An effective fluid model for landmine detection using acoustic to seismic coupling

A model is developed to describe the response of the ground to airborne sound in the presence of a buried landmine. The model describes both the near-field acoustic wave phenomena as well as the mechanical dynamics of the mine itself. Since buried landmines are typically close to the surface of the soil, the induced ground vibration is the result of near-field scattering so that classical asymptotic scattering theory cannot be used. Instead, the near-field problem is solved in a right circular waveguide with rigid walls. The waveguide contains air in the upper half, soil in the lower half, and a buried mine placed concentrically on the waveguide’s axis. The advantage of a waveguide model over infinite space models is that the resulting computer models are much more straightforward to code and run much more quickly. As the radius of the waveguide increases, the results of the waveguide model converge to those of the infinite space model. For low frequencies (a few hundred Hz) this convergence is quite rapid: it is found that a waveguide radius of 10 times the mine radius is more than sufficient. The results obtained are found to qualitatively explain the phenomena observed in the field.