Absorption term in time-varied-gain functions

Abstract

A potential source of error in acoustic measurements of fish density is the absorption part of time-varied-gain functions. This should be determined from the 1977 Fisher and Simmons equation. Use of its predecessor, the 1962 Schulkin and Marsh equation, is shown by comparison to introduce large errors in fish density estimates. Adjustment of the absorption term with changing hydrography is also necessary. This is supported by an analysis of errors due to deviations from an assumed reference temperature. The various computations are performed for six echo sounder frequencies used in fisheries surveying; namely, 30, 38, 49.5, 70, 105, and 120 kHz. A broad hydrographical range is considered.