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The attenuation of low-frequency sound in the sea increases with pH. In most ocean areas, the value of pH changes significantly with depth, and must be included in an accurate attenuation computation. Previous determinations of other parameters, such as surface loss, which were based on older attenuation formulas, are therefore inaccurate. An analysis of a previously reported surface loss formula indicates that the predicted values are too high. >

Low‐frequency propagation measurements in sound channels often show a frequency‐independent excess loss that is evidently caused by internal scatter from large‐scale inhomogeneities. Using the Garrett–Munk internal wave spectrum (GM 75) as the scattering model, we have obtained an approximate solution of the ray‐diffusion equation on the deep sound‐channel axis. The estimated attenuation coefficient, ∠5×10−4 dB/km, is consistent with the smaller experimental values reported.Subject Classification: [43]30.20; [43]20.20; [43]30.40.

The deep ocean sound channel is used to obtain very long range (typically >2,000 km) acoustic transmission1 via totally refracted propagation paths (SOFAR propagation2). Such experiments can therefore determine the acoustic transmission properties of large areas of ocean3. Those acoustic propagation experiments can be used to locate major ocanographic changes4 and to identify specific water masses5. A 10,000-km underwater sound transmission experiment conducted between New Zealand and Peru to obtain an acoustic cross-section of the South Pacific Ocean is described here. Three distinct regions of transmission loss were found. The highest attenuation, which is attributed to Antarctic intermediate water, occurred in the central South Pacific Ocean.

In a previous paper (D. G. Browning et al J. Acoust. Soc. Am. 61, S10(A) (1977)) a large oceanographic feature, designated as a swirl, was identified in the Western Mediterranean Sea. Acoustic propagation loss was predicted based on archival environmental data using both single and multi‐profile models. These results are now compared with improved predictions using detailed environmental data and with corresponding experimentally measured propagation loss. It is found that the change in sound speed structure across the swirl significantly effects the propagation of sound. (Work supported by NUSC)

Low frequency propagation measurements in sound channels often show a frequency‐independent attentuation that is evidently caused by scatter from internal inhomogeneities. Using the Garrett‐Munk internal wave model [J. Geophys. Res. 80, 291–297 (1975)] as the scatter mechanism, approximate solutions for the ray‐diffusion equation are obtained for typical inhomogeneous sound channels with source and receiver on the axis. The resulting attentuation coefficients are in good agreement with experiment. [Work supported by NUSC.]

A persistent large (200‐km‐diam) warm core eddy has been reported [S. Tabata, J. Phys. Oceanogr. 12, 1260–1282)] in the Northeast Pacific Ocean off Sitka, Alaska (centered at 57° N Lat., 139° W. Long.). Not associated with a strong boundary current, this eddy differs from a previously observed South Pacific warm‐core eddy [P. Scully‐Power et al., J. Acoust. Soc. Am. 63, 1381–1388(1978)]. It appears to be formed from an influx of warm water at 100–500 m which moves northward along the North American Coast. At these high latitudes the surrounding water mass is relatively cold with a deep sound channel axis at 200 m and a secondary channel axis at 100 m. It was found, as in other warm core eddys, that there is a broadening of the secondary sound channel. For this eddy there is also a significant change in the depth of the deep sound channel axis (400 m at center of eddy, 200 m outside the eddy). An analysis was conducted using a range dependent propagation modeling program for a sound speed cross‐section of the eddy. Results which show the relative effect on sound propagation are given for various source and receiver depths. Computations are also made of the frequency dependence of both the deep and secondary sound channels and their interdependence as suggested by Hall [M. Hall, J. Acoust. Soc. Am. 66, 1102–1107 (1979)].

The axial arrival pattern at low frequencies (below 100 Hz) and relatively long ranges (200 km) from a sound channel propagation experiment in Baffin Bay shows an extensive series of arrivals after the refracted arrival pattern. Since the bottom at this location was relatively lossless, the hypothesis that these later arrivals represented energy that was diffracted from the sound channel was examined using the Fast Field (Modeling) Program. A comparison of sound‐propagation characteristics was made for the frequency range 1–100 Hz, assuming both high and low loss bottoms. The results show that diffraction does not significantly change the average propagation loss at frequencies above 5 Hz.

There are three principal causes of shallow or secondary sound channels in the North Pacific Ocean: Temperature inversions, water mass intrusions, and dynamic interleaving. Our analysis shows temperature inversions to be widespread throughout the Subarctic North Pacific, water mass intrusions and dynamic interleaving are more associated with specific locations. Examples are given of each type of shallow sound channel, showing their acoustic and oceanographic characteristics. Estimates of the distribution and duration of these channels is given.

An analysis of two extensive sound speed profile surveys (winter and summer) of the subarctic region (above 40° N) in the Northeast Pacific Ocean shows that the depth of the deep sound channel axis is symmetric about the Alaskan Gyre (centered at 53° N, 153° W). The axis depth ranges from 100 m or less in the Gyre to 400 m or more along the coast of North America. A model for axis depth contours is developed which has some interesting results. For example, a north‐south track along 140° W has little change of axis depth with latitude, while an east‐west track along 53° N would have a significant change in axis depth. An estimate of the yearly variability of the model is made from long‐term oceanographic data obtained at weather station “Papa” (50° N, 145° W). Sound propagation loss is computed for selected tracks using a range‐dependent acoustic prediction model and these results are compared to experimental data.

