APPENDIX A 



NARRATIVE DESCRIPTION OF EXPERIMENT 



The organization of the surface duct acoustical transmission experiment was 

 established in NEL Project Plans 25-65 and 26-65. Its purpose was to deter- 

 mine (1) the effect of internal waves on acoustical transmission in a surface 

 layer and (2) the validity of normal mode theory in the analysis of surface duct 

 transmission. Accordingly, acoustical transmission loss was measured in two 

 ocean areas, each containing a well defined quasi -isothermal layer but differing 

 in that the water mass in one area was stable whereas it was unstable in the 

 other. The experiments were performed in December 1964, in ocean areas 

 between 200 and 300 miles SW of Los Angeles (Figure 1) . 



The USS Marysville surveyed the ocean areas of interest and selected two sites 

 in deep water for the acoustical measurements. The first site was at the boun- 

 dary between Eastern North Pacific Central Water and Transition Water flowing 

 southward along the coast of California . The second area was located within the 

 Transition Water to the east of the Boundary Region in an area described by Lee, 

 Reference (1), as the more stable of the two. The Marysville employed a therm- 

 istor chain to determine the thermal profiles (Appendix B), upon which its site 

 recommendations to the USS Rexburg and USNS Charles H . Davis were based. 

 The thermal surveying was completed between 10 and 24 hours before the other 

 two ships arrived on station to perform the acoustical experiment . The latter 

 obtained their thermal profile data from hourly bathythermograph casts . 



The USS Rexburg received and recorded on magnetic tape the acoustic signals 

 transmitted by the USNS Charles H . Davis . The Rexburg used three hydrophones: 

 a shallow hydrophone (Channel 1) at a depth of 25 or 50 feet, an intermediate hy- 

 drophone (Channel 2) at 80 feet, and a deep hydrophone (Channel 3) at 300 feet. 

 The outputs of the hydrophones were recorded on Visicorder and magnetic tape 

 by means of the circuitry illustrated in Figure C-l . A local clock recorded half 

 second time markers at five -second intervals and a local microphone was pro- 

 vided for the recording of verbal information from the scientist in charge . 



The USNS Davis was equipped with projectors that radiated half-second pings at 

 700 cps, 1300 cps, and 3000 cps in sequence at 10-second intervals. The acous- 

 tical ping was accompanied by a synchronizing radio pulse transmitted to the 

 Rexburg and recorded on the magnetic tape . For identification, every second 

 ping at 1300 cps and the corresponding radio pulse were doubled in length. 



At the two locations at which the experiment was performed the same pattern 

 was followed. At the site chosen by the Marysville, the Rexburg lay to and 

 drifted with the current . The Davis moved radially away from the Rexburg 



A-l 



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