UC San Diego

The methods and tools available to make geodetic measurements offshore are still limited but increasingly important for studies of underwater subduction zones, volcanoes, and more. One method is to use ocean bottom pressure sensors, which can be used to measure vertical seafloor motion. While most pressure sensors inherently drift at rates that can exceed tectonic signals, some drift correction methods have been developed.

Self-calibrating pressure recorders (SCPR) correct sensor drift using a piston-gauge calibrator (PGC) that produces a reference pressure whose value can be determined with a high level of accuracy. Changes in the reference pressure observed by the pressure sensors are attributed to sensor drift and are removed from the seafloor pressure. The resulting drift-free pressure record can be used to infer seafloor height. We successfully demonstrated an SCPR at Axial Seamount between 2013 and 2015, and later connected it to real-time cabled infrastructure in 2018.

We also used an SCPR in campaign-style surveys to make absolute pressure measurements on seafloor benchmarks. In this absolute SCPR (ASCPR) we make accurate determinations of the parameters that affect the reference pressure to determine its absolute value. We use the true, absolute reference pressure and the difference between the observed reference and seafloor pressures to determine the absolute seafloor pressure. The measurements provide instrument-independent, fiducial values for calibrating nearby sensors or as individual points in long-term time series. We developed the campaign calibrated pressure measurements and conducted four surveys between 2014 and 2017 in the Cascadia subduction zone to measure the vertical deformation occurring offshore. The results serve as initial reference values that will be useful far into the future for marine geodesy and future surveys could be used to estimate the secular vertical deformation rate.

I investigated additional methods, models, and avenues related to seafloor pressure measurements and marine geodesy. I computed elastic half-space boundary element models for scenarios we may expect to see in Cascadia, which highlight the need for geodetic measurements offshore. I also conducted tests of structure-from-motion photogrammetry as a cost- and time-effective method to monitor small changes within geodetically instrumented seafloor sites. Finally, I explored ocean models and satellite data as potential solutions to the oceanographic noise problem present in many marine geophysical measurements.