REFLECTING SEISMIC HORIZON AT THE BASE OF THE SUBMARINE ZONE OF GAS HYDRATE STABILITY

Analysis is given to the world data on the reflecting seismic horizon BSR (Bottom Seismic Reflector), limiting from below the submarine ther mobaric zone of HC gas hydrate stability, as well as to data on its ex tension, seismic characteristics and signs sufficient for identificati on on time sections. On the basis of analysis of the observed values o f the BSR reflection coefficient and the existing models for the effec t of hydrates and free gas on velocities of seismic wave travel it is shown that the BSR is due to the presence of free gas immediately bene ath the bottom of the zone of gas hydrate stability. In the general ca se free gas appears as a product of hydrate decomposition when the bot tom of the thermobaric zone of hydrate stability is displaced upward r elative to the sedimentary section. As a result, the amount of hydrate and free gas above and below the BSR, respectively, must be equivalen t. It is reasonable to suppose that this amount makes up few percent o f the porous volume of host deposits, while the thickness of the gas-b earing bed does not exceed few tens of meters. The thickness of the hy drate-bearing bed above the BSR has virtually no effect on the dynamic characteristics of reflection.