Acoustics and Eddies

Project Team Member

LCDR David M. Ruth, USN

Major Findings

The acoustic environment of an ocean basin is significantly perturbed (locally) by the presence of eddies.The literature reviewed for this presentation examines the effects of a cyclonic ring in the Sargasso Sea on sound transmission.Two important factors affecting sound transmission through an eddy are sound speed variation and current speeds.

The vertical displacement of isotherms in a cold-core ring alters the local sound velocity profile in such a way that the SOFAR axis is deflected to a shallower depth inside the ring than in the surrounding Sargasso Sea water, and sound speed along the axis is slower inside the ring.For sound transmitted from (relatively) shallow depth at the cold-core ring center, the associated convergence zone is deformed and shifted closer to the transmission source compared to sound transmitted from the same depth in the Sargasso Sea.Transmission loss is also greater for sound traveling through the ring due to stronger downward refraction of rays.

Figure 1. Sargasso Sea ray trace

Figure 2. Ray trace with origin at cyclonic ring center

Figure 3. Transmission loss through Sargasso Sea (solid) and cyclonic ring (dashed)

[Figures 1-3 from Vastano (1973)]

Current speeds are a less obvious, but important, factor affecting sound transmission.Maximum eddy current speeds are great enough to noticeably delay the arrival of a sound signal that is transmitted "upstream" (against the current) compared to one transmitted in the absence of a current.The combination of sound speed variation and high current speeds results in a shift in signal phase as source-receiver (S-R) position and orientation vary within the ring.

Figure 4.Phase shift as a function of S-R position

Figure 5.Phase shift as a function of S-R orientation

[Figures 4-5 from Henrick (1980)]

As long as an interest exists in underwater detection in environments where rings are present, studying and understanding the effects of rings on sound transmission will be of interest to the Navy.

References

Henrick, R.F., W.L. Siegmann, and M.J. Jacobson, 1977: "General analysis of ocean effects for sound transmission applications." J. Acoust. Soc. Am., Vol. 62, 860-870.

Henrick, R.F., M.J. Jacobson, and W.L. Siegmann, 1980: "General effects of currents and sound-speed variations on short-range acoustic transmission in cyclonic eddies." J. Acoust. Soc. Am., Vol. 67, 121-134.

Vastano, A.C., and G.E. Owens, 1973: "On the Acoustic Characteristics of a Gulf Stream Cyclonic Ring." J. Phys. Oceanogr., Vol. 3, 470-478.

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