Our topic will include three parts. First, we will give a brief introduction of developments of seismic methods in the studies of marine gas hydrates. Field investigations, data processing and analyses, amplitude versus offset analysis and full waveform inversions are discussed. Then we will present research results by my copartners and me. Finally we will review seismic researches on gas hydrate in China.

　　An anomalous reflector was first recognized in 1970 by Mark, and since DSDP Leg 11, it has been named to the bottom-simulating reflector (hereafter BSR) and inferred to be related with the base of gas hydrate-bearing sediment zone. Later the explanation was confirmed by DSDP and ODP. The base limit of the hydrate-bearing sediment zone on continental margins is often marked by BSR on seismic profiles, then the distribution of marine gas hydrates is easily mapped by seismic survey. Gas hydrate-related BSRs are widely distributed on the continental margins. The BSR, which mimics the shape of the sea floor, often cuts the dominant stratigraphy, and is characterized by a strong reverse-polarity event. The BSR is usually interpreted as a reflection at the boundary between the high-velocity gas hydrate-bearing sediments and the underlying low velocity gas-bearing sediments.

　　In 1970s, scientists recognized BSRs on seismic profiles, studied their reflection characteristics and distribution, and focused on their geometrical features. In 1980s, there was less works of seismic studies of gas hydrates. Since 1990s, seismic studies of gas hydrates have developed vigorously. Numerous studies involving forward modeling and inversion of seismic reflection have been carried out. These studies are post-stack reflection amplitude analysis, post-stack reflection waveform modeling, calculation of interval velocities from RMS stacking velocities，travel time inversion，post-stack acoustic impedance inversion，amplitude-versus-offset analysis, attenuation analysis and full waveform inversion. These researches are focused on extraction of physical properties, better understanding of the distribution of gas hydrates and free gas, and discussion about origin of BSRs.

　　We will give an example of seismic data processing for BSRs in western Nankai accretionary prism, a typical gas hydrate distribution region. Seismic data processing is proved to be important to obtain better images of BSRs distribution. Studies of velocity structure of hydrated sediments are useful for better understanding the distribution of gas hydrates. Using full waveform inversion, we successfully derived high-resolution velocity model of a double BSR in eastern Nankai Trough area. Gas hydrate occurs above the first BSR and free gas occurs beneath the second BSR. Between the first and second BSR, gas hydrate also exists.

　　Recent survey and research show that gas hydrates occur in the marine sediments of the South China Sea and East China Sea. But we would like to say seismic researches on gas hydrate in China are very preliminary.