INTRODUCTION

Gas hydrate is a crystalline solid，composed of water and gas.Natural gas hydrate is usually methane hydrate.It is stable within a certain high-pressure low-temperature range and can exist in permafrost regions and continental margins where sea depth exceeds 300m［1］.About 164m3methane can be released from 1m3methane hydrate at ambient temperature and pressure.Methane in gas hydrate probably contains twice as much organic carbon as all fossil fuels on earth and therefore may become a major energy resource.Small changes in temperature or pressures can cause breakdown of hydrate，so that it has significant effect on sea floor sediment stability，in that it is the cause of collapse and landsliding.The release and absorption of large volumes of methane from sea-floor hydrates may have had major impacts in modifying the Earth's climate［2］.(https://www.daowen.com)

Abnormal reflectors were first recognized by Markl［3］in 1970，and they were named bottom simulating reflectors（BSRs）and they have been related to marine gas hydrates since DSDP（Deep Sea Drilling Project）leg-114］.Recent development of ODP（Ocean Drilling Project）and seismic surveys have revealed that gas hydrates are widely distributed in continental margin sediments.The base limit of the hydrate stability zone（GHSZ）is often marked by a very strong reflector called a bottom-simulating reflector on seismic profiles，so that the distribution of marine gas hydrates is easily mapped from seismic surveys carried out in the early stage of research［5］.Characteristics of BSR include the fact that they are parallel to the seafloor，oblique to bedding plane，the presence of high amplitude strong reflectors，reverse polarity and sometimes amplitude blanking above them.Along most continental margins，BSRs are usually oblique to the bedding planes of sediments.However，in the South China Sea it is difficult to recognize BSRs［6，7］because bedding planes are usually parallel to the seafloor.Velocity information provided by seismic experiments has been useful to infer possible hydrate distribution because pure gas hydrates have much higher velocity（3.3～3.8km/s）than normal sediments at the depths of the hydrate stability zone.Sediments with free-gas can also occur beneath the gas hydrates stability zone and have lower velocity and Poisson's ratio.So a BSR，which exhibits a strong reflector with negative polarity，results from the negative acoustic impedance contrast between hydrate-bearing strata and free gas-bearing strata or saturated-water strata.BSR，amplitude blanking zone and velocity inverse from high velocity to low velocity are useful tools that can thus be applied in the studies of marine gas hydrates.