Modeling of Broadband Signal Propagation in Different Types of Bottom Sediments for Detection of Small-Sized Objects

This paper presents numerical simulation of broadband ultrasonic pulse propagation in the frequency range of 15…60 kHz to optimise the detection of small-sized underwater objects in bottom sediments of different nature (sand, silt, gravel). The chosen frequency range provides a compromise between the spatial resolution required for identification of objects with small angular dimensions and the depth of penetration of acoustic energy into dispersed media with different acoustic impedances. A mathematical model based on acoustic equations has been constructed, which takes into account dissipative energy losses during propagation in water medium and bottom sediments, dependence of acoustic velocity and density of the medium on its granulometric composition and porosity, influence of bottom topography on coherent and incoherent signal scattering, as well as the effect of multipath propagation leading to temporal and spatial interference of acoustic waves. A comparative study of the effectiveness of linear-frequency-modulated (LFM) signals and noise-like signals with pseudo-random phase modulation in the context of detection tasks is carried out, with emphasis on analysing their resolution and ability to suppress multiplicative interference and reverberation. The results of numerical experiments demonstrate a significant influence of propagation medium parameters (in particular, acoustic impedance and attenuation coefficient) and the shape of the probing signal on the probability of correct detection of small-sized objects, which determines the requirements to the algorithms for processing the received signals.