★ This program revised the description of the SASW method in the R (surface) wave method on May 07, 2018 (3.6.2)
3.6.2 R (surface) wave method
The basic principle of the surface wave method is to first calculate the wave velocity of the surface wave (usually Rayleigh wave) at different frequencies, and the frequency to wave velocity curve is also called the dispersion curve. Then, according to the spectrum curve, considering the relationship between the Rayleigh wave wavelength and the depth of influence, the Rayleigh wave velocity of the concrete at different depths is estimated.
There are many methods for analyzing the dispersion curve, and the first method can be divided into the steady state method and the transient method according to the excitation method. Among them, the analysis method of steady-state surface waves is relatively simple and intuitive. The analysis principle of transient surface waves is relatively complicated. There are two commonly used analysis methods: SASW method and fk method, among which the SASW method (Spectral Analysis of Surface Wave) uses two signals. The Fourier spectrum of the cross-correlation function is relatively simple as an analytical benchmark. On the other hand, we propose a transient surface wave method based on excellent wavelength, called the "excellent wavelength method". The SASW method and the superior wavelength have their own advantages and disadvantages and can be used together.
1) SASW method
The wave velocity can be calculated by
In the formula:
For a specific frequency, Hz;
The surface wave velocity corresponding to the frequency, m/s;
For the phase difference of the two signals after the frequency is expanded, rad is equal to the phase angle of the mutual power spectrum of the two signals at the same frequency;
For the distance between the two sensor tracks, m.
The wavelength λ is calculated by:
2) Excellent wavelength method
The method directly seeks the propagation speed of the first wave at a superior frequency by exciting Rayleigh waves of different excellent frequencies. The test principle is simple and clear, as shown in the following figure:
The dispersion curve can be obtained by changing the size of the excitation hammer and exciting the wave velocity and wavelength of the surface wave. According to the shape of the dispersion curve, the R wave velocity of concrete of different depths can be reversed according to the following formula:
1) When the average R wave velocity of the medium increases with depth:
2) When the average R wave velocity of the medium decreases as the depth increases:
3) Regardless of the average R wave velocity of the medium as a function of depth:
Where, the depth of the nth, n-1 points (taking into account the Poisson's ratio of the concrete, may take 0.6 times the wavelength);
The average R wave velocity above the depth of the nth and n-1 points can be considered as the test speed of the wavelength;
The media R wave velocity to the depth interval.
The SASW method and the superior wavelength method each have advantages and disadvantages. The SASW test is relatively simple, and a single excitation can obtain a continuous dispersion curve. However, its quality requirements for signals are high, analysis is complex, and the experience and level of technicians are very high. The principle of superior wavelength is the opposite, the test principle and calculation analysis are relatively simple, and the stability of the analytical results is also good. However, this method needs to change different excitation hammers, the test efficiency is low, and the continuity of the dispersion curve cannot be guaranteed.