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High-Frequency Sonar Performance

Active sonars that operate at frequencies above about 100 kHz are used in shallow water ocean environments to find objects located in the water column and on the ocean bottom. Sonar imaging capability improves at higher frequencies where wavelengths are smaller, but sonar detection range decreases due to increased sound absorption at higher frequencies. Additionally, sonar performance is limited by back-scattered sound from the ocean bottom, surface, and volume, as well as noise from wind and thermal effects. Sonar performance is measured in terms of the signal-to-noise ratio (SNR). High SNR corresponds to situations where the desirable target echo power exceeds the background reverberation and noise power. Sonar SNR is controlled by a variety of factors including the sonar source level, frequency, depth, tilt angle, and beam width. The sonar SNR is also influenced by target reflectivity, bottom type, and local wind conditions. This Demonstration shows those areas near the sonar where the SNR measured on a decibel scale is positive. Warmer colors correspond to higher SNR values.

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DETAILS

The intensity level of the sound radiated by a high-frequency sonar at distance from the sonar and angle measured with respect to the sonar transmit-receive array axis is
,
where is the projector source intensity, is the projector beam pattern on a power scale, and is absorption in dB per unit length. The received target echo from a scattering object located at distance from the sonar is
,
where is the receive beam pattern and is the decibel (dB) target strength of the reflecting object.
When the sonar projector-receiver array axis is pointed directly at the target, then and both the projector and receiver beam pattern functions are unity. In this case the target echo level on a decibel scale reduces to
,
where is the projector source level in dB at unit distance from the source and is the transmission loss. In isovelocity water, sound spreads spherically and the transmission loss is .
The signal-to-noise ratio for a high-frequency sonar that transmits a short continuous wave (CW) pulse and employs incoherent signal processing can be written in the form
,
where is the sonar pulse length, is the directivity factor of the sonar transmit-receive array and is the ambient noise spectral intensity measured in units of power per Hz. The numerator in this equation is the signal intensity and the denominator the mask intensity. The mask is a measure of the amount of unwanted sound that is received by the sonar. The specific terms in the mask are the bottom, surface, and volume reverberation intensities and the receiver noise intensity. The combined effect of these four terms is often just referred to as the noise. Under normal circumstances, reliable detection can be achieved whenever (dB) exceeds about 12 dB.
When is the dominant reverberation component, becomes
,
where is the bottom back-scatter function, is the speed of sound, and is the effective azimuthal beam width of the sonar in radians. If the projector source level is sufficiently large, then the reverberation will dominate the noise and
.
In this case, no further gains in performance can be achieved by using a larger source level. Active sonars are usually operated in a reverberation-limited mode in order to maximize sonar detection range.
References
[1] R. J. Urick, Principles of Underwater Sound, New York: McGraw–Hill, 1983.
[2] C. Clay and H. Medwin, Acoustical Oceanography, New York: John Wiley & Sons, 1977.
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