Clark and Yarnall 



surfaces here would be influenced by the bottom boundary. 



It is tempting to extrapolate the results discussed above 

 to the H43 propagation path, i.e. , to picture a spatially coherent 

 diurnal disturbance of the thermal structure extending completely 

 across the Straits and accounting for the phase measurements at both 

 H3 and H43 . Such a situation could be set up by a cross-stream tidal- 

 ly resonant internal seiche. 20 a glance, however, at Figure 13, might 

 generate suspicion about extrapolating results obtained for the H3 

 portion of the path to the complete path. There is little doubt, 

 over the full time span of LCT-1, that dramatically different influ- 

 ences are dominating the signal at H3 and H43 . In addition, exper- 

 imental data exists in regard to the cross-channel component of cur- 

 rent which must be taken into account at H43. 



Very simple calculations involving the trajectory of a 

 "direct ra^' show that variations in the Florida Current flow compo- 

 nent which is perpendicular to the acoustic path will produce a 

 negligible response in acoustic phase. Cross-channel components of 

 the flow, however, add algebraically to sound speed along the hypo- 

 thetical direct ray and will have appreciable effect. 



Data relating to the measured cross-channel component has 

 been provided by Operation Strait Jacket.* Figure 23 shows four sets 

 of depth averaged velocity measurements transecting the Straits taken 

 in the period 24 May to 24 June, 1965.2-'- The fifth curve, the dashed 

 half- sinusoid, is a convenient approximation of the measured currents 

 used in calculations of the acoustical effects.** The dashed curve 

 may be understood to represent a current which is uniform from top to 

 bottom and which peaks at 0.5 knots approximately in the center of the 

 Straits. The calculation consists of assuming first a non-current 

 condition and then obtaining the total acoustic phase change in the 

 transition to a current condition represented by the dashed line. 

 Unfortunately the propagation model is not adaptable to this cal- 

 culation, and it was necessary to fall back upon the assumption of a 

 direct ray. Since, on the average, the ratio of path length to water 

 depth in the Straits is greater than 100:1 the calculations based on 

 a direct ray should give useful order of magnitude values. The re- 

 sult of the calculation is a phase change of 2.44 cycles. For a 

 cross- channel component, which can swing also to a maximum in the 

 opposite direction, the peak to peak change would be 4.88 cycles. 

 These numbers are to be compared to measured values of 5 to 15 cycles. 

 There is order of magnitude correspondence in the results. However, 

 in view of the approximations Involved, they will be used to support 



* 



Florida Current studies performed under the direction of 

 Dr. William S. Richardson, now of Nova University, Ft. 

 Lauderdale, Florida. 



** These calculations were performed by Mr. Richard A. Altman of 

 the University of Miami Ocean Engineering graduate program. 



328 



