Velocity Modelling Results Of A Pockmark In The Nyegga Region, Norwegian Sea

Pockmarks are a common indicator of fluid flow through the seafloor at continental margins. They are thought to be of global significance as pathways for the escape of methane from the sediments to the water column beneath continental margins, and also as habitats for chemosynthetic communities of biota. Their mechanism of formation and internal dynamics remain poorly constrained, partly due to a lack of proper three dimensional imaging of their internal structure. Numerous fluid escape features provide evidence for an active fluid-flow system in the Nyegga region of the Norwegian continental margin. In June-July 2006 we conducted a high-resolution seismic experiment using ocean bottom seismometers (OBS) to investigate the detailed 3D structure of a 250-m-wide pockmark in this region, named G11 and hence to determine the distribution of gas and gas hydrate in and around the pockmark . An array of 14 OBS were deployed across the pockmark. Shots fired from two 35 cu. in. mini GI guns were recorded on these OBS and on a near surface hydrophone streamer. Shot and OBS locations were determined with c. 1 m uncertainty using water wave arrivals. The OBS and reflection data reveal many interesting features of the subsurface geology of the chimney. A group of bright reflectors underlies the pockmark at a travel time of c. 1.4 s and at deeper depths some of the reflectors show strong attenuation indicating the presence of gas in the sediments. A pipe ascends from this gas charged zone to where it terminates in the investigated G11 pockmark. We have analysed data from five OBS lying along a line running NE-SW across the pockmark, using both raytracing and reflection tomography. The raytraced forward model, which incorporates signals from six subsurface reflectors, shows, a gradual increase in velocity between the seafloor and the gas charged zone, lying c. 300 m depth below the seabed. The travel-time fit is improved significantly if velocities in the pipe beneath the pockmark are higher than those in the surrounding sediments. The maximum velocity anomaly is c. 6%. An initial tomographic inversion using signals from a single subsurface reflector at the top of the gas-charged zone also recovers a velocity anomaly of c. 6%. This velocity anomaly may be attributed to the presence of increased saturations of methane hydrate beneath the pockmark.