A Thirty-Month Seafloor Test of the A-0-A Method for Calibrating Pressure Gauges

Abstract

Geodetic observations in the oceans are important for understanding plate tectonics, earthquake cycles and volcanic processes. One approach to seafloor geodesy is the use of seafloor pressure gauges to sense vertical changes in the elevation of the seafloor after correcting for variations in the weight of the overlying oceans and atmosphere. A challenge of using pressure gauges is the tendency for the sensors to drift. The A-0-A method is a new approach for correcting drift. A valve is used to periodically switch, for a short time, the measured pressure from the external ocean to the inside of the instrument housing at atmospheric pressure. The internal pressure reading is compared to an accurate barometer to measure the drift which is assumed to be the same at low and high pressures. We describe a 30-months test of the A-0-A method at 900 m depth on the MARS cabled observatory in Monterey Bay using an instrument that includes two A-0-A calibrated pressure gauges and a three-component accelerometer. Prior to the calibrations, the two pressure sensors drift by 6 and 2 hPa, respectively. After the calibrations, the offsets of the corrected pressure sensors are consistent with each other to within 0.2 hPa. The drift corrected detided external pressure measurements show a 0.5 hPa/yr trend of increasing pressures during the experiment. The measurements are corrected for instrument subsidence based on the changes in tilt measured by the accelerometer, but the trend may include a component of subsidence that did not affect tilt. However, the observed trend of increasing pressure, closely matches that calculated from satellite altimetry and repeat conductivity, temperature and depth casts at a nearby location, and increasing pressures are consistent with the trend expected for the El Niño Southern Oscillation. We infer that the A-0-A drift corrections are accurate to better than one part in 105 per year. Additional long-term tests and comparisons with oceanographic observations and other methods for drift correction will be required to understand if the accuracy the A-0-A drift corrections matches the observed one part in 106 per year consistency between the two pressure sensors.

Geodetic Monitoring at Axial Seamount Since Its 2015 Eruption Reveals a Waning Magma Supply and Tightly Linked Rates of Deformation and Seismicity

“…All the BPR data presented in this paper are either drift‐corrected or did not need correcting. Other more recent approaches to quantifying BPR drift use modified sensors with a known reference pressure to compare with the ambient pressure over time (Cook et al., 2019; Manalang et al., 2019; Sasagawa & Zumberge, 2021; Sasagawa et al., 2016; Wilcock et al., 2021), some of which are being tested at Axial, but we do not employ these methods here. However, these self‐calibrating BPRs could be used as a reference site for MPR measurements in the future.…”

Using Seafloor Geodesy to Detect Vertical Deformation at the Hikurangi Subduction Zone: Insights From Self‐Calibrating Pressure Sensors and Ocean General Circulation Models

“…Our study shows that sensors equipped with self‐calibration allow for effective minimization of sensor drift, as shown in Figure 4, reducing the problem of unknown drift of seafloor sensors—as also demonstrated by Wilcock et al. (2021). Therefore, the range of displacement estimates we find for the regular pressure sensor data based on different drift correction periods (variations of 0.2–5.8 cm, Table S4 in Supporting Information S1) does not exist for the self‐calibrated data.…”

“…The largely drift-free A-0-A observations revealed similarities between the long-period predictions from the OGCMs and the reference site-corrected seafloor pressure data (Figure 7). Using a regional OGCM to reproduce ocean circulation in Monterey Bay, Wilcock et al (2021) also identified long-period pressure variations in self-calibrated pressure data due to ocean variability signals.…”

SMART Subsea Cables for Observing the Earth and Ocean, Mitigating Environmental Hazards, and Supporting the Blue Economy

“…The longest period signal that could be observed would be half the record length, so with the cabled sensors this could be a decade or so. Methods to overcome this with in-situ calibration are in development (Sasagawa and Zumberge, 2013;Wilcock et al, 2021). Shinohara et al (2021) conclude that "Through the evaluation of records of tides and a tsunami, it is estimated that the buried pressure gauge records data with the same quality and amplitude as the pressure gauge on the seafloor.…”

“…The addition of other sensor types, including hydrophones, conductivity sensors, inverted echosounders, as well as acoustic or optical modems capable of relaying data from free swimming sensors, has been considered. A new in-situ calibration method called ambient-zero/internal pressure case-ambient (A-0-A; Wilcock et al, 2021) would approach accuracies required for detecting longer-term (secular) vertical deformation signals, seafloor geodesy, and absolute sea level rise. Given that the SMART repeater will provide a general interface, in principle it should be possible to add these and others once the initial concept has been successfully demonstrated.…”