Previous long range acoustic experiments show that the Mid‐Atlantic Ridge, a major topographic feature rising to the deep sound channel axis, can have a significant effect on SOFAR propagation. [R. J. Urick, J. Acoust. Soc. Am. 35, 1413 (1963)]. In order to quantify this effect, data has been analyzed from a recent SOFAR experiment which deployed SUS charges during several transits across the Ridge. The signals were received on a hydrophone located near Bermuda, a distance of approximately 2500 km. These results are compared with data from Atlantic seamounts of similar height and ridges in other oceans. [K. M. Guthrie, J. Acoust. Soc. Am. Suppl. 1 68, S52(A) (1980)]. The enhancement or shadowing of SOFAR propagation is presented as a function of source depth and frequency for various geometries. [Work supported by NAVSEA.]

A series of long range propagation runs were analyzed to determine attenuation coefficients for the frequency range 25–1000 Hz. Above 200 Hz, the dominant cause of attenuation was boron absorption; below 200 Hz, the attenuation was independent of frequency indicating a scattering mechanism as the cause. In the Labrador Basin, absorption was close to that predicted by the Thorp formula; in the Davis Strait and Baffin Bay, it was significantly higher (1.3–1.5 Thorp). This increase, corresponding to higher pH values, can be attributed to two possible causes, the influence of the oxygen‐rich Labrador current and the higher pH values encountered as the sound channel axis shallows. The frequency independent component was similar in the Labrador Sea and Davis Strait but lower in Baffin Bay confirming results reported earlier [R. H. Mellen et al., J. Acoust. Soc. Am. 57, 1201–1202(L) (1975)] and suggesting less scattering in Baffin Bay. (Work supported by NUSC)

During Project LADLE a series of large explosive charges were detonated in the ocean near the Puerto Rico trench. Signals were received on hydrophones and a seismograph at a range of approximately 1500 km. An analysis is made of travel times and sound intensities. The results show the relative importance of water‐borne, water‐borne/seismic, and seismic transmission paths.

A low frequency (50–800 Hz) sound‐propagation experiment was conducted along a 1400‐km path running eastward from Bermuda toward the Mid‐Atlantic Ridge. SUS charges were detonated at depths of 18, 154, 615, and 1230 m. The receiver was located at the axis of the deep sound channel (1250 m) at a range of approximately 1300 km. The acoustic path crossed several seamounts of the Corner Seamount Group. The highest of these peaks rose to the sound axis. This paper presents the relative enhancement of signal level for SOFAR propagation due to these seamounts as a function of source depth and frequency. The enhancement was minimal for the 1230‐m shots, while the greatest enhancement occurred for the 18‐m shots at the 50‐Hz filter band. This implies these seamounts and other topographic features such as the Mid‐Atlantic Ridge can significantly increase the coupling of low‐frequency ship‐generated noise into the deep sound channel. [Work supported by NAVSEA.]

Two low frequency attenuation mechanisms have recently been identified; a boron relaxation process and scattering from oceanographic inhomogeneities. It is now possible to obtain at least semiquantitative predictions of attenuation of sound in the sea for all frequencies of practical interest. Unfortunately, the available sonar slide rules are based on out dated or incomplete attenuation formulas. This paper suggests new engineering formulas based on present at‐sea and laboratory data applied to realistic propagation situations. [Work partially supported by Naval Underwater Systems Center.]

Sound‐channel propagation measurements in the Ionian basin of the Mediterranean Sea have been analyzed to determine the attenuation coefficients in the frequency range of 50–3200 Hz. Concurrent measurements of sound‐speed, temperature, salinity, and pH show a strong sound channel having a broad minimum below 100 m and highly uniform properties over the 600‐km path. Explosive sources were detonated near the channel axis. The results obtained from hydrophones located near the axis are compared with predictions of the temperature/pH‐dependent relaxation‐absorption model (components: MgSO4, B(OH)3>, and MgCO3). Scattering loss is found to be minimal and agreement is good. Attenuation coefficients are compared with earlier values from the Ligurian Sea over a single refraction path approximately 35 km long.

Owing to the pressure dependence of the temperature of maximum density in fresh water, the sound channel in Lake Superior is extremely weak. The fast field program (FFP) is applied to determine if the bottom loss is significant for sound channel propagation in the kilohertz region. The values of attenuation are computed for the frequency range 500–10 000 Hz and compared to values obtained from the Lake Tanganyika experiment.

The fast field program (FFP) is used to study sound propagationin a deep ocean sound channel for the frequency range 1–200Hz. Typical sound velocity profiles from the Atlantic and Pacific Oceans are used. Both the velocity profile and bottom depth are assumed to be invariant with range. The channel cutoff frequency is determined and the bottom loss contributionto the measured values of attenuation is estimated. Comparison is made with recent experiments.