Analysis and application of in situ pore pressure measurements in marine sediments

Abstract

Recent measurements of in situ subseafloor pore pressure in sediment ponds on the flank of the Mid‐Atlantic Ridge using a Pop Up Pore Pressure Instrument have provided important information regarding fluid exchange processes at the seafloor as well as in situ estimates of physical properties of marine sediments. The in situ pore pressure recordings of 4–5 days duration contain three distinct components. The early part of the record is dominated by the decay of a pressure pulse associated with probe penetration. The rate of decay of this pulse is used to estimate in situ permeabilities, which range from 3.1 × 10−16 m2 to 6.5 × 10−15 m2. The recording also exhibits a low‐amplitude oscillation that is associated with tidal pressure variation on the seafloor. A study based on Biot's theory shows that amplitudes and phases of tidally induced pore pressure oscillations are determined by both the frame compressibility and the permeability of the sediments, and thus the oscillation components are used to estimate these parameters. The frame compressibilities and the permeabilities estimated from tidal oscillations range from 1.2 × 10−8 Pa−1 to 4.1 × 10−8 Pa−1 and from 1.1 × 10−15 m2 to 4.8 × 10−15 m2, respectively. Laboratory‐measured permeabilities range from 7.6 × 10−16 m1 to 1.2 × 10−15 m2. Permeabilities determined by these three methods are comparable. The third component is the ambient equilibrium pore pressure. Negative pore pressure gradients were identified on all deployments. When combined with the permeabilities of the sediments the estimated average rates of drawdown of water through the sedimentary layers in the ponds are 2–7 mm/yr. The pore pressure gradient near the seafloor can be used to construct a pore pressure profile through the sediment. Extrapolated pore pressure at the top of the basement is about 80 kPa (0.8 bar) below the hydrostatic pressure.