Sound Velocity Structure Research Articles

Aspects of the Mid-Pacific Transition Zone

In the central North Pacific, the transition between subarctic and subtropical water masses takes place between 42° and 32°N. The width of the zone and the sharpness of its boundaries are largely determined by the wind stress distribution at the sea surface. The northern boundary is characterized by numerous temperature inversions, the disappearance of the subarctic halocline, gravitational instabilities, and a complicated sound velocity structure. The southern boundary is marked by sharp thermohaline gradients and instabilities in the upper-most layer. The vertical distribution of properties in the transition zone is characterized by a deep salinity minimum of about 33.95‰ at 500 meters, by a sound velocity minimum of slightly less than 1480 m sec−1 at 800 meters, and by an almost linear temperature gradient in the upper 600 meters. Both the salinity and sound velocity minimum rise northward and lose their identity at the northern boundary. The baroclinic and barotropic modes of motion in the transition zone are very slow with speeds rarely exceeding 5 cm sec−1. A strong westward setting current with surface speeds in excess of 100 cm sec−1 was found 22 km north of Kahuku Point, Oahu, Hawaii. The width of the high-speed core was about 6 km, and the relative vorticity due to horizontal current shear reached values in excess of 2×10−4 sec−1.

Fluctuation in Horizontal Acoustic Propagation over Short Depth Increments

Underwater acoustic-propagation measurements have been made off Key West, Fla., for the past 3 yr. Two or three deep-submergence vehicles were allowed to sink freely and simultaneously through the water to depths as great as 5000 ft while transponding acoustically with each other. Pulses at frequencies of 10, 20, and 40 kHz and horizontal ranges of 1700 to 8000 yd were used in the experiment. Because direct acoustic signals showed amplitude fluctuations of approximately 15 dB for depth changes of 15 20 ft, a method of rapid pulsing was installed. This allowed the emission of discrete signals 0.25 sec apart over a period of 5 sec periodically during an operation. Amplitude fluctuations for depths changes of only a few inches could be measured. Pulse-to-pulse amplitude differences of up to 15 dB were found for depth changes of as little as 10 in. These fluctuations are discussed in terms of transmission loss and are compared to the losses suffered for the larger depth changes and for those predicted by ray theory on the basis of the measured sound-velocity structure of the water. It is concluded that refraction conditions can account for the fluctuation.

Fluctuation in Underwater Acoustic Propagation Over Short Depth Increments

Underwater-acoustic-propagation measurements have been made off Key West, Florida periodically for the past 2 yr. Two or three deep-submergence vehicles were allowed to sink freely through the water to depths as great as 5000 ft while transponding acoustically with each other. Pulses at frequencies of 10, 20, and 40 kHz and ranges of 1700 to 4000 yd were used in the experiment. Because direct acoustic signals showed amplitude fluctuations of the order of 20 dB for depth changes 20–30 ft, a method of rapid pulsing was installed so that discrete signals 0.25 sec apart could be transmitted periodically for a period of 5 sec. This allowed a measure of the fluctuation for depth changes of only a few inches. Pulse-to-pulse amplitude differences of up to 10 dB were found for depth changes of as little as 10 in. These fluctuations are discussed in terms of transmission loss and are compared to the losses suffered for larger depth changes and those predicted by ray theory on the basis of the measured sound velocity structure of the water. [This research was supported by the U. S. Naval Ordinance Systems Command.]

Long-Range CW Sound Transmission

An attempt was made to propagate pulsed CW sound coherently over thousand-mile distances in the mid-Atlantic using the focusing properties of the sound channel. The sound field emanating from a 440-cps lumped source at the sound channel depth off Barbados was studied at stations 159 and 975 mi. away in the NNE direction. This field was probed in depth with a 3-dimensional array of hydrophones with the source on for 8- and 9-min periods followed by 2- or 1-min silent periods. While a moderately coherent field existed at 159 mi., inter-hydrophone correlation measurements indicated a sharply rippled wave front which was not at all stable in time. Individual hydrophone records showed large random instabilities with variation of intensity by a factor of 3 occurring within periods of a few minutes. At 975 mi. no coherent signal at all was detected on individual hydrophones although the source strength was eight times greater. A barely detectable inter-hydrophone correlation was observed. The negative results of this attempt are at least in part due to the irregular sound velocity structure of the sound channel as described in the accompanying abstract (F1), (This work was supported by the Office of Naval Research.)

Observations of the Stability of a Normal Mode Sound Field in an Intermediate Scale Model

Spectra of sound pressure amplitude versus frequency in the range 1.1–2.4 kc have been obtained at regular intervals in shallow (10 ft) water during two periods of extended observation. The acoustical data have been presented in the form of contour diagrams of sound pressure amplitude versus frequency and time as coordinates and compared with water conditions. The form of the contour diagrams permits differentiation between field structure variation and variation in attenuation, although differentiation between the associated processes has not been possible. Temporal variation in the vertical sound velocity profile produced only small changes in the gross features of the field structure—as inferred from the contour diagram—for up to the first three modes of propagation. For frequencies above 1.6 kc (i.e., when three or four modes were stimulated) the variations in field structure were prominent. Attempts have been made, with little success, to explain periods when the received signal was strongly attenuated in terms of vertical sound velocity structure.