Inference of material properties of zooplankton from acoustic and resistivity measurements

Abstract

A laboratory apparatus has been developed and used to infer the sound speed and density contrasts of live zooplankton. The sound speed contrast is determined from acoustic measurements of travel time (time-of-flight) and from the resistivity measurements of volume fraction. The density can then be inferred by applying the phase-compensated distorted wave born approximation (DWBA) model based on the attenuation measurement. For the decapod shrimp (Palaemonetes vulgaris), the inferred sound speed contrast found by using three different methods, namely the two-phase ray model (time average), the compressibility model (Wood's equation), and the DWBA model (scattering theory), is quite consistent, while the inferred density contrast agrees with the measured density reasonably well. The influence of ambient pressure on the sound speed and density contrasts has also been measured using a pressure vessel. The results indicate that the density contrast remains essentially unchanged under different pressure, but the sound speed contrast increases about 2.0% with pressure changing from 0 dbar to about 350 dbar. Although this 2.0% change in sound speed contrast only causes a moderate change in estimating biomass for a decapod shrimp, it could cause a much larger bias for weaker scatterers with the same amount of change in sound speed contrast (up to 20 dB). The most important advantage of this newly developed material properties measuring system is its potential applicability to the in situ determination of acoustic properties of zooplankton.