Deep-ocean piezometer probes were developed and extensively tested in the laboratory, hyperbaric chamber, and the Pacific Ocean Basin environment at ambient pressures of 1 to 545 atm. The probes are small diameter (8 mm) and utilize single sensor differential transducers to measure very slight changes in excess pore-water pressure caused by induced mechanical and thermal stresses in the surrounding sediment. Pore pressure increases can cause time-dependent changes in the physical and mechanical properties of the sediment and thus affect the integrity of the sediment's “isolation” capacity. A unique system design, which exposes the transducers' external and internal (line pressure) components to ambient hydrostatic pressures, permits the measurement of extremely small pressure changes with high sensitivity and precision. The pressure sensors exhibited excellent sensitivity and stability during deep-ocean simulated laboratory tests and in-situ systems performance tests.
Simulation tests were performed with piezometers inserted in reconstituted illitic marine clay for a period of 30 days. Two deep-ocean deployments were conducted in 5500-m water depths off the Pacific Ocean Basin in red clay deposits. Small differential excess pore pressures were generated in response to both mechanically and thermally generated forcing functions. Ambient pressure changes of less than 0.14 kPa (0.02 psi) were measured with precision at total hydrostatic pressure of 545 atm.
In addition to the measurement of ambient and dynamic pore pressure, the piezometer insertion and dissipation data can be used to effectively derive the sediment shear strength and permeability. These are critical parameters in the geotechnical assessment of geological formations considered for subseabed waste disposal sites.