Validating Pressure Gauges for Seafloor Seismology and Geodesy
Seafloor pressure gauges have long been an essential tool for oceanographic observations and are now a critical sensor deployed with seafloor seismometer systems. Three different sensors, the hydrophone, differential pressure sensor (DPG) and the high-resolution absolute pressure gauges (APG), are all used because no one sensor covers all frequencies necessary to record all seismically or oceanographically relevant pressure signals. A long-standing problem has been that there has been no means to accurately calibrate these sensors at frequencies relevant to seismology and at appropriate temperatures and pressures. A system for inter-calibration of the sensors will be built and installed within a high-pressure facility to make calibrations of the different sensors. The new calibrations can be applied to many previous data sets archived by the scientific community and to future seafloor data enhancing the usefulness of the data. A second component of the research will be to analyze data from recent seafloor deployments of a new system for recalibrating pressure gauges at the seafloor to remove long term variations in the calibration (drift). The development of drift free or nearly drift free methods for measuring seafloor pressure will open heretofore impossible observations of faulting processes relevant to seismic hazard and of oceanographic currents helping to validate long term changes in oceanography. An early career, postdoctoral scientist will lead much of the research.
A system for calibrating marine pressure gauges at frequencies relevant to seafloor seismology (0.001 to 50 Hz) will be constructed and installed in a laboratory high pressure facility to enable accurate inter-comparison of the responses of DPGs, APGs and hydrophones. The calibration of APGs is currently well known at low frequency but unknown at short period. Hydrophone calibrations are only accurate at high frequency. DPG calibrations are uncertain at all frequencies due to temperature and pressure dependencies of the sensor response and possible variations related to mechanical compliance. The research seeks to eliminate any remaining uncertainties in our understanding of the response of these gauges by calibrating the sensors completely over all relevant frequencies for the first time and searching for possible problems in sensor function. An analysis of signals from large earthquakes on seismic and pressure sensors recorded during recent deployments in New Zealand and Alaska will be used to confirm sensor calibrations. Application of pressure measurements to the study of oceanographic currents and to seafloor geodesy is limited by drift - the slow variation in calibration of the pressure gauges with time. A system A-0-A for calibrating absolute pressure gauges at the seafloor was installed on 5 instruments recently recovered from year-long deployments offshore of Alaska and New Zealand. The inputs to dual pressure gauges are periodically switched to measure the pressure within the instrument housing which is verified using data from a barometer. The drift at high (seafloor) pressure is expected to follow the drift measured at low pressure and thus the drift can be subtracted from the high-pressure data. The data from the dual pressure gauges (which drift at different rates) will be compared to assess the accuracy of this new method for the removal of drift.