Seismic velocity studies of a gas hydrate bottom‐simulating reflector on the northern Cascadia continental margin: Amplitude modeling and full waveform inversion

Abstract

On the northern Cascadia subduction margin, the multichannel seismic amplitude‐versus‐offset (AVO) behavior of a bottom‐simulating reflector (BSR) suggests a P wave velocity change from high‐velocity hydrate‐bearing sediment to lower velocity sediment containing a small amount of free gas. The observed nonlinear AVO behavior, constant or slightly decreasing amplitudes at near‐to‐mid offsets and a large‐amplitude increase at far offsets, can be reproduced in the models if an S wave velocity enhancement is assumed as expected from hydrate cementation. The AVO behavior of the hydrate BSR is found not to be as useful as was earlier thought for determining the amount of free gas below the BSR. This is because Poisson's ratio change below the BSR due to gas is likely very small in high‐porosity unconsolidated sediments. The uncertainty in the models is large, as there is no reliable S wave velocity information for the sediments containing hydrate or gas. AVO modeling alone is not sufficient to distinguish different velocity models across the BSR. Our interpretation of the BSR amplitude behavior is that a P wave velocity increase above the BSR is the main cause of the BSR reflection amplitude increase at large incidence angles. Caution must be taken in applying AVO analysis, as little is known about S wave velocities. However, subtle differences in BSR amplitude behavior and reflection waveform can provide constraints through very careful full waveform inversion. A well‐defined reference velocity‐depth profile is also required to represent water‐saturated sediment unaffected by either hydrate or free gas. Using full waveform velocity inversion, a high‐resolution velocity model for the hydrate BSR has been derived. The best fit model for the seismic data near the Ocean Drilling Program (ODP) sites 889/890 consists of a high‐velocity zone above the BSR and a thin low‐velocity layer below, in agreement with the ODP downhole velocity data.