An insight into shallow gas hydrates in the Dongsha area,South China Sea

Previous studies of gas hydrate in the Dongsha area mainly focused on the deep-seated gas hydrates that have a high energy potential,but cared little about the shallow gas hydrates occurrences.Shallow gas hydrates have been confirmed by drill cores at three sites(GMGS208,GMGS209 and GMGS216)during the GMGS2 cruise,which occur as veins,blocky nodules or massive layers,at 8–30 m below the seafloor.Gas chimneys and faults observed on the seismic sections are the two main fluid migration pathways.The deep-seated gas hydrate and the shallow hydrate-bearing sediments are two main seals for the migrating gas.The occurrences of shallow gas hydrates are mainly controlled by the migration of fluid along shallow faults and the presence of deep-seated gas hydrates.Active gas leakage is taking place at a relatively high-flux state through the vent structures identified on the geophysical data at the seafloor,although without resulting in gas plumes easily detectable by acoustic methods.The presence of strong reflections on the high-resolution seismic profiles and dim or chaotic layers in the subbottom profiles are most likely good indicators of shallow gas hydrates in the Dongsha area.Active cold seeps,indicated by either gas plume or seepage vent,can also be used as indicators for neighboring shallow gas hydrates and the gas hydrate system that is highly dynamic in the Dongsha area.

Quantitative estimation of bubble volume fraction of submarine seep plumes by modeling seismic oceanography data

Submarine seep plumes are a natural phenomenon in which different types of gases migrate through deep or shallow subsurface sediments and leak into seawater in pressure gradient.When detected using acoustic data,the leaked gases frequently exhibit a flame-like structure.We numerically modelled the relationship between the seismic response characteristic and bubble volume fraction to establish the bubble volume fraction in the submarine seep plume.Results show that our models are able to invert and predict the bubble volume fraction from field seismic oceanography data,by which synthetic seismic sections in different dominant frequencies could be numerically simulated,seismic attribute sections(e.g.,instantaneous amplitude,instantaneous frequency,and instantaneous phase)extracted,and the correlation between the seismic attributes and bubble volume fraction be quantitatively determined with functional equations.The instantaneous amplitude is positively correlated with bubble volume fraction,while the instantaneous frequency and bubble volume fraction are negatively correlated.In addition,information entropy is introduced as a proxy to quantify the relationship between the instantaneous phase and bubble volume fraction.As the bubble volume fraction increases,the information entropy of the instantaneous phase increases rapidly at the beginning,followed by a slight upward trend,and finally stabilizes.Therefore,under optimal noise conditions,the bubble volume fraction of submarine seep plumes can be inverted and predicted based on seismic response characteristics in terms of seismic attributes.