Acoustic reflections from arctic ice at 15–300 kHz

Abstract

Underwater acoustic systems operating in the Arctic are generally used to detect ice or objects in an ice background. In either case, knowledge of the reflections from ice, both undisturbed and ridged, is necessary. For the past few years, the authors have been measuring monostatic reflections near normal incidence from arctic ice to advance this knowledge. In 1988, reflections at 20-80 kHz were measured from the ends of four cylindrical blocks of flat arctic ice with diameters of 27-84 cm. The blocks were individually depressed below the surface so that their reflections were separable from those off the underside of the ice canopy. The transducer was moved horizontally beneath the block to measure the angular response pattern.Examination of the bottom of the block after it was removed showed that the surface was flat except for some roughness with a standard deviation less than 3 mm. Compared with solid ice, the skeletal layer reduced normal-incidence reflection by 8-11 dB. The results were consistent for all four blocks, confirming that the results were valid. Measurements of reflection from the underside of the ice canopy at 15-300 kHz gave similar results and extended the frequency range. Measurements of the physical properties of the ice were helpful in analyzing the nature of the reflection.

Laboratory measurements of high-frequency, acoustic broadband backscattering from sea ice and crude oil

“…However, the sound speed contrast at the oil/ice interface is much greater than the density contrast so the model is relatively insensitive to a realistic range of densities. In the absence of oil, the reflection coefficient from the water/ice interface is R w;ice ¼ 0.05, which is consistent with previously reported values for sea ice at high-frequencies [5][6][7][8]14 and the data in Fig. 1(c) prior to the introduction of oil.…”

“…The skeletal layer and bulk properties of the ice vary based on their structure and brine content, so sea ice can be modeled acoustically using sound speed profiles dependent on the range from the water/ice interface. [7][8][9]14 Because of the high frequencies used in this work, reflection from the sea ice is modeled based on the impedance mismatch at the interface where the ice properties are c ice ¼ 1700 m/s and q ice ¼ 960 kg/m 3 , which are similar to values previously used to model reflections from ice. [7][8][9]14 The properties of oil, c oil ¼ 1475 m/s and q oil ¼ 885 kg/ m 3 , are based on empirical formulations for the properties of crude oil using the API gravity at 0 .…”

“…[7][8][9]14 Because of the high frequencies used in this work, reflection from the sea ice is modeled based on the impedance mismatch at the interface where the ice properties are c ice ¼ 1700 m/s and q ice ¼ 960 kg/m 3 , which are similar to values previously used to model reflections from ice. [7][8][9]14 The properties of oil, c oil ¼ 1475 m/s and q oil ¼ 885 kg/ m 3 , are based on empirical formulations for the properties of crude oil using the API gravity at 0 . 15 At the time of deployment the crude oil was approximately 5 C; no temperature measurements of the oil are available post-deployment.…”

“…The physical scales associated with the complex structure of the skeletal layer suggest that high-frequency (>10 kHz) acoustic scattering techniques are well suited to studies aimed at remotely characterizing sea ice. [5][6][7][8][9] The results presented here demonstrate that high-frequency, broadband techniques can be applied in both the temporal and frequency domains to identify a thin layer of crude oil, approximately 4 cm thick, beneath laboratory-grown congelation ice. Additionally, the measurements performed here suggest that the structure of the skeletal layer is significantly modified by the presence of even a very thin layer of oil, thinner than the oil layer thickness resolvable given the available bandwidth.…”

Near-normal incidence scattering from rough, finite surfaces: Kirchhoff theory and data comparison for arctic sea ice

“…Our study was motivated by observations of acoustic reflection from the bottom of arctic ice blocks by Garrison et al (1991). Figure 1 shows their generic experimental setup.…”