Sound pressure levels measurements and transmission loss models for explosions in coastal waters

Abstract

Sound propagation in shallow waters can have great variability due to many different characteristics of these environments and its described by numerous interactions with both the sea surface and sea floor which influence the propagation of acoustic energy through reflection, scattering and absorption. Here we used the basic spreading loss acoustic models and a semi empirical shallow water model known as Colossus for transmission loss predictions and compared with in situ measurements of explosions originated from underwater rock blasting activities performed in shallow waters of a coastal bay area in Brazil. During explosion events measurements were taken from stationary boats using custom calibrated hydrophones and record systems in three different sites of known distance. Transmission loss observed was compared with models considering range, frequency and additional parameters input for the semi empirical model. Explosions were characterized as broadband impulsive sounds with energy content concentrated in low frequency bands of 63 to 500 Hz. Sound pressure levels in these bands often exceeded 200 dB re 1pPa around 200–300 meters distance from the source. A single small charge with no clipped signal was used to measure transmission loss and comparison with models showed that all models overestimate the transmission loss in the area. Cylindrical transmission loss had the best prediction from in situ measurements but errors up to 20 dB were observed. Results, and variables that can affect them, are discussed in light of shallow water propagation modeling and experimental data.