High resolution 3D seismic survey “Tokai-oki to Kumano-nada” was conducted for methane hydrate exploration in the eastern Nankai Trough by METI in 2002. Our study focuses on zigzag-shaped specific reflectors which occur on BSR margins on the 3D data. We call the reflectors “Foldback Reflectors (FBRs)” in this study. From the edge of BSR, the 1st FBR generally extends down to the lower formation below the BSR and cross sedimentary horizons. The following FBRs extend down from the edge of the upper FBR forming accordion-like shape. The 1st FBR indicates normal polarity, and the following FBRs change their polarities alternately. FBRs are mostly developed in the well stratified formation but not in the area showing frequent fractures and major lateral lithological change. Dip direction of each FBR is probably controlled by that of crossing formations. FBR generally corresponds to lateral seismic facies boundaries between the BSR distribution area and the area outside the BSR distribution. The formation beneath the BSR shows dimmed seismic facies characterized by relatively low amplitude and lack of high frequency components with relatively low velocity in contrast to the area outside the BSR with normal seismic facies. The lowest FBR does not cross over major unconformities, which often exhibit negative polarity suggesting fluid bearing strata. In addition, high amplitude layers are sometimes described at foldbacks convex to the area outside the BSR. These high amplitude layers probably having higher permeability are interpreted as conduits for outward migration of gas-related fluids which are distributed in the low velocity area. The shapes of FBRs are possibly related to layer-parallel migration in strata with wide range of permeability. From these observations, FBR can be regarded as an important proxy indicating migration front of gas-related fluid.
