Seismic Velocity Techniques and Bottom Simulating Reflections

2 Seismic Velocity Techniques and Bottom Simulating Reflections 

Seismic velocity techniques for hydrate detection have two components (1) translation of seismic signals to velocity and (2) translation between velocity and detection of hydrates. The first component is beyond the scope of this monograph. However, a brief consideration will be given to advances in translating velocity to the detection of hydrates. 

For hydrates in ocean sediments, the technology for detecting the BSR was determined in 1953 with the development of a precision ocean depth recorder (Hamblin, 1985, p. 11). In this technique a sonic wave penetrates (and is reflected from) the ocean floor, with the time recorded for the return of the reflected wave to the source. Velocity contrasts beneath the ocean floor mark a change in material density, such as would be obtained by hydrate-filled sediments overlying a gas. BSRs related to hydrates are normally taken as indications of velocity contrasts between velocity in hydrated sediments and a gas, marked by a sharp decrease in sonic compressional velocity (Vp) and a sharp increase in shear velocity (Vs) (Ecker et al., 1996). 

Hyndman and Spence (1992) indicated that about one-third of the pore space should be filled with hydrate to give a BSR impedance contrast at the Cascadia 

Margin the sediments contain 15-20% (volume) hydrate. This 33% of the total might be taken as a maximum for the hydrate indicated by a BSR. Hyndman and Davis (1992) indicated that the Vp decrease (1) suggested a gradational boundary with the thickest hydrate at the BSR and lesser hydrate concentrations above the BSR and (2) that the BSR did not require a gas layer beneath the hydrate. 

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