Development of an in situ self calibrating ocean bottom pressure meter (OBP)

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Monitoring hydrostatic pressure at the seafloor plays a key role in several marine disciplines like marine geodesy, marine hydrocarbon reservoir monitoring, oceanography and the validation of satellite gravity products. But since its initial days more than 40 years ago down until today the high resolution sensors unchangedly suffer from a lack of long-term stability of the sensor calibration, preventing the unequivocal detection of long-term trends in all these disciplines.Our goal is to develop, test and build an in situ self-calibrating ocean bottom pressure meter, which is able to monitor and unequivocally recognize long-term pressure variations at the seafloor due to its built-in drift-quantification with a precision of better than 10 Pa i.e. 1 mm water column equivalent through the length of survey (one or multiple years) and therefore to resolve the expected trends - a task, which no instrument is able to cope with presently. Our method is to measure the difference between the environmental pressure and a frozen-in reference pressure. The reference pressure is captured in a chamber by closing the valve to the environment after the instrument was deployed at the seafloor. The innovative aspect of this concept for long-term monitoring is the combined usage of a differential pressure sensor with a temperature compensated reference pressure chamber, a strategy combining the advantages of significantly less drift and simple in situ calibration.Three orders less instrumental drift will be achieved, as the dynamic range of relative pressures is three orders less compared to absolute seafloor pressures (tidal height instead of full ocean depth) thus allowing smaller full scale of the sensor (drift amplitude scales with the full scale of the sensor). Quantification of the drift will be gained shorting both ports of the differential pressure sensor at the beginning and at the end of the survey, as the physical pressure difference then is zero. The challenge is to reduce temperature-caused noise in the reference pressure by intelligent choice of materials, geometry and temperature monitoring and to avoid rigorously even minimal potential leaks in the reference system.Our strategy has four crucial advantages over the only known competing approach, the in situ calibration of an absolute pressure sensor by means of a deadweight tester: Less sensor drift, more robustness and minor cost with the resulting option to install the instruments in geo-sensor networks.We plan to design, test and investigate a very basic system, to evaluate strengths and weaknesses as well as the options for an improvement (proof of concept). In a second step with a second proposal we want to implement the new insights and experiences from this prototype to build an instrument which has the potential to become the new standard for long-term in situ pressure observations in the deep sea.