We present a theoretical and experimental analysis of the mirage detection and measurement of a semi-infinite horizontal buried slab of air within opaque solids. A detailed study on the effects introduced by the mirage parameters (pump and probe beam sizes, probe beam height) as well as by the surface features is discussed. We use pump and probe beams orthogonally superimposed, and detection is made of the amplitude and phase of the normal component of the mirage deflection. Air slabs with a thickness (e2) as low as 0.1 μm and located down to a depth (e1) of 0.3√ks (ks is the thermal diffusivity of the material) can be easily observed. Detection improves as the pump radius (a) and probe height (z0) increase, although a compromise is necessary because of signal attenuation. Determination of e1 and e2 through the model fitting, requires an independent control of a and z0. The use of low frequencies avoids the effects of surface features on the slab detection. However, for surface planar horizontal steps, their height must be incorporated into the model as a variation of z0 in order to characterize the slab.