Читать книгу Monument Future - Siegfried Siegesmund - Страница 291

These non-destructive methods based on elastic wave measurements are commonly used in stone conservation investigations in both laboratory and field experiments. The study of compressional and shear wave velocities is considered a reliable method of determining the elastic, physical-mechanical, and durability properties of studied samples. Elastic wave velocities are frequently investigated either individually or in combination. The compressional or primary (P) wave velocity, V_p, (also termed as ultrasonic pulse velocity) is a frequently studied parameter that is simple to measure in the laboratory. In addition, V_p is also used as an indicator of rock strength and degree of weathering. Obtained in combination with V_p, shear or secondary (S) elastic wave velocity, V_S, is used when calculating the Young and Poisson dynamic elastic moduli.

Modification of microstructural properties of rock caused by salt weathering, combined with the presence of crystallised salts and water, strongly affects the rocks elastic wave velocities. (Benavente et al., 2018). The determination of P-velocities in weathered stone samples could present difficulties due to waveform attenuation, which results in output signals showing a low signal-to-noise ratio (SNR). On the other hand, the generation and acquisition of pure S-waves in rocks is difficult (Wang et al., 2009). The most critical problems come from the contamination of S-waveforms by P-waves (P- wave appears before the S- wave arrivals), the reduction of waveform amplitude (lower SNR) and increase of wavelength.

As a result, picking of onset time of the S-wave, for both fresh and weathered samples, and the P-wave in highly weathered samples, becomes a complicated process. All of these difficulties limit the use of S-waves, particularly when calculating the dynamic elastic moduli, which considers a tabulated value of the Poisson’s ratio, yielding dubious estimations of elastic rock properties. Manual picking is a tedious and time-consuming process, which is subject to human error. Furthermore, manual operation relies on the attention of a trained technician, which can be a disadvantage when analysis of large volumes of data is required.

In this paper, we record P- and S-waveforms in three sandstones used in the Scottish heritage and obtain the P- and S-velocities as well as the calculation of wavelength of the P-waves in both fresh and salt weathered samples. We use an automatic methodology for the calculation of onset time that includes the signal pre-processing and the analysis, in the time-domain, of the first pulse symmetry, amplitude and duration criteria. This triple check provides greater confidence in the obtained results. This method limits human error, while improving the accuracy and reproducibility of P and S wave onset time estimations.