Correlations between sound speed and density in seabed sediment cores collected in Norwegian waters

In modelling of underwater sound propagation, knowledge about the seabed’s acoustic properties is essential, especially sound speed and density. These two properties are highly correlated, and standard empirical regressions are often used to predict one property from the other. The Norwegian Defence Research Establishment (FFI) has a large database of sediment sound speed and density measurements from laboratory analysis of seabed gravity cores. This paper presents the results from 47 cores collected from FFI’s research vessel M/S H.U. Sverdrup II on the Norwegian continental shelf and in the Barents Sea. The densities measured in the cores range from 1.35 to 2.4 g/cm3 and sound speeds from 1410 to 1750 m/s (corrected to 23°C, and 1 atm. pressure). An empirical regression between density and sound speed derived from FFI’s data set deviates significantly from standard empirical regressions found in the literature, as bulk densities are higher and sound speeds lower than expected. Another study from the Barents Sea also reported high bulk densities in seabed sediments and suggested that clay mineralogy might be the cause.The importance of applying an appropriate sound speed-density relationship will be demonstrated using the acoustic ray tracer Lybin developed by the Royal Norwegian Navy and FFI.