High Frequency Acoustic Properties of Urea Ice

摘要

The objective of this experiment was to describe the high frequency acousticbackscatter from simulated arctic sea ice as a function of the physical properties of the ice. In addition, diffraction experiments were performed on the edge of an open lead. These edges are acoustically interesting because saline ice both supports shear waves and is highly absorptive. Experiments were carried out on urea ice grown under controlled laboratory conditions in the cold pit at the Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire. Normal incidence sonar echo amplitude data were collected at kilo-Hertz carrier frequencies from the underside of a growing ice sheet. Using a plane-wave reverberation model, the velocity of the acoustic wave, attenuation and normal incidence reflection coefficient of the ice were measured at 15, 17, 20, 25 and 190 kHz. Analysis showed that the urea ice had reflection coefficients of about 0.4 in the tens of kHz range, significantly higher than saline ice. Attenuation at those frequencies was about 0.3 dB/cm. At higher frequencies, the acoustic properties of the urea ice begin to approach those measured on saline ice. For the diffraction phase of the experiment, a slot was cut in in the ice and the transducers were moved laterally under the edge of the lead. The diffracted and reflected echo arrivals were resolved and the amplitude of the diffracted signal was measured. A good comparison was found between the diffracted arrival values and Helmholtz-Kirchoff theory.