| Summary: | More than 70% of Earth's surface is ocean covered. This generally creates challenges for geophysicists largely because it is much easier to set up observatories on land than on the seafloor. However, seawater's electromagnetic signals can be utilized by geophysicists to study the solid Earth, the oceans themselves, and natural hazards such as tsunamis. Earth's salty oceans produce electromagnetic fields because seawater is an electrically conducting fluid with a mean electrical conductivity of 3–4 S/m traveling through Earth's main magnetic field (~50,000 nT). This thesis focuses on using Earth's oceanic electromagnetic signals for electromagnetic sensing, monitoring ocean circulation, and improving natural hazard warning systems. Magnetic signals due to ocean tides may be used to probe the electrical conductivity of Earth's interior. Electrical conductivity is sensitive to both temperature and chemical composition, thus such studies provide important data complementary to seismic studies of Earth's interior. I explore this application of oceanic electromagnetic tides, guided by the question of how much ionospheric signals may impede this application. I compare semi-diurnal lunar tidal (M2) magnetic signals estimated from 64 global observatories to physics-based forward models of the ionospheric M2 magnetic field and the oceanic M2 magnetic field. Overall, I find that the agreement between the physics-based model predictions and the observations is very encouraging for electromagnetic sensing applications, especially since the predicted ionospheric vertical component is very weak. Additionally, the agreement between the physics-based model and the observations constitutes a primary validation of the physics-based model used. Unlike ocean tides, humanity's understanding of ocean circulation mostly relies on near-surface data from buoys and satellite altimetry. Indeed, measuring depth-integrated ocean flow is very challenging, but the oceans transport a huge quantity of heat, so monitoring ocean transport is very important for understanding both our current ocean, as well as how the oceans will change with global warming. I address the question of whether seafloor voltage cables (such as those originally used for telecommunication purposes) can provide an additional data source of ocean circulation's transport. I process the cable data to isolate the seasonal and monthly variations, and evaluate the correlation between the processed data and numerical predictions of the electric field induced by ocean circulation. I find that the correlation between cable voltage data and numerical predictions strongly depends on both the strength and coherence of the transport flowing across the cable, as well as the length of the cable. The results are encouraging for using intentionally placed seafloor voltage cables to monitor oceanic transport in challenging areas such as the Antarctic Circumpolar Current. Tsunamis are a marine natural hazard caused by solid Earth processes such as earthquakes. Their velocities may be constrained from seismic or geodetic studies of the earthquake's rupture zone, and also from magnetic data from a sea floor observatory within the tsunami's travel path. I explore the time-frequency characteristics of tsunamis, motivated by the question of whether tsunami magnetic signals can be used to improve natural hazard warning systems. I develop a method to separate the data's noise from the tsunami signal by using additional data from a remote magnetic observatory. I focus on four Pacic Ocean events of varying tsunami signal amplitude: 1) the 2011 Tohoku, Japan event (M9.0), 2) the 2010 Chile event (M8.8), 3) the 2009 Samoa event (M8.0) and, 4) the 2007 Kuril Islands event (M8.1). I find possible tsunami signals in high-pass filtered data and successfully isolate the signals from noise using a cross-wavelet analysis. The cross-wavelet analysis reveals that the longer period signals precede the stronger, shorter period signals. The results are very encouraging for using tsunami magnetic signals in warning systems.
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