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Seismo-acoustic wave propagation in the Rade of Hyères (France) generated by counter-mining of explosive devices: comparison between numerical simulations and real experiments
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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This paper presents a brief introduction to the basic fundamentals of underwater explosions, including discussion of the features
of explosive charge detonation, the formation and characterization of the associated shock wave, bulk cavitation effects,
gas bubble formation and dynamics, surface effects and shock wave refraction characteristics. Illustrations of each of these
fundamental aspects of underwater explosion (UNDEX) loadings are made with a set of videos from a variety of experimental
testing events. In addition, analyses of associated measured loading and dynamic response data, as well as descriptions of
supporting numerical simulations of these events are presented. At the conclusion of this paper, each of these UNDEX effects
are tied together with a summary discussion and illustration.
We present the spectral element method to simulate elastic-wave propagation in realistic geological structures involving complicated free-surface topography and material interfaces for two- and three-dimensional geometries. The spectral element method introduced here is a high-order variational method for the spatial approximation of elastic-wave equations. The mass matrix is diagonal by construction in this method, which drastically reduces the computational cost and allows an efficient parallel implementation. Absorbing boundary conditions are introduced in variational form to simulate unbounded physical domains. The time discretization is based on an energy-momentum conserving scheme that can be put into a classical explicit-implicit predictor/multi-corrector format. Long-term energy conservation and stability properties are illustrated as well as the efficiency of the absorbing conditions. The associated Courant condition behaves as Δt C < O (n el-1/nd N -2), with n el the number of elements, n d the spatial dimension, and N the polynomial order. In practice, a spatial sampling of approximately 5 points per wavelength is found to be very accurate when working with a polynomial degree of N = 8. The accuracy of the method is shown by comparing the spectral element solution to analytical solutions of the classical two-dimensional (2D) problems of Lamb and Garvin. The flexibility of the method is then illustrated by studying more realistic 2D models involving realistic geometries and complex free-boundary conditions. Very accurate modeling of Rayleigh-wave propagation, surface diffraction, and Rayleigh-to-body-wave mode conversion associated with the free-surface curvature are obtained at low computational cost. The method is shown to provide an efficient tool to study the diffraction of elastic waves by three-dimensional (3D) surface topographies and the associated local effects on strong ground motion. Complex amplification patterns, both in space and time, are shown to occur even for a gentle hill topography. Extension to a heterogeneous hill structure is considered. The efficient implementation on parallel distributed memory architectures will allow to perform real-time visualization and interactive physical investigations of 3D amplification phenomena for seismic risk assessment.
The present study utilizes the Israel Seismic Network (ISN) as a spatially distributed multichannel system for the discrimination of low-magnitude events (ML < 2.5), namely earthquakes and underwater explosions in the Dead Sea. In order to achieve this, we began with the application of conventional single-station methods, such as spectral short-period ratios. We then applied a newly developed, network-oriented algorithm based on different spectral features of the seismic radiation from underwater explosions and earthquakes, i.e. spectral semblance statistics.
Twenty-eight single-shot underwater explosions (UWEs) and 16 earthquakes in the magnitude range ML = 1.6–2.8, within distances of 10–150km, recorded by the ISN, were selected for the analysis. The analysis is based on a smoothed (0.5Hz window) Fourier spectrum of the whole signal (defined by the signal-to-noise criterion), without picking separate wave phases. It was found that the classical discriminant of the seismic energy ratio between the relatively low-frequency (1–6Hz) and high-frequency (6–11Hz) bands, averaged over an ISN subnetwork, showed an overlap between UWEs and earthquakes and cannot itself provide reliable identification.
We developed and tested a new multistation discriminant based on the low- frequency spectral modulation (LFSM) method. In our case the LFSM is associated with the bubbling effect in underwater explosions. The method demonstrates a distinct azimuth-invariant coherency of spectral shapes in the low-frequency range (1–12Hz) of short-period seismometer systems. The coherency of the modulated spectra for different ISN stations was measured by semblance statistics commonly used in seismic prospecting for phase correlation in the time domain. The modified statistics provided an almost complete separation between earthquakes and underwater explosions.
Results of a study of hydroacoustic, acoustic, and seismic effects from a series of three large-scale chemical explosions
of 0.5, 2, and 5 tons, conducted in November, 1999 in the Dead Sea, are presented. The shots were detonated at a water depth
of 70 m (485 m below the ocean level). The main objective of the experiment was calibration of seismic stations of the International
Monitoring System in the Middle East, using accurate travel times and source phenomenology features of underwater explosions.
The largest shot provided magnitude about 4 and was recorded at distances up to 3500 km. Near-source seismic and hydroacoustic
observations obtained were utilized to estimate source parameters of the conducted explosions. Based on the curve-fit equation
of the time-pressure measurements, the direct shock wave energy was estimated as 30.8% of the total explosive energy. The
TNT equivalent to the 5000 kg charge of the explosive used (Chenamon) was determined as 4010 kg, corresponding to the manufacturer’s
estimate of the Chenamon energy as ≈80% TNT.
Experiments have been carried out to measure the source levels of standard 0.82 kg SUS (Signal Underwater Sound) charges at nominal depths of 18, 91, and 180 m. The source levels are reported in this paper for 1/3 octave bands from 5 to 500 Hz. Approximately 30 charges for each depth were included in the analysis, and the standard deviation was less than 1 dB for each frequency band. The source levels estimated from these data were compared to the previous results of Gaspin and Shuler which were based on hand drawn pressure waveforms. Their values are generally within 2 or 3 dB of the measured source levels but there are some greater differences. Reasons for the disagreement between the two sets of values are suggested. In addition to the measured source levels, source levels for 0.82 kg SUS at 244 and 610 m, and for 22.7 kg (50 lb) charges at 100 and 200 m have been estimated using a scaling technique [R. Hughes, in Underwater Acoustics and Signal Processing (Reidel, New York, 1981), pp. 87–91]. The spectra of standard SUS measured at 91 and 180 m were scaled, and the source levels obtained in this way are accurate for frequencies up to three or four times as large as the bubble frequency. These results are also presented in 1/3 octave bands, and are compared to previously published values.
We present an introduction to the spectral element method, which provides an innovative numerical approach to the calculation of synthetic seismograms in 3-D earth models. The method combines the flexibility of a finite element method with the accuracy of a spectral method. One uses a weak formulation of the equations of motion, which are solved on a mesh of hexahedral elements that is adapted to the free surface and to the main internal discontinuities of the model. The wavefield on the elements is discretized using high-degree Lagrange interpolants, and integration over an element is accomplished based upon the Gauss–Lobatto–Legendre integration rule. This combination of discretization and integration results in a diagonal mass matrix, which greatly simplifies the algorithm. We illustrate the great potential of the method by comparing it to a discrete wavenumber/reflectivity method for layer-cake models. Both body and surface waves are accurately represented, and the method can handle point force as well as moment tensor sources. For a model with very steep surface topography we successfully benchmark the method against an approximate boundary technique. For a homogeneous medium with strong attenuation we obtain excellent agreement with the analytical solution for a point force.
Underwater explosions
- Robert H Cole
- Royal Weller
COLE, Robert H. and WELLER, Royal. Underwater explosions. Physics
Today, 1948, vol. 1, p. 35.