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