From Geology to Impedance
Learning objectives
- Explain that seismic responds to impedance, not to facies directly
- See a channel sand fade from the section as its impedance nears the shale
- Understand why fluid, which moves impedance, can make or break an anomaly
- Identify the geology-to-impedance step as the one a synthetic must get right
Seismic Sees Impedance, Not Rock
It is tempting to think of a seismic section as a picture of geology. It is not. Seismic responds only to acoustic impedance, and geology enters the picture solely through the impedance it happens to carry. Two different rocks with the same impedance are seismically identical; the same rock with two different fluids can look completely different. This section makes that concrete with a channel sand cut into shale.
The Vanishing Channel
On the facies map the channel is unmistakable, a clean sand body in shale, and it never changes as you move the slider. The section is another story. Slide the channel's impedance toward the shale value and its reflection fades to nothing. At the point where sand and shale impedance match, the channel is invisible on the section while sitting, obvious, in the geology beside it. Give the sand a gas fill and its impedance drops far below the shale, so it brightens into a strong anomaly.
Two lessons follow. First, the step a synthetic absolutely must get right is geology to impedance, the rock-physics link, because a section built on the wrong impedance is a confident picture of the wrong thing. Second, because fluids move impedance, the same sand body can be a bright anomaly in a gas field and invisible in a wet one. That fluid sensitivity is the seed of AVO, which reads how the reflection changes with angle to separate fluid from rock, and which Part 6 builds. For now, hold the idea that the section is a map of impedance contrast, and geology reaches it only through that lens.