Application of high-order surface waves in shallow exploration : An example of the Suzhou river, Shanghai

The Frequency-Bessel transform method (abbreviated as F-J method) is a new tool to analyze microtremors. This method utilizes frequency vector wave-number transformation based on a stationary microtremor source in space and time and a flat multi-layered isotropic elastic model, so it can extract clear fundamental and higher-mode Rayleigh dispersion curves in theory. While there is no field experiment to study and apply the F-J method to engineering exploration so far. In this paper, we use the F-J method to effectively generate frequency-phase velocity spectrograms from urban microtremor recordings in the Suzhou River, Shanghai and then extract multi-mode dispersion curves. Then, the particle swarm algorithm is used to invert fundamental and higher-mode Rayleigh dispersion curves, yielding the S-wave velocity structure in depth range of 0 similar to 70 m. And the results are validated by borehole data. In addition, the comparison with traditional SPAC (Spatial Autocorrelation) method shows that the F-J method has advantages in imaging dispersion spectrogram. The results show that (1) The F-J method can effectively extract clear fundamental and higher-mode Rayleigh dispersion curves from the vertical component of urban microtremor recordings. (2) The higher-order dispersion curves extracted by the F-J method are clearer than that extracted by the traditional SPAC method, especially at high frequency (>13 Hz). (3) The inversion results using fundamental and higher-order dispersion curves are more accurate. It can identify velocity interface and low velocity anomalies in Quaternary sediments with small physical property difference. The F-J method has good prospects for imaging fine structures in shallow subsurface of urban areas.