Inversion Analysis of Time-Domain Controlled-Source Electromagnetic Data to Characterize Offshore Freshened Groundwater in the Canterbury Bight, New Zealand, and Offshore Maltese Islands

Offshore freshened groundwater (OFG) refers to low-salinity water stored in marine sediments beneath the seafloor. These water resources are gaining attention due to their potential role in future freshwater supply and their connection to past sea-level and climate changes. Marine controlled-source electromagnetic (CSEM) methods are effective in delineating the spatial extent of OFG bodies by using electrical resistivity as a proxy. Integrating resistivity models with subsurface properties, such as host-rock porosity derived from in situ measurements or seismic data, enables estimates of pore-water salinity, while correlation with seismic reflection profiles allows broader geological interpretation. However, deterministic inversion approaches remain limited in their ability to quantify pore-water salinity and associated uncertainties. This thesis investigates the capability of time-domain (TD) CSEM methods to characterize OFG systems across different geological settings. Deterministic and probabilistic inversion approaches are applied and integrated with seismic and borehole data to resolve resistivity structures associated with OFG and assess pore-water salinity and its uncertainties. Two regional case studies are examined: Canterbury Bight (New Zealand), representing a siliciclastic shelf environment, and offshore Gozo (Maltese Islands), located on a carbonate platform setting. In Canterbury Bight, Bayesian inversion of CSEM data reveals a low-salinity OFG body within silty and sandy sediments located approximately 20–60 km offshore, with pore-water salinity distributions indicating a freshening trend toward the coast within a permeable sand layer 40–150 m below the seafloor. In offshore Gozo, inversion results integrated with seismic reflection data reveal resistive anomalies within the Lower Coralline Limestone formation at 210–250 m below sea level and beginning at 350–400 mbsl.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Christian-Albrechts-Universität zu Kiel

Year

2025

Author dc:creator

• Faghih, Zahra

Contributors dc:contributor

• Berndt, Christian
• Bücker, Matthias

• Controlled-source electromagnetic (CSEM)
• Offshore groundwater (OFG)
• Trans–dimensional Bayesian inversion
• Probability density distribution
• Pore-water salinity estimation

Repository record source_url

https://macau.uni-kiel.de/receive/macau_mods_00008080

OAI identifier oai:identifier

oai:macau.uni-kiel.de:macau_mods_00008080