveys to map offshore groundwater using EM methods.