Ambient Noise Vertical Directionality and Reflection Loss Estimates for a Shallow Tropical Site with a Fast Sound Speed Bottom

Abstract

For high frequency ocean acoustic modeling applications, seabed reflection loss is a useful alternative input compared to conventional geoacoustic model parameters. Reflection loss can be estimated by comparing the noise intensity of the up and down components of the ambient noise vertical directionality pattern. The potential of this method is demonstrated with experimental data spanning one week, collected off shallow east coast of India using a 21 element vertical hydrophone array. The compact and easily operable vertical array has been designed for high frequency directionality estimation in the band 2–10 kHz. The ambient noise data are beam formed to arrive at the vertical directionality pattern. Further reflection loss values as a function of frequency and grazing angle have been estimated for 1/3 octave bands for a sandy sea bed in warm tropical waters. This has been compared with modeled reflection loss estimates using OASR reflection loss module of OASES. This will serve as inputs to propagation models for applications such as inverse techniques, ambient noise modelling, and sonar system performance prediction.

Keywords:

• Reflection loss
• vertical directionality
• vertical hydrophone array

Acknowledgements

The authors gratefully acknowledge the support extended by the Director, National Institute of Ocean Technology in carrying out this work. We thank the two anonymous reviewers for their careful reading and critical review which greatly improved the manuscript. Mrs. A. Malarkodi and the Acoustic Test Facility Team are thanked for the testing and calibration of hydrophones, which is integral to this research. The field team of Mr. A. Thirunavukkarasu, Mr. M. Ashokan, Mr. Edwards Durai, Mr. K. Nithyanandam, Mr. M. M. Mahanty, and Mr. C. Dhanaraj are gratefully acknowledged for their efforts during system deployment and retrieval at sea.