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Apr 1985

Volume 77, Issue S1, pp. S1-S108

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back to top Session F. Underwater Acoustics I: Propagation
Contributed Papers
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Pulse propagation at long ranges in an underwater surface duct (A)

E. Topuz and L. B. Felsen

J. Acoust. Soc. Am. Volume 77, Issue S1, pp. S13-S13 (1985); (1 page)

Online Publication Date: 12 Aug 2005

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In a previous communication, we have presented a hybrid‐ray‐mode theory for coping with convergence difficulties occurring near the source depth in the ray acoustic model of long range propagation in a surface duct. The hybrid formulation replaces the problematic acoustic ray fields with a uniquely defined group of surface ducted modes, while retaining the legitimate ray fields intact. Detailed numerical calculations for a high frequency Gaussian pulse propagating in a model environment with exponential velocity profile in depth have now revealed the inherent features of the hybrid scheme. With a solution obtained by modal summation as a reference, the hybrid form is found to be accurate and numerically efficient. It is also capable of explaining in cogent physical terms the features of the received signal by blending automatically and self‐consistently the ray arrivals dominant at early times with mode arrivals dominant at later times. The interpretations follow from separate examination of ray fields and mode fields, with clear indications of ray field failure in the transition region noted above. [Work supported by ONR Underwater Acoustics.]
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Air‐suspended shallow‐water acoustic waveguide (A)

Jacques R. Chamuel and Gary H. Brooke

J. Acoust. Soc. Am. Volume 77, Issue S1, pp. S13-S13 (1985); (1 page)

Online Publication Date: 12 Aug 2005

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No experimental data have been published to date on the air/water/air acoustic waveguide with flat, sloped, or rough boundaries. In the conventional air/water/solid acoustic waveguide, the presence of the solid introduces the Scholte mode. The solid surface roughness causes not only energy leakage from the water waveguide to the solid, but it causes spreading of the energy over a wide time window. In order to study the normal modes interference and coupling phenomena in a rough waveguide, the problem would be greatly simplified if we could eliminate the Scholte mode and the energy leakage to the solid. A novel experimental technique is described which uses a thin stretched rubber membrane covered with a water layer from above. Controlled pressurized air below the membrane is used to counteract the weight of the water layer and obtain a flat surface at the bottom of the water waveguide. The water layer has a free top surface and a free bottom surface. Ultrasonic broadband pulses are propagated in the air‐suspended water waveguide. Theoretical and experimental results are compared for the free flat parallel boundaries waveguide ease. Scattering objects are placed on the surface of the membrane to form a rough or periodic boundary. Experimental findings are presented providing physical insights into the shallow‐water acoustic waveguide with rough boundaries.
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A new eigenfunction expansion with applications to waveguide acoustics (A)

Ronald F. Pannatoni

J. Acoust. Soc. Am. Volume 77, Issue S1, pp. S13-S13 (1985); (1 page)

Online Publication Date: 12 Aug 2005

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A new type of eigenfunction expansion has been developed for analysis of sound propagation in a lossless waveguide having an uneven boundary that is acoustically hard. The method of expansion is novel in that two essentially independent functions are expanded simultaneously. Expansions of this kind provide complete representations of the exact pressure and velocity fields at the boundaries of the waveguide as well as in the interior of the waveguide. The eigenvalue problem that generates the expansion functions has a boundary condition that contains the eigenvalue, and this fact accounts for the ability of the expansions to be valid at the uneven boundary of the waveguide. It also causes the eigenvalues to be complex even though the waveguide is lossless. As functions of range, the coefficients in the expansions of the acoustic fields satisfy a linear system of first order ordinary differential equations that resemble coupled mode equations that arise in the study of plane‐parallel waveguides. These equations have been integrated numerically and validated in a few test problems that can be solved by alternative approach involving conformal mapping. [Work supported by ONR.]
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Computation of normal modes for the seismo‐acoustic problem (A)

Michael B. Porter and Edward L. Reiss

J. Acoust. Soc. Am. Volume 77, Issue S1, pp. S13-S13 (1985); (1 page)

Online Publication Date: 12 Aug 2005

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In a recent paper the authors presented a fast and accurate method for computing the normal modes in a stratified ocean with a perfectly rigid bottom [M. Porter and E. L. Reiss, J. Acoust. Soc. Am. 76, 244–252 (1984)]. We describe the extensions which allow for the computation of modes in an ocean overlying a subbottom with depth‐dependent P‐ and S‐wave velocities. The effect of the ocean sub‐bottom is incorporated by replacing the rigid bottom boundary condition by a numerically computed impedance condition which is a function of frequency, horizontal wavenumber, and the material properties of the ocean sub‐bottom. We illustrate the method by computing the modes for an ocean with a Munk sound speed profile overlying an ocean sub‐bottom with linearly increasing P‐ and S‐wave velocities and terminated by a rigid basement. [Work supported by NSF and ONR.]
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Normal mode cycle distance and beam displacement, time delay, etc., for low (non‐WKB) frequencies (A)

