shallow water propagation with variable depth

not be uniquely related (the WKB result is fortuitous). Instead, some aux- illiary 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.]

10:05

F6. Sound channel formation in the Strait of Juan de Fuca. David

G. Browning (New London Laboratory, Naval Underwater Systems Center, New London, CT 06320) and J. W. Powell {Defence Research Establishment Pacific, FMO, Victoria, British Columbia V0S lB0, Canada)

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 evolu- tion 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.]

10:20

F7. Measurement of sound propagation, down-slope to a bottom-limited sound channel. William M. Carey (NORDA, 800 North Quincy Street, Arlington, VA 22217-5000), Estvan Gereben (TRW, McLean, VA), Burlie A. Brunson (PSI, McLean, VA 22102), and Marshall R. Bradley (PSI, Slidell, LA 70458)

Signal transmission loss and spatial coherence data for source-receiv- er 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 moorcA Webb sound source at 988-m depth driven at 175 Hz. Environ- mental data such as the range dependent bathymetry and sound velocity profiles were measured. The 67-Hz data showed a persistent sound trans- mission 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 fiat bottom calculations. Pairwise coherence data show the combined effects of multipath interference and signal-to-noise ratio. Esti- mates of signal coherence length from the coherent summation of stream- er 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 fiat portion of the measurement tracks when measured geoacoustic pro- files or related bottom loss curves were used. The implicit finite difference parabolic equation calculations were consistent with range-averaged data for the fiat portion of the track as well as on the slope. These results show that if proper qualitative description of the sub-bottom velocity profiles are used, then computations with either a parobolic equation or normal mode technique are consistent with experimental results.

10:35

FS. Model exi•riments on mode propagation in a shallow water wedge. H. Hobaek, a) J. Lindberg, and T. G. Muir {Applied Research Laboratories, The University of Texas at Austin, P.O. Box 8029, Austin, TX 78713-8029)

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 fillcA tray, l0 m long, 1 m wide, and 20 cm deep. The tray is suspend- CA beneath the water surface, with one end pivoted so as to vary the angle

of the wedge from 0 ø-- 10 ø. 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 func- tion 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 tl•eory. [Work supported by the Office of Na- val Research.] al On leave from Department of Physics, Bergen Universi- ty, Norway.

10:50

F9. Propagation loss measurements in a region of complex bathymetry over the continental slope. J. Syck and R. Chapman (Defence Research Establishment Pacific, FMO, Victoria, BC V0S lB0, Canada)

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 geome- tries. An enhancement in the propagation loss was observed for the downslope run, with the loss decreasing by up to l0 dB for shots at the crest of the slope, whereas the loss increased with range faster than cylin- drical spreading for the upslope run. Also, an optimum frequency of prop- agation was observed at 50 Hz for both geometries. In contrast, the propa- gation loss increased with both range and frequency for the deeper shots. The measurements have been modelcA using a wide-angle parabolic equa- tion 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.

11:05

F10. Shallow water acoustic modeling over a sloping bottom. C.T. Tindle and G. B. Deane (Physics Department, The University of Auckland, Private Bag, Auckland, New Zealand)

Ray theory with beam displacement gives an approximate method of finding the acoustic field in shallow water. It was shown to be quite accu- rate 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.

11:20

FII. Shallow water propagation with variable depth. Daniel N. Dixon and Stephen K. Mitchell (Applied Research Laboratories, The University of Texas at Austin, Austin, TX 78713-8029)

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 accountcA 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. Conse- quently, 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 interfer- ence 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 presentcA. [This work was supported by the ARL:UT IR&D Program.]

S14 J. Acoust. Soc. Am. Suppl. 1, Vol. 77, Spring 1985 109th Meeting: Acoustical Society of America S14

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