Ground fi ssure is a geological hazard that poses a great threat to human life and,property and the environment.Therefore,it is necessary to detect shallow underground fi ssures eff ectively.In this paper,a time-frequency analysis of Rayleigh waves based on the wavelet transform is proposed to detect shallow underground fi ssures.The arrival time of the directed Rayleigh waves and the diff racted Rayleigh waves from the underground fi ssure is extracted from the time-frequency spectrum of any two traces.Furthermore,the locations of the underground fissures are calculated according to the time difference relation.Four sets of fracture models and one set of fi eld data were used to test the eff ectiveness of the wavelet transform of Rayleigh waves.Moreover,the detection results of the actual data are compared with that of the high-density electrical method to further prove its detection eff ect.The fi eld investigation shows that using the wavelet transform of Rayleigh waves to detect shallow underground fi ssures is feasible and eff ective.
Rayleigh wave imaging is efficient in estimating the shear- (S) wave velocity in near-surface exploration. The key is to accurately extract the dispersion of Rayleigh wave. We propose a method to calculate the dispersion of the active-source Rayleigh wavefield by using the Aki formulation. The spectrum after the cross correlation of two-channel records in the frequency domain is expressed by the Bessel function. Using the corresponding relation between the zero point of the spectrum real part and the Bessel function root, the phase velocity at the discrete frequency point is obtained and the dispersion curve is extracted. First, the theoretical basis and calculation method used in the active-source Rayleigh wave data are introduced. Then, three sets of theoretical models are calculated by this method and the results are consistent with the theoretical dispersion. Finally, we process a group of real Rayleigh wave data and obtain the phase velocity profiles and compared them with the results obtained by the multichannel surface wave analysis method. The effectiveness and applicability of the Aki method in active-source data processing are verified. Compared with multichannel wave processing, the advantage of the Aki method lies in the use of two- channel data in a single-shot record. When the number of acquisition channels in a shot gathers is insufficient or there is a bad channel, the quality of the extracted dispersion is guaranteed.