Acoustic wave scattering from dynamic rough sea-surfaces using the finite-difference time-domain method

Abstract

Numeric models for underwater acoustic propagation typically assume the sea-surface to be either perfectly smooth or rough but “frozen” in time. For long sonar signals on the order of tens of seconds, the sea-surface can interact at many different wave displacements over its duration. This causes anomalies in the received signal which introduces additional transmission losses and Doppler effects. The impact of including roughness and motion of the sea-surface on sonar systems is investigated using the finite-difference time-domain (FDTD) method. The FDTD method is a numeric technique that is well suited for modeling boundary roughness and motion. This is due to its ability to directly configure complex boundary conditions in the surrounding simulation grid and full pressure wave propagation in the time-domain. The rough, time-evolving sea-surface is modeled using a Pierson-Moskowitz (PM) frequency spectrum which is simple to implement and defined using just wind speed and direction. The results from FDTD simulations of static rough sea-surfaces are compared to a previously established integral solution method to evaluate the validity of the approach. Agreement is also demonstrated for FDTD simulations of a dynamic rough sea-surface and a theoretic statistical model. [Work supported by the Office of Naval Research.] Numeric models for underwater acoustic propagation typically assume the sea-surface to be either perfectly smooth or rough but “frozen” in time. For long sonar signals on the order of tens of seconds, the sea-surface can interact at many different wave displacements over its duration. This causes anomalies in the received signal which introduces additional transmission losses and Doppler effects. The impact of including roughness and motion of the sea-surface on sonar systems is investigated using the finite-difference time-domain (FDTD) method. The FDTD method is a numeric technique that is well suited for modeling boundary roughness and motion. This is due to its ability to directly configure complex boundary conditions in the surrounding simulation grid and full pressure wave propagation in the time-domain. The rough, time-evolving sea-surface is modeled using a Pierson-Moskowitz (PM) frequency spectrum which is simple to implement and defined using just wind speed and direction. The results from FDTD...