Numerical Simulation of Multiple Scattering by Hidden Cracks under the Incidence of Elastic Waves

In this paper, the scattering of P-waves by hidden cracks located near to a free surface is investigated. The Indirect Boundary Element Method was used in order to study the wave propagation phenomenon in a halfspace considering various crack models. A near free-surface crack generates scattered surface waves whose amplitude spectra show conspicuous resonance peaks. Such effect has been attributed to local resonances of a virtual layer formed by the free surface and the up crack´s face. For the case of two-near-free-surface cracks the amplitude spectra show additional peaks, which can be associated to the presence of the second crack. Given similar sizes between these two cracks, the characteristic resonance frequency measured at the free surface corresponds mainly to the equivalent layer formed by the shallowest crack and the free surface. However, when the deepest crack becomes sufficiently large with respect to the shallow crack, two characteristic resonance frequency peaks appear. To illustrate the response and multiple scattering effects due to the presence of a second crack, results in frequency domain are provided. For the case of a near free-surface crack, results obtained by means of the formulation shown here have been verified against those obtained by using an analytical solution and good agreement was obtained.