New Constraints on Gas and Gas Hydrate Concentration Estimates on the Cascadia Margin from Long-Offset Multichannel Seismic Data.
Abstract
Recent length extensions to the hydrophone streamer towed by R/V Marcus G. Langseth have resulted in significantly more accurate sound speed profiles than were previously possible, particularly in areas of deep, relatively flat lying sediments. However, due to the crossing reflection and refraction arrivals at far offsets (typically greater than about 6 km), taking full advantage of the far offsets requires a velocity inversion beyond that of the traditional semblance scan. We have developed such an inversion and apply it to data acquired in the summer of 2021 (MGL2104) offshore Vancouver Island, Canada in an area well studied and drilled for gas and gas hydrates (IODP Leg 311). We use this new dataset to quantify P-wave velocity profiles and we track the uncertainty reduction from the inversion as offset increases. We analyze velocity profiles at each individual CMP along MGL2104 Line PD04 which transects the area drilled during IODP Leg 311. We use sonic logs from the drilling expedition to verify the seismic velocity inversion. The accuracy increase and uncertainty reduction are greater in the shallower section, less than a few hundred meters below the seafloor, where the angular coverage is greatest. These improved P-wave velocity profiles provide tighter constraints on the distribution and concentration of gas and gas hydrate in the subsurface in this area. Using these constraints, we were able to identify an isolated velocity-amplitude anomaly (VAMP), likely associated with a methane gas accumulation, in about 2600 m water depth, far from accreted sediments that were targeted during drilling [work supported by ONR].
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFMOS12C0762W