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Minerals Report - International Seabed Authority

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ottom simulating reflections (BSR) are present on seismic profiles,<br />

although they need to be ascertained by in situ sampling.<br />

4.1.2. Amplitude blanking<br />

The amplitudes of seismic reflections from within the hydrate<br />

stability zone are generally much lower in the area where there is an<br />

observed BSR. Thus, amplitude blanking is a useful seismic attribute to<br />

locate the presence of gas hydrate 39 . Blanking is the reduction of the<br />

amplitude of seismic reflections caused by hydrate cementation. The<br />

amplitudes of reflections are much smaller above the BSR (where<br />

sediments, are cemented by gas hydrates) than they are below the BSR.<br />

This phenomenon consistently appears in sediments containing gas<br />

hydrates, indicating that the changes in acoustic impedance between the<br />

strata are much reduced by hydrate cementation.<br />

4.1.3. Velocity inversion<br />

A third characteristic of gas hydrates is the abrupt decrease in<br />

acoustic velocity across the boundary between hydrate-cemented<br />

sediments above the BSR and non-cemented sediments below that are<br />

water or even gas-filled. This downward velocity decrease, known as an<br />

“inversion”, is detectable with some seismic techniques (Dillon, et al. 4 and<br />

references therein).<br />

The BSR and velocity inversion are only related to the bottom<br />

boundary of the hydrate zone, whereas the top of the hydrate zone<br />

(within sediments) does not produce a sharp reflection, as it has no such<br />

precisely defined boundary 4 .<br />

4.2. Non-geophysical proxies<br />

Apart from the BSRs, which are the most important geophysical<br />

indicators of offshore hydrate deposits, several geological and biological<br />

parameters may be useful tools for reconnaissance.<br />

INTERNATIONAL SEABED AUTHORITY 530

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