مشخصات مورفولوژی هیدرات گاز از داده های لرزه ای در دریای شمال چین Characterization of gas hydrate morphology from seismic data in the northern South China Sea

1. Introduction Gas hydrates are formed due to the high hydraulic pressure present under the cold seabed over long periods of time. Gas hydrates are stable under the temperature and pressure conditions typical of water depths greater than 500 m in oceanic sediments along the continental margins. In the South China Sea, gas hydrates were found by the Guangzhou Marine Geologic Survey (GMGS) in 2007 (Zhang et al., 2007). Gas hydrates can be summarized as pore-filling or fracturefilling based on their morphologies in sediments. In the process of exploiting hydrates, the gas hydrates existence in sediments must first be known. The exploitation methods would be different according to the hydrate morphology. In 2013, GMMS once again conducted a drilling program that targeted the gas hydrate deposits in the northern South China Sea. A total of 23 holes were drilled at 13 sites (Zhang et al., 2014b). The logging data and core analysis indicated the occurrence of gas hydrates from approximately 5 m to 220 m below mudline (BML). Gas hydrates occur as solid nodules, disseminated within pore spaces of sediments and fracture fillings in veins (Yang et al., 2014; Zhang et al., 2014a, b). Nowadays, many researchers have studied the presence and amounts of gas hydrate. Still, we cannot identify the morphology of gas hydrate in sediments before we drill the site. There’s no such research to give a way to solve the problem. In this paper, we will discuss how to identify the gas hydrate morphology in sediments based on the frequency dependent reflection coefficient. 2. Mesoscopic-loss mechanism Previous studies found that the major cause of attenuation occurs at three scales: macroscopic, mesoscopic, and microscopic. At macroscopic scale, the Biot theory (Biot, 1962) gives the attenuation mechanisms. The drawback of this is that the macroscopic-flow mechanism underestimates the velocity dispersion and attenuation in rocks. At the microscopic scale, the so-called squirt flow is incapable of describing the measured levels of dissipation at seismic frequencies. Pride et al. (2004) studied the mesoscopic loss mechanism and found that the mesoscopic model provides the proper attenuation to explain the field data, and therefore, we use the mesoscopic theory to study the reflection coefficient varying with frequency at BSR.