Modeling results on detectability of shallow tunnels using Rayleigh‐wave diffraction

Abstract

Rayleigh waves have been widely employed in nearsurface high-resolution seismic investigations since the development of multi-channel analysis of surface waves (MASW). Usually, detecting near-surface features such as voids and tunnels by inverting Rayleigh-wave phase velocities is challenging because the resolution of inverted shear (S)-wave velocity profiles is limited by a geophonespread length. As a human-made structure, a shallow tunnel can generate strong diffractions of seismic waves due to its angular shape and the high contrast elastic modulus around a tunnel surface. Since Rayleigh-wave energy is dominant in the near-surface P-SV wavefield, the diffraction of Rayleigh waves is also significant on a shotgather, which allows a tunnel to be located directly by using a hyperbola curve matching technique. However, the size and depth of the tunnel can influence the diffraction energy dramatically. In case where the diffraction energy is too weak, it is usually impractical to recognize the hyperbolic pattern on a shotgather even with F-K filtering. In this paper, we present finite-difference numerical modeling results illustrating the variation of Rayleigh-wave diffraction energy for different size tunnels. The modeling results reveal that the amplitude of Rayleigh-wave diffraction decreases at different rate depending on tunnel size The diffraction energy decreases more rapidly when the size of a tunnel is smaller than 3/16 of the central depth of the tunnel. When a tunnel size is smaller than 1/32 of its depth, the diffraction energy of Rayleigh-wave is less than 1% of the direct Rayleigh-wave energy, which makes the hyperbola difficult to be recognized on a shotgather. Considering the noise and nearsurface attenuation in a real data, the ratio of size to depth of a detectable tunnel could be much larger than these modeling based results.