Offset VSP data is often undersampled because the sources and/or receivers are not properly placed to provide adequate reflection coverage of the target. The quality of an offset VSP seismic image is a direct function of the source locations, receiver locations, target depth, velocity function at depth, and the near-surface velocity. Of these factors only the source and receiver locations are under the survey planner's control. Unfortunately there is no simple rule of thumb to follow in planning the optimal source and receiver placement for an offset VSP.

We have found that the most efficient and lowest-risk way to plan an offset VSP survey is to begin with a 2D velocity model of the survey area, like the model in Figure 1, and compute finite difference shot records with very dense source and receiver coverage. The coverage is much more dense than would be practical in a real survey.

 

Reflections in the far offset shot (left) and mid-range shot (center) have strong interference from near-surface reverberations. Some data recorded in the upper 1/3 of geophones probably could not be used for processing.

A Modeling/Processing Example

In addtion to creating accurate estimates of reverberation noise, finite difference modeling, combined with migration, gives a more accurate prediction of imaging results than simple illumination modeling. Figure 3 shows the velocity model in Figure 1 overlayed by the image we obtained by migrating shots with a source spacing of 1000 ft and receivers from 100 ft depth to 5000 ft depth at a 50 ft interval.

The reflection image is dim in the area around the receiver well, particularly the interface sloping upward and to the left of the receiver well. By adding source points at an interval of 400 ft within 3000 ft of the receiver well the sloping area becomes much more clear (Figure 4). We also learned that the receiver depth range could be reduced to 500 through 3500 ft and still maintain the image quality.