Gas hydrates off Eastern Canada: Velocity models from wide-angle seismic profiles on the Scotian Slope

Abstract

Coincident single-channel reflection and wide-angle ocean bottom seismometer (OBS) reflection/refraction profiles were taken on the central continental slope off Nova Scotia in a region of possible gas hydrate occurrence, as evidenced by recent identification of bottom simulating reflectors (BSRs). Joint travel time inversions of wide angle reflections and refractions in the OBS data result in three similar velocity models, which all display anomalies consistent with the presence of gas hydrate and underlying free gas. A high velocity zone (HVZ), 60–120 m thick, has velocity increases of 70–130 m/s above a low velocity zone (LVZ), 150–260 m thick, where velocities decrease by 50–75 m/s. Sensitivity analysis indicates that the refracted phase from the HVZ offers better velocity resolution than the reflected phase from the top of the LVZ. Comparison of synthetic seismograms for alternate models can further help to resolve details in the structure of the HVZ and LVZ layers. Depth migrations of the reflection profiles using the velocity models show that the depths of the velocity inversions and BSRs are consistent. Preliminary predictions of 2–6% bulk gas hydrate and <1% free gas concentrations are made using an effective medium model for the best constrained OBS velocities. These values are smaller than similar predictions for ODP drill sites on the Blake Ridge and Vancouver Margin; but they are consistent with generally lower porosity within the normally consolidated, clay-rich upper sediment layers of the Scotian Slope. Similarity of velocity profiles above the BSR indicate that gas hydrate concentration is similar at all three OBS locations. However, dissimilarity of the LVZ models, amplitude variations of the BSRs, and variations in reflectivity within the LVZ suggest that the free gas concentration is laterally heterogeneous. Reflection profiles indicate that higher concentrations of gas may occur beneath the Mohican Channel and an adjacent sediment slump scarp. These coincidences suggest that previous sediment failures on the margin, which occurred during recent glacial periods, may have been influenced by the presence of gas hydrate and/or have resulted in the release of methane gas.