Poroelastic model for acoustic landmine detection
Acoustic waves can be a viable tool for the detection and identification of land mines and unexploded ordnance (UXO). Design of acoustic instruments and interpretation and processing of acoustic measurements call for accurate numerical models to simulate acoustic wave propagation in a heterogeneous soil with buried objects. Compared with the traditional seismic exploration, high attenuation is unfortunately ubiquitous for shallow surface acoustic measurements because of the loose soil and the fluid in its pore space. To adequately model such acoustic attenuation, we propose a comprehensive model to simulate the acoustic wave interactions with land mines and soils based on the Biot theory for poroelastic media. The finite-difference time-domain method is then used to solve the Biot equations. For the truncation of the computational domain in the FDTD method, we extend the acoustic and elastic perfectly matched layer (PML) to poroelastic media. Numerical experiments show that, with only 10 cells of PML medium, a high attenuation of about 50 dB can be achieved for outgoing waves. The numerical model is validated by comparison with analytical solutions. Unlike the pure elastic wave model, this efficient PML-FDTD model for poroelastic media incorporates the interactions of waves and the fluid-saturated pore space. The difference between elastic model and poroelastic model is investigated by studying surface wave amplitude variation with offset (AVO) in three different ground media: dry sand, fully water saturated sand and partly water saturated sand. The interaction of elastic wave with a plastic mine buried in dry sand is simulated. The results show that the surface wave is significantly influenced by the existence of a mine-like object. The diffraction of the surface wave can serve as an acoustic target signature.
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