Читать книгу Monument Future - Siegfried Siegesmund - Страница 293

The ultrasonic measurements were carried out by means of the transmission method, which consists of two piezoelectric sensors coupled to the sample at constant pressure. Compressive (P) and shear (S) waves were measured using polarised Panametric transducers (1 MHz). Emitting-receiving equipment (Panametrics-NDT 5058PR) and an oscilloscope (TDS 3012B-Tektronix) were used to acquire and digitalize the waveforms to be displayed, manipulated and stored. Two types of visco-elastic couplants were used to achieve good coupling between the transducer and the sample: one of a fluid consistency for the P-wave transducers (eco-gel), and another of a more viscous consistency for the S-waves (SWC, shear wave couplant, GE Panametrics).

In this work, we have applied a new method for automatic characterisation of the first pulse in both compressional and shear waves described in Benavente et al., (2020). Firstly, the recorded signal is pre-processed in the wavelet domain. This step filters noise and removes low-frequency disruption. Secondly, the recorded signal is analysed in the time-domain. This step identifies and characterises the first pulse, allowing for an estimation of onset time. Using the automatic approach allows for all detected pulses in the output signal to be analysed, where the first pulse of the P or S wave is determined based on criteria related to symmetry, amplitude and duration. The method outlined above is carried out using Matlab based on Galiana-Merino et al. (2013), where parts of the pre-processing stage are carried out using the Wavelet Toolbox. This triple check provides greater confidence in the results. We obtain the automatic onset time from recorded P- and S- waveforms and they are compared to manual picking, which is considered as a true or reference value.

Finally, the wavelength, λ, was calculated using the velocity, v, and frequency, f, of the first pulse as λ=v/f.