Fresnel zones for broadband data

For monochromatic waves, the term “Fresnel zone” is well‐defined even though different authors use different terminology. Most authors use the definition originating from optics. There, the first Fresnel zone is defined as the area of a circular hole in a screen between a light source and an observation point that produces maximum light intensity in the observation point (Figure 1). If the radius of the hole is enlarged, minima and maxima in light intensity alternate. The first maximum is reached if the raypath difference between the direct ray and the ray traveling via the edge of the hole equals half a wavelength. The extension of the definition to energy reflected from a circular disk is straightforward (if we restrict ourselves to ray theory and neglect the angle dependency of the reflection coefficient) and is illustrated in Figure 2 (see also Sheriff, 1991).