Temporal variability of gas seeps offshore New Zealand: multi-frequency geoacoustic imaging of the Wairarapa area, Hikurangi margin

Cold seeps on Opouawe Bank, situated in around 1000 m water depth on the Hikurangi Margin offshore North Island. New Zealand, were investigated using multibeam bathymetry, 75 and 410 kHz sidescan sonar imagery, and 2–8 kHz Chirp sediment echosounder data. Towed video camera observations allowed ground-truthing the various geoacoustic data. At least eleven different seep locations displaying a range of seep activity were identified in the study area. The study area consists of an elongated, northward-widening ridge that is part of the accretionary Hikurangi Margin and is well separated from direct terrigenous input by margin channels surrounding the ridge. The geoacoustic signature of individual cold-seep sites ranged from smooth areas with slightly elevated backscatter intensity resulting from high gas content or the presence of near-surface gas hydrates, to rough areas with widespread patches of carbonates at the seafloor. Five cold seeps also show indications for active gas emissions in the form of acoustic plumes in the water column. Repeated sidescan sonar imagery of the plumes indicates they are highly variable in intensity and direction in the water column, probably reflecting the control of gas emission by tides and currents. Although gas emission appears strongly focused in the Wairarapa area, the actual extents of the cold seep structures are much wider in the subsurface as is shown by sediment echosounder profiles, where large gas fronts were observed.