INTRODUCTION

The base of the hydrate stability zone on continental margins is usually identified by the occurence of a bottom simulating reflector（BSR）［1］.Gas hydrate related BSRs are widely distributed on continental margins［1～3］.The BSR is similar in shape to sea floor，often cuts the dominant stratigraphy，and is characterized by a strong，reversed polarity event.The BSR is usually a reflection at the boundary between the high-velocity gas hydrate-bearing sediments and the underlying low-velocity gas-bearing sediments［4］.(https://www.daowen.com)

A full waveform inversion method was first used to study velocity structure of hydrate-related BSR in 19934］.Their inversion result showed that a low-velocity free gas-bearing zone of 30m thickness exists beneath BSR off Vancouver Island and it was confirmed by the drilling of ODP Leg 146［5，6］.Since then full waveform inversion has been extensively used in the study of gas hydrates on the continental margins in the world，such as the Cascadia margin［4，68］，Offshore Columbia［9］，Offshore Peru［10］，Blake Ridge［11］，Offshore Costa Rica［12］and the Makran accretionary wedge of Arab Sea［13］.Velocity structures from those inversions have been used to study the distribution of gas hydrates and free gas within the sediments and to estimate the thickness，concentration and amounts of methane within gas hydrate-bearing sediments.In the field of seismic study of gas hydrate，the full waveform inversion has been proved to be a very useful tool.In this paper we propose a full waveform inversion of gas hydrate-related BSR and apply it to study velocity structure of a double BSR in the eastern Nankai accretionary wedge offshore Japan.