It is important to understand the relations between the formation of methane hydrate in shallow sediments and seafloor manifestations accompanied by methane discharges to delineate the following issues concerning the exploitation of methane hydrate: (1) establishing methane hydrate exploration method through geological and geochemical surveys of the seafloor; (2) understanding methane hydrate system; (3) clarifying relations between methane hydrate-bearing formations and global warming; and, (4) understanding production problems during methane hydrate development.
 As a preliminary study to solve the above-mentioned issues, we attempted to clarify the relations between methane hydrate-bearing formations and various seafloor manifestations accompanied by methane releases from the seafloor, such as pockmarks and carbonate precipitations, using 3D seismic data in the three survey areas of the eastern Nankai Trough. Bathymetric and seafloor amplitude maps constructed from high-resolution 3D data provided extensive information on the seafloor. We also constructed BSR depth anomaly maps to interpret geothermal gradient and structure of the P-wave velocity in shallow formations because methane hydrate is sensitive to temperature change.
 It is likely that methane hydrate-bearing formations and seafloor manifestations have a strong relationship with geological migration conduits of methane-bearing fluid in the eastern Nankai Trough, e.g., permeable sandy sediments, and large and shallow faults. New geological and geochemical surveys of the seafloor are required to clarify the relationship.