Lev A. Ostrovsky
Alexander M. Sutin
Inst. of Appl. Phys., Nizhni Novgorod, Russia
The hypothesis that cracks in ice may provide anomalously strong acoustic nonlinearity is experimentally verified. Two series of experiments were performed. One of them was done in situ, on a freshwater lake covered by a 40-cm-thick ice. Flexural waves were excited in ice by a vibrator in frequency range 0.2--2 kHz and received by accelerometers. Air temperature was -5 and +3 (degrees)C for two different series of measurements. A high level of nonlinearity was registered. In particular, a pronounced subharmonic signal was generated which testifies for parametric instability. The second experiment was performed in laboratory conditions, when a ``crack'' was created artificially, as a contact between two pieces of ice. We studied modulation of ultrasound (about 26 kHz) by low-frequency (30--90 Hz) vibrations. For vibration acceleration amplitudes up to 0.1 and 0.3 g a pronounced modulation spectrum appeared; moreover, modulation frequency subharmonics were observed in this case. Some theoretical considerations of the effects observed are also given. It is believed that nonlinear effect can be effectively used for Arctic ice characterization. [sup a)]Now at Univ. of Colorado, CIRES, ETL/ERL/NOAA, 325 Broadway R/E/ET, Boulder, CO 80303.