Amplitude Variation with Offset and Lam? Parameter Analysis of Sediments at the Toe of the Cascadia Accretionary Prism

The Cascadia convergent margin lies off the west coast of North America where the Juan de Fuca oceanic plate is being subducted beneath the North American continental plate. West of Vancouver Island a large accretionary wedge has evolved as sediments have been scraped off the subducting oceanic plate and welded on to overriding North American plate. At the toe of the accretionary complex, sediments are squeezed by tectonic forces leading to decreases in porosity and expulsion of pore fluids. Expelled fluids migrate to other layers where they may become trapped and can lead to 'over pressuring' of layers. Porosity and pore pressure variations within sediments lead to changes in elastic properties such as hp, pp, Poisson's ratio and Up. Changes in these properties affect the angle dependent P-wave reflectivity of a reflector bounding the base or top of a layer. By fitting a linearised approximation to the Zoeppritz equations to P-wave reflection amplitude variation with offset (AVO) data it is possible to extract changes in elastic properties within layers, and thus infer regional changes in porosity and pore pressures. Extracted reflectivities and inverted elastic impedances (and thus hp and pp) are highly dependent on the available background P- and S-wave velocity information. Currently S-wave velocities are unknown and can only be estimated approximately; thus, extracted values are not quantitatively accurate, and only qualitative interpretations can be made. A 2-D seismic reflection line was processed to extract P and S-wave reflectivities from Pwave AVO. These reflectivities were subsequently inverted for kp and pp. These values show that porosities in the Juan de Fuca basin decrease approaching the deformation front. A package of sediments towards the base of the basin sediments is highly fractured. This package has a low pp relative to surrounding sediments. At the top of this layer a thin bed of extremely low pp is evident. This layer appears not to be decreasing in porosity at the same rate as surrounding layers. This thin bed shows evidence of having high pore fluid pressures and may act as a trap for fluids escaping the fractured layer below.