Stacking and Interval Velocity

The velocity analysis of the data is performed in Promax, separately from the processing. The raw data is gained and decimated at the far-offsets to eliminate the traveltime kink in the reflections. I then generate supergathers at every 25th CMP location by combining 10 adjacent gathers. Since the sediment structure at the Blake Outer Ridge is fairly simple, a conventional NMO stacking velocity analysis is used to obtain good RMS velocities. The resulting 2-D velocity field can be seen in Figure .

 

vrms
Figure 6 RMS stacking velocities.

 

vint-ann
Figure 7 Interval velocities.

Using Dix's equation Dix (1955), the RMS velocities are converted into a physical interval velocity model, which is displayed in Figure . The velocity is characterized by an increase to approximately 1.9 km/s above the BSR. Such a velocity increase due to the presence of hydrate is a commonly observed characteristic of sediments containing methane hydrates Andreassen et al. (1995); Minshull et al. (1994); Rowe and Gettrust (1993); Singh et al. (1993); Yuan et al. (1996). Underneath the BSR, the velocity drops to approximately 1.7 km/s, which might be indicative of the presence of free gas beneath the BSR. Consequently, the flat reflector underneath the BSR might be the transition zone from gas-saturated sediment to brine-saturated sediment. The observed drop in velocity at the BSR is supported by the negative reflection polarity at the BSR, which already suggested a velocity reversal at the transition from hydrate-bearing sediment to the sediment underneath.

Between a lateral distance of 0 and 25 km, where no BSR is visible, the velocity is characterized by a steady increase with increasing depth. No pronounced velocity anomaly is present.