The field tests have been accomplished acquiring tomographic data on living trees which showed signs of decays and that were later knocked down to extract wood disks and cubic samples to perform laboratory tests. The ultrasonic equipment used both for laboratory and for field measurements is a Pundit (Portable Ultrasonic Non-destructive Digital indicating Tester) with operating frequency of 54 kHz.

Before performing tomographic inversion, statistical pre-processing has been carried. out both on field and on laboratory- results, allowing to define the reliability of the data.. to optimise the tomographic processing and to carry on a correct interpretation of tomographic images. The pre-processing has contribute to point out some important aspects to he considered, in particular: the problem of the signal attenuation, the problem of physical resolution, that determine the minimum size of the detectable anomalies; the problem of anisotropy that has to be considered during the final interpretation.

• attenuation. particularly strong on bark and in case of wood decays, affects the signal to noise ratio, especially for higher frequency energising pulse, and the detection of the travel time values with good accuracy needs the application of signal processing techniques;
• resolution has been estimated for all the tomographic data sets to optimise the discretisation of the investigated section for the tomographic processing and to define the minimum size of the detectable anomalies. Considering both mathematical and physical resolution, for the most favourable cases, the best resolution obtained has been about 5 em..
• since longitudinal, radial and tangential velocities are, in some cases, sensibly different. velocity anisotropy on the investigated sections could generates on the tomographic image slow velocity.. peripheral zones that do not correspond to the real mechanical properties distribution;
• direct check of the tomographic results has shown that, in some cases, the velocity contrasts between healthy wood and decays are underestimated by the tomographic inversion.

In spite of this, the results obtained show that the developed methodology allows to detect, very often with good accuracy, the geometry and the extent of internal decays. In the figure is showed an example of tomographic imaging of a Plane compared with the trunk section picture: the low velocities zones are related to decays.