Target phase: an extra dimension for fish and plankton target identification.

The acoustic signal backscattered from a fish in water, Lbs(f), at a frequency, f, differs from the incident signal in both magnitude, /Lbs(f)/, and phase, arg[Lbs(f)], and it has been common practice for many years to use the backscatter magnitude from individual fish as an aid to species identification. However, very little use has been made of the phase of the backscattered signal relative to that of the incident acoustic pulse. If the gross phase changes due to propagation through water are compensated for, the residual phase signature is found to contain useful target-specific information. The phase signature can be characterized by estimating the rate at which the echo phase changes, relative to the transmitted pulse, during the echo from a fish. Clear groups are produced when single fish targets from in situ data are plotted in complex target space (target strength versus target rate of change of phase) and this is explored here by computing the acoustic backscatter, in complex target space, from a series of simple large and small model fish targets, both with and without gas-filled swimbladders, using a Monte Carlo technique. It is shown that all the features found in the in situ data can be explained in terms of the size and attitude of the fish. The modeling was carried out at the frequency of 38 kHz, which pertained to the in situ data, and the specific results only apply to this frequency. However, the complex target approach is generally applicable to fish target strength analysis independently of frequency.