Interpreting in situ pressure and hydraulic properties with borehole penetrometers in ocean drilling: DVTPP and Piezoprobe deployments at southern Hydrate Ridge, offshore Oregon

[1] Two borehole penetrometers, Fugro-McClelland’s Piezoprobe and the Ocean Drilling Program’s (ODP) DVTPP, were deployed 50 m below seafloor at Site 1244 on ODP Leg 204 to measure formation pressure at southern Hydrate Ridge, offshore Oregon. Pore pressure is interpreted to be hydrostatic and the sediment’s coefficient of consolidation is interpreted to lie between 6.92 and 7.8 10 7 m/s, which is in approximate agreement with laboratory measurements. The Piezoprobe pressure reaches 90% of dissipation 14 times sooner than the DVTPP. The observed and modeled pore pressure responses illustrate how penetrometer geometry impacts our ability to interpret in situ properties and demonstrate under what conditions these tools can be effectively used. Because of its narrow tip, the Piezoprobe disturbs a narrower zone than the DVTPP does. This generates a narrower zone of pressure increase around the piezoprobe, which dissipates much faster than the DVTPP. As consolidation proceeds, pressure dissipation of the Piezoprobe is retarded and forms a ‘‘bench,’’ or flat spot, on the dissipation curve. Owing to its distinct two-radius geometry, it is possible to apply a consistent method to estimate in situ pressure from partial dissipation record based on the position of the ‘‘bench.’’