10 June: Today marks the sixth day of our seismic reflection survey.

Dylan Baker & Andrew Gorman, University of Otago

In a marine seismic reflection survey, an airgun is used to generate seismic waves that propagate outwards into the ocean and subsurface. Seismic waves travel until they encounter a medium with different seismic impedance (seismic impedance is the product of density and seismic velocity). At these boundaries, some wave energy is transmitted and some wave energy is reflected. Waves which have been reflected towards the surface can be recorded by the long streamer that is being towed behind the ship. The streamer contains hydrophones that convert pressure changes caused by reflected seismic waves into an electrical pulse. By determining where each wave has been reflected from, a picture of the subsurface can be developed.

The last two and a half days of our seismic reflection survey have targeted various gas hydrate systems along the southern coast of the North Island. Gas hydrate is an ice-like substance which is made up of a central gas molecule trapped in a cage of hydrogen bonded water molecules. Gas hydrate is stable at low temperatures and high pressures within the gas hydrate stability zone (GHSZ). Gas hydrate can be inferred in seismic data primarily from a bottom simulating reflection (BSR). BSRs are an anomaly mostly caused by gas that accumulates beneath the GHSZ causing a contrast in seismic impedance. It is important to study gas hydrates as they play a significant role in hydrocarbon storage, climate change, and geohazard generation. Our work in this region is focusing on constraining the extent and dynamics of three previously identified gas hydrate anomalies using the high-resolution GI-gun seismic system on board the Roger Revelle.

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The science team deploying the seismic line.

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Andrew and Dylan (advisor and Master’s student, respectively) pause for a photo during deployment.

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