"Seismic Experiments in Accretionary Wedges" by Harold Tobin

Estimating pore pressure in accretionary wedge faults using seismic reflectivity and core petrophysics ODP drilling at many subduction zone accretionary wedges worldwide has shown that the sediments in and below the wedges are generally highly overpressured at depths as shallow as a few hundred meters. Extreme overpressures have been inferred, especially in the basal decollement faults, based on log and core porosity anomalies, seismic velocity and reflectivity, and limited well tests. Proposed sources of overpressure include compaction disequilibrium and fluid generation at depth through smectite dehydration. One approach to quantifying overpressure has been to measure velocity-porosity-effective stress functions on core samples taken from these zones, then model seismic reflectivity to derive estimates of acoustic impedance structure. These data are combined to estimate in situ pore pressure. Results from the Barbados Ridge and offshore Oregon suggest pore pressure of 90-99% of lithostatic. This kind of indirect approach has generally been necessary due to the lack of well control, RFT data, and the usually difficult drilling conditions in accretionary wedges.