International standards for the measurement of underwater noise from vessels: Numerical modelling in support of a shallow water standard

ANSI and ISO have issued standards for the measurement of the radiated noise level of surface vessels in deep water. In addition, a draft standard for the conversion of such measurements into a monopole source level, as required for environmental noise models, is at an advanced stage. However, many operators and investigators are constrained to making measurements in shallow water where both sea-surface and seabed effects are important. In order to study the influence of these effects a numerical study has been performed using both an image source model and OASES, with very good agreement. The goal of this work has been to understand the variability of the correction factor required to convert measurements of received pressure, at short range, into the equivalent source level in such an environment. These calculations have included the effects of reflection at both the sea-surface and an attenuating seabed. The influences of the water depth, seabed type, distance of closest approach, and number and location of the hydrophones have been investigated. The results show how the correction factor varies and indicate the extent to which simplified approximations can be used for the correction factor.