Ultrasonic tomography: non-destructive evaluation of the weathering state on a marble obelisk, considering the effects of structural properties

The use of ultrasonic methods for a non-destructive investigation of immovable, high-ranking cultural heritage, landmark buildings, and sculptures made of natural stone, has developed into the state-of-the-art testing throughout the last 20 years. This routinely applied transmission method gives information about the rock as a whole, but no detailed information concerning the internal structures. Onsite measurements have been executed in a tomographic approach for a marble obelisk in the Neuer Garten, Potsdam (Germany) to obtain an assessment on its internal weathering conditions. Detailed mapping of macroscopically visible structures and weathering phenomena has been performed on a scale of 1:1, to prove the validity of this data. The rock properties for the same marble variety were examined under laboratory conditions to gain data on their directional dependence, influencing factors like water content and artificial weathering behaviour. These results were used to cross check the ultrasonic measurements onsite and the tests under laboratory conditions, to obtain a reliable interpretation. The ultrasonic velocity distributions measured under defined conditions, revealed a possible anisotropy between 9 and 30%, which is a basic input parameter for the calculated tomograms. The synthetic tomograms clearly show the great impact of anisotropy considering the velocity distribution modelled for the measured planes. Based on the laboratory data, an amount of 20% anisotropy was applied to the tomograms, which improved the ability to distinguish the velocity variation due to deterioration from that caused by rock fabric. The results demonstrate that the rock fabric and its anisotropy need to be considered for interpreting the tomographic investigation. Before an adequate assertion can be made, the above influences must be considered as a basis for conservation purposes.