Investigating Surficial Alterations Of Natural Stone By Ultrasonic Surface Measurements (Vol 16, Pg 257, 2017)

The surface of buildings and monuments is essential for their visual appearance and their protection. Usually the surface is carefully designed and strongly related to the purpose of the object. Moreover, it is carrying information on the artistic and technical skills of the builders. It is however also prone to weathering and deterioration. A prerequisite for a careful restoration of the surface of historic buildings and monuments is a thorough analysis of surficial alterations. Quantitative measurements support visual inspections, but nondestructive characterization of material properties in the uppermost centimeters of an object represents a challenge. Here, the potential of ultrasonic surface measurements for the quantification of surficial alterations of natural stone is demonstrated by forward modeling, laboratory tests, and case studies. The fundamental Rayleigh mode is shown to dominate the waveform in the case of surface measurements. It is sensitive to changes in the shear wave velocity with depth and in anelastic damping of shear wave propagation. We report on ultrasonic surface measurements at facades made of marble (Neptungrotte, Park Sanssouci, Potsdam), sandstone (Porta Nigra, Trier), and tuff (Campidoglio, Rome). Average Rayleigh wave velocities are roughly proportional to measured P-wave velocities and are related to the overall state of the object in the uppermost centimeters. Rayleigh wave group velocities indicate changes in the material properties with depth, and waveform inversions allow for estimating depth profiles of velocity and damping of shear waves. The strong variability of weathering of natural stone is illustrated.