Robert A. Koch and Jo B. Lindberg

J. Acoust. Soc. Am. Volume 77, Issue S1, pp. S13-S14 (1985); (2 pages)

Online Publication Date: 12 Aug 2005

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The work of Tindle et al., on the relationship between mode and ray quantities, is extended to the low frequency regime where WKB is invalid. Given the WKB dispersion relation, the eigenvalue of the horizontal wavenumber can be “differentiated” with respect to the mode number. Furthermore, mode cycle distance is equivalent to mode normalization, which in ray terms is the sum of the water column horizontal path length and a lateral displacement along the bottom. In this paper the analogous relationships for low frequency are developed from the impedance boundary condition mode identification of Koch, Vidmar, and Lindberg. The result shows the mode number derivative and mode normalization should not be uniquely related (the WKB result is fortuitous). Instead, some auxilliary condition, such as Tindle's identification of bottom loss per bounce and mode attenuation per cycle distance, must be invoked to uniquely define cycle distance (as opposed to the periodicity of interference between two modes) as a single mode quantity. As a bonus, a straightforward eigenvalue estimator is suggested for numerical eigenmode calculations. [Work supported by ONR.]
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Sound channel formation in the Strait of Juan de Fuca (A)

David G. Browning and J. W. Powell

J. Acoust. Soc. Am. Volume 77, Issue S1, pp. S14-S14 (1985); (1 page)

Online Publication Date: 12 Aug 2005

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Two distinct oceanographic layers exist in the Strait of Juan de Fuca: a reduced salinity surface layer, approximately 100 m thick, which contains seaward flowing freshwater runoff; and a deeper layer which has access to the North Pacific but is blocked to landward by a sill at the head of the Strait. The annual temperature cycle in each layer is distinctly different, for example, the surface layer attains its highest temperature in July when the deeper layer reaches its lowest. This contributes to a complex evolution of the sound channel during the year, which we describe. In general, we find the sound channel axis to be located in the shallow layer during the winter and in the deeper layer during the summer. [Work supported by NUSC and DREP.]
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Measurement of sound propagation, down‐slope to a bottom‐limited sound channel (A)

William M. Carey, Estvan Gereben, Burlie A. Brunson, and Marshall R. Bradley

J. Acoust. Soc. Am. Volume 77, Issue S1, pp. S14-S14 (1985); (1 page)

Online Publication Date: 12 Aug 2005

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Signal transmission loss and spatial coherence data for source‐receiver separations between 100 and 250 km were acquired in the Gulf of Mexico with a calibrated seismic measurement system (400 m deep), a towed projector (100 m deep) which emitted 67 and 173‐Hz tones, and a moored Webb sound source at 988‐m depth driven at 175 Hz. Environmental data such as the range dependent bathymetry and sound velocity profiles were measured. The 67‐Hz data showed a persistent sound transmission loss of approximately 90 dB whereas the 173 Hz showed several pronounced loss minima between 100–90 dB. Slope enhancements were found to be on the order 2–4 dB at 67 Hz and 6 dB at 173 Hz when compared to flat bottom calculations. Pairwise coherence data show the combined effects of multipath interference and signal‐to‐noise ratio. Estimates of signal coherence length from the coherent summation of streamer hydrophones yield coherence lengths between 70–300 m at a frequency of 173 Hz. Fast asymptotic coherent and normal mode transmission loss calculations produced results consistent with measured data for the deep flat portion of the measurement tracks when measured geoacoustic profiles or related bottom loss curves were used. The implicit finite difference parabolic equation calculations were consistent with range‐averaged data for the flat portion of the track as well as on the slope. These results show that if proper qualitative description of the sub‐bottom velocity profiles axe used, then computations with either a parobolic equation or normal mode technique are consistent with experimental results.
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Model experiments on mode propagation in a shallow water wedge (A)

H. Hobaek, J. Lindberg, and T. G. Muir

J. Acoust. Soc. Am. Volume 77, Issue S1, pp. S14-S14 (1985); (1 page)

Online Publication Date: 12 Aug 2005

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Guided mode propagation in a wedge bounded by the ocean surface and a sloped bottom is modeled in a 80‐kHz experiment conducted in an indoor tank, under controlled conditions. The bottom is simulated by a sand filled tray, 10 m long, 1 m wide, and 20 cm deep. The tray is suspended beneath the water surface, with one end pivoted so as to vary the angle of the wedge from 0°–10°. A vertical line array of seven elements is used as a source to preferentially excite low ordered modes in the waveguide. The field of propagating modes is studied with a probe hydrophone as a function of range and depth in both the water column and the sand sediment. Measurements on mode conversion in propagation out of the wedge are discussed and compared to theory. [Work supported by the Office of Naval Research.]
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Propagation loss measurements in a region of complex bathymetry over the continental slope (A)

