Seismic Velocity and Porosity Distribution within Underthrust Sediments at the Toe of the Nankai Subduction Zone: Inferences from PreStack Depth Migration Analysis

At the Nankai subduction zone offshore SW Japan, it has been inferred that rapid burial combined with the low permeability of underthrust sediments result in excess pore pressure, controlling the decollement strength. 2D PreStack Depth Migration (PSDM) was performed on seismic data collected in the seaward portion of the offshore Cape Muroto survey area within the Nankai subduction zone to obtain robust interval velocities for the incoming sediment on the subducting plate, prism, and underthrust sediments. The objectives of mapping the underthrust section's velocity are porosity computation and ultimately mapping pore pressure distribution beneath the decollement. Robust PSDM interval velocities are obtained by applying several iterations of residual move-out (RMO) velocity corrections to flatten reflectors as a function of source-receiver offset on depth migrated gathers. Formation velocities and horizon depths acquired from shipboard core and downhole measurements of Ocean Drilling Program's (ODP) sites 808 & 1173 were also used as a constraint for the seismic processing. Image quality (optimal collapse of reflectors in the pre-stack migration) and borehole depth ties were both used as constraints to guide the velocity analysis. Careful balancing of reasonable velocity gradients was performed when strict RMO analysis produced large velocity differences between adjacent inlines. In this way we have obtained the best constrained velocity model to date for this accretionary prism. Processing was run on a total of 30 inlines composed of 250 m line spacing and 25 m CDP spacing. We have produced a fine 2D grid of underthrust section interval velocity. Mapping of this velocity structure shows generally increasing velocity with depth and distance landward of the deformation front however, the underthrust section is overlain by higher velocity prism material. Superimposed on this trend, localized variations within the underthrust section velocity structure do exist, suggesting possible small-scale variability in porosity. We then applied an empirically derived velocity-porosity transform, permitting a calculated quantitative distribution of porosity for the Shikoku Basin underthrust sediments. Preliminary analysis of this porosity mapping indicates inhibited dewatering of most of the underthrust section, implying superhydrostatic pore pressure persisting 10-15 km landward beneath the decollement.