Quality assessment of seafloor pressure data from RBR loggers for applications to seafloor geodesy

Continuous observations of seafloor water pressure, which enable detection of vertical seafloor motion, are an essential technique in seafloor geodetic observations. For such geodetic applications, observation instruments equipped with Digiquartz sensors manufactured by Paroscientific Ltd. are commonly used because of their extremely high accuracy, resolution, and stability. Downsizing of observation instruments are important for making dense network allowing detection of subtle crustal deformation; however, the relatively high power consumption of Digiquartz makes it difficult to downsize self-pop-up mobile observation systems.

 

Our group has used RBRduo3, temperature–pressure loggers as auxiliary instruments to monitor environmental conditions in other seafloor observations, such as seafloor acoustic ranging. Given that these products are compact, lightweight, and capable of easily acquiring continuous records over periods exceeding one year, we investigated their potential applicability to seafloor geodetic observations. To this end, we conducted parallel observations with ocean-bottom pressure recorders (OBPRs) equipped with high-precision Digiquartz sensors and RBR loggers.

 

Pressure records from RBR loggers exhibit large transient variations immediately after deployment on the seafloor, with amplitudes up to about an order of magnitude larger than the typical transients observed in Digiquartz sensors. However, except for approximately the first three days after installation, this behavior can be well approximated by a time-dependent function combining an exponential term and a linear term, and no other irregular fluctuations are observed.

 

Results from parallel observations at sites with water depths exceeding 5,000 m show that, aside from the initial post-deployment transients, pressure time series obtained by RBR loggers agree well with those from Digiquartz sensors. In contrast, at shallower sites with water depths less than about 2,000 m, the pressure time series from the two instruments differ substantially. The time series of the pressure differences closely resembles the temperature time series, suggesting that these discrepancies arise from insufficient temperature correction of the RBR pressure data.

 

Assuming that apparent pressure variations caused by temperature changes dominate the short-period components of the pressure fluctuations recorded by the RBR logger, we estimated a coefficient for temperature correction by minimizing the power of the fluctuations. Using this coefficient, we removed the temperature-correlated component over the entire frequency band. As a result, we obtained pressure time series that agree with those from the Digiquartz sensors within approximately 0.2 hPa in the parallel observations. This demonstrates that, with appropriate temperature correction, it is possible to obtain pressure variation data from RBR loggers that are comparable in quality to those from Digiquartz sensors.

 

By taking advantage of their compact size and low power consumption, RBR loggers could be applied as add-on instruments to ocean bottom seismometers and similar seafloor observation instruments, and are expected to contribute to an increased number of observation points in mobile seafloor observation networks.