ANALYSIS AND APPLICATION OF INSITU PORE PRESSURE MEASUREMENTS IN MARINE-SEDIMENTS

Recent measurements of in situ subseafloor pore pressure in sediment p onds on the flank of the Mid-Atlantic Ridge using a Pop Up Pore Pressu re Instrument have provided important information regarding fluid exch ange processes at the seafloor as well as in situ estimates of physica l properties of marine sediments. The in situ pore pressure recordings of 4-5 days duration contain three distinct components. The early par t of the record is dominated by the decay of a pressure pulse associat ed with probe penetration. The rate of decay of this pulse is used to estimate in situ permeabilities, which range from 3.1 x 10(-16) m2 to 6.5 x 10(-15) m2. The recording also exhibits a low-amplitude oscillat ion that is associated with tidal pressure variation on the seafloor. A study based on Biot's theory shows that amplitudes and phases of tid ally induced pore pressure oscillations are determined by both the fra me compressibility and the permeability of the sediments, and thus the oscillation components are used to estimate these parameters. The fra me compressibilities and the permeabilities estimated from tidal oscil lations range from 1.2 x 10(-8) Pa-1 to 4.1 X 10(-8) Pa - 1 and from 1 .1 x 10(-15) m2 to 4.8 x 10(-15) m2, respectively. Laboratory-measured permeabilities range from 7.6 x 10(-16) m2 to 1.2 x 10(-11) m2. Perme abilities determined by these three methods are comparable. The third component is the ambient equilibrium pore pressure. Negative pore pres sure gradients were identified on all deployments. When combined with the permeabilities of the sediments the estimated average rates of dra wdown 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 constr uct a pore pressure profile through the sediment. Extrapolated pore pr essure at the top of the basement is about 80 kPa (0.8 bar) below the hydrostatic pressure.