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Earth and Space Sciences · Glossary

What is Fresnel zone?

Definition 2.8 University Earth and Space Sciences — Year 3 · Chapter 2 — Exploration Seismology

The (first) Fresnel zone of a reflector, for a coincident source and receiver at height zz above it, is the disc of reflector around the normal-incidence point from which the two-way path is longer than the normal path by less than half a wavelength: its reflections arrive in phase and build the recorded reflection together.

The first Fresnel zone (schematic): the part of the reflector whose distance from a coincident source and receiver exceeds the normal distance z by less than /4 (dashed arc: the sphere of radius z + /4), so that the two-way excess path is under half a wavelength and its echoes arrive in phase. The scale of is exaggerated.
The first Fresnel zone (schematic): the part of the reflector whose distance from a coincident source and receiver exceeds the normal distance zz by less than λ/4\lambda/4 (dashed arc: the sphere of radius z+λ/4z + \lambda/4), so that the two-way excess path is under half a wavelength and its echoes arrive in phase. The scale of λ\lambda is exaggerated.
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