UC San Diego

This dissertation presents research exploring the application of marine electromagnetic methods toward studying the nearshore continental shelf. The shallow-water controlled-source electromagnetic (CSEM) method has been shown to be a useful tool to study continental shelves. This marine CSEM method uses a man-made source of EM energy that passes through seawater and propagates into the seafloor and to towed-receivers which measure the resulting electric fields. These fields are processed into amplitude and phase data and then inverted to image subseafloor electrical resistivity. Electrical resistivity, while not a unique identifier, can be indicative of porosity and pore fluids, mineral chemistry, melt, and temperature. Thus, the CSEM method is well-suited to identify and characterize a variety of features and systems, both anthropogenic and naturally occurring, within continental shelves. Offshore San Diego, surface-towed CSEM data were collected to detect the possible offshore extent of the county’s onshore aquifer. Little was known of the offshore character of the aquifer, making it vulnerable to over-extraction and saltwater intrusion. Thus, this survey mapped pore-fluid salinity and groundwater pathways offshore to better constrain the freshwater-bearing formation. The results mapped a previously unidentified aquifer extending offshore San Diego which contains considerable volumes of fresh-to-brackish water, doubling the known groundwater volume of the county, in both continuous lenses and isolated pockets that appear influenced by fault systems and shallow stratigraphy. Near Santa Barbara, California, a surface-towed CSEM survey was used to target marine hydrocarbon seeps (MHS) within Coal Oil Point seep field (COP) at intermediate depths (<400 m). The results show significant spatial variability of MHS within COP and indicate at least two previously unidentified subseafloor accumulation sites. The depth and lateral extent of these accumulation sites could constrain overall seep-emission models for COP. From these studies, it became evident that the resolution and sensitivity of marine CSEM systems should be formally tested. Thus, rigorous and practical resolution and sensitivity studies were conducted to better constrain the depth of inference for several CSEM systems. The results from these tests indicate that the depth of inference for CSEM systems is deeper than previously thought. Finally, motivated by the search for archeological sites submerged offshore, a new CSEM system capable of detecting subtle and small targets in culturally and biologically sensitive regions was developed. Initial inversions from first deployments of the new system offshore the northern Channel Islands, California show significant improvement in resolution when compared to surface-towed CSEM systems.