ACOUSTIC EFFECTS OF FRONTS

The following changes can be of significant importance to acoustics as a front is crossed:

. Near-surface sound speed can change by as much as 100 ft/sec. Although this is due to the combined effect of changing temperature and salinity, temperature is usually the dominant factor.

. Sonic-layer depth (SLD) can change by as much as 1,000 feet from one side of a front to the other during certain seasons.

. A change of in-layer and below-layer gradient usually accompanies a change in surface sound speed and SLD.

. Depth of the deep sound-channel (DSC) axis can change by as much as 2,500 feet when crossing from one water mass to the other.

. Increased biological activity generally found along a front will increase reverberation and ambient noise.

. Sea-air interaction along a frontal zone can cause a dramatic change in sea state and thus increase ambient levels.

. Changes in the vertical arrival angle of sound rays as they pass through a front can cause towed array bearing errors.

It is clear that any one of these effects can have a significant impact on ASW operations. Together they determine the mode and range of sound propagation and thus control the effectiveness of both short- and long-range acoustic systems. The combined effect of these characteristics is so complex that it is not always possible to develop simple rules for using ocean fronts for ASW tactics. For example, the warm core of the Gulf Stream south of Newfoundland will bend sound rays downward into the deep sound channel, thereby enhancing the receiving capability of a deep receiver. The same situation with a slightly shallower bottom south of Maine may create a bottom-limited situation, and the receiving capability at the same hydrophore will be impeded. In view of this, the acoustic effects of a front must be determined for each particular situation by using multiprofile (range-dependent environment) acoustic models. The input for these models can come from detailed oceanographic measurements, or from historical data in combination with surface frontal positions obtained from satellites.

DETERMINING FRONTAL POSITION USING SATELLITE DATA

Most fronts exhibit surface-temperature signatures that can be detected by satellite infrared (IR) sensors and are used in determining frontal positions. Figure 9-17 is an example of a satellite IR image obtained by the TIROS-N showing the location of the Gulf Stream and formation of a warm ring. Because surface-temperature gradients are not always reliable indicators of the subsurface front, satellite images must be interpreted by a skilled analyst, preferably in combination with data from other sources such as BTs. Automatic interpretation of satellite data is also being developed using techniques generally known as automatic imagery-pattern recognition or artificial intelligence. Now lets discuss oceanographic effects on mine warfare (MIW). Environmental Effects on Weapons Systems and Naval Warfare (U), (S)RP1, provides further detail on this subject.

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