Figure 20. Acceleration power spectrum for September 7, 2017.

Figure 16. Records for the September 7, 2017 (local time) Chiapas, Mexicoearthquake.

Figure 15. Records for the September 3, 2017 North Korea nuclear test.Figure 14. Records for the July 6, 2017 Lincoln, Montana earthquake.

Figure 17. Accelerations for July 27, 2017 collected with a low-pass InfiniteImpulse Response (IIR) counting filter corner frequency set to 0.5 Hz.Horizontal accelerations show variations in the tilt of the instrumentpackage of up to 0.001° due to ocean currents.

Figure 18. Acceleration power spectrum for July 27, 2017. Figure 19. Accelerations for September 7, 2017 with a IIR counting filtercorner frequency set to 8 Hz. “Fish-bumps“ or “Bio-bumps” dominate thishigher frequency data

Earthquake Records and Accelerometer Data

Figure 1. Geodetic and Seismic Sensor Module(GSSM) comprising the following suite of Digiquartz®sensors: two pressure sensors, a barometer, and a 3-component accelerometer. A valve, actuated fromshore, periodically switches the pressure sensorsbetween ambient seawater pressure and internalhousing pressure of ~1 atmospher in order to correctfor long-term drift through comparison with thebarometer. This process is termed “A-0-A”.

Figure 2. Remotely operated vehicle (ROV),Ventana prepares to deploy the GSSM on June13, 2017.

Figure 4. Cabled GSSM in place on the seafloornear the MARS cabled observatory node. Pressureand accelerations are transmitted in real time toshore.

(Left) Figure 3. Two image of the ROV Ventanadeploying the cabled GSSM at a depth of 880m.

AcknowledgementsParoscientific, Inc. provided guidance and engineering support for the effort to add remote control andoperation capabilities to the GSSM.The MARS team at MBARI have provided engineering support throughout the design, testing, deployment andongoing operations of the GSSM.

Deployment

Figure 9. Calibration measurements (left axis) and internal housingtemperature (right axis) plotted against time. The offset in temperature nearthe start of the deployment reflects a change in the operation of the valveactuator from power cycling to permanently on.

Figure 11. Difference in the spans of the two pressure sensors (left axis)and housing internal temperature (right axis) plotted against time.

Figure 7. Example of a 5-minute calibration. The offsets between thepressure sensors and the accurate barometer are measured by averagingdata from 180 s to 240 s into the test.Figure 5. Day of pressure data that includes an A-0-A calibration plotted at

full scale to show the calibration (left axis) and magnified to show tides (rightaxis) The inset figure shows 10 minutes of data in which infragravity wavesand microseisms are clearly visible.

Figure 6. One day of pressure data that does not include an A-0-Acalibration. The difference between the two pressure sensor readingsremains constant over the day to ~0.1 hPa (1 mm)

Figure 8. All calibrations for pressure sensor 1 color coded by the internaltemperature of the housing. The differences in calibration pressures reflectdifferences in temperature and sensor drift.

Figure 10. Calibration measurements after removal of a linear temperaturecorrection and then fit by curves of the form ∆p = C + Bt + A exp(-t/t0) wheret is time and C, B, A and t0 are constants chosen to optimize the fit. TheRMS misfit for each pressure sensor is ~0.1 hPa (1 mm of water).

Figure 12. Difference in the spans after applying a linear correction fortemperature. This plot shows that at the times of the calibrations, the twocalibrated external pressures are consistent to a standard deviation of 0.01hPa (0.1 mm of water).

Figure 13. External pressure just before each calibration (left axes) and thedifferences in external and internal pressure readings for the two sensors(right axes). The remarkable similarity between the latter two curves is aresult of the constant span.

A-0-A Pressure Calibrations

KeyFigures

Abstract. Seafloor geodetic observations are critical forunderstanding the locking and slip of the megathrust in Cascadia andother subduction zones. Differences of bottom pressure time series havebeen used successfully in several subduction zones to detect slow-slipearthquakes centered offshore. Pressure sensor drift rates are muchgreater than the long-term rates of strain build-up and thus, in-situcalibration is required to measure secular strain. One approach tocalibration is to use a dead-weight tester, a laboratory apparatus thatproduces an accurate reference pressure, to calibrate a pressure sensordeployed on the seafloor by periodically switching between the externalpressure and the deadweight tester (Cook et al, this session). The A-0-Amethod replaces the dead weight tester by using the internal pressure ofthe instrument housing as the reference pressure. We report on the firstnon-proprietary ocean test of this approach on the MARS cabledobservatory at a depth of 900 m depth in Monterey Bay. We use theParoscientific Seismic + Oceanic Sensors (SOS) module that is designedfor combined geodetic, oceanographic and seismic observations. Themodule comprises a three-component broadband accelerometer, twopressure sensors that for this deployment measure ocean pressures, A,up to 2000 psia (14 MPa), and a barometer to measure the internalhousing reference pressure, 0. A valve periodically switches betweenexternal and internal pressures for 5 minute calibrations. The seafloortest started in mid-June and the results of >70 calibrations collected overthe first 6 months of operation are extremely encouraging. Aftercorrecting for variations in the internal temperature of the housing, theoffset of the pressure sensors from the barometer reading as a functionof time, can be fit with a smooth curve for each sensor with a RMS misfitof ~0.1 hPa (1 mm of water). A comparison of the external pressures justprior to each calibration with the pressures recorded during thecalibration, show the difference in span of the two pressure sensorsremains constant to ~0.01 hPa (0.1 mm of water) after applying a lineartemperature correction. We present the A0A calibrations results and alsoshow examples of data from the accelerometer.

A Seafloor Test of the A-0-A Approach to Calibrating Pressure Sensors for Vertical GeodesyWilliam S. D. Wilcock1, Dana Manalang2, Michael Harrington2, Geoff Cram2, James Tilley2, Justin Burnett2, Derek Martin2, and Jerome M. Paros31School of Oceanography, University of Washington, Seattle WA 98195, 2Applied Physics Laboratory, University of Washington, Seattle WA 98195, 3Paroscientific, Inc. & Quartz Seismic Sensors, Inc., Redmond, WA 98052

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