FIG. F-25. Fresnel zones. (a) For coincident source and receiver at S, the first Fresnel zone radius is R1 (perpendicular to h). The second Fresnel zone is the annular ring. Higher-order zones (not shown) are also annular rings. The dominant wavelength is λ. Another way of thinking of this is that a reflecting point in the subsurface influences a similar Fresnel-zone region on the surface. (b) The build-up of energy integrating outward from the reflecting point. (c) Migration collapses the Fresnel zone to a much smaller area, but 2-D migration collapses it in only one direction.