VELOCITY STRUCTURE OF A METHANE HYDRATE STABILITY ZONE, OFFSHORE VANCOUVER ISLAND

Abstract

The occurrence of methane hydrates within marine sediments of the accretionary prism of the Cascadia Margin, offshore Vancouver Island has been inferred by the occurrence of a bottom simulating reflector (BSR), which is believed to mark the phase boundary between methane hydrate and methane gas. A seismic study in May 1997 was undertaken in order to use wide-angle seismic data, with deployment of ocean bottom seismometers (OBSs), to model velocity-depth gradients within the methane hydrate stability zone. Wide-angle seismic data from a single OBS station is analyzed from 40 cu. in. and 300 cu. in. airgun sources along one line (Line 1) along a ridge structure where a strong BSR is known to occur. Velocity-depth modelling of the hydrate zone consists of 2 stages: 1) forward modelling of the layers to fit travel-time curves based on the given reflectors from the 40 cu. in. airgun source, and 2) using reflected and refracted phase arrivals from the 300 cu. in. airgun source to constrain the velocity gradients. The BSR displays variability with source frequency, with discontinuities in the BSR at vertical incidence observed with the higher frequency airgun data. The proposed model infers a low velocity gradient to depths slightly above the BSR, and a thin zone of high velocities (-32m) directly above the BSR ranging from 1.79 to 1.86 km/s, which has a higher amount of hydrate concentration ( -15-20% ). Existence of a relatively thin layer of free gas (-22m) with low velocities ( -1.65 km/s) is postulated to exist below the BSR based on a strong reflector which occurs below BSR depths that might represent the base of the free-gas zone. The velocity models proposed support the proposition that the relative strength of the BSR might be the result of more than the amount of hydrate concentration within the methane hydrate stability field; instead, the continuity and strength of the reflector may be more closely linked to the nature of the underlying free gas zone.

Keywords: methane hydrates, BSR, Cascadia Margin, wide-angle seismics, velocity modelling