ASA 126th Meeting Denver 1993 October 4-8

5aUW12. Frequency-dependent sound attenuation in the northeastern Pacific Ocean below 400 Hz.

Gerald B. Morris

Code 7172, Naval Res. Lab., Stennis Space Center, MS 39529

Signals from SUS explosives were received at four hydrophones suspended in the deep ocean from the Research Platform FLIP. Energy flux densities were determined for approximately 700 of these broadband sources at ranges from 70 to 1250 nmi. The normal approach in determining attenuation is to calculate the transmission loss with no frequency-dependent attenuation as a function of range using a numerical model, subtract the calculated spreading loss and attribute the remaining loss to frequency-dependent attenuation. Because of the large losses due to geometric spreading and small losses from the attenuation together with the strongly range-dependent environment calculating the spreading or transmission loss with high accuracy is difficult. An alternate technique was used. Differences in measured propagation losses at various frequency bands up too 400 Hz were used to calculate relative losses referenced to a band near 50 Hz. Assumptions were then made regarding the functional form of the frequency dependency of these losses and the resulting frequency-dependent attenuation losses determined. The resulting values are consistent with a boric acid relaxation frequency of 1 kHz, but the loss coefficient is lower than those predicted from Thorp's equation. These results are in agreement with published values determined for higher frequencies from cw acoustic transmissions in the same region. [Work supported by ONR.]