ESTIMATION OF THE AMOUNTS OF METHANE IN HYDRATED...
For a better understanding of energy potential and effects on global climate and submarine instability of gas hydrates，it is crucial to study the amounts and！distribution of gas hydrates.The base limit of the hydrate stability zone is often marked by a very strong reflector called a bottom-simulating reflector on seismic profiles，then the distribution of marine gas hydrates is easily mapped by seismic survey in the early stage of research.Characteristics of BSR include its parallel to seafloor，oblique to bedding plane，high amplitude strong reflector，reverse polarity and sometimes amplitude blanking above it（Fig.1）.
Fig.1 BSR shown in single-channel seismic data on Blake Ridge，velocity functions（right panel）are determined from traveltime inversion of vertical seismic profile data［4］(https://www.daowen.com)
Velocity information provided by seismic experiments has been useful to infer possible hydrate distribution because pure 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 which maybe occur beneath gas hydrates stability zone have lower velocity and Poisson ratio.Therefore velocity and elastic coefficients associated with the formation of gas hydrate in marine sediments can be used to obtain semiquantitative estimate of hydrate concentration.Then the amounts of methane in hydrated sediments can be estimated with other parameters as thickness and area of gas hydrates distribution.
In the Blake Ridge，the BSR distribution area is 24000km2，thickness of gas hydrate in deposits is 250m and thickness of solid hydrates（3layers）is 2m.Concentration of gas hydrates in sediments is estimated as 2.5%based on data of velocity structure and chloride concentration.Then in the Blake Ridge the amount of methane is calculated to be 351012m3in hydrates sediments，while free methane beneath gas hydrates stability zone is estimated to 321012m3.And total amount of methane in gas hydrate and free gas is estimated to be 1001012m3in Nankai Trough.
Although it is well known that the BSRs are associated with the base of the stability field of gas hydrates and are caused by a sharp elastic contrast between the sediments containing gas hydrates and the underlying sediments without them.But it is argued whether the main controlling factor of BSR is gas hydrate or free gas beneath gas hydrates stability zone.It is found that gas hydrates also exist in regions without the BSR，e.g.at site 994of Leg 164.And there are two formation models：the hydrate wedge model and the free gas model.So studies of seismic velocity and elastic coefficient structure of hydrated sediments are useful for better understanding the distribution，amounts and formation mechanism of gas hydrates