A multiple-frequency method for potentially improving the accuracy and precision of in situ target strength measurements

The effectiveness of a split-beam echosounder system to reject echoes from unresolvable scatterers, thereby improving the measurements of in situ target strengths (TS) of individuals, is dramatically enhanced by combining synchronized signals from two or more adjacent split-beam transducers of different frequencies. The accuracy and precision of the method was determined through simulations and controlled test tank experiments using multiple standard spheres and 38- and 120-kHz split-beam echosounders. By utilizing the angular positional information from one of the split-beam transducers, additional corresponding TS measurements were shown to be obtainable from a juxtaposed single-beam transducer. Both methods were utilized to extract in situ TS measurements of Antarctic scatterers simultaneously at 38, 120, and 200 kHz. The ultimate efficiency of the multiple-frequency technique is shown to be limited by phase measurement precision, which in turn is limited by the scattering complexity of targets and the receiver bandwidth. Imprecise phase measurements also result in significant beam-compensation uncertainty in split-beam measurements. Differences in multi-frequency TS measurements provided information about the identity of constituents in a mixed species assemblage. The taxa delineation method has potential, but is limited by compounding measurement uncertainties at the individual frequencies and sparse spectral sampling.