Comparison with VSP Data

In order to further quantify these possible errors in interval velocities, I compare them with two vertical-seismic-profiling (VSP) velocity functions obtained in wells 995 and 997 (see Figure ), both of which penetrated a strong BSR in the region of the Blake Outer Ridge. The comparison can be seen in Figure , where the measured interval velocity of the CMP gather in Figure is overlain by the VSP velocity measured in well 995 (dashed line) and the one in well 997 (dotted line). Since the VSP measurements are from a region of less seafloor and, consequently, BSR depth, the high and low-velocity layers are slightly shifted in depth with regard to the one from surface seismic.

 

velcomp-ann Figure 11 Measured interval velocity (solid line) compared with VSP velocity from well 995 (dashed line) and the one from well 997 (dotted line).

The velocity functions displayed by the VSP data confirm the presence of a high-velocity hydrate zone and a low-velocity zone underneath. The velocity increase due to the presence of hydrate is approximately 100 m/s less in the VSP data compared to the seismic data. The VSP data, on the other hand, show a significantly stronger decrease in velocity underneath. These discrepancies in velocities can be caused by several factors. First, the VSP and seismic measurements were not conducted in the same region; thus neither the hydrate nor the sediment structure need to be equivalent. Furthermore, both measurements are done with different frequency spectra, resulting in different resolutions. The comparison shows, however, that both the VSP and seismic data are characterized by an increase in velocity in the hydrate zone and a subsequent decrease underneath. This velocity structure at the Blake Outer Ridge was additionally confirmed by Wood et al 1994, Katzman et al. 1994 and Korenaga et al. 1997.