A sonar operator could do everything and not hear anything useful. That has become an inherent part of the water in the Strait of Hormuz: a slender balancing act between basins and shallow shelves alongside deep slopes, an acoustic environment that is not so much an open ocean but more of an echoing, shifting passage.
The present-day antisubmarine systems are based on two principles of sound traveling predictability and targets generating recognizable acoustic signatures. The Strait chastises both assumptions. Its geometry collimates sound paths, its seafloor and surface boundaries reflect many times, and there is seldom any “background” noise. Practically, contact classification is a question of isolating weak and distorted signals in a field of competing arrivals, reverberation and transient events which appear as machine till they do not.
The speed of sound in seawater is the most technically significant variable that is largely controlled by temperature, salinity, and pressure. The oceanographic measurements and modeling of diffusive convection (DC) structure in the western Strait, a type of a double-diffusive mixing, manifests itself by the presence of colder and fresher water above the warmer and saltier water. The stratification of that is not continuously stratified; it takes the form of step-like configurations and changes of boundaries layers that change the curvature of acoustic rays and the concentrations of energy. The variations in sound speed in the mentioned study area are reported to be 1552 m/s extending to 40 m depth, and around the vicinity, the increase of sound speed to 1555 m/s is observed due to rising of warm water. In the case of sonar, those little figures count: they bend the refraction, change shadow areas and convert the “good” performance of yesterday into blank water today.
According to the same work, sound transmission loss increases by 5–15 dB in the localities of high DC. It is not a large penalty in a world where the detector is often reliant on marginal signal to noise ratios to get a detection. The specifications also chop against workarounds. The frequency increases the frequency-driven loss, but frequency systems usually sacrifice range against resolution, and clutter in the shallower waters can cancel out this benefit. The paper also observes that on small propagation angles, DC can scale propagation steps and wavelengths, and that increases until 2x as source depth increases, another way the water column is quietly re-writing the sonar equations.
Noise is an environmental phenomenon as well as self-inflicted. Traditional submarines use diesel-electric designs where diesel engines operate on the surface or in the near-surface to replenish batteries with propulsion being done by electric motors in the submerged sections. Snorkeling facilitates such battery-recharge system whereby with the snorkel mast the submarine draws air on the surface and the submarine stays at periscope depth. The same mechanism which maintains endurance, however, makes markations and local acoustics chaos: machine noise, flow noise around the mast, and the fact that certain onboard sensors become almost useless whenever the engines are on.
Then there are also signals which are not a “naval” signals. Seismo-acoustic studies in the Persian Gulf indicate that the camera used by an ordinary diver recorded acoustic phases of an earthquake which could be modelled as being shallow water T phases whose frequency and group velocity was dependent on bathymetry. The practical point concerning sonar work is simple: in a shallow basin, the activity of acoustic arrivals well beyond the image of the tactical scene can be generated by sources out of the tactical view, and they can pass large areas of the water in such a manner as to mimic operational activity.
“Better sonar” is not a single solution in that environment. Ocean physics and reflection at boundaries, as well as competing sources of acoustic energies, are common contributors to the Strait of Hormuz, and they frequently overwhelm the clean detection ideas, transforming the contemporary sonar into a form of an on-going environmental interpretation practice.
