OOI-RCA 2024

Axial Acoustic Array

Axial Volcano is one of the most extensively studied sites on the global network of mid-ocean ridges. Three eruptions have been observed in 1998, 2011 and 2015 with a near continuous 20-year record of vertical geodesy from bottom pressure sensors in the caldera recording cycles of inflation and rapid syn-eruptive deflation. Axial volcano has also been the focus of several seismic imaging studies including a recent three-dimensional seismic reflection experiment that provide a remarkably detailed view of the magmatic system. In comparison, the geodetic models of inflation are limited in complexity and resolution by the small number of geodetic sites and the difficulty distinguishing distributed inflation from fault motion on the buried outward dipping ring faults using vertical geodesy alone. Here we propose to install a 10-year 4 station acoustic network at Axial that range between each pair of transponders and include a pressure gauge, temperature, salinity and velocimetry sensors. One station will be connected to the OOI Regional Cabled Array enabling real time acquisition of data. The primary scientific objective is to measure horizontal extension rates across and within the caldera that can be combined with the other geodetic observations to (1) discriminate between distributed inflation and motion on the buried outward dipping faults beneath the east and west caldera walls and thus, understand the pattern of seismic coupling on these faults throughout the eruptive cycle; (2) contribute data that can enable more complex models of inflation and (3) obtain measurement of the dike with for dikes that propagate across the acoustic baseline. A secondary objective is to provide an acoustic network that can support future efforts to demonstrate and optimize remote communication and navigation of autonomous underwater vehicles. _____ The A-0-A method is a relatively 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 deployment of a 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), the Sonardyne AZA calibrated pressure recorder on a cabled FETCH unit and the relative pressure measurements obtained from repeat closed-loop mobile pressure recorder surveys, all of which have an uncertainty of ~1 cm of water. The cabled sensor can then be used for future vertical geodetic studies elsewhere, such as those in subduction zones aimed at understanding patterns of fault locking, creep and slow slip.