ANDERSON: BOTTOM PROPERTIES FOR LONG-RANGE PROPAGATION PREDICTION 



receiving point and then converge on an eigenray. Once the eigenrays 

 are identified, one way of treating the influence of the boundaries 

 and in particular the bottom is through an interface reflection co- 

 efficient, a Rayleigh plane-wave reflection coefficient. 



Either method of treating the bottom requires more detailed in- 

 formation about the physical properties of the bottom sediments. These 

 physical properties include acoustical properties such as speed of 

 propagation and attenuation and are combined in what Hamilton (1974) 

 calls a geoacoustic model. 



In some cases when the boundary is treated as a reflecting inter- 

 face, we can go through an intermediate model, feeding the geoacoustic 

 model information into a mathematical model for computing bottom loss. 

 An alternative is to structure the measurements of bottom loss into an 

 empirical model. 



BOTTOM -LOSS MODELS 



Figure 1 illustrates some of the bottom -loss models. Standard 

 empirical bottom-loss models consist of tables of bottom loss versus 

 grazing angle. Probably the earliest of these came from the AMOS 

 program, another set was developed at Fleet Numerical Weather Central 

 based on the MGS data, and some have been based on the FASOR data. 

 NAVOCEANO also has a set. Other measurement programs have produced 

 what can be considered as empirical bottom-loss models at various 

 frequencies . 



Mathematical models progress through a series of increasing 

 complexity using plane interfaces, plane layers, plane waves. Models 

 with liquid layers can progress to layered models that support shear 

 waves. More complex models may have gradients of the acoustical 



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