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The goal of the 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 unexploded historical ordnance (UXO) of large weights in variable shallow water environments with a water depth less than 50 m. The risk assessment is quantified through the seismic magnitude on the Richter scale, induced by the detonation of charges of different weights (between 80- and 680-kg TNT-equivalent). This metric is investigated experimentally using a coupled seismo-acoustic approach within the framework of a UXO clearance (countermining) campaign in the Mediterranean Sea. Analysis of real acoustic and seismic data shows that, compared to a charge detonation in water, a similar detonation on the seabed generates seismic signals of lower frequencies and higher amplitudes that propagate in the seabed. The larger the charge weight, the higher the seismic amplitude. Besides the explosion-coast distance, the ground properties also affect the signals. The sediments favor a longer signal duration and the presence of late dispersive and very low-frequency signals with a large amplitude, whereas the rocky grounds better preserve the high-frequency energy propagation. For the local environment considered in this study, a charge detonation on the seafloor generates seismic events of higher magnitudes compared to a detonation in water. However, these magnitudes are likely low enough to prevent any large damage in the nearby inland infrastructures. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The capacity to monitor suspended sediment concentrations (SSCs) in the ocean, from surface to bottom, using data acquired by the future Meteosat Third-Generation (MTG)/flexible combined imager (FCI) satellite sensor has been quantified by observing system simulation experiments (OSSEs). The “true” ocean state for these experiments is based on a 15-month numerical simulation of hydrodynamic and sediment transport, configured to represent the highly dynamical waters of the English Channel under the influences of tides and waves. Simulated MTG/FCI hourly averaged acquisitions at a given location near the Isle of Wight have been processed via hidden Markov model combined with a statistical classification—based on self-organizing maps—of predicted vertical SSC profiles. The resulting experiments demonstrated that MTG/FCI images, despite their high temporal resolution, and because of many gaps due to nights and clouds over the English Channel, still require spatial interpolations to enhance the amount of information available at a given location. For an accurate determination of particle concentrations, time series of the main forcing (wind, tides, and waves) need to be included in the process: 1) as a crucial parameter correlated with the dynamics of large particles (sands) and 2) as an equally important parameter as satellite data themselves in the correlation with the dynamics of fine particles (silts). [ABSTRACT FROM AUTHOR]

A mine burial field experiment was carried out on two sandy seafloors between January and April 2004 in the Bay of Brest, France. Burial recording mines (BRMs) were used to measure burial and mine orientation at 15-rain intervals. Sonar and bottom photographs were also used to characterize sediment morphology and mine burial. These observations are compared with the predictions of mine burial using the following three models: a momentary liquefaction model, a current-induced scour model, and a wave-induced scour model. Analysis combines mine burial data, sediment data, seabed observations, and hydrodynamic measurements. At the first site, "Rascas," the seabed dynamics are dominated by tides and river runoff. Almost no mine burial was measured during the experiment which is in agreement with predictions of mine burial models (current-induced scour and liquefaction). Dynamics at the second site, "Bertheaume," are driven by tides and ocean waves. A long storm lone week) and several swell events were experienced and significant mine burial was observed in conjunction with high significant waveheights. Mine burial models suggest that burial at "Bertheaume" was dominated by wave-induced scour rather than current-induced scour or momentary liquefaction. [ABSTRACT FROM AUTHOR]