PORE PRESSURES AND PERMEABILITIES MEASURED IN MARINE-SEDIMENTS WITH A TETHERED PROBE

A new probe for measuring pore fluid pressures in marine sediments has been constructed and tested in two hydrologic environments on the Juan de Fuca Ridge. The instrument utilizes a highly incompliant high-sensitivity differential pressure transducer, and a small (4mm) diameter sensing probe that telescopes within a larger diameter 1.5-m-long (currently 2.5 m) strength member. The small probe diameter and the low compliancy serve to minimize the time required for pressures to approach equilibrium after penetration. Observed decay times are short enough to permit the instrumement to be used in a tethered mode. The mechanical configuration enables the small diameter probe to penetrate the sediments without buckling and to be pulled out without bending. This allows multiple penetrations to be completed during a single instrument lowering. Ten measurements were made during the development and testing of the instrument in fine-grained turbidite sediments on the Juan de Fuca Ridge. Four penetrations were uninterrupted by mechanical disturbances or instrumentational problems, and although equilibrium conditions were approached during only one penetration (2.7 hours maximum undisturbed decay period), all provided useful constraints on equilibrium pore pressures. Values at two sites where sediment cover is thick and heat flow relatively low were unresolvably different from hydrostatic. Values estimated from two penetrations located within a few hundred meters of a hydrothermal vent field within the sedimented axial valley of the northern Juan de Fuca Ridge, Middle Valley, exceeded hydrostatic and yielded pressure gradients of roughly 0.5 kPa m-1. Permeabilities were estimated from the rates of decay of the penetration transients. Values ranged from 5 x 10(-16) to 1 x 10(-14) m2 and agreed well with values measured on core samples collected near the probe penetrations which ranged from 8 x 10(-16) to 1 x 10(-14) m2. Using the values determined from the probe measurements from Middle Valley, the implied rate of fluid flow upward through the sediments in the vicinity of the vent field is roughly 5 x 10(-10) m s-1, or 15 mm yr-1. With the current practical limit of resolution of roughly 10(-10) m s-1 (3 mm yr-1) this direct measurement technique provides a means of determining pore fluid flow rates that is roughly 1 order of magnitude more sensitive than the method of estimating the rate of flow from the perturbation of the conductive thermal regime measured by typical marine heat flow instruments.