Hello,
Has anyone noticed or can explain the apparent baseline increase in the “Tidal Seafloor Pressure” data from Low-Power JBox (LJ01B) at the Southern Hydrate Summit 1 Seafloor (OOI Regional Cabled Array – Oregon Margin) around August 10, 2017?
There seems to be a sudden and persistent increase in the pressure baseline during that week, as shown in the attached screenshot.
The weekly mean pressure shifted from ~793.1 dBar to ~793.3 dBar and has remained elevated since then.
Was this change due to a sensor recalibration, a maintenance event, or possibly a real geophysical or oceanographic process?
Any insights or documentation would be appreciated. Thanks!
Hi Wendi,
Thanks for your response. I’d appreciate it if you could clarify a few more details about this maintenance. Specifically:
How many types of instruments in the box were swapped, Did the maintenance involve replacing the seismic sensor?
Was the instrument box relocated?
Could any mechanical activities during the maintenance have significantly altered the surrounding seafloor structure?
The reason I’m asking is that I’ve been using pressure data from the tidal pressure gauge along with the co-located seismic sensor to calculate seafloor compliance. I’ve observed a significant and persistent change in compliance amplitude before and after the maintenance. Similar to the baseline shift in absolute pressure, this change has remained over time.
Interestingly, the coherence between vertical seismic and pressure data remains high and relatively stable, which suggests that the calculated compliance is still reliable. However, I find it unlikely that a change in pressure amplitude alone could account for such a large shift in compliance. Therefore, I’m trying to gather more context to better understand what might be driving this difference.
Thanks again, and I look forward to any additional information you can provide.
Hi Matt,
Thanks for your input. In addition to this tidal pressure gauge, another usable pressure sensor is the pressure component of the OBS station (NSF SAGE: MDA : OO : HYS14 : -- : HDH). However, this sensor is a hydrophone.
I have tried to use it for seafloor compliance calculations, but the results were not very promising-- low coherence between the vertical and pressure component. Based on this, I suspect the hydrophone’s instrument response may not be reliable at low frequencies.
After our discussion at the RCA Data Team office hours call, we pulled a series of images from the 2016 and 2017 summer maintenance cruise ROV dives and plotted up data from the seafloor pressure sensor (PREST), a nearby PI CTD deployed by the German MARUM group, and data from other PREST instruments at the Slope Base (2900 m water depth) and Axial Base (2600 m) sites.
I have collected the findings here (let me know if you are unable to access the slides):
The takeaway points are that the instrument was not moved significantly between deployments, there was no evidence of environmental disturbance or changes in the sediment from 2016 to 2017, but there was a definite shift in internal temperature and pressure between the two instruments. We also spoke to Sea-Bird, and the SBE 54 recovered in 2017 did fail calibration checks when it was returned to the vendor. However it’s not clear from the documentation what caused the electronics board to fail, and the instrument is no longer supported by the vendor.
Regardless, it does appear that the data from 2017 onwards are more reliable, and hold true from deployment to deployment. I will be adding an annotation to the 2016 data to indicate the calibration failure and shift in pressure. We did not see any nonlinear drift in the data, so it’s possible that applying a simple offset could correct the 2016 data to bring them into alignment with the rest of the dataset.
Please let us know if you have further follow-up questions or comments on the PREST or other datasets.
Thanks,
Mike Vardaro, Wendi Ruef, Mariela White, and RCA Data Team
Hi Mike,
Thank you for the detailed information and for taking the time to look into this.
I have a few questions as follows:
The temperature shift seems to have originated from the instrument itself (please correct me if that’s not the case). If so, could the elevated internal heat levels prior to maintenance have somehow influenced or leaked into the surrounding environment, potentially affecting seabed conditions?
I also did some calibration using the signal from big earthquakes, and the results suggest that coherence between the pressure and seismic data improved after the maintenance activity.
Thanks again for your insights—I’d appreciate any further thoughts you might have.
I was curious about what might have caused the observed temperature difference. Was it due to abnormal heat generation from the instrument itself? Or could external heat sources have impaired the instrument’s heat dissipation or sealing integrity, leading to internal heat accumulation, was it possible to turn the instrument into an additional heat source?
We have limited information from the vendor about the definite causes of the elevated temperature and lower pressure readings from the instrument deployed from 2014-2017. That said, here is what I can report:
The temperature shift in 2017 was due to the initial sensor (SBE 54, S/N 5471540-0031) being replaced by a new instrument (SBE 54, S/N 5471540-0047). It does not appear to be a real change in the seafloor temperature signal.
It’s unclear if the elevated temperature reading from instrument #0031 (which originated from an internal housing thermistor) was because the instrument was generating additional heat or if the readings were just 0.35 degC offset from the actual internal temperature due to an electronics failure.
The new instrument (#0047) had been deployed for a year prior to being turned on, right next to the previous instrument. It did not show an elevated initial temperature when it was activated, nor was there any visual evidence of destabilization of the nearby seafloor. Also, the instruments are mounted on a cradle that sits several cm above the seafloor, so the powered-on instruments were never in direct contact with the sediment.
If the first instrument (#0031) was functioning incorrectly (as the vendor reported when the instrument was returned) then it makes sense that you would see improved pressure/seismic coherence following the deployment of the new instrument.
Sorry we don’t have more information about that sensor. I’ll let you know if we find any additional documentation or hear anything more from the vendor (which is unfortunately unlikely, as the SBE 54 has been discontinued and they do not seem to have many people who are experts on that instrument remaining on staff at Sea-Bird).
