Numerical modeling of underwater parametric propagation to detect buried objects

Abstract

In underwater acoustics, detection of buried objects in sediments (cables, mines,…) is a complex problem. One reason is that acoustic attenuation in these sediments increases with frequency. To ensure sufficient penetration depth in marine sediments, low frequencies have to be used, implying a low resolution. A solution proposed to solve this problem is the parametric emission based on the nonlinear properties of the propagation medium. This method can generate a low frequency wave from two directional high frequencies beams. The parametric propagation is simulated in seawater and marine sediments. The model developed is based on the fractional-step numerical method introduced by Christopher and Parker [1]. In this method, the normal particle velocity is calculated plane by plane from the surface of the transducer to a specified distance. The effects of nonlinearity, attenuation and diffraction are calculated independently for each spatial step. Moreover, to reduce the number of spatial steps, a second order operator splitting scheme is used. The diffraction computation is based on a method of angular spectrum in the frequency domain where the field across a source plane is described by a spatial frequency distribution. To improve code stability, the effects of nonlinearity and attenuation are calculated and associated in shorter propagation substeps. At the interface between water and marine sediments, the transmission conditions are applied. Several tests have been carried out in different configurations (changing the primary frequencies, the parametric frequency, the source geometry, the inclination of the source with the interface, the focal distance,…). The 3D velocity field is calculated in each case, thereby allowing to know the directivity of the source, the velocity amplitude in sediments and the performance.