Frequency based noise coherence-function extension and application to passive bottom-loss estimation.

Area of Science:

• Ocean acoustics
• Geophysical signal processing

Background:

• Accurate acoustic propagation modeling is vital for SONAR performance prediction, especially in shallow waters where bottom reflection loss is significant.
• Passive estimation of bottom reflection loss using vertical line arrays of hydrophones is a key technique, but array length limitations exist for lower frequencies.

Purpose of the Study:

• To demonstrate that shorter hydrophone arrays can effectively estimate the coherence function of longer arrays for improved bottom loss estimation.
• To enable accurate seabed characterization using compact systems on small autonomous underwater vehicles.

Main Methods:

• Utilizing natural surface noise (wind and wave) recorded by vertical line arrays.
• Applying a technique to approximate the coherence function of a longer array using data from a shorter array.
• Validating the method on simulated and experimental data, including complex bottom types.

Main Results:

• Shorter arrays (below 2m) can approximate longer array coherence functions for frequencies below 10kHz, improving bottom loss estimation.
• The technique enhances angular resolution and allows use of frequencies above the array's design frequency.
• The method proves effective for both simple half-space and complex bottom types.

Conclusions:

• The proposed technique significantly improves reflection loss estimates, making seabed characterization feasible with small, autonomous systems.
• Dispensing with active sources, this passive method offers a compact and efficient solution for underwater acoustic surveys.