Calibration of Pressure Sensors for Ocean-Bottom Seismology

ABSTRACT Three types of pressure sensors are used for seismic and oceanographic observations in seafloor instruments: the hydrophone, the differential pressure gauge (DPG), and the high sampling rate absolute pressure gauge (APG). There has not been a good way to accurately calibrate these sensors across the entire frequency range of importance to seismologists (5 mHz to 200 Hz). We constructed a system to introduce time-varying pressure signals into a pressure test chamber to accurately calibrate each sensor at seafloor pressures. The gain decreased by more than 40% and the capacitance of the Hitech HTI-U-90 hydrophone by 25% with depth from 0 to 5000 m effecting the response. The DPG gain increases linearly with log frequency by 3% per decade from 0.1 mHz to about 10 Hz due to the viscoelastic deformation of the o-rings in the overpressure relief valves. The DPG gain changes with temperature and pressure. The time constant for the DPG high-pass capillary leak mechanical filter increases with depth (pressure) and decreases with temperature, increasing from about 100 s at 10 m and 20°C to about 300 s at 5000 m and 2°C. Future work will be required to determine the upper frequency limit of the response of the DPG. Our results show good results to 125 Hz, and suggest the DPG could fully replace hydrophones for many seismic observations at the seafloor. The APG has a simple, stable response ideal for observing large dynamic range signals, but the APG inlet piping acts as a low-pass filter with a corner frequency that may depend on pressure and temperature.