Problems in ADCIGs

Although the angle domain allows us to avoid multipathing, in complex media it will still be plagued by problems such as poor illumination. Poor illumination can be caused by energy being lost by leaving the survey area, energy becoming evanescent at boundaries such as salt interfaces, and focusing/defocusing caused by the complex overburden. In the angle domain, energy leaving the survey may do so at some middle range of reflection angles. Figure  demonstrates this with raytracing through a simple velocity model provided by The Rice Inversion Project. The maximum offset recorded in this model is 3000 meters, so rays that are separated by more than 3000 meters at the surface represent energy that is lost. The velocity model consists of one horizontal layer beneath a strong low velocity lens. It is clear that some of the energy is leaving the bounds of the survey. This will result in the shadow zones in the common ray parameter section and the holes in the angle domain CIG seen in Figure . The poor illumination can be seen most clearly in the depth slice (top panel of Fig. ). If there were no illumination problem, the triangle would be solid black. The white streaks through the triangle are caused by the lost energy.

 

symesray Figure 6 Raytracing through a velocity model with a strong low velocity lens. The maximum offset recorded in this model is 3000 meters, so rays that are separated by more than 3000 meters at the surface represent energy that is lost.

 

holey Figure 7 Migration result from the velocity model shown in Figure . The common phx section (left panel) has shadow zones positive and negative 600m in CRP. The ADCIG (right panel) has clear ``holes'' in the events caused by energy leaving the bounds of the survey, as demonstrated by Figure . The depth slice (top panel) shows the illumination problem as white streaks through the black triangle.