High-resolution OBS modeling beneath Honeycomb and Glendhu ridges on the southern Hikurangi subduction margin reveals concentrated gas hydrate accumulations in unprecedented detail

Gas hydrates are relevant to global carbon cycling, climate change and ocean acidification. In particular, hydrates play an important role in sub-seafloor fluid migration because they reduce the porosity and permeability of sediments. Gas hydrates, and their associated underlying free gas zones, have also been linked to sediment failure and submarine mass transports. The active Hikurangi Margin hosts New Zealand’s largest gas hydrate province, with concentrated accumulations generally focused below accretionary thrust ridges.

Recently acquired high-resolution Ocean-Bottom-Seismometer (OBS) data at the southern Hikurangi Margin images highly reflective layers beneath the accretionary Honeycomb Ridge. This ridge is of particular interest as it is thought to host a concentrated gas hydrate system. Unlike previous surveys, we have the advantage of being able to record converted shear waves that help us identify the nature of the highly reflective layers in the gas hydrate stability zone. In March 2023, we deployed 20 OBS from R/V Tangaroa with a USBL-wired system to position each OBS with 100 m spacing along an existing 2D seismic profile. A 150 in³ GI-gun was fired at a shot rate of 7 s, to ensure for excellent lateral and vertical resolution. This setup allows us to present an updated high-resolution seismic velocity model and inversion of Honeycomb Ridge, and partially Glendhu Ridge.

OBS data were processed in Seismic Unix and Vista 2023. In Vista 2023, the data were flattened, filtered with Ormsby bandpass, FK-filter and a threshold median noise attenuation and reduction (THOR) filter. Reflection and refraction phases were picked with PASTEUP and used for forward modeling with MODELING (RAYINVR). The detailed P-wave forward model served as input for the 2D tomography inversion (TOMO2D). The tomography for 8 iterations results in a χ² of 2.1 and RMS-fit of 30 ms.

The P-wave tomography confirms a low velocity zone below the BSR in both ridges. Higher velocities are resolved in the landward limbs of the ridges compared to seaward limbs in agreement with previous findings. The areas of higher velocities correspond to high-reflectivity layers in the seismic data. We suggest that the anomalously high-reflectivity layers above the BSR in the ridge represent concentrated gas hydrate accumulations, fed by underlying free gas via stratigraphic pathways that enable fluid migration into the system. We also aim to test whether positive and negative polarity reflections within the regional gas hydrate stability zone are due to simultaneous presence of gas hydrates and free gas, respectively. Supplementary analysis of S-waves will allow us to test our hypothesis that free gas is injected into the hydrate stability zone and remains, at least partially, in the gaseous phase. Our detailed study contributes to a better understanding of how gas hydrate systems and fluid migration pathways evolve at active margins.