Abstract:
Since the first installation of a CORK at hole 857D (Middle Valley, Juan de Fuca Ridge) in 1991 Paroscientific Digiquartz Broadband Pressure sensors have played a key role in monitoring formation and seafloor pressures at CORK installations. Both pressure measurements provide information about the hydrological and seismological processes at time scales from seconds to years. Therefore sensor in situ performance e.g. effective resolution and long-term stability needed to be evaluated. Starting with the installation of the NOAA tsunami early warning system DART in 1983, the Paroscientific Digiquartz Broadband Depth Sensor has been deployed for long-term ocean bottom pressure measurements in numerous marine investigations. The sensor turned out to be very reliable with very good accuracy within a broad measuring range and an extremely high resolution at deep sea ambient pressures. However, up to now its long-term drift, noise level, and effective resolution under in situ conditions at the seafloor are only known from a few published studies and for a few sensors. In this study we analyze 118 long-term seafloor pressure time series (longer than 2 months and up to a maximum time period of 9 years) to investigate effective resolution and long-term drift under in situ conditions. The data are from DART (NOAA, USA), IFM-GEOMAR (Germany), LOLEM (MAR monitoring project), and CORK (ODP/IODP) stations. Sensor drift is well described by an initial exponential part and a subsequent linear drift. In situ mean long-term drift is -0.88+ or -0.73 kPa/a, determined from all available data with a slight increase of drift with deployment depth. Unfortunately long-term sensor drift cannot be determined by short-term laboratory experiments before deployment nor predicted based on results from previous deployments. For the first time, we are able to quantify effective pressure resolution and drift of the widely used Paroscientific pressure gauges under in situ conditions, based on a large data base and numerous deployments. Our results provide important constrains for the interpretation of pressure records made with the Paroscientific sensors. Whereas relative resolution of the sensors is adequate to quantify transient seafloor and formation processes, long-term pressure changes caused by subsidence or uplift cannot be distinguished from sensor drift unless rates of vertical motion are large (tens of cm/yr). Therefore, in situ self-calibration procedures for the Paroscientific sensors are needed in order to benefit from the sensor's high effective resolution and long-term stability for the monitoring of geodynamic processes at the seafloor.