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Multipath pulse shapes in shallow water: theory and simulation.
Chris H Harrison1, Peter L Nielsen
1NATO Undersea Research Centre, Viale San Bartolomeo 400, 19126 La Spezia, Italy. harrison@nurc.nato.int
The Journal of the Acoustical Society of America
|April 6, 2007
Summary
This study extends sound pulse shape analysis in shallow waters to include refraction. The findings reveal a predictable exponential decay in pulse tails, useful for sonar design and geoacoustic inversion.
Area of Science:
- Underwater acoustics
- Acoustic propagation modeling
- Geophysical signal processing
Background:
- Shallow water sound propagation is significantly affected by boundary losses, particularly at steeper ray angles.
- Sound intensity, viewed as a function of angle, can be transformed to represent multipath pulse shapes based on travel time.
Purpose of the Study:
- To extend the closed-form isovelocity pulse shape to scenarios with upward or downward refraction.
- To analyze the characteristics of multipath pulse shapes in shallow water environments.
Main Methods:
- Converting sound intensity from a function of angle to a function of travel time.
- Applying numerical interpolation to invert angle-dependent functions into time-dependent functions.
- Calculating two-way paths using numerical convolution.
- Utilizing the C-SNAP wave model for validation.
Main Results:
- The envelope of earliest arrivals exhibits a trapezoidal shape with a delayed peak from near-horizontal refracted paths.
- Pulse tails decay exponentially (linearly in decibels) with a constant decay rate.
- This decay rate depends solely on bottom reflection properties and water depth, independent of travel time.
- Numerical solutions align with depth-averaged multipath arrivals in broadband cases.
Conclusions:
- The derived pulse shape characteristics offer valuable insights for geoacoustic inversion and sonar system design.
- The analytical and numerical methods provide accurate solutions for multipath pulse shapes under refraction.
- The findings demonstrate the utility of the model in understanding complex underwater acoustic phenomena.
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