Underground cavity detection: a new method based on seismic rayleigh waves

摘要

We demonstrate the effectiveness of Rayleigh waves for detecting and locating underground cavities, first through modelling and then through two field experiments. To study wave propagation in elastic media, we used 2D numerical simulations based on the finite-difference method. Model parameterization consisted in introducing a cavity in the medium by partitioning the staggered grids in several domains connected by boundary conditions. Analysis of the simulated wavefronts and corresponding shot gather shows that the diffracted Rayleigh waves reflect the cavity location along the profile. Moreover, the direct Rayleigh wavefront seems to be filtered by the cavity, which affects the spectral band according to the cavity depth. The features observed on synthetic data enabled us to define optimum measurement and processing protocols to obtain information about the cavity position by comparing dispersion diagrams and developing an imaging technique using Common Receiver Gathers (CRG). The protocols were then applied to two field experiments: one at Jargeau (Loiret, France) and the other at May-sur-Orne (Calvados, France). The experiment carried out at Jargeau concerns a cavity with a concrete masonry surround. The maximum amplitude curve on the dispersion diagrams shows a discontinuity due to the presence of the cavity. Furthermore, the stacked time-domain CRG images improve the signal-to-noise ratio and clearly highlight diffraction phenomena due to the masonry surrounding. Consequently, the lateral cavity position can be detected fairly easily from the diffraction patterns observed on the stack. A depth profile was obtained after convolution with a sweep signal having a frequency component that increases linearly with time and by using a frequency-to-depth relationship; it shows lateral variations of amplitude that are generally related to the presence of both the save and the absorbing material. The feature provides additional information on the site environment, such as the distribution of decompressed zones which could be induced by the cave. In the second experiment, distribution of decompressed zones which could be induced by the cave. in the second experiment, at May-sur-Orne, the cavity has no masonry surround and the Rayleigh waves were not diffracted.