Computed narrow-band azimuthal time-reversing array retrofocusing in shallow water.

Area of Science:

• Underwater acoustics
• Wave propagation
• Signal processing

Background:

• Acoustic time reversal (TR) focuses sound waves back to their source in reciprocal environments.
• Vertical time-reversing arrays (TRAs) are predicted to create horizontally confined foci, especially with volume refraction.
• Shallow water environments present unique challenges for acoustic focusing due to complex sound propagation.

Purpose of the Study:

• To validate and extend predictions of vertical TRA retrofocus confinement to shallow water environments.
• To investigate the impact of environmental randomness (bottom roughness, internal waves) on the azimuthal extent of the retrofocus.
• To determine power law scalings governing retrofocus width under various conditions.

Main Methods:

• Monochromatic Monte Carlo simulations using parabolic equation computations (RAM).
• Modeling of shallow-water sound channels with random bottom roughness.
• Modeling of shallow-water sound channels with random internal-wave-induced sound speed fluctuations.

Main Results:

• Randomness in shallow water environments significantly reduces the azimuthal width of the vertical TRA retrofocus, to as little as several degrees.
• This narrowing effect is observed for source-array ranges of 5-20 km and frequencies of 500 Hz-2 kHz.
• Power law scalings were identified that collapse retrofocus width data across various environmental parameters and source depths.

Conclusions:

• Vertical TRAs can achieve highly directional focusing in shallow water, even with environmental complexities.
• Environmental randomness plays a crucial role in determining the angular resolution of acoustic time reversal.
• The findings provide a scalable framework for understanding and predicting acoustic focusing in realistic shallow-water scenarios.