A geophysical study of a pockmark in the Nyegga region, Norwegian Sea

Over the last decade pockmarks have proven to be important seabed features that provideinformation about fluid flow on continental margins. Their formation and dynamics are stillpoorly constrained due to the lack of proper three dimensional imaging of their internal structure.Numerous fluid escape features provide evidence for an active fluid-flow system on theNorwegian margin, specifically in the Nyegga region. In June-July 2006 a high-resolutionseismic experiment using Ocean Bottom Seismometers (OBS) was carried out to investigate thedetailed 3D structure of a pockmark named G11 in the region. An array of 14 OBS was deployedacross the pockmark with 1 m location accuracy. Shots fired from surface towed mini GI gunswere also recorded on a near surface hydrophone streamer. Several reflectors of high amplitudeand reverse polarity are observed on the profiles indicating the presence of gas. Gas hydrateswere recovered with gravity cores from less than a meter below the seafloor during the cruise.Indications of gas at shallow depths in the hydrate stability field show that methane is able toescape through the water-saturated sediments in the chimney without being entirely convertedinto gas hydrate. An initial 2D raytraced forward model of some of the P wave data along a linerunning NE-SW across the G11 pockmark shows, a gradual increase in velocity between theseafloor and a gas charged zone lying at ~300 m depth below the seabed. The traveltime fit isimproved if the pockmark is underlain by velocities higher than in the surrounding layercorresponding to a pipe which ascends from the gas zone, to where it terminates in the pockmarkas seen in the reflection profiles. This could be due to the presence of hydrates or carbonateswithin the sediments