Modeling of acoustic reverberation from a diurnally migrating fish aggregation with depth-dependent resonance.

Abstract

The depth dependence of the resonance frequency of fish with swimbladders is affected by both the change in volume (physostomes only) and pressure of the swimbladder (physostomes and physoclists). Due to these physical processes, the migration of fish upward or downward the water column can cause significant variation in the magnitude of the echoes received by an active sonar, most notably in narrowband applications where the resonance frequency may pass through the sonar band during migration. This variation in received echo level is further complicated by waveguide effects in long-range acoustic scenarios. The sonar simulator toolset, a ray-based sonar simulator, is used to simulate the reverberation of a narrowband, FM pulse by a vertically migrating aggregation of fish over a range of conditions. We examine the results of varying range, depth, and other waveguide parameters on the matched filter output. The predictions distinguish the effect of swimbladder resonance from acoustic waveguide effects on the scattering. A key result illustrated is a 28-dB variation in the received echo due to the resonance frequency passing through the band of a 3-kHz sonar during diurnal migration of the Atlantic herring.