Formation-scale hydraulic and mechanical properties of oceanic crust inferred from pore pressure response to periodic seafloor loading

摘要: Observations of fluid pressure variations in young igneous oceanic crust have been made five sealed and instrumented Ocean Drilling Program boreholes on the flanks Mid-Atlantic Juan de Fuca Ridges. The holes penetrate locally well sedimented, hence hydrologically well-sealed crust, are situated 1 to 85 km from areas where water can flow freely through seafloor at extensive basement exposures. Amplitudes phases formation determined relative tidal nontidal measured simultaneously for periods ranging 4.8 hours 14 days. Formation reduced amplitudes between 98% 28% those shifted phase by up 20°. Simple theory porous media response periodic loading predicts that scale diffusive signal propagation locations outcrop buried parts should he proportional permeability inverse square root period variation. This behavior is consistent with observations, characteristic half wavelength defined data sites near exposures diurnal periods. If signals propagate a simple one-dimensional manner, this requires formation-scale 1.7 X 10 m 2 . No constraints provided thickness material characterized permeability, hut high-permeability pathway must be laterally continuous. At two hulk modulus rock matrix estimated elastic component pore 3 GPa. Where regionally sediment, yields 16 increase age distance an observed crustal alteration, seismic velocity, decrease Observed lateral gradients pressure, coupled suggest amplitude semidiurnal volumetric flux (Darcy velocity) exceeds -6 s -1 ; particle excursions would reach 0.25 if full volume contained 10% porosity were involved. channelized along discrete pathways, tidally modulated velocities greater. velocity similar buoyancy-driven hydrothermal convection, direction generally different. Thus may enhance water-rock interactions significantly. Energy dissipated manner approach 0.3 μW