Seismic images of gas conduits beneath vents and gas hydrates on Ritchie Ridge, Hikurangi margin, New Zealand

Recently acquired seismic reflection data across the southern edge of Ritchie Ridge, a prominent bathymetric high on the Hikurangi margin, display zones of high amplitudes and reflections that crosscut strata. We interpret the latter as bottom-simulating reflections which are commonly associated with gas beneath gas hydrates. An analysis of reflection strength indicates the high-amplitude zones are caused by free gas in the pore space of sediments, probably migrating upward along layers. One of the high-amplitude regions is situated beneath the projected location of a known gas vent site. The seismic data appear to image the conduits that supply this vent site with gas. The seafloor in most of the study area is likely to be within the zone of gas hydrate stability, depending on bottom water temperatures and hydrate cornposition. Hence, gas appears to be venting through the gas hydrate stability zone, favouring locally high concentrations of gas hydrates.