Thus in a real body of water, the speed profile varies with seasonal changes and currents. A typical deep-water profile is shown in figure 7a. In the limit where the water depth is small compared to the horizontal distance, grazing rays will reflect and refract. This phenomenon can be exploited in several ways. Military submarines utilize this phenomenon by cruising below the thermocline, the depth where the sound speed reaches a minimum, thus hiding their acoustic trace from surface ships.
Marine biologists use similar arrays in tracking animal life such as whales.
Geologists are able to detect submarine movement of magma and seabed materials. Indeed, high quality side-scan units are commercially available now for a modest price to practically anyone. It typically operates in the hundreds of kilohertz range providing good azimuthal and range measurements. As the towfish is pulled through the water, it emits a sonar pulse in a fan-pattern covering a line of seabed. The strength of the return plotted against the delay can then be interpretted as a the illumination of a source as a function of distance.