Broadband acoustic backscatter from crude oil under laboratory-grown sea ice

Abstract

In ice-covered seas, traditional air-side oil spill detection methods face practical challenges. Conversely, under-ice remote sensing techniques are increasingly viable due to improving operational capabilities of autonomous and remotely operated vehicles. To investigate the potential for under-ice detection of oil spills using active acoustics, laboratory measurements of high-frequency, broadband backscatter (75-590 kHz) from crude oil layers (0.7-8.1 cm) under and encapsulated within sea ice were performed at normal and 20 incidence angles. Discrete interfaces (water-oil, oil-ice, and ice-oil) are identifiable in observations following oil injections under the ice and during the subsequent encapsulation. A one-dimensional model for the total normal incidence backscatter from oil under ice, constrained by oil sound speed measurements from À10C to 20 C and improved environmental measurements compared to previous studies, agrees well with preencapsulation observations. At 20 incidence angles echoes from the ice and oil under ice are more complex and spatially variable than normal incidence observations, most likely due to interface roughness and volume inhomogeneities. Encapsulated oil layers are only detected at normal incidence. The results suggest that high-frequency, broadband backscatter techniques may allow under-ice remote sensing for the detection and quantification of oil spills.

Direct inference of first-year sea ice thickness using broadband acoustic backscattering

“…The modeled or measured backscatter from both interfaces is strongly dependent on the transmitted frequency due to the sound speed profile, roughness of the ice, and the backscattering and attenuation within the ice. Bassett et al (2016) show broadband measurements of backscattering from the water-ice interface as a function of frequency. At frequencies greater than 200 kHz, backscatter becomes highly variable and at even higher frequencies penetration into the ice is limited by the scattering and high attenuation rates.…”

“…At frequencies greater than 200 kHz, backscatter becomes highly variable and at even higher frequencies penetration into the ice is limited by the scattering and high attenuation rates. Low SNRs may have been a limiting factor in Bassett et al (2016) at the highest frequencies. These results, which are derived from the same data presented here, show that at the lowest frequencies measured (75-130 kHz) echoes from both the water-ice and ice-air interfaces were observed with ice thicknesses up to approximately 80 cm.…”

“…The experiments were performed at the Ice Engineering Facility Testing Basin at the Cold Regions Research and Environmental Laboratory (CRREL) in Hanover, NH, between October 23, 2014, andJanuary 23, 2015. Additional details about the facilities, experiment, and instrumentation can be found in Bassett et al (2016) and Pegau et al (2017).…”

“…All backscattering measurements of the ice were obtained outside of the oil containment skirts that had been placed in the tank. System 1 measured broadband acoustic backscattering with a modified Edgetech sidescan sonar system (Bassett et al, 2016;Lavery et al, 2010) and custom Airmar Technology Corporation (35 Meadowbrook Drive, Milford, NH) broadband transducers operated in a monostatic configuration. The transducer used in this analysis transmitted a 75-130 kHz linear frequency modulated (LFM) chirp.…”

“…(1) the transition in acoustical properties across the skeletal layer and (2) the physical structure of the skeletal layer itself. At high frequencies (f > 150 kHz) where the acoustic wavelengths are typically small compared to the thickness of the skeletal layer (i.e., ka > 1, where k ¼ 2p=k is the acoustic wavenumber and a is a representative geometric scale related to the roughness), the physical structure (e.g., dendrite size) of the skeletal layer becomes important and scattering is less coherent (Bassett et al, 2016;Stanton et al, 1986). On the other hand, at ka < 1, the scattering is less variable and specular reflections are dominated by the strong sound speed gradient across the skeletal layer (Garrison et al, 1991;Mourad and Williams, 1993;Wen et al, 1991;Winebrenner, 1992).…”