ACOUSTIC REFLECTIONS OF SHALLOW GAS ON THE NORTHERN SLOPE OF SOUTH CHINA SEA AND IMPLICATIONS FOR GAS HYDRATE EXPLORATION

Submarine gas seepage trapped in shallow strata occurs widely in the marine environment. It may change the physical properties of the sediment and leave some prints on the acoustic profiles. The sub-bottom profiles and multi-channel seismic reflection data collected from the northern slope of the South China Sea were processed and analyzed in this paper. Acoustic blank, acoustic turbidity, enhanced reflection and velocity pulling-down are discovered on sub-bottom profiles, and gas chimney or mud diapia, bright spot, velocity pulling-down and enhanced reflection on seismic profiles. The Bottom Simulating Reflector(BSR), indicator of gas occurrence on seismic profiles, is a boundary, which separate the gas system into two parts. The bright spot, velocity pulling-down and enhanced reflection caused by gas chimney or mud diapia, occur under the BSR, and the gas hydrate above the BSR may prevent gas migration upward, and force it to move laterally along the BSR. The continuous gas migration upward at the weak zone of the seal may cause the formation of acoustic blank, acoustic turbidity, enhanced reflection and velocity pulling-down. Therefore, the acoustic blank in the shallow strata, may be regarded as a right place for accumulation of block-hydrate and thus a potential exploration target.