A Unified Method of Broadband Acoustic Characterization for Fish Schools, Channels, and Range-Extended Targets

Fish schools and long-range propagation channels can be described by a scattering function, i.e., by the expected distribution of Doppler shifts and delays that are introduced by a collection of moving point scatterers. Acoustic inhomogeneities such as man-made objects and minerals on or below the ocean floor can be characterized by the frequency dependence of sound absorption and reflectivity, and by the positions of highlights, or rapid changes in acoustic impedance. Highlight structure and frequency dependence can be obtained from the time-frequency energy density function of the target impulse response. Both the scattering function and the time-frequency energy density function can be represented by the spectrogram of an echo. Spectrogram analysis therefore provides a unified framework for echo analysis. Experimental results are given in order to illustrate the use of spectrograms for detection and maximum likelihood echo classification. Pattern recognition techniques are used to determine the features of the echo spectrogram that are most important to a classifier.