Comparative studies and field measurements of the acoustic absorption coefficient in seawater

The absorption coefficient of seawater is an important component to the sonar equation underlying fisheries and zooplankton acoustic investigations. The equations currently considered most accurate and widely used for calculating the coefficient are the three decades-old work of Francois and Garrison (1982, J. Acoust. Soc. Am. 72: 896- 907, and 72: 1879-1890). However, there is evidence to suggest that these equations are inadequate for the higher frequency applications that have become increasingly important in ecosystem-based acoustic surveys and management. Comparative studies among the different formulas will be presented. In situ measurements of acoustic absorption were conducted onboard the R/V G. O. SARS in the Norwegian fjords from 2011 to 2014. It was found that the absorption coefficient at 333 kHz was about 18.5 dB/km more than that given by Francois and Garrison's work, which could lead to an uncertainty in biomass estimates of about 50% more at a range of 100 m. In addition, inaccurate absorption coefficients will also introduce a depth-dependent bias on frequency response and impair acoustic echo characterization and species identification/classification that strongly rely on relative frequency response.