MULTI-FREQUENCY IMAGING OF SHALLOW GAS HYDRATES

A large amount of methane may migrate upward to the seafloor and then form shallow gas hydrates deposits there. This type of gas hydrates helps understand how cold seeps and gas hydrates respond to the change of environments. Approaches for detecting the shallow gas hydrate as such has not been well established so far since the shallow gas hydrates occur too close to the seafloor and often in small scale. Taking the GMGS-08 site as an example, we use multi-frequency method to characterize the shallow gas hydrates. The data used includes multi-channel seismic data (~ 65 Hz), primary high frequency (22 kHz) and secondary low frequency (4 kHz) sub-bottom profiler data. On the seismic data, BSR occurs in a discontinuous way, suggesting perturbations due to the ascent of fluid. On the sub-bottom profiler, acoustic blanking, enhanced amplitude and weak seafloor amplitude are observed and related to cold seep systems. In addition, on the seismic data, the seafloor is characterized by two close strong event (time distance of ~16 ms), rather than a single one and this is different from the normal seafloor reflection. Calculation shows that the interface corresponding to the second event lies ~12 m below the seafloor. On the secondary low frequency profile, the top of the gas chimney lies ~13.5 m below the seafloor. Accounting the errors during the calculation, we infer that the top of the gas chimney correspond to the second event of the seafloor reflection. Moreover, these two depths are consistent with the interval where shallow gas hydrates are recovered and high velocity anomaly observed. The latter depths are 9-22 m below the seafloor. Our results show that multi-frequency imaging is effective in charactering the shallow gas hydrate and is useful in speculating the formation process.