Using Core Properties and Seismic Reflectivity to Estimate Pore Pressure in an Active Decollement Fault

Harold J. Tobin and J. Casey Moore

In the decollement zone of the Barbados accretionary prism, a 3-D seismic image exhibits patchy high-amplitude negative polarity reflections, which have been attributed to large overpressures confined to the fault zone. We collected laboratory P-wave velocity and porosity vs. pore pressure data, using core samples from and adjacent to the decollement zone at ODP Site 948. Logs constrain density and velocity through the decollement zone at Site 948. We use these data to calibrate the reflectivity of the fault zone to pore pressure through waveform and amplitude models of the fault plane reflections.

Modeling of the positive polarity Site 948 reflection indicates that it can be explained by a lithologic boundary coincident with the decollement, without anomalous fault properties. By contrast, the dominantly-negative polarity waveform of the reflection approx. ~2 km arcward (beneath Site 947) is best modeled by inserting a 16-19 m thick zone of extremely low impedance into the Site 948 impedance structure, with a gradational return to "normal" impedance just above the positive boundary. Relative amplitudes in this reflection indicate a larger impedance contrast than can be accounted for at sub-lithostatic fluid pressure, based on the core properties data. We conclude that lithostatic pore pressure with attendant hydraulic dilation of the fault zone is required to generate the negat ve-polarity reflections. Mapping of these reflections thus delineates zones of elevated fluid content and zero effective stress in the fault zone.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California