Broadband acoustic scattering from oblate hydrocarbon droplets

Abstract

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.

Shipboard Acoustic Observations of Flow Rate From a Seafloor‐Sourced Oil Spill

“…A further aid to classification of oil droplets and gas bubbles would be the addition of broader frequency range, especially lower frequencies, to the echosounders used in surveying the site. With a broader range of frequencies it may be possible to detect the peak and null structure of individual droplets which would aid in determining the size as well as the identity of oil droplets (Loranger et al, 2019). At low frequencies, the difference between the target strength of oil droplets and gas bubbles is much more pronounced.…”

“…Density and sound speed were modeled according to Loranger et al (2018). Oil droplets within the ellipsoidal regime (radius < 0.5 mm Clift et al, 1978) were assumed to have an eccentricity of 0.6, the average eccentricity for an oil with similar API gravity reported by Loranger et al (2019).…”

“…The backscatter for individual gas bubbles, , was modeled according to Clay and Medwin (1977) with a correction by Ainslie and Leighton (2009), while the backscatter for individual oil droplets, , was modeled using the distorted wave Born approximation (Loranger et al, 2019; Stanton et al, 1998). The radius of bubbles a j and droplets b j from Step 1 was used to calculate and in 0.5‐m‐depth bins.…”

“…The backscatter for individual gas bubbles, bs,n ( ), was modeled according to Clay and Medwin (1977) with a correction by Ainslie and Leighton (2009), while the backscatter for individual oil droplets, bs,m ( ), was modeled using the distorted wave Born approximation (Loranger et al, 2019;Stanton et al, 1998). The radius…”

“…Broadband acoustics have potential to overcome this ambiguity by using the shape and amplitude of the backscatter over a range of frequencies to differentiate between oil and gas. The shape and amplitude of broadband acoustic backscatter has previously been used to differentiate between and identify different targets in the water column (Holliday, 1972; Lavery et al, 2010; Loranger et al, 2019; Medwin & Breitz, 1989; Stanton et al, 2010; Terrill & Melville, 2000; Thompson & Love, 1996; Vagle & Farmer, 1992). Broadband acoustic echosounders also improve range resolution (Weidner et al, 2019) over narrowband systems which helps to separate oil and gas into difference‐ensonified volumes.…”