Deepwater high‐resolution expanding spread and split spread seismic profiles in the Nankai Trough

Abstract

In deep water the source‐receiver offsets that are required for accurate velocity determinations cannot be achieved with single‐ship multichannel seismic methods. Two ships, one equipped with a multichannel receiving array and the other with a seismic source, have previously been employed to acquire common midpoint, expanding spread profiles, principally to determine deep crustal velocity structures. We extend this method to higher resolution in space and time to determine the velocities of sedimentary layers in deep water offshore Japan in the Nankai Trough. This high‐resolution two‐ship data acquisition method used a 13.1‐L water gun source array; a 1.6‐km, 96‐channel receiving array with 0.0166‐km active group; and shore‐based navigation. Analysis of the data was performed in the τ‐p domain by successive downward continuation of the plane wave data. Interactive velocity analysis methods for both one‐dimensional and two‐dimensional Earth models are described for both common source/receiver and common midpoint profiles. Results in one of two areas surveyed show a low‐velocity zone below the subduction decollement which is consistent with models of low wedge taper, high pore fluid pressure, and reflection polarity reversal described previously by other researchers. The velocity profiles show the expected landward increase in velocity assumed to be due to lateral strain and porosity decrease, but the effect is small, only slightly greater than would be expected in an area of no lateral strain.