Determination Of Pressure Field And Hydraulic Diffusivity Of Sub-Sea Formations By Continuous Measurements Of Pore Pressure

Potential field and hydraulic properties of sub-sea formation govern the groundwater flow below sea bottom, and hence, control the location and flux of Submarine Groundwater Discharge (SGD). Because SGD is considered to be one of the important pathways of material transport from land to coastal sea, determination of both potential field and hydraulic properties can improve our understanding of land-sea interaction processes. In this study, we developed a new devise to continuously measure pore pressure and seafloor pressure, and applied the theory of poroelasticity to obtain hydraulic properties from measured data. The developed devise has multi-depth pressure ports; one is on the sea floor and the others are set at two different depths in the sub-sea formation, and measures pore pressure data synchronously. The devise was set up for a month at the water depth of 20m, offshore Kurobe alluvial fan, Japan, where fresh ground water discharge has been discovered and studied. The depths of the pressure measurements were at 0.50 meters below sea floor (mbsf) and 0.84 mbsf. Pressure data were acquired every 30 minutes throughout the experiment. To evaluate the effect of water wave loading, we measured pressure data with 2 Hz frequency two times per day. The resolutions of measured data were 0.01 Pa for the lower frequency measurement and 2 Pa for 2 Hz frequency measurement, respectively. At 0.84 mbsf, pore pressure value was about 0.6 to 0.7 kPa higher than hydrostatic pressure. On the other hand, it was nearly hydrostatic pressure to 0.3 kPa higher than hydrostatic pressure at 0.50 mbsf. These data showed the upward hydraulic gradient of at around 0.1 ∼ 0.2, which was consistent with the existence of fresh groundwater discharge at the location. The hydraulic diffusivities were estimated using two phenomena; one was pore pressure recovery process due to the disturbance of pore pressure field by installation of the devise, and the other was pore pressure response to the water wave loading. The analysis of pressure recovery at 0.84 mbsf indicated that the hydraulic diffusivity was on the order of 10^-9 m²/s. At 0.50 mbsf, pressure recovery was not observed, suggesting the higher hydraulic diffusivity at this depth. The difference of the recovery patterns suggested that the formation was made up of at least two layers. Using the two layer model of the pore pressure response to water wave (Mu et al, 1999), the hydraulic diffusivity of upper layer was estimated to be 0.3 ∼ 1.5 m²/s. According to the direct observation by diving survey, the sub-sea formation at the location was composed of sands overlying the clay layer. Thus, the obtained hydraulic values are consistent with those expected from the observed geology. Reference Mu, Y., A. H-D. Cheng, M. Badiey and R. Bennett, 1999. Water Wave Driven Seepage In Sediment And Parameter Inversion Based On Pore Pressure Data. Int. J. Numer. Anal. Meth. Geomech., 23, 1655-1674.