Attenuation Tomography On Historical Buildings Through Spectral Analysis Of Sonic And Radar Data

Sonic and radar investigations are frequently used in conjunction with other NDT techniques to<br>assess the safety conditions of historical buildings, to understand the construction techniques and to<br>locate internal in-homogeneities or hidden wood or metal elements. Both sonic and radar equipment can<br>be used to produce tomographic sections that show the velocity map within a construction element like a<br>wall or a pier. Radar data can be also processed by back-projecting the received energy in order to<br>produce attenuation maps that can be interpreted to understand the absorption and scattering properties<br>of the construction materials. Unfortunately, this method is not applicable to sonic data because sonic<br>amplitudes are influenced by too many factors besides absorption. A promising alternative method<br>consists of back-projecting the frequency downshift effect produced by absorption. The reliability of this<br>approach is discussed for the specific application to historical buildings by presenting real data<br>examples. The discussion shows the additional information that can be extracted from these sonic<br>attenuation maps.<br>By extending the approach to radar data, new attenuation maps can be obtained and compared to<br>those derived from amplitude back-projection. Since the two inversion methods are based on the<br>analysis of different effects produced by attenuation, i.e., frequency downshift and amplitude decay<br>respectively, the attenuation maps are not necessarily equivalent and the analysis of similarities and<br>differences can be quite useful to constrain the final interpretation of all the tomographic images.