Measuring vessel source level in shallow water using the smoothed semi-coherent image method.

Area of Science:

• Acoustics
• Oceanography
• Marine Engineering

Background:

• Standardizing measurements of underwater sound from ships in shallow waters is challenging.
• Current methods, like the seabed critical angle (SCA) method (ISO 17208-3 draft), may underestimate source levels (SL) above 500 Hz.
• Accurate source level derivation from sound pressure level (SPL) measurements requires precise propagation loss (PL) approximations.

Purpose of the Study:

• To present and evaluate an alternative method, the smoothed semi-coherent image (SSCI) method, for calculating propagation loss (PL) and source levels (SL) in shallow waters.
• To assess the performance of the SSCI method against analytical and empirical data.
• To offer a more accurate and robust approach for underwater radiated noise (URN) measurements.

Main Methods:

• Developed the smoothed semi-coherent image (SSCI) method for calculating underwater propagation loss (PL).
• Evaluated the SSCI method using analytical scenarios and empirical data from the SATURN project.
• Conducted URN tests with multiple instrumentation deployment strategies, depths, and distances.

Main Results:

• The SSCI method demonstrated enhanced accuracy in calculating source levels (SL) across a broad frequency range compared to the SCA method.
• The SCA method showed a general underestimation of SL above approximately 500 Hz.
• The SSCI method maintains robustness and seeks to preserve the simplicity of the SCA approach.

Conclusions:

• The SSCI method provides a more accurate and reliable approach for determining ship source levels (SL) in shallow waters.
• This method addresses the limitations of existing standards like ISO 17208-3 for shallow water URN measurements.
• The SSCI method offers a promising alternative for standardizing shallow water acoustic measurements.