An interface (reflector) refers to the boundary between two rock layers, each with their respective elastic properties. The rock layers can be reservoir lithologies, non-reservoir lithologies or litho-fluids.

When defining multiple interfaces, the upper layer is typically kept the same and is often a non-reservoir lithology. In some instances, it can be useful to have two different upper-layer non-reservoir lithologies.

The lower layer is typically a litho-fluid combination.