We present observations from two field deployments of a calibrated tiltmeter that we name the Self-Calibrating Tilt Accelerometer (SCTA). The tiltmeter is based upon a triaxial quartz crystal accelerometer; the horizontal channels measure tilt and are periodically rotated into the vertical to obtain a measurement of the acceleration of gravity. Changes in the measured total acceleration are ascribed to drift in the vertical channel and used as calibrations for removing that same drift from the tilt time series observed between calibrations. Changes in the span (sensitivity) of the accelerometer channels can also be measured by calibrating them pointing up and down. A 3-year test on the seafloor at Axial Seamount show that the calibrations are consistent with a linear-exponential model of drift to a RMS residual of ~0.5 μg (#&956;rad). The calibrated tilt time series was impacted by platform settling for the first 2 years, but after repositioning the tiltmeter, the calibrated observations were consistent for the final year with the tilt observed on a nearby LILY tiltmeter, within an assumed level of drift for the unconstrained LILY sensor. A separate 15-month test in a stable vault at Piñon Flat Observatory was complicated by seasonal temperature variations of >5°C; the calibrations are consistent with a linear-exponential model of drift to ~2 μg RMS when temperature and temperature time-derivative dependence is included. Similarly, the calibrated tilt time series was impacted by thermal deformation of the SCTA assembly. A future test in a thermally and tectonically stable borehole will be required to assess the accuracy of the SCTA.

Approximately six years of underwater noise data recorded from the Regional Cabled Array network are examined to study long-term trends. The data originate from station HYS14 located 87 km offshore of Newport, OR. The results indicate that the third-octave band level centered at 63 Hz and attributable to shipping activity is reduced in the spring of 2020 by about 1.6 dB relative to the mean of the prior five years, owing to the reduced economic activity initiated by the COVID-19 pandemic. The results are subtle, as the noise reduction is less than the typical seasonal fluctuation associated with warming ocean surface temperatures in the summer that reduces mode excitation support at typical ship source depths, causing a repeated annual level change on the order of 4 dB at shipping frequencies. Seasonality of the noise contribution near 20 Hz from fin whales is also discussed. Corroboration of a COVID-19 effect on shipping noise is offered by an analysis of automatic identification system shipping data and shipping container activity for Puget Sound, over the same six-year period, which shows a reduction in the second quarter of 2020 by ~19% and ~17%, respectively, relative to the mean of the prior five years.

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.