Results

I applied the fitting goals (4) and obtained a $\bf \Delta s$ (Figure ). The change introduced by tomography was a spatially low frequency increase in the velocity below the second tomography reflector. The decrease in slowness agreed with previous 2-D tomography tests Clapp and Biondi (2000). After adding in the slowness I ended up with an updated velocity field (Figure ). The new velocity field is very consistent with the initial model (Figure ).

 

ds-3d
Figure 6 Changes to velocity computed from first iteration of tomography.

 

elf3d.vel1
Figure 7 Velocity after one iteration of tomography.

Using the updated velocity field I applied CAM. Figure  shows the migration result. Generally, the image quality is about the same or slightly improved. Note that image is worse at `B' (compared to the initial migration (Figure ). At `A' the initial image shows better coherence but the new migration has a higher frequency content. A sharper image can also be observed at `C' - `F'. If we compare common reflection point gathers (Figure  and ) we get a similar story. At `A', `B', and `E' the gathers are flatter and higher frequency. At `C' and `D' where the gathers curved up initially they now curve down.

 

elf3d.mig0.v2
Figure 8 Migration result using initial velocity model. Overlaid are several locations where image quality changes with the new velocity model.

 

elf3d.mig1.steer
Figure 9 Migration result after one iteration of tomography. Note that image is worse at `B' (compared to the initial migration (Figure ). At `A' the initial image show better coherence but the new migration is higher frequency content. A sharper image can also be observed at `C' - `F'.

 

gather.vel0
Figure 10 Every 20th CRP from an inline section of the initial migration.

 

gather.vel1.steer
Figure 11 Every 20th CRP from an inline section of the migration after one iteration of tomography. At `A', `B', and `E' the gathers are flatter and higher frequency than the gathers in Figure . At `C' and `D' where the gathers curved up initially they now curve down.