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Hydee-OBS Support of marine hydrate volume estimates by converted shear wave records

Hydee-OBS Cruise Report

Scientific Discipline: Geophysics

Research Area: Hikurangi Margin, North Island, New Zealand

Research Vessel/Equipment: RV Tangaroa

PI Name and Affiliation: Dr Joerg Bialas, GEOMAR Helmholtz Center for Ocean Research Kiel

Date: 21 - 31 March 2023

Citation from PI: Eurofleets+ Program Oceans provides access to research vessels for small projects that would otherwise be difficult to undertake on regional or ocean class vessels. Engaged program managers at all engaged offices helped a lot to realise the project.

Abstract & Main objectives: The overarching motivation of this cruise is to move towards much improved estimations of gas and gas hydrate distribution and concentration in a typical active subduction margin. Estimates of the global gas hydrate reservoir vary widely as do geophysics-based estimations of hydrate saturation at local-scale gas hydrate reservoirs. The first aim is to test the hypothesis that: The co-existence of gas and gas hydrate within gas hydrate accumulations is the reason for the discrepancy in predicting hydrate saturation from seismic-based and CSEM (controlled source electromagnetic) -based methods. The converted S-wave data will be acquired using a high-frequency source (150 cu-in GI airgun), short shot intervals (⁓10 m) and closely spaced OBS nodes (100 m spacing). The second aim is to determine the dominant mechanism(s) driving concentrated gas hydrate formation within an accretionary wedge setting. The hypothesis that concentrated deposits are formed by free gas being injected into the gas hydrate stability zone (GHSZ) by “long-range” gas migration rather than by “short-range” methane migration will be investigated. Existing seismic reflection data show indications of gas injection, but seismic resolution limitations in those data make it unclear how free gas is distributed and whether “tuning effects” partially cause the seismic reflectivity observed.

Map of the existing 2D multichannel seismic network along the Hikurangi Margin.
The RV Tangaroa leaving Wellington port in good weather
Figure 3: Layout of ocean-bottom seismometers (OBS) plotted above multichannel seismic images of two hydrate formations (a) and (b). Seismic events interpreted to represent gas injection into the GHSZ at the landward (NW) termination of the gas hydrate deposits are labelled “free gas”. The less complicated structure in (a) is the primary target of the proposed experiment. The more complicated feature in (b) is our secondary target. Here positive reflection events cross-cut the layering at three different depth levels.
A variety of photos showing the ocean-bottom seismometers (OBS) being prepared, deployed and recovered during the Hydee-Obs cruise.
Photo 1 shows the streamer being prepared; the streamer cable is connected section by section. After confirmed test routine the section is coiled on to the winch drum. Photo 2 shows the streamer cable and airgun (red bulb) being towed behind the vessel.
RV Tangaroa returning to port in not so favourable weather conditions.