Coincidence echo statistics

Abstract

Two scatterers at similar range give an echo which may appear to be due to a single scatterer. Methods for determining target strength that depend on resolving single scatterers may fail in this instance. Statistics associated with the described special case of coincidence are derived and illustrated by theoretical computation for the SIMRAD EK500 echo sounder system with the ES38B split-beam transducer resonant at 38 kHz. Connections to angle measurement in radar and swath bathymetry and to bottom-scattering-strength measurement are noted.

Hydroacoustic studies on the commercially important squid Sthenoteuthis oualaniensis in the South China Sea

“…Unfortunately, current methods of single-target detection are imperfect; even though the split-beam systems are designed to detect fish that are far enough away from neighbours to avoid overlapping echoes (Foote, 1996), errors may still occur through the false detection of multiple targets. This error can be small when individuals are at low density, though the problem increases as fish density increases.…”

Potential improvements to current methods of recognizing single targets with a split-beam echo-sounder

“…This is essentially the phase detection method implemented in the SIMRAD EK500 echo-sounder (Version 4.01). Echoes are rejected as being multiples if the average absolute deviation in successive phase sample (P dev ), taken within 6 dB of the peak, exceeds a preset limit in either the athwartships or alongships direction (Bodholt, 1990;Foote, 1994;Soule et al, 1995, H. Nes, pers. comm).…”

“…In these peak-density regions, the performance of the single-target detection algorithms may well bias the TS estimate. This bias, which may be large (Foote, 1994), is due to the acceptance of multipletarget echoes, aggravated by algorithm-specific selection biases as reported by Soule et al (1995).…”