FIELD DATA EXAMPLE

This section describes an exploratory study of a marine dataset recorded over a salt structure. The dataset is from Conoco, and was donated to SEP by Western Geophysical Co. that did most of the preprocessing. It contains 239 shots, each consisting of 84 geophones. The geophone spacing equals the shot spacing, and is 30 m. The near-offset distance is 75 m. The time sampling interval is 4 ms, the total time 3 s.

Figure  shows two shot gathers, Figure  displays two CMP gathers at about the same location. The reflections at 1.3 s and 1.8 s are reflections from the top and bottom of a salt layer. The extent of the salt structure is shown on the near-offset (Figure ) and stacked section (Figure ). The velocity function used for the stack is displayed in Figure .

The top and bottom of the salt are reasonably well-defined in the left and right part of the section, but do not show up clearly in the middle part. The salt has flowed upward in this middle part, bending and faulting the sediments above it. These warped sediments cause lateral velocity variations, which become even more pronounced below the salt layer because of variable salt thickness. The moveout of the reflection events is therefore non-hyperbolic as can be seen in Figure , and stacking-velocity analysis breaks down.

Figures  and  show the result of such a velocity analysis at two midpoints. The salt top reflection is mostly hyperbolic in the left part of the section (the semblance peak at 1.2 s in Figure  is well-defined), whereas the reflection becomes non-hyperbolic at the dome: different parts of the moveout curve stack in at different velocities, broadening the semblance peaks in the velocity panels. The non-hyperbolicity is caused by lateral velocity variations in the bended sediments above the dome, and probably also by discontinuities in the salt top. The salt bottom reflection is much less apparent in the velocity analysis; not only do lateral velocity variations distort the moveout of this reflection, but it is also much weaker: the high reflection coefficient at the salt boundary prevents seismic energy from penetrating the salt structure.