Measurements of Acoustic Reflection from the End of a Cylindrical Block of Arctic Ice

• Main Authors: Garrison, G. R., Francois, R. E., Wen, T., Stein, R. P.
• Other Authors: WASHINGTON UNIV SEATTLE APPLIED PHYSICS LAB
• Format: Text
• Language: English
• Published: 1986
• Subjects: Acoustic Detection and Detectors; Acoustics; *TARGET STRENGTH; *SEA ICE; *ACOUSTIC REFLECTION; CONFIGURATIONS; PATTERNS; MEDIA; ACOUSTIC SCATTERING; IMPEDANCE; NEAR FIELD; ELECTROACOUSTIC TRANSDUCERS; ARCTIC REGIONS; CYLINDRICAL BODIES; SURFACES; MEASUREMENT; ISOLATION; Arctic; Sea ice
• Online Access: http://www.dtic.mil/docs/citations/ADA229601; http://oai.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=ADA229601

In the fall of 1984 an experiment was conducted at an arctic ice camp to examine 20-80 kHz acoustic reflections from an isolated ice block. The results are compared with a simple model of ice block reflections that has been used in conjunction with an ice block configuration for pressure ridge keels. The study of the reflections from these keels is important because they form an interfering background for acoustic equipment operating under the ice. A cylindrical block of ice 0.84 m in diameter was cut from the flat surface of a floe and depressed so that, for a transducer placed below, the reflection from the block would arrive before that from the surrounding ice. A transducer 15-30 m below the block was moved horizontally in steps to measure changes with aspect. The returns from the block are compared with those predicted by the model, which includes a loss based on the bulk impedance properties of the two media. When near field effects are included, the measured returns at 20 kHz are similar to those predicted; but as the frequency is increased, the return to normal incidence is lower than predicted and the response pattern is broadened and smoothed. The complex structure of the growing sea ice, producing both volume and surface acoustic scattering, is the most likely cause of the discrepancy. (JHD)