The PIs propose to develop a new subseafloor pore pressure instrument similar to the Pop Up Pore Pressure Instrument (PUPPI) developed by Peter Schultheiss in the eighties. The instrument package consists of an upper command unit attached to a lance and ballast. It is deployed by free-fall from a ship and embeds the lance in the sediment. One or more ports on the lance are connected to a high resolution pressure transducer and the initial insertion pressure pulse, decay, and final equilibrium are recorded along with the bottom pressure. The difference between the equilibrium value and the hydrostatic gradient give the state of over- or underpressure and ultimately the state of stress and nature of fluid flow. The instrument can be left on the bottom for months or years for a continuous record of pressure or can be recovered after reaching equilibrium. An acoustic release mechanism separates the command unit from the lance and ballast and the command unit floats to the surface for recovery.
Pore pressure is a fundamental physical parameter for determining the current state of stress and the rate of transport of fluid and energy within the sediment. In spite of this, direct measurements of pore pressure by the marine hydrogeology community have been rare. This has primarily been due to the lack of instrumentation with adequate resolution and other important qualities. Recent advances in various technologies have made such measurements possible and there has been a resurgence of interest for such measurements within the international marine geoscience community.
Broader Impacts
Development of the seafloor piezometer will require international collaboration as well as collaboration with industry. As the PIs point out, this instrument could become an integral part of a number of national and international programs (RIDGE, MARGINS, IODP, NEPTUNE), and could be used to address many of the questions that represent the stated goals of these programs. In addition, pore pressure data is valuable for researchers studying problems with societal relevance, such as earthquake responses, gas hydrate formation, and mass wasting events. The proposal includes support for a graduate student in instrument design.
to NSF by PI Michael Tryon Part 1: Background Pore fluid pressure is the most fundamental physical parameter for determining the current state of hydrogeologic processes such as fluid and energy flow and the state of stress within marine sediments. It is also a primary indicator of transient tectonic processes ranging from earthquakes to aseismic creep. The PUPPI-II is a seafloor instrument designed to measure pore pressure in the near subsurface and is based on the successful design philosophies of the original PUPPI piezometer designed by Peter Schultheiss in the 80s but significantly updated based on new technology. The instrument consists of two sections: a lower section consisting of the disposable weight stack and lance with pressure port and an upper instrument section which houses the pressure transducers, acoustic release, logger, acoustic modem, and the floatation and recovery aids. During deployment the instrument free-falls to the seafloor and penetrates up to 8m. At that point it can measure and record formation pore pressure near the probe tip for up to two years. Upon acoustic command the hydraulic and mechanical links between the two sections disengage and the upper section floats to the surface for recovery. The current PUPPI-II design consists of the following (see figure 1): 8 m by 5 cm steel lance with one port near the tip weight stack consisting of a variable number of 100 lb. barbell weights which fit over the lance a strength member that holds the weight stack in place and gussets the intersection of the lance with the weight stack and upper instrumentation package a depth of penetration arm a release mechanism which separates the weight stack and lance from the instrumentation package based on an aerospace frangibolt design 2 Paroscientific Digiquartz absolute pressure sensors with Bennest Enterprises pressure period counter; one monitoring bottom pressure and the second monitoring formation pressure zero dead volume valve for switching the formation pressure sensor to bottom pressure periodically for calibration acoustic release based on ORE Offshore (EdgeTech) electronics Benthos acoustic modem Applied Geomechanics 900 series clinometer Persistor Instruments CF2 computer based controller and data logger syntactic foam floatation Part 2: Field Testing A number of instrument lowerings and two full deployments were done to test the instrument’s capabilities. With the exception of modem problems, the tests went very smoothly and the instrument performed as expected. Equipped with 500 lbs of ballast, the instrument was found to free-fall at a rate of 6.35 m/s and penetrate turbidite basin fill to a depth of 6.25 m. The initial pressure spikes were 20-50 kPa and decayed to about 1.5 kPa on the long deployment (fig. 2). Pressure at the instrumentation package and probe tip were recorded every 4 s during freefall and the first hour, and then every minute thereafter. Tidal loading response at 6.25 m depth was as predicted by theory (see fig. 10a of Wang and Davis, 1996). Tidal amplitude at the probe tip was 0.95 that of the tide and differential pressure was in phase but of opposite sign (Fig. 3). Average differential pressure was 1.7 kPa, within the range of what one would expect for this environment. Upon release, the instrument rose at 1.68 m/s (11 minutes at our 1140 m site), faster than expected. The first full deployment lasted about 4 hours and the second was 6 weeks. Reference Kelin Wang and Earl Davis, Theory for the propagation of tidally induced pore pressure variations in layered subseafloor formations, Journal of Geophysical Research, v. 101, n. B5, p. 11,483-11,495, May 10, 1996.
