Phase stability of convergence zone propagation at mid-frequency

Area of Science:

• Underwater acoustics
• Oceanography
• Signal processing

Background:

• Mid-frequency acoustic transmissions (1.5-7.5 kHz) are crucial for underwater communication and sensing.
• Ocean internal waves and thermocline fine structure can significantly impact acoustic propagation.
• Understanding acoustic phase behavior is key to predicting signal coherence and integration times.

Purpose of the Study:

• To analyze the phase behavior of mid-frequency acoustic transmissions in a realistic ocean environment.
• To predict and validate coherent integration times based on acoustic propagation models.
• To investigate the correlation between signal phase, ocean dynamics, and source motion.

Main Methods:

• Statistical calculations of propagation parameters (Φ and Λ) for a representative background environment.
• Simulated acoustic propagation using a split-step parabolic equation solver with a detailed sound speed field.
• Analysis of experimental data from the Philippine Sea, correlating received signal phase with source tow body motion.

Main Results:

• Predictions indicate partially saturated propagation for mid-frequency transmissions.
• Calculated coherent integration times range from ~370s at 10 kHz to ~1370s at 1 kHz, influenced by internal waves.
• Experimental data shows strong correlation between signal phase and source vertical excursions, limiting integration to 128s at 5.5 kHz.

Conclusions:

• Acoustic phase variations in the ocean are influenced by internal wave fields and thermocline structures.
• Source motion, rather than solely ocean dynamics, appears to be the primary limitation for coherent integration times in practical scenarios.
• The study validates acoustic propagation models and highlights the importance of considering source motion in underwater acoustic systems.