Modelling of BSRs as prime indicator of gas hydrates

摘要

To understand the complex phenomena associated with the formation of bottom simulating reflector (BSR), the main identifier for the presence of gas hydrate, we have generated synthetic responses of different seismic velocity (hydrate-free gas) models to draw meaningful reflection characteristics. The synthetic response of the model of high-velocity (hydrated) sediment layer underlain by low-velocity free-gas layer indicates a normal polarity reflector generated at the top edge of the hydrated layer and a strong reverse polarity reflector (BSR) at the interface of hy-drated/free-gas layer (base of the gas hydrate). The synthetic response of high-velocity hydrated layer sandwiched in the normal oceanic sediments produces reversely polarized reflector at its base (BSR). The BSR is quite distinguishable when the acoustic velocity of hydrated sediments is appreciably high. Model studies indicate that high-amplitude reverse polarity reflector (BSR) can be generated with only the presence of low-velocity (free-gas) layers overlain by normal sediments at a depth of a few hundred metres below the seafloor. There is appreciable increase in the strength of this reflector with lowering of velocity free-gas layer. The studies indicate that BSR may be generated in most of the cases where a strong impedance contrast exists due to either high-velocity gas hydrates or free-gas layer, and hydrates underlain by free-gas. Out of all three different configurations, hydrates underlain by free-gas produce BSRs with higher strength. The results from drill sites where BSRs were not found associated with gas hydrate may indicate that the BSR might have been produced due to the presence of free-gas only. Places where gas hydrate has been found without BSR may suggest that the gas hydrate may occur in lenses or very thin layers, not giving rise to appreciable impedance contrast for the generation of the BSR.