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The Hikurangi Margin, east of the North Island of New Zealand, contains a significant gas hydrate province. However, the distribution, concentration and dynamics of hydrate accumulations in the southern portion of the margin (the Pegasus Sub-basin) off the northeastern coast of the South Island are poorly constrained due to a lack of data. In late 2009 and early 2010, a seismic dataset consisting of approximately 3000 km of long-offset 2D seismic data was collected in the Pegasus Sub-basin. Bottom-Simulating Reflections (BSRs) are widespread in the data, and they are supplemented by other features that may indicate the presence of free gas and gas hydrates in zones of high concentration. We present the results of reprocessing and analysis of the data that has focussed on the identification of such specific gas-related features.

Methane is one of the most aggressive greenhouse gases driving climate change. Occurrence of marine gas hydrates depends on temperature, pressure, available gas and fresh water. Therefore changes in pressure and bottom water temperature will influence the formation or dissolution of gas hydrates. In general both parameters vary slowly and hence changes do not result in large Methane contribution to the atmosphere. An exception could be a sudden dissolution of larger quantities of gas hydrate with a related expulsion of Methane. Such focused fluid flow appears as funnel-shaped depressions at the seafloor, so called “pockmarks”. Typical dimensions are within a few hundreds of meters. However, five to twelve kilometre wide “giant pockmarks” (GP) are known as well. The mechanism of formation of GPs is not fully understood. New 2D and 3D multichannel seismic data have been acquired across two fields of giant pockmark-like features to better understand the formation of these depressions and potential for catastrophic methane release in the past.

Pockmarks have been observed on the Chatham Rise east of New Zealand over an area of >20,000 km2. Their formation has been linked primarily to gas hydrate dissociation during glacial-stage sealevel lowstands. We here show first images from a recent high-resolution 3D seismic images targeting the sub-surface structure of a giant pockmarks. Results show in particular that the pockmark may be underlain by a large gas chimney.