Acoustic sampling volume versus equivalent beam angle

Abstract

Irnprovements in target strength measurement have shown the equivalent beam angle (EBA) to be a worrisome source of error in echo integrator surveys. The reason is that the EBA is generally defined as a purely physical characteristic of the transducer. In fact, it depends on both the beam pattern and the detectability of fish, hence target strength, behaviour, and detection threshold. This is clearly seen through the concept of acoustic sampling volume, which is herein defined explicitly and evaluated numerically in several examples. In order to preserve the EBA in the echo integration equation, the EBA is generalized. Several methods for measuring this quantity in situ are described.