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UNDERWATER ACOUSTIC PROPAGATION MODELING & DATA ANALYSIS

Propagation Models
ESL has developed several propagation models that are used within the laboratories and at other research facilities in the world.

The normal mode model ORCA was developed by Dr. Evan K. Westwood while he was a visiting scientist at the Defense Research Establishment Pacific in Victoria, British Columbia. It has become widely used in the underwater acoustics research community, as well as within ESL. For a given ocean environment, specified by the sound speed profile in the water column and a geoacoustic profile of the ocean bottom, ORCA finds the normal modes and computes the acoustic field. The model includes the effects of sound speed gradients in the water and the bottom layers, shear waves in the bottom layers, steep-angle propagation represented by leaky modes, and attenuation in the bottom layers. It may be used to predict narrowband or broadband propagation. The model is unique among underwater acoustic propagation codes because it is largely automatic: the user does not need to guess at any obscure convergence parameters such as depth- or range-sampling resolutions. Several graphics from the journal articles describing ORCA are given below [1,2].

Broadband propagation from ORCA

The ray model GAMARAY was also developed by Dr. Westwood. It performs the same calculations as ORCA using the same type of inputs, but uses ray theory instead of normal mode theory. For short-range and/or deep-water scenarios, the ray model provides an intuitive and accurate picture of how the sound propagates from the source to the receiver[3,4].

Multipath propagation from GAMARAY

Data Analysis
One of the strengths of ESL is that we combine the ability to understand and model underwater acoustic propagation with the ability to perform advanced signal processing on measured data. Using our modeling and signal processing tools, we simulate measured data (usually at the time series level), process the simulated and measured data in the same manner, and often are able to demonstrate remarkable agreement in the resulting data products. Such agreement is extremely valuable in understanding the performance of sonar systems and in developing and improving the signal processing algorithms that are at the heart of those systems.

As an example of a data/model comparison, we examined the broadband correlation structure of data measured on two bottom-mounted hydrophones separated by about 450 m in the shallow water of the English Channel. As a ship passed nearby the receivers, the multiple paths of acoustic energy to the two receivers produced a complex correlation structure as a function of time delay and time [5].

Another area of data analysis involves the technique of source localization by way of matched field processing (MFP), in which the field measured at a set of receivers is matched with a set of fields computed from a propagation model. The simulated fields are for a number of hypothesized source positions, and the output of the matched field processor is an ambiguity surface over hypothesized source position. Using a vertical array of receivers, the hypothesized source position is a function of range and depth only. An example output of a broadband MFP technique, where the hypothesized source depth has been fixed, is shown [6,7].

References
1. E. K. Westwood, C. T. Tindle, and N. R. Chapman, "A normal mode model for acousto-elastic ocean environments," J. Acoust. Soc. Am., 100, 3631-3645 (1996).

2. E. K. Westwood and R. A. Koch, "Elimination of branch cuts from the normal mode solution using gradient half spaces," J. Acoust. Soc. Am., 106, 2513-2523 (1999).

3. E. K. Westwood and P. J. Vidmar, "Eigenray findingand time series simulation in a layered-bottom ocean," J. Acoust. Soc. Am., 81, 912-924 (1987).

4. E. K. Westwood and C. T. Tindle, "Shallow water time series simulation using ray theory," J. Acoust. Soc. Am., 81, 1752-1761 (1987).

5. E. K. Westwood and D. P. Knobles, "Source track localization via multipath correlation matching," J. Acoust. Soc. Am., 102, 2645-2654 (1997).

6. D. P. Knobles, E. K. Westwood, and J. E. LeMond, "Modal time-series structure in a shallow water environment," IEEE J. Oceanic Eng., 23, 188-202 (1998).

7. E. K. Westwood, "Broadband matched field source localization," J. Acoust. Soc. Am., 91, 2777-2789 (1992).

 

Data model comparison
Matched field processing image
 

 

 


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