3D DISCRETE ELEMENT SIMULATIONS OF ACOUSTIC DISPERSION IN SEDIMENTS

Sound speed dispersion and frequency dependence of attenuation in marine sediments are important for any undersea activities using acoustics, such as remote sensing and target detection. Current established models use grain shearing and fluid viscosity as the source of loss in sediments, both of which may not be significant given the relatively small strains imposed by most acoustics' signals. In this paper, we analyze a source of attenuation due to compressional losses in grains, modeled with a dashpot term. Discrete element modelling (DEM) using Large Scale Atomic Molecular Massively Parallel Simulator (LAMMPS) was used to model motions of distinct particles in the system. A small amplitude pressure wave was introduced into the channel via an oscillating boundary wall and its effect on each discrete particle was measured to obtain the sound speed and attenuation coefficient. The measurements with sets of varying frequencies for system size of up to 50000 particles exhibit similar relationships with experimental data used for comparison in this paper. In particular, we were able to recover the observed frequency dependence of attenuation, following a power law of frequency squared at low frequencies and the square root of frequency at high frequencies for all system sizes used in our analysis. The small number of parameters used in our theory present a much more tractable and parsimonious problem for geo-acoustic inversion than the more complicated established models.
Major, Republic of Singapore Navy
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