Seismic Methods for Detecting and Quantifying Marine Methane Hydrate/Free Gas Reservoirs

Abstract

Seismic methods are the most widely used approach for indirect detection and quantification of gas hydrate in marine sediments. Historically, the presence of methane hydrate has been inferred on the basis of bottom simulating reflections (BSRs), which mark the phase boundary between hydrate and the underlying free gas zone (e. g. Shipley et al., 1979). In addition to their association with BSRs, hydrates and the underlying free gas affect the elastic properties of the host sediment in ways that are seismically detectable. Partial replacement of pore fluid by rigid gas hydrate causes an increase of both compressional wave velocity (Vp) and shear wave velocity (Vs) (Chapter 20), while the presence of free gas will strongly decrease Vp. Compressional- and shear-wave attenuation (Q_p ^-1, Q_s ^-1) may also prove to be hydrate/gas indicators: hydrate may increase both Q_p and Q_s, while gas certainly decreases Q_p (Wood and Ruppel, 2000). Accurate, detailed images of the elastic properties of hydrate deposits therefore hold great promise for remotely quantifying hydrate and gas occurrence and concentrations.