Broadband acoustic quantification of mixed biological aggregations at the New England shelf break.

At the New England shelf break, cold, less saline shelf water collides with warmer saltier slope water to form a distinct oceanographic front. During the Office of Naval Research Sediment Characterization Experiment in 2017, the front was mapped by narrowband (18 and 38 kHz) and broadband (70-280 kHz) shipboard echo sounders. The acoustically determined cross-shelf velocity of the front ranged in amplitude from 0.02 to 0.33 m/s. Acoustic surveys revealed aggregations of scatterers near the foot of the front. Acoustic backscatter in conjunction with Northeast Fisheries Science Center bottom trawl surveys identified longfin squid (Doryteuthis pealeii) and mackerel (Scomber scombrus) as the most likely scatterers in the aggregations. A mixed species scattering model was developed and further refined by the use of a matching method used for distribution of the lengths of each species. The mean length of squid and mackerel, respectively, using the matching method was 4.45 ± 1.00 and 20.25 ± 1.25 cm compared with 6.17 ± 2.58 and 22.76 ± 1.50 cm from the trawl data. The estimated total biomass of the aggregation was a factor of 1.64 times larger when using the matching method estimated length distribution compared to the trawl length distribution.

Broadband acoustic scattering from oblate hydrocarbon droplets.

Improved in situ quantification of oil in the marine environment is critical for informing models of fate and transport and evaluating the resiliency of marine communities to oil spills. Broadband acoustic backscatter has been used to quantify a variety of targets in the water column; from fish and planktonic organisms to gas bubbles and oceanic microstructure, and shows promise for use in quantifying oil droplets. Quantifying water column targets with broadband acoustic backscatter relies on accurate models of a target's frequency dependent target strength (TS), a function of the target's acoustic impedance, shape, and size. Previous acoustic quantification of oil droplets has assumed that droplets were spheres. In this study, broadband (100.5-422 kHz) acoustic backscatter from individual oil droplets was measured, and the frequency dependent TS compared to a model of acoustic scattering from fluid spheres and two models for more complex shapes. Droplets of three different crude oils, two medium oils, and one heavy oil were quantified and all droplets were oblate spheroids. The impact of the deviation from sphericity on the accuracy of each model was determined. If an inversion of the model for spherical droplets was used to estimate flux from acoustic observations, errors in the predicted volume of a droplet were between 30% and 50%. The heavy oil also showed deviations in predicted volume of 20%-40% when using the two models for more complex shapes.

Acoustically relevant properties of four crude oils at oceanographic temperatures and pressures.

Inversions of models of broadband acoustic scattering to detect and quantify weakly scattering targets, such as oil droplets in seawater, require precise knowledge of the physical properties that determine scattering. When the characteristic impedance contrast between a target and the surrounding medium is weak, small differences between the true and modeled impedance can cause significant errors in modeled scattering. For crude oil, currently available empirical models of density and sound speed are derived from measurements made at reservoir conditions (high temperature and pressure), which may not be relevant to oceanographic conditions due to phase changes in the oil. Measurements of the density and sound speed, as well as thermal characterization of phase changes via differential scanning calorimetry, of four crude oils at oceanographically relevant temperatures and pressures were made and compared to a commonly used empirical model for sound speed and density. Significant deviations between the measured and modeled values were found and different empirically fit models were developed. A literature review of sound speed data was also performed, and the innovative empirical model shows improvement over the commonly used empirical model for both the data measured here and the measurements in the literature.