Jones, Andrew T., Greinert, Jens , Bowen, David A., Klaucke, Ingo , Petersen, Carl Jörg, Netzeband, Gesa and Weinrebe, Reimer Wilhelm (2010) Acoustic and visual characterisation of methane-rich seabed seeps at Omakere Ridge on the Hikurangi Margin, New Zealand. Marine Geology, 272 (1/4). pp. 154-169. DOI 10.1016/j.margeo.2009.03.008. Date online first: 2009

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Abstract

Six active methane seeps and one cold-water reef that may represent a relict seep were mapped at Omakere
Ridge on New Zealand's Hikurangi Margin during cruises SO191 and TAN0616. Hydroacoustic flares,
interpreted to be bubbles of methane rising through the water column were identified in the area. The seep
sites and the cold-water reef were characterised by regions of high backscatter intensity on sidescan sonar
records, or moderate backscatter intensity where the seep was located directly below the path of the
sidescan towfish. The majority of sites appear as elevated features (2–4 m) in multibeam swath data. Gas
blanking and acoustic turbidity were observed in sub-bottom profiles through the sites. A seismic section
across two of the sites (Bear's Paw and LM-9) shows a BSR suggesting the presence of gas hydrate as well as
spots of high amplitudes underneath and above the BSR indicating free gas. All sites were ground truthed
with underwater video observations, which showed the acoustic features to represent authigenic carbonate
rock structures. Live chemosynthetic biotic assemblages, including siboglinid tube worms, vesicomyid clams,
bathymodiolin mussels, and bacterial mats, were observed at the seeps. Cold-water corals were the most
conspicuous biota of the cold-water reef but widespread vesicomyid clam shells indicated past seep activity
at all sites. The correlation between strong backscatter features in sidescan sonar images and seep-related
seabed features is a powerful tool for seep exploration, but differentiating the acoustic features as either
modern or relict seeps requires judicial analysis and is most effective when supported by visual observations.

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