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BSRs (Bottom Simulating Reflectors) are special reflective layers in the seafloor that indicate the transition from stable gas hydrates to free gas. Some gas hydrate provinces have been showing a remarkable phenomenon for decades: double or even multiple BSRs. These structures could indicate the presence of different types of gas hydrate with different stability ranges. In many cases, however, the deeper BSRs appear to be remnants of an earlier boundary of the gas hydrate stability zone. This was formed in the geological past under different pressure and temperature conditions.
The persistence of these ancient BSRs is unusual and could have two main causes:
(a) Dissolution of gas hydrates is very slow, even when environmental conditions change.
(b) Residual free gas remains even after the gas hydrates have completely dissolved.
Despite progress in the study of gas hydrates, uncertainties remain about their distribution and concentration. Predictions based on seismic methods using compressional and shear wave velocities often differ significantly from the results of electromagnetic measurements using electrical resistivity.
This is where the HYDEE OBS project comes in: Using high-resolution seismic measurements and ocean-bottom seismometers, it is investigating a prominent BSR structure on the Hikurangi continental margin off New Zealand. These measurements provide converted shear wave data, which are sensitive to changes in the shear modulus. This can be used to determine whether gas hydrates are filling pores or acting as cement.
The results are used in rock physics modelling and laboratory experiments to better estimate hydrate saturation. Converted shear waves also offer a higher resolution than conventional seismic images, providing a detailed view of the layers beneath the seafloor.
In addition, high-frequency signals will be used to better understand possible differences in hydrate distribution. For example, it could be investigated whether there is a higher concentration of hydrate in landward-sloping sedimentary layers due to cementation - a pattern that may not be present on the seaward sides of ridges.