Structural investigation of historical structures by Laser Doppler Vibrometry

The determination of the state of conservation of historic structures by optical techniques is still not used so widely as it ought to be, and this is clearly reflected also in national norms that make references only to well established techniques such us ultrasound, radar and sonic tests. However, the clear advantages of such techniques, and especially laser based ones like the Laser Doppler Vibrometry (LDV), are driving research in this direction; the most important feature of optical techniques is their non-invasiveness, so that they can be harmlessly employed on items characterized by delicate structures and precious surfaces. For the scope of this paper, LDV has been employed to acquire mechanical waves travelling in sample structures, in order to characterize the degree of connection among different parts of the structure and evaluate the presence and importance of defects. Different case studies will be presented, regarding both laboratory samples and real structures: as regards the field tests, investigations have been conducted in Schwäbisch Gmünd (Holy Cross Minster) and Berlin (Pergamon Museum), and the potential of LDV as a monitoring tool for short term structural evaluation will be also presented using sample walls installed at the University of Bologna.