J. Syck and R. Chapman

J. Acoust. Soc. Am. Volume 77, Issue S1, pp. S14-S14 (1985); (1 page)

Online Publication Date: 12 Aug 2005

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Measurements of propagation loss over a sloping bottom have been obtained in experiments on the continental slope off the west coast of Vancouver Island. Shot runs were carried out to ranges of 100 km using 18 m and 180 m SUS charges in upslope and downslope geometries. The data were processed in 1/3‐oct bands from 12.5–630 Hz. The 18‐m shots were bottom‐limited in these experiments, and the effect of the interaction with the seafloor was observed for these charges in both experimental geometries. An enhancement in the propagation loss was observed for the downslope run, with the loss decreasing by up to 10 dB for shots at the crest of the slope, whereas the loss increased with range faster than cylindrical spreading for the upslope run. Also, an optimum frequency of propagation was observed at 50 Hz for both geometries. In contrast, the propagation loss increased with both range and frequency for the deeper shots. The measurements have been modeled using a wide‐angle parabolic equation method which is capable of accounting for the interaction with the sloping bottom. The modeled results provide an accurate description of the features observed in the measurements.
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Shallow water acoustic modeling over a sloping bottom (A)

C. T. Tindle and O. B. Deane

J. Acoust. Soc. Am. Volume 77, Issue S1, pp. S14-S14 (1985); (1 page)

Online Publication Date: 12 Aug 2005

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Ray theory with beam displacement gives an approximate method of finding the acoustic field in shallow water. It was shown to be quite accurate for a horizontally stratified two fluid (Pekeris) model by comparison with normal mode results. [C. T. Tindle, J. Acoust. Soc. Am. 73, 1581–1586 (1983)]. The method is extended to the sloping bottom situation by simple geometric arguments and without further approximation. Results show that even small bottom slopes have a dramatic effect on the sound field. The results are compared with those for the adiabatic normal mode approximation and agreement is good for higher frequencies. At lower frequencies differences are attributed to normal modes passing through cutoff, a process which is ignored in simple adiabatic mode theory.
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Shallow water propagation with variable depth (A)

Daniel N. Dixon and Stephen K. Mitchell

J. Acoust. Soc. Am. Volume 77, Issue S1, pp. S14-S14 (1985); (1 page)

Online Publication Date: 12 Aug 2005

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This paper presents calculations of acoustic propagation in a shallow depth‐varying ocean environment using both the simple adiabatic and the uniformly valid adiabatic (UVA) normal mode theory of Desaubies [J. Acoust. Soc. Am. 76, 624–626 (1984)]. In the UVA approximation, modal coupling effects are accounted for through a second order correction term in the phase of the sound field; only the phase relationships between the modes are affected, and the modal amplitudes remain unchanged. Consequently, the approximation is adiabatic in that no energy is exchanged between modes. This expansion technique, which Desaubies applied to a range‐varying sound speed profile, is generalized here to the variable depth waveguide problem. In the shallow water example considered, it was found that the phasing effects can significantly change the interference patterns of propagation loss curves. In general, the phase changes are found to become more significant with greater bottom slopes and higher frequencies. Comparisons of calculations and data are presented. [This work was supported by the ARL:UT IR&D Program.]
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Ray finding and time series simulation in a layered‐bottom ocean (A)

Evan K. Westwood and Paul J. Vidmar

J. Acoust. Soc. Am. Volume 77, Issue S1, pp. S15-S15 (1985); (1 page)

Online Publication Date: 12 Aug 2005

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This paper outlines the capabilities and methods of a computer program which finds ray paths between a source and recover in an ocean/layered‐bottom environment and simulates the time series at the receiver due to an arbitrary pressure waveform at the source. The user specifies a range‐independent environmental model that may include piecewise‐linear profiles of the sound velocity and the frequency‐dependent attenuation in an arbitrary number of bottom layers. The ray paths may have multiple surface and bottom interactions, as well as arbitrary numbers of bounces in each of the bottom layers. The program finds rays efficiently by computing the range dependence of the Snell invariant in each layer. For each of the rays found, the arrival time, accumulated reflection/transmission coefficient, geometric spreading loss, and frequency‐dependent attenuation factor are calculated and used to form the frequency transfer function. The summation of the products of the source spectrum and the ray path transfer functions yields the receiver spectrum and thus the received time series. [Work supported by Naval Ocean Research and Development Activity.]
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Transmission loss displays using color contours for model evaluations (A)

Ruth Eta Keenan

J. Acoust. Soc. Am. Volume 77, Issue S1, pp. S15-S15 (1985); (1 page)

Online Publication Date: 12 Aug 2005

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Transmission loss as a function of depth and range are presented in color contour displays. Black and white contour plots are not as effective compared to the color plots which make the information easier to assimilate. However, the color selection is an important consideration in that it can bias interpretation. The color transmission loss displays can be effectively used to judge quality and significance of the predictions. This is demonstrated in this paper by model comparisons using PE, RAYMODE, and FACT. Predictions from all these models at 500 Hz and 15 kHz in a surface duct environment are presented. The PE model is used to illustrate the effects of sound velocity and bathymetry range dependence. [Work supported by WHOI PO 29518 under NORDA Code 270 Contract N00014‐80‐C‐0381.]
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