Knowledge of the occurrence of in situ GH is very incomplete, and is either based on limited direct evidence (hydrate samples) or inferred from other data. In perma­frost regions, direct evidence of gas hydrate is provided by ongoing R&D programs (discussed below), and by analysis of industry 3-D seismic data and data obtained during the drilling and logging of conventional oil and gas wells. In marine environ­ments, most of the inferences of GH occurrence are based on indirect indicators involving interpretation of relatively low-quality 2-D seismic data. Direct GH detec­tion and characterization from marine 3-D seismic data have recently been reported by Dai et al. [32]. The use of four-component ocean bottom seismic surveys has also shown great promise [4, 14].