Читать книгу Monument Future - Siegfried Siegesmund - Страница 296

This paper addresses determination of the picking of the onset of P- and S-waves in transmitted output waveforms on weathered sandstones. Stones weathered by salt crystallisation show an increase in surficial roughness and their microstructural properties are strongly modified by crystallisation pressure and the presence of remained salts. The wavelength of output signals depends on grain size and weathering, while microstructural components of stones and the presence of salts operate as a wavelength filter. Manual picking of the onset time for elastic waves becomes more difficult with increase in wavelength, although this observation is less important for P-waves than S-waves. Particularly, as grain size and stone alteration increase, the determination of arrival time of S-waves becomes more problematic due to the contamination of S-waveforms by P-waves, a lower signal-to-noise ratio and an increase of wavelength.

Figure 1: P signals for fresh and weathered samples for (a) Doddington sandstone, D; (b) Forest of Dean, F, measured in the parallel direction to bedding; and (c) St. Bees sandstone, BC.

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Figure 2: S signals for fresh and weathered samples for (a) Doddington sandstone, D; (b) Forest of Dean, F, measured in the parallel direction to bedding; and (c) St. Bees sandstone, BC.

Figure 3: Comparison of P (a) and S (b) wave velocities obtained manually (manual) and automatically (authomatic).

The automatic onset time from recording P- and S- waveforms is compared to manual picking, which is considered as a true or reference value. The discrepancies between automatic and manual measurements are within the experimental error of the onset time picking measurements.

This methodology is recommended for fresh and weathered stones with a medium-coarse grain size (0.5–1 mm). This methodology may be particularly helpful in samples where the quality of the S-waveform signals is poor, resulting in difficulties with manual picking of the onset time. The great advantage of this methodology is the accuracy and reproducibility of the obtained results, which do not depend on human subjectivity.