2016 - Wavelet analysis
Up to now, the sun-pulse recorded during the heating (day) and cooling (night) phases has not yet been analyzed by using the infrared thermography (IRT) method through the complex wavelet transform (CWT) technique.
CWT can be used with the sun-pulse data in a similar way as the discrete Fourier transform (DFT).
In addition, CWT preserves the time information of the signal both in the phasegrams and in the amplitudegrams. In this work, a mosaic sample containing artificial flaws positioned at different depths was inspected into the long wave IR spectrum. It is possible to observe that by comparing defective and defect-free areas, a difference in phase during the thermal diffusion appears.
The signal reference, measured on the defect-free area, was subtracted from the other measurement points.
The resulting signal thermal contrast, representing the difference of the temporal evolutions of the surface temperature above the defective and defect-free positions, was also plotted. Subsequently, the wavelet phase contrast was computed.
The solar radiation influencing the sample was estimated bearing in mind the sun path in the sky, the mosaic orientation and the inclination with respect to its local geographical coordinates. Finally, the ambient parameters have been recorded by a control unit.
Although the CWT technique did not provide a sound visualization of the shape of the flaws, it permitted to reflect on the heat release coming from the bituminous material behind the statumen layer.
Indeed, it is not atypical to find inclined mosaics to be restored.
(a) Amplitude of the DFT inherent to the whole time sequence of thermograms at ∼2 h as periodicity. The areas corresponding to the flaws are marked by “x”, while the defect-free areas are pointed out by “+”; (b) the average of the magnitude square (γ2), and (c) the phase (Φ) of the WC calculated between the areas corresponding to flaws and the defect-free areas. The white curves in the bottom panels correspond to the cone of influence (COI).
Stefano Sfarra and Mauro Regi