the Self-Calibrating Tilt Accelerometer: a method for observing tilt and correcting drift with a triaxial accelerometer

These data were acquired from a novel calibrated tiltmeter design that we name the Self-Calibrating Tilt Accelerometer (SCTA), deployed in a vault at the Scripps Institution of Oceanography's Cecil and Ida Green Pinon Flats Observatory in southern California (33.611 N, 116.455 W). The SCTA uses a Quartz Seismic Sensors, Inc. triaxial accelerometer with a full scale of ± 3g and a resolution of parts per billion. The accelerometer works by measuring changes in the oscillation frequency of three isolated quartz crystal resonators as they are strained by accelerations in orthogonal directions. The oscillation frequency of a fourth unstrained quartz resonator measures temperature and is used to provide temperature corrections to the frequency outputs of the accelerometers. Such quartz crystal resonators are highly sensitive to changes in load, but are well known to undergo significant instrumental drift. The SCTA is designed to be capable of measuring this drift. Any of the accelerometer's channels in the vertical measure the acceleration due to gravity, g, while any in the horizontal are highly sensitive to changes in tilt. We use this fact to obtain tilt observations in perpendicular horizontal directions and periodically rotate those channels into the vertical to obtain calibrations against g. Any change in the measured value of g through time is attributed to drift of that channel and the tilt time series can be corrected accordingly.

Methods
Data were collected at 40 Hz, from October 18, 2018 – January 16, 2020. There are 5 principal data fields: X acceleration, Y acceleration, Z acceleration, total acceleration (root-sum-square), and temperature. The instrument was deployed with the X-channel aligned with north, the Y-channel aligned with west, and the Z-channel aligned with vertical up. Periodically, the instrument was rotated such that X and Y were temporarily aligned with the vertical. Prior to August 9, 2019, the rotation sequence was +X, +Y, then +X again in the vertical, before returning to +Z in the vertical. From August 9, 2019 onward, the rotation sequence was updated to instead be +X, +Y, -Y, +X, then -X in the vertical, before returning to +Z vertical. There were five data outages: one from February 14 – March 28, 2019 that was the result of a failure of the local computer (during this interval the instrument continued to run but no calibrations were performed), and shorter gaps from March 31 – April 3, 2019, April 27 – 29, 2019, September 8, 2019, and December 25 – 29, 2019 that were each the result of local power outages at the site.