SONAR (SOund NAvigation Ranging) has been in use for decades to detect submerged objects, but researchers are finding how to extract new information from its echoes. With the help of multiphysics modeling software, a group of researchers at the NATO Undersea Research Centre in La Spezia, Italy, are studying how lowfrequency echoes can determine what an object is made of.

SONAR uses sound waves traveling through water to detect and identify objects. The technique is similar to the more widely known RADAR (RAdio Detection And Ranging), which is based on electromagnetic waves instead of acoustic waves. RADAR is not used underwater because radio waves cannot reach very far in that medium due to absorption. During WWII, SONAR was used primarily to detect submarines; today these techniques are used to look for undersea objects such as shipwrecks and for measuring fish abundances and distributions. Similar acoustic techniques are being applied to wide-ranging applications such as ultrasonic NDT (nondestructive testing), acoustic-transducer design, medical acoustics, and geoacoustics.

SONAR traditionally has used frequencies for which the wavelengths are far smaller than the size of the objects being studied. This makes it possible to discriminate the shape of objects rather well, and leads to advanced applications such as underwater acoustic cameras. However, it is difficult to identify the material of an object using high-frequency signals. Thus researchers are turning their attention to low-frequency schemes (see Figure 1).

Figure 1: SONAR Frequency Response of a scuba tank (modeled as an empty cylindrical shell) computed with a model implemented in COMSOL Multiphysics. TS (target strength) is a logarithmic measure of the echo at a large distance from the target.

Researchers have found out that the low-frequency echoes also can contain information that describes other properties of a submerged object, such as the physics of its materials. This is because solids, unlike liquids, support not only longitudinal vibrations, but also transverse, or shear, vibrations, and thus can propagate sound in a number of modes. Particularly useful is the Lamb wave, a complex wave that travels through the entire thickness of a material layer. Propagation of these waves depends on the material's density as well as its elastic and material properties; they are also influenced by the selected frequency and material thickness.