Page 38 - Summer2022

WEIRD DATA IN UNDERWATER ACOUSTICS Figure 4. Interactions with the surface and bottom change acoustic propagation. The ocean surface and bottom can have varying roughness. Ice on the surface can also cause reflections. Layering in the ocean bottom can also cause subbottom reflections. Bottom topography, in particular, can cause acoustic arrivals at the receiver from unexpected angles, at large intensities and unexpected time delays. Peter Brodsky of the Applied Physics Laboratory, University of Washing- ton (APL-UW), Seattle, described a particularly startling reflection he experienced: “Years ago (many) I was on a Navel Oceanographic Office ship — the USNS Lynch — performing seismic surveys in the South Atlantic. Our typical acous- tic sources were airguns and sparkers, but in really deep water we’d use explosives. On one occasion we dropped a final charge, heard it go off, then went off to get dinner. A minute or so later there was a BIG bang that shook our soup bowls in the mess. The Chief Engineer (CE) ran out and raced down to the engine room, cursing about some incompetent oiler who let the engine throw a rod (again). Turns out it was a reflection of the last charge off a submerged ledge of some kind; far away but oriented perfectly to direct the sound right back at us. The CE never showed up at regular mealtimes again” (personal email, 2022, used with permission). Volume Effects The second major impact of the underwater environment on propagation is that sound energy refracts (i.e., curves) due to changing sound speed with depth and range. This creates convergence zones, shadow zones, ducted sound, and long-range propagation of low-frequency energy. Figure 5 shows some of the ways the sound speed profile affects the refraction of sound in the ocean. Sound curves toward a lower sound speed, and this causes the complex behavior observed in ocean propagation. This includes SOFAR paths (Figure 5, green) that are refracted above and below to channel around the sound speed minimum, deep convergence paths (Figure 5, purple) that are caused by steeper launch angles and refract at deeper and shal- lower points than the SOFAR paths, surface ducts (Figure 5, orange) where a local sound speed minimum can trap sound near the surface, and downward refracting pro- files that commonly cause increasing bottom interaction and signal attenuation in shallow water. Another feature of environmental ocean propagation is the creation of shadow zones where little energy is received due to the bending of rays. Changes with Time Fading of signals and/or amplification of noise due to changing environmental conditions is another common underwater acoustics challenge to signal interpreta- tion. Temporal variability in sound speed is caused by submesoscale to mesoscale (i.e., 1-100 km) variations in temperature and salinity, such as eddies, warm core rings, internal waves, and buoyancy fluctuations (Colosi, 2016). Figure 5. Examples of volume propagation effects due to sound speed changes with range in the ocean. Three different sound speed profiles are shown (yellow dotted lines): a summer shallow-water sound speed profile (left), a deepwater sound speed profile (center), and a deepwater sound speed profile with a surface duct (right). Green, so-called SOFAR or deep sound channel paths; purple, convergence zone (cz) paths; orange, convergence zone/surface duct paths; magenta, downward-refracting paths that result from a shallow-water summer sound speed profile; gray box, shadow zone where little acoustic energy is received. 38 Acoustics Today • Summer 2022