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

Abstract

Over the last decade pockmarks have proven to be important seabed features that provide
information about fluid flow on continental margins. Their formation and dynamics are still
poorly 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 the
Norwegian margin, specifically in the Nyegga region. In June-July 2006 a high-resolution
seismic experiment using Ocean Bottom Seismometers (OBS) was carried out to investigate the
detailed 3D structure of a pockmark named G11 in the region. An array of 14 OBS was deployed
across the pockmark with 1 m location accuracy. Shots fired from surface towed mini GI guns
were also recorded on a near surface hydrophone streamer. Several reflectors of high amplitude
and reverse polarity are observed on the profiles indicating the presence of gas. Gas hydrates
were 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 to
escape through the water-saturated sediments in the chimney without being entirely converted
into gas hydrate. An initial 2D raytraced forward model of some of the P wave data along a line
running NE-SW across the G11 pockmark shows, a gradual increase in velocity between the
seafloor and a gas charged zone lying at ~300 m depth below the seabed. The traveltime fit is
improved if the pockmark is underlain by velocities higher than in the surrounding layer
corresponding to a pipe which ascends from the gas zone, to where it terminates in the pockmark
as seen in the reflection profiles. This could be due to the presence of hydrates or carbonates
within the sediments.

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Identifiers

ORCID for T.A. Minshull: orcid.org/0000-0002-8202-1379

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