Assessment of Risks Induced by Countermining Unexploded Large-Charge Historical Ordnance in a Shallow Water Environment—Part II: Modeling of Seismo-Acoustic Wave Propagation

The goal of this work presented in a two-companion paper is to pave the way for reliably assessing the risks of damage to buildings on the shore, induced by the detonation of large-charge historical ordnance (i.e., countermining) in variable shallow water environments. Here, we focus on the impact of the marine environment, more specifically the unconsolidated sedimentary layer, on detonation-induced seismo-acoustic wave propagation. We rely on a multidisciplinary cross-study including real data obtained within the framework of a countermining campaign, and numerical simulations of the seismo-acoustic propagation using a spectral-element method. We first develop a strategy relying on physical insights into the different kind of waves that can propagate in a coastal environment, to provide clues for a computational cost reduction. The geological surveys and the hydroacoustic measurements provide input data for the 3-D axisymmetric modeling of wave propagation. The numerical simulations, obtained for one specific source–receiver path with a variable sedimentary facies, are compared with the real seismic data induced by the detonation of a charge either on the seabed, or in the water column, and recorded on the coast. Numerical analysis sheds light on the strong interaction between surface waves and the sedimentary facies. The short-scale and deep sedimentary basins favor a local wave-amplitude amplification and a frequency shift toward the low-frequency domain. However, the seismo-acoustic waves are globally attenuated during their propagation because of intrinsic attenuation and geometrical spreading, which generally prevents any large damage to nearby buildings on the shore.