Feature images from biologically inspired acoustic imaging

A broadband, broad-beam sonar with two receiving arrays has been constructed at the SPAWAR Systems Center in San Diego. The sonar emulates the transmitted pulse, projector beam width, and receiver beam width of the Atlantic bottlenose dolphin. The sonar output can be used to compute a pixel value in an acoustic image by summing echo samples from a chosen point in the environment for various aspect angles. The echo samples usually correspond to reflectivity, e.g., the sampled output of a pulse compression filter. Other quantities, however, can be summed over multiple aspects in the same way. A sum of squared samples, together with the original sum, can be combined to form an image that represents the aspect-induced variance of the echo from each point on an object. This variance is large for smooth, specular surfaces and small for rough surfaces. Another feature image is obtained from a maximum likelihood bearing estimator, using the outputs of the two "ears". The relevant sample from each echo corresponds to the location of the chosen point on a range-bearing plot. Summation of these samples over different aspects produces an interaural correlation image. Rough/smooth feature images and interaural correlation feature images are useful for object detection and classification.