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System-noise level includes self noise generated by the vessel, ambient noise that exists in the water, and reverberation noise which is a function of the propagation paths and configuration of the acoustic system.

An appropriate value of self noise generated by the vessel should be determined for the installation considered. It is usually the self-noise level that limits bathymetric system performance. Self noise is expressed as a spectrum level and specified in dB//? bar in a 1Hz band. It then becomes necessary to increase the spectrum level by a bandwidth correction factor (N_BW) determined by the bandwidth of the receiving system. The bandwidth correction factor is expressed in dB as:

where

N_BW = bandwidth correction factor

BW = bandwidth of receiving system in Hz.

Self-noise levels are generally assumed as isotropic sources. This assumes that noise received at the hydrophone has no directional characteristics, and consequently, the directivity of the transducer will act to reduce the self noise that passes into the receiving system. The total noise term in the sonar equation is then expressed by:

Equation -8    L_N = L_a + N_BW - N_DIR

where

L_N = Total self noise in the receiver bandwidth

L_a = Spectrum level of self noise in a 1 Hz bandwidth

N_BW = Bandwidth correction factor

N_DIR = Directivity index of receiving transducer.

It should be remembered that self noise, like ambient noise, is not always isotropic in its directional characteristics. Consequently, the use of the directivity index as an exact reduction of the noise level entering the receiving system is not always satisfactory. For example, if a highly directional noise source exists within the beamwidth of the receiving system, then the directivity index will be almost meaningless as a measure of discrimination against noise.

Contributions of the electronics in a bathymetric system to the self-noise level usually occurs only at high frequencies; potential electrical noise sources in the receiving system should be reduced to a value well below the limiting ambient acoustic noise level.

Ambient noise is that background noise level that exists in the ocean due mainly to marine life and man-made noise. Man-made noise will vary in its origin e.g., for surveys in a harbor, man-made noise may be predominately originated on land. Whereas, for bathymetric operations near shipping lanes, man-made noise will be the aggregate of nearby and distant shipping noise.

The lower limit of ambient noise is that generated by the movement of the water itself. Water noise may be generated in a number of ways, again depending on where the bathymetric operations are being performed. Measurements of water noise in a wide variety of locations, and under varying conditions, have yielded statistical estimates as to its magnitude. As might be expected, the resulting averages have been found to vary with the state of the sea. Values for the spectrum level of water noise as a function of frequency and sea state⁶ are shown in Figure 1. The significance of the noise levels shown in Figure 1 is that the dominant noise level (self noise) that is present during bathymetric operations is often given relative to the spectrum levels shown. Table 4-4 presents the sea-state numbers, and a description of the conditions they represent.

Another source of noise in bathymetric systems is that caused by reverberation. Reverberation can be defined as the re-radiations of sound by the sum of all the unwanted scattering contributions from all scatterers not pertinent to the bathymetric measurement. The reverberation that occurs can come from the water surface, re-radiation from a body of marine life scatterers (volume reverberation) in the sea, or from the bottom. Bottom reverberation is a significant noise source consideration in active sonar systems. For bathymetric applications, bottom reverberation is considered only when the application is to discriminate some feature that extends above or is buried in the bottom and might be obscured.

Table 4. State of the Sea

Recognition Differential (N_RD )

An acceptable value of recognition differential has been arrived at empirically by many observers using various types of displays in bathymetric applications. The two principal display techniques for bathymetric systems are the dry paper precision recorder and digital. Current design practice dictates that when a signal-to-noise ratio of 10 dB is used for the recognition differential for a dry paper recorder, and a signal-to-noise ratio of 14 dB is used for digital systems, reliable data recognition is achieved by the system.

Acoustic Interface Loss (N_D)

When the transducer transmits in any manner other than directly into the sea, or final propagation medium, then an acoustic interface loss (two-way transmission loss) must be considered when using the general form of the sonar equation. The technique for calculating this loss is explained in detail in Section 3.2.3.