Forward Modeling and Identification of Shallow Gas in the Seabed

Abstract The accumulation of shallow gas in the stratum will form high-gas bag and reduce the strength of soil, which will endanger the construction of marine engineering. So it is of great significance to find out the distribution of shallow gas in the work area. Shallow gas in soil will increase porosity and decrease formation sound velocity, which has absorption and attenuation effect on high frequency components of sound wave. Based on the drilling data around an oil platform in Bohai Bay, a formation model of shallow gas is established and the forward analysis is carried out, the result show that at the interface between the shallow gas and the formation, the phase of reflected wave reverses and the events at the lower part of the shallow gas is pulled down. The stratigraphic profile around the platform is investigated, and the amplitude, phase and frequency attributes of acoustic stratigraphic profile data are analyzed. The shallow gas reflection is identified by combining the features of weak amplitude clutter reflection, phase reversal, phase axis pull-down and high frequency missing, and the shallow gas reflection is columnar and the top to clay layer of the seafloor. The shallow gas is agglomerated and may come from the deep.