Sound exposure level and energy spectral density of underwater explosions in shallow water over a coral substrate off the southern coast of O`ahu, Hawai`i

Abstract

This work presents the sound exposure levels (SEL) and energy spectral densities (ESD) from underwater explosions measured in shallow water (10-18 m) at distances of 500 to 1500 m at the Pu’uloa Underwater Detonation Range off the southern coast of Oahu. Nine explosive charges, with TNT-equivalent weights of 2.2 to 8.6 kg, were detonated on a seabed characterized by a thin sand layer over limestone. The ESD of the measurements are characterized by high propagation loss in the frequency ranges 50 to 350 Hz. Previous studies have shown that this is a common characteristic of this environment with the upper and lower frequencies directly related to the geoacoustic properties of the seabed and the waveguide geometry. Unlike measurements collected in sandy environments, where SEL agreed with empirical predictions, the SEL for these measurements differ by up to 30 dB. To understand the mechanisms responsible for these lower than expected levels, a geoacoustic model for the seabed is developed using these frequencies and utilized in broadband modelling simulations. The SEL will also be calculated for these simulations and compared to the measured levels. [Research supported by Pacific Fleet with partial support from the Office of Naval Research.]