Ph.D. Research Seminar: Electromagnetic Characterization of Freshened Offshore Aquifers on the Formerly Glaciated Continental Shelf of Prince Edward Island
Fernando Córdoba Ramírez
Ph.D. Candidate
Department of Earth and Environmental Sciences
Dalhousie University
Title: Electromagnetic Characterization of Freshened Offshore Aquifers on the Formerly Glaciated Continental Shelf of Prince Edward Island
Abstract: Freshwater scarcity is an increasing problem humanity has to face. Presently, 1.42 billion people live in areas of high or extremely high water vulnerability. By 2050, three out of four people worldwide could face drought impacts. In this panorama, freshened offshore aquifers (FOAs) have emerged as an important unconventional groundwater resource in coastal areas. FOAs consist of freshened water (<33 PSU) hosted in unconsolidated sediments or rocks in the seafloor within ~50 km of the coast. However, questions regarding the geometrical, physical, and lithological characteristics of FOAs are still the subject of study. This research hypothesizes that ice sheet dynamics and landscape evolution during the Last Glacial Maximum contributed to formation and/or recharging of FOAs in highly fractured consolidated sandstones on the continental shelf of Prince Edward Island (PEI), Canada. To test this hypothesis, we conducted a Time-Domain Controlled-Source Electromagnetic (TD-CSEM) survey in the Gulf of Saint Lawrence offshore PEI. TD-CSEM is the most effective tool for mapping FOAs, as it quantifies the electrical resistivity of materials, a key indicator influenced by porosity, permeability, pore fluid temperature, and salinity. To accurately assess the electrical resistivity of the seafloor and evaluate the presence of FOAs, precise knowledge of the geometry between the transmitter and receiver is necessary. For my first research chapter, I tested commonly used methods to quantify the orientation of receivers placed on the seafloor measuring the electromagnetic fields. I then proposed a new method that provides accurate orientation estimates with uncertainties as low as 5°. My second research chapter is focused on solving optimization problems using the TD-CSEM data to compute 1D and 2D Earth models, providing the bulk electric resistivity distribution of the seafloor and hence the salinity of the pore fluid. I identified high resistivity anomalies (>20-ohm m) at a depth of about 70 m below the sea bottom, at a location near gravity cores that exhibit freshened water (~25 PSU). My third research chapter concerns the integration of electrical resistivity models with other available information. This includes constraining 2D electrical models with seismic reflection profiles, the integration with shallow dome-shaped features from parasound data, and the comparison with the onshore-offshore groundwater flow models. This approach will allow us to estimate the geometry of FOAs and identify lithologies that are prone to host them offshore PEI.
Biography: Fernando holds a BSc degree in Geophysical Engineering (2015) from the National Autonomous University of Mexico and an MSc degree in Applied Geophysics (2018) from the Ensenada Center for Scientific Research and Higher Education. During his undergraduate studies, he was introduced to the Controlled-Source Electromagnetic (CSEM) method. Impressed by its theoretical and technical complexities, he decided to focus his academic career on this method. His master’s project, in cooperation with the Marine EM Lab at Scripps Institution of Oceanography, focused on the application of the Frequency-Domain CSEM method to evaluate the geothermal potential near the Guaymas Basin in the Gulf of California, Mexico.
Fernando believes that professional experience greatly enhances the geophysicist’s knowledge. Therefore, alongside his academic development, he spent years working in different geophysical exploration companies, applying mainly, but not exclusively, electromagnetic methods. In 2021 he decided to pursue a PhD degree in the Department of Earth and Environmental Sciences at Dalhousie University. His current research, in collaboration with GEOMAR Helmholtz Centre of Ocean Research Kiel, focuses on applying the Time-Domain CSEM method to identify freshened offshore aquifers in this overpopulated planet.
Time
Location
Milligan Room, 8th Floor Biology-Earth Sciences Wing, Life Sciences Centre, Dalhousie University