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.