Conference Paper

Seismo-acoustic wave propagation in the Rade of Hyères (France) generated by counter-mining of explosive devices: comparison between numerical simulations and real experiments

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Abstract

Explosive devices from World War II are discovered every week on the French coasts. In a short time after their discovery, they must be destroyed by the French Navy Mine Warfare Office. If the risks are well known by mine warfare experts, the consequences of counter-mining on the marine environment are much more complex to evaluate and expertise is then required. Depending on the environment geology, the explosive charges and their localization, the seismo-acoustic waves generated by the explosion may cause damage to infrastructures located on the coast, and under specific conditions, small submarine landslides. The POSA project, led by SHOM, and which includes LMA, Géoazur and LPG Nantes, focuses on the Mediterranean coast and proposes to address the upstream hazard management issue of such counter-mining operations in the marine field. The goal is to identify these risks for specific configurations and to develop a decision support tool for their control. The POSA project also contributes to a lesser extent to civil research by improving the natural seismicity catalogs and allows the distinction between the anthropogenic part linked to the counter-mining of explosives and the natural part of regionally recorded micro-seismicity. The main originality of the POSA project lies in the approach used to assess the risks arising from the counter-mining of explosive devices. This approach combines numerical simulations of seismo-acoustic wave propagation and the coupling between acoustic data recorded within the coastal zone at sea and seismic data recorded on the coast, together with sedimentary measurements. We propose here to present the results of such a methodology applied to the counter-mining campaigns which occurred in the Rade of Hyères (France) in December 2018. We focus more specifically on the numerical simulations of seismo-acoustic wave propagation in this area. As a first step, from topographical and sedimentary measurements performed in the Rade of Hyères, physical and geometrical characteristics of the marine seabed (i.e., sediments and rock basement) have been carefully selected in order to become input data for numerical simulations. In addition, particular attention has been paid to the characteristics of the wave source (i.e., the explosion of the device). The source time functions of explosions, together with their source directivity and frequency spectrum, have been estimated from real acoustic signals recorded by hydrophones located around the counter-mining location. Taking into account these input data (i.e., characteristics of the seabed and the source), numerical simulations of seismo-acoustic wave propagation, from the source to several seismometers deployed on the coast of Hyères and surrounding islands, has been then conducted using a spectral-element method which is a full-wave method combining the accuracy of a pseudo-spectral method with the flexibility of a finite-element method. The impact of the explosive device (source) charge and its location (on the seabed or in the water column), together with the impact of the marine environment properties, on the simulated signals have been studied. Finally, the numerical results have been compared with the real signals recorded by the seismometers during the counter-mining campaigns which took place in November 2016 and December 2018. The risks of land degradation and triggering of underwater avalanches have been evaluated afterward.

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Underwater explosions
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COLE, Robert H. and WELLER, Royal. Underwater explosions. Physics Today, 1948, vol. 1, p. 35.