ASA 126th Meeting Denver 1993 October 4-8

2pUW3. Low-frequency acoustic transmission at a shallow water site in the Gulf of Mexico.

Peter G. Cable

BBN Systems and Technol., Union Station, New London, CT 06320-6147

During Area Characterization Test I (ACT I), conducted in September 1992 in 100-fathom water on the West Florida Shelf, acoustic transmission measurements were made covering the band 50 Hz to 1 kHz using explosive charges detonated at 90-m depth. The test site had a sand--silt--clay bottom with a gentle slope of 0.135(degrees), and up, down, and cross-slope transmission data to ranges of 40 km were collected on collocated 25-element vertical and horizontal hydrophone line arrays. The low sea states and downward refracting acoustic conditions prevalent during ACT I ensured that transmission loss was controlled by geometric spreading and bottom interactions. Transmission loss as a function of range, frequency, and receiver depth has been determined and compared favorably with predictions for both the flat and sloping bottom cases. In addition, the multipath temporal structure and the vertical arrival structure of the received signals have been analyzed and have indicated the role of bottom and subbottom characteristics to the properties of the received signal. These analyses of the signal structure have highlighted transmission mechanisms not readily apparent in transmission loss data and have emphasized the importance of knowing subbottom structure to a detailed understanding of shallow water transmission over sand bottoms. [Work supported by the ARPA Adverse Environments Program.]