ABSTRACT
As shown in Chapter 9, reflection methods can yield what is considered to be a focused image provided there is normal incidence (Sect. 9.1.3). For the common GPR measurement geometry, there will be normal incidence when the reflecting surfaces are planar such as a horizontal (Fig. 9.11) or a gently sloping interface (Fig. 9.12). It must be reiterated that this is only true for planar interfaces. There can also be normal incidence for curved surfaces, such as a circular reflecting object. However, this target is manifested in the GPR data as a hyperbola (Fig. 9.16) and, unlike a planar interface, this object's shape is not replicated in the radargram. Objects of this type are deemed to be out-of-focus. In geophysical measurements, focus is taken to mean that the shape of a buried object is somehow 'recovered' either directly m the acquired data or in its subsequent manipulation.
