On the compliance method and the assessment of three-dimensional seafloor gas hydrate deposits

Abstract

Marine gas hydrates, prevalent in offshore sediments in Canada and Japan, are a possible hydrocarbon resource, a hazard to drilling and the source of a major greenhouse gas. Quantitative estimates of hydrate concentrations in deep sea sediment are difficult to obtain by conventional methods. Our group has sought novel techniques specifically designed for assessment such as the compliance method where ocean surface gravity waves are used as a source. Compliance is the transfer function between the vertical displacement of the seafloor and the corresponding pressure expressed as a function of wavelength. It is sensitive to the elastic parameters of the underlying sediments, particularly the shear modulus which is probably increased in zones containing hydrate due to cementation. Our group has demonstrated a connection between a compliance measurement and the amount of hydrate present in the hydrate stability zone for layered models. Here, we develop a 3-D numerical finite-difference code using control volume discretization to predict the compliance response over non-layered structures. Among the features of the algorithm are an ability to handle sharp contrasts in elastic moduli with a low usage of computer memory. The compliance ‘anomaly’ over such structures has a signature and resolution not unlike the corresponding gravity anomaly. The average response over heterogeneous structures is sensitive to the bulk hydrate content but not the connectivity pattern. Gravity waves with different polarizations crossing markedly anisotropic structures produce statistically the same average stiffness value.