Developing a new calibrated pressure sensor for the Ocean Observatories Initiative Cabled Array and other applications

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Measurements of seafloor pressure are useful for understanding the earth and the ocean because they are sensitive to changes in the height of the seafloor and the weight of the overlying water and atmosphere. The best pressure sensors drift unpredictably so their output cannot be used to resolve long terms signals such as the steady buildup of strain between subduction zone earthquakes and long term changes in the mass of the oceans. Drift also makes existing pressure sensors less sensitive to shorter-term signals such as those from slow-slip earthquakes and ocean eddies. A new method has been developed to remove drift using the internal pressure inside the instrument housing as a reference and has been tested successfully with a seafloor prototype. This project will redesign the prototype so that the instrument is more robust, better calibrated, more compact and lower power. It will be attached to the Ocean Observatories Initiative Cabled Array on Axial Seamount, a volcano that is inflating rapidly following an eruption in 2015. The engineering effort will be led by a female engineer in a field where women are underrepresented and the project will train a graduate student in a technical project where they will learn to work with both engineers and scientists. The cruises that service and deploy instruments on the Ocean Observatories Initiative Cabled Array incorporate a large number of undergraduate students, and one will be engaged to conduct a project related to this instrument.The A-0-A method is a new approach to calibrating pressure sensors to remove drift. Periodically, a valve is used to switch the pressure sensor for a short interval from measuring the pressure in the ocean to measuring the pressure inside the instrument housing. The drift can then be determined by comparing the pressure readings to an accurate barometer. The A-0-A method has the potential to substantially reduce the drift of long-term seafloor pressure records in a much more economical way than current approaches. The first non-proprietary seafloor tests at 900 m in Monterey Bay on the MARS cabled observatory suggests that A-0-A calibrations may reduce pressure sensor drift to as little as 1-2 mm/year in 1000 m of water. However, design improvements are required to remove known sources of error and a side-by-side seafloor test is essential to verify the technique against established methods of drift removal. The first deployment of the redesigned instrument on Axial Seamount will allow comparisons with the absolute pressure measurements of the Scripps Institution of Oceanography Self-Calibrating Pressure Recorder (an instrument that uses a dead weight tester for periodic calibrations) and the relative pressure measurements obtained from repeat closed-loop mobile pressure recorder surveys, both of which have an uncertainty of ~1 cm of water. The cabled sensor can then be used for future vertical geodetic studies of the Cascadia subduction zone megathrust aimed at understanding patterns of fault locking, creep and slow slip.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.