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## Gayana (Concepción)

##
*versión impresa* ISSN 0717-652X*versión On-line* ISSN 0717-6538

### Gayana (Concepc.) v.68 n.2 supl.TIIProc Concepción 2004

#### http://dx.doi.org/10.4067/S0717-65382004000300013

Gayana 68(2): 385-388, 2004
Naval Service of Research and Development (SENID), 327 Libertador Ave., 1638, Vicente López, Argentina. m_milou@yahoo.com, ssalvadores@yahoo.com, silblanc@yahoo.com.
Up and down slope acoustic propagation measurements were conducted in 1989 [Milou
An excellent analysis of works dealing with the application of the parabolic wave equations to underwater sound has been reported in It provides not only historical references but also enables understanding of the steps followed by researchers in this wave-theory technique since early 1970 (Hardin In this work up-dated versions of Range-dependent Acoustic Model, RAM These codes use the Padé series for numerical implementation of the wide-angle parabolic equation which enables to achieve a combination of accuracy and stability in the obtained theoretical results. The current knowledge about the bottom characteristics in the studied area RAMS A better fitting between experimental data and theoretical results is even reasonable to be expected due to the improvement in the seabed geoacustic representation.
According to the description of the down-slope experiment (Milou In the already mentioned article (Milou Calculus is simplified here using the Reciprocity Principle valid for parabolic wave equations (Nghiem-Phu, 1977). Accordingly, source and receiver positions can be exchanged and a receiver moving away from a fixed source is considered, thereby avoiding the necessity for re-computing the field for each different source position. A Padé number 5 is used as input data for running RAMS A shear wave velocity, c For down-slope acoustic propagation, computed range dependent coherent TL values, using RAMS RAMS
Coherent TL, here computed through RAMS
Comparison of experimental data and theoretical predictions leads to conclude that recent computed values with the up-dated code, provide a better fitting for all frequencies in the studied interval. This result could be explained by the combination of two factors: (i) several significant improvements incorporated in the updated code, and (ii) a modified geoacoustic bottom description instead of the one originally used.
Regarding the first factor (i), it is currently well known by the scientific community that the PE method has undergone extensive development along the two last decades since it was first introduced in the field of underwater sound (Tappert, 1977). Since the PE solutions are approximations, interest in their accuracy has permanently given rise to different attempts to improve it. On the other hand, the efficiency problems associated to numerical solutions of PE techniques cannot be neglected when range-dependent ocean environments have to be handled. Precisely the PE updated code used in this work differs from the previous one since it includes a higher-order energy-conservation correction at vertical interfaces that is accurate for problems involving large ocean bottom slopes, large depth and range variations in sound speed; very wide propagation angles and piece-wise continuous density variations (Collins Concerning the second factor (ii), inclusion of shear waves propagating into unconsolidated marine sediments layers overlying a solid substrate with their corresponding estimated phase velocity and absorption coefficient, might also contribute to increase the agreement between current computations and experimental data.
The authors acknowledge Dr M. Collins and Dr L. Fielkowski's valuable discussions on RAM vers. 1.5, RAMS vers. 0.6s and RAMGEO vers. 2.0g updated codes during a visit to the Naval Research Laboratory on 2002 (ONRIFO-VSP4063-2002). They also thank Dr G. Parker with the Marine Geological Dept. - Argentinean Hydrographic Service
Blanc, S. & Novarini, J. C., 1987, Dependencia con la frecuencia del coeficiente de reflexión de las ondas acústicas en el lecho marino, Revista Latinoamericana de Acústica, 1, 22-28. [1] Collins, M. D., 1989, A higher-order parabolic equation for wave propagation in an ocean overlying an elastic bottom, Journal of the Acoustical Society of America, 86, 1459-1464. [2] Collins, M. D., 1989, Applications and time-domain solution of higher-order parabolic equations in underwater acoustics, Journal of the Acoustical Society of America, 86, 1097-1102. [3] Collins, M.D., 1993, A split-step Padé solution for the parabolic equation method, Journal of the Acoustical Society of America, 93, 1736-1742. [4] Hardin, R. H. & Tappert F. D.,1973, Application of the split-step Fourier method to the numerical of nonlinear and variable coefficient wave equation, SIAM Rev,15, 423. [5] Jensen, Finn B., Kuperman, William A., Porter Michel B. & Schmidt Henrik, 1994, Milou, M. E. de & Blanc, S., 1990, Mediciones acústicas en plataforma y talud continental. Tech. Rep. AS1/90. Naval Service of Research and Development, 1-55. [7] Milou, M. E. de & Blanc, S., 1994, Sound range-dependent propagation: an experiment on down slope propagation over the Argentinean continental slope, GEOACTA, 21, 127-136. [8] Nghiem-Phu L. & Tappert F., 1985, Modeling of reciprocity in the time domain using the parabolic equation methods, Journal of the Acoustical Society of America, 78, 164-171. [9] Tappert, F. D., 1977, The parabolic approximation method, in |