Abstract: As a common technical means of hydrate research, high-precision OBS surveys can obtain the fine-scale velocity structure of hydrate orebody, which is important for studying gas hydrate saturation, prediction and development of hydrate resources. However, forward simulation of OBS velocity structure is a tedious and time-consuming process, and constructing a reasonable initial model is an important prerequisite for subsequent fast fine-scale structure imaging. Based on data from18 hydrate areas in the world where velocity structures have been identified, this study analyzed the common characteristics and influencing factors of P-wave velocity in hydrate orebodies, and obtained the empirical formula of P-wave velocity in hydrate orebody zone with seawater depth and sediment thickness. Based on the empirical formula, OBS data and multi-channel seismic profiles, an initial model of Shenhu hydrate area is established, and a one-dimensional P-wave velocity structure of Shenhu sea area is obtained by ray tracing and travel-time fitting. The results show that Shenhu hydrate orebody has high P-wave velocity (1.83-1.92 km/s), and free gas has low P-wave velocity (1.60-1.70 km/s). In addition, the empirical formula can serve as an important reference for the simulation of the velocity structure in Shenhu area, and it is expected to provide a reliable initial model for obtaining the two-dimensional / three-dimensional velocity structures in Shenhu sea area so as to guide fine-scale explorations and resource assessments of hydrate.

Key words: gas hydrate, division of different zones, empirical formula, initial model, travel-time fitting, velocity structure