A New Quantitative Procedure to Determine the Location and Embedment Depth of a Void Using Surface Waves

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Detecting underground cavities beneath construction sites and urban areas is a crucial task for many engineering projects. Each year, subsidence and surface soil failure due to underground voids cause substantial damage around the world. Most of the seismic methods currently used for cavity detection can successfully locate a void but not its embedment depth. In spite of successful case histories, void detection is still a challenging problem because of the lack of a standard, quantitative void-detection technique. In addition, existing non-destructive techniques do not consider the effect of lateral inhomogeneities, i.e., cavities, in the wave propagation. Thus, the detection of underground cavities needs further study. This paper presents the results of numerical simulations of the multi-channel analysis of surface waves (MASW) in a laterally non-homogeneous medium. First, the Lamb solution is used to calibrate a homogeneous model, subsequently, voids with different dimensions and embedment depths are included in the medium. Analysis of the resulting surface responses shows that time and frequency domain parameters are sensitive to the location, embedment depth, and size of voids; which interact with the incoming wave front causing reflection of Rayleigh waves and strong attenuation of transmitted waves. The power-spectral-density functions clearly show patterns of attenuation and amplification. The authors propose a new analysis procedure to determine not only the location but also embedment depth of a void; this procedure is based on the attenuation analysis of Rayleigh waves (AARW). The new method uses the frequency spectra of recorded signals to compute a spectral-energy parameter and a modified logarithmic-decrement parameter. Numerical results of the AARW method applied to different conditions, including noisy signals, show that these parameters indicate successfully the location and embedment depth of underground voids.