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.