Searching for Dead Bodies with Sonar

Abstract

Sonar is very useful to locate shipwrecks, human bodies and other objects underwater and assess the context in which they drowned. A sonar combines acoustic sensors with sophisticated electronics and is streamlined to avoid vibrations during deployments. They generally are of metric sizes and can be used at all depths, from less than 1 m to the deepest parts of the ocean, which is more than 11 km (Fig. 182.1). Sonars project focused beams of sound, “pings”, into the water and record echoes from the bottom and any strong reflector (Fig. 182.2). The acoustic reflectivity from objects on the seabed, and sometimes the topography, are presented as two-dimensional maps (Fig. 182.3). Rocks or metal returns strong acoustic signals, whereas water-logged objects show lower acoustic contrasts compared to the surrounding water [1]. As human bodies contain a large amount of water, they are often better found from their acoustic shadows or from acoustically reflective objects on or near them [2]. Sonar images will sometimes show human bodies as distinct targets in a cluttered background (Fig. 182.3). The shadow from a recent victim will generally be small and increase only if, due to warm water, decomposition and subsequent bloating of the decomposed body affect the overall size and lift it above the bottom [3]. In cases of more advanced decomposition, a body partially or completely skeletonised would be identified in the same way, but the smaller size of the bones requires much higher resolutions than traditionally used. This reduces the range of the surveying area. Human bodies are therefore hard to image, and even the best sonars cannot provide a 100 % certainty of detection, especially in complex terrains.