3D Pre-stack depth imaging of the Nankai Trough accretionary prism off Shikoku Island, Japan

During 1999 we acquired an 8x90 km 3D seismic dataset across the toe of the Nankai Through accretionary prism south of Shikoku. Previous processing steps have focused on 3D pre-stack time migration of the entire survey and 2D pre-stack depth migration (PSDM) of two in-lines that cross the Leg 190/196 drill sites. In this study, we conducted 3D PSDM of the seaward half of the data set to improve structural images and to derive the velocity structure of the underthrust sedimentary section in order to better understand its 3D compaction and dewatering history. Velocities derived from pre-stack depth migration are considered to most accurately reflect actual in-situ formation velocities. Our processing procedure started with pre-stack time migration in the cross-line direction to image the data into 2D inlines, allowing us to use 2D migration velocity analysis (MVA) techniques to update the velocity field. 3D imaging of target volumes of data around the leg 190/196 drill holes using several distinctive reflections as depth marker horizons provided constraints for the migration input velocity model. We then 2D MVA on every 5th inline (total of 32 lines), using a top-down, layer stripping technique with Residual Move Out picking to iteratively update the velocity model and flatten the Common Reflection Point (CRP) gathers. We also compared CRP gathers with image gathers in order to detect dipping events and velocity anisotropy. We then used the resulting 3D velocity field as input to a full 3D PSDM of the entire data set. The depth image clarified the accretionary prism's structure, including the numerous thrust faults, the basal décollement, and the underthrusting Shikoku Basin sedimentary unit. The thickness of the underthrust section decreases landward because of compaction. The velocity model shows that the underthrust section's velocity increases about 20% over the first 15 km landward. Along strike variations in velocity are generally less than about 5-10%.