Determination of the Hydraulic Properties of Sub-Sea Formations Using Continuous Measurements of Pore Pressure at Submarine-Groundwater-Discharge Area

The potential field and the distributions of hydraulic properties of sub-sea formation govern the groundwater flow, and control the location and the flux of the submarine groundwater discharge (SGD). In this study, we developed a new method to determine the ambient pore pressures and to estimate the hydraulic diffusivity of sub-sea formation using in-site continuous measurements of pore pressures at two different depths and pressure on the sea floor. The device was set up in an unconfined sand layer. The pore pressures were measured at three water depths; 9m (site A), 6.6m (site B) and 5.1m (site C), offshore Kurobe alluvial fan, Japan, where fresh ground water discharge has been discovered. The depths of the pore pressure measurements were 1.0 meters below sea floor (mbsf) and 2.0 mbsf at the Site A, and 0.5 and 1.0 mbsf at the sites B and C. Pressures were measured every 30 minutes for a month, and to evaluate the effect of water wave loading, we measured pressures data with 2 Hz frequency two times per day. The pore pressures in excess of hydrostatic pressure were determined to be 0.12 to 0.55 kPa at all locations except at 1.0mbsf of the site B. The average upward hydraulic gradients are 0.023 at the site A, 0.032 at the site B and 0.027 at the site C, which was consistent with the existence of fresh groundwater discharge at the location. The hydraulic diffusivities were estimated to be 1.3 m²/s at site A, 0.21 m²/s at site B and 0.17 m²/s at site C from the analysis of the pore pressure responses to the water wave loading based on the theory of poroelasticity. The fluxes of submarine groundwater discharge were estimated to be 5.1_~10^-6 to 3.6_~10^-4 m/s from the average hydraulic gradients, the hydraulic diffusivities and the assumed specific storage of the formation, which was consistent with the direct measurement (1.6_~10^-6 to 3.9_~10^-5 m/s) using the chemical-mixing based seepage meter.