Читать книгу Monument Future - Siegfried Siegesmund - Страница 290

In this investigation, we determined the onset of P- and S-waves considering signal pre-processing and the analysis of the recorded signals. For most stone conservation investigations, P-waves are easy to determine. However, the measurement of pure S-waves in fresh and weathered building materials presents critical experimental problems.

We recorded P- and S- waveforms in three sandstones used in the Scottish heritage. Doddington sandstone (D) is a quartz-arenite commonly used throughout Scotland and is currently utilised as a replacement stone at Jedburgh Abbey, Scotland. St. Bees sandstone (BC) is a dark-red lithic arkose and is currently used as a replacement stone at Arbroath Abbey. BC is quarried on the west coast of Cumbria, England and exhibits planar bedding. Forest of Dean (F) is a grey-green (sub)litharenite quarried in Gloucestershire and used throughout the UK, including in the restoration of Dunkeld Cathedral, Scotland. We obtained the P- and S-velocities as well as the calculation of wavelength of the P-waves in both fresh and salt weathered samples.

The recorded signals highlight that the microstructural components of rocks and their modification by salt crystallization affect the output signal. P-wave signals present a high signal/noise ratio and their arrival-times are clear. The wavelength increases with grain size and also the degree of salt crystallization. However, as grain size increases, the determination of S-wave arrival time becomes more problematic due to the contamination of S-waveforms by P-waves, a lower signal-to-noise ratio and an increase of wavelength. These difficulties are more prevalent in the weathered samples, where the manual picking of the onset-time becomes more difficult and time-consuming. In general, the automatic method obtains S-wave values slightly lower than the manual method and their discrepancies are, on average, lower than 5 %.

Keywords: Wavelet analysis, Building stones, Salt weathering, Primary wave velocity, Shear wave velocity Ultrasound, Non-destructive testing