Answers.Body: <p>Target-strength in dB, Transmission loss in dB, Source level in dB//something are values that are meant to be used in the Sonar Equation which is designed for long range performance assessment and as OP indicates are meaningless are very close ranges (close with respect to the size of sound source, objects, etc)</p>
  <p>For such close ranges one needs to integrate the size of the source, the scattering object etc, into the wave-equation to account for all interactions and complexities.</p>
  <p>Industries do that by applying a full-blown finite-element approach (or similar) to solve the wave equations. This would certainly be an overkill in the bioacoustics world.</p>
  <p>IMO, a simpler model to describe the interaction (scattering by nets) could be developed when the sonar functionality of buzzing is better described and understood in bio-physics term. This would need lab-type experiments to have controlled conditions.</p>
  
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Answers.Body: <p>As OP points out, target strengths are meant to be used to get a 'broad' idea of object reflectivity in the far field. However, when the net is so close to the animal, the situation begins to move towards a near-field type scenario.</p>
  <p>While looking for ways to measure target strength measurement I found van Ruiten 1989. Here the author proposes a method used to detect objects that are large in comparison to the sensor/detector (e.g. underwater pipes).</p>
  <p>Instead of using the classical single number target strength, the author proposes using a 'reflectivity matrix' where the matrix entries have the reflection coefficients across different frequencies and angles.</p>
  <p>While the problem of cetacean click directionalities remains, hopefully the reflectivity matrix will provide some kind of framework to describe object detection without having to resort to simplified target strengths or detailed numerical simulations?</p>
  <p>References</p>
  <ul>
  <li>van Ruiten, C.J.M., 1989, Near field target strength measurements, in <em>Underwater acoustic data processing</em>, pp177-182, Kluwer Academic Publishers</li>
  </ul>
  
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Body: <p>In the context of mitigating bycatch, a lot of research has gone into making nets more detectable for echolocating toothed whales by making them more reflective.</p>
  <p>The magnitude of the backscattering properties of a target is expressed as the target strength: the dB difference between the received level on the target and the echo level 1m from the target on the same acoustic axis. This assumes that the target of interest acts as point target which is true for a fish, an insects or even a submarine if ensonified at long enough range. However, a fishing net does not full-fill those requirements as a toothed whale will never be far enough away from the net to act as a point target and still provide a detectable echo. Rather the toothed whale will likely often be well closer to the net than the net is long meaning that directional beam will ensonify more or less of the net depending on the range between the net and the whale.</p>
  <p>So why is a single TS unit for nets often used to discuss detectability in the context for bycatch despite that the metric is clearly meaningless? what to do about that problem?</p>
  
Tags: <echolocation>
Title: What is the target strength of a fishing net?
ThreadText: # What is the target strength of a fishing net?
  
  (Asked by: username_0 on 2022-07-11)
  
  In the context of mitigating bycatch, a lot of research has gone into making nets more detectable for echolocating toothed whales by making them more reflective.
  
  The magnitude of the backscattering properties of a target is expressed as the target strength: the dB difference between the received level on the target and the echo level 1m from the target on the same acoustic axis. This assumes that the target of interest acts as point target which is true for a fish, an insects or even a submarine if ensonified at long enough range. However, a fishing net does not full-fill those requirements as a toothed whale will never be far enough away from the net to act as a point target and still provide a detectable echo. Rather the toothed whale will likely often be well closer to the net than the net is long meaning that directional beam will ensonify more or less of the net depending on the range between the net and the whale.
  
  So why is a single TS unit for nets often used to discuss detectability in the context for bycatch despite that the metric is clearly meaningless? what to do about that problem?
  
  **Tags:** echolocation
  
  
  ---
  
  ## 2 answers
  
  ### Answer by username_1 on 2022-07-11. Score: 3
  
  Target-strength in dB, Transmission loss in dB, Source level in dB//something are values that are meant to be used in the Sonar Equation which is designed for long range performance assessment and as OP indicates are meaningless are very close ranges (close with respect to the size of sound source, objects, etc)
  
  For such close ranges one needs to integrate the size of the source, the scattering object etc, into the wave-equation to account for all interactions and complexities.
  
  Industries do that by applying a full-blown finite-element approach (or similar) to solve the wave equations. This would certainly be an overkill in the bioacoustics world.
  
  IMO, a simpler model to describe the interaction (scattering by nets) could be developed when the sonar functionality of buzzing is better described and understood in bio-physics term. This would need lab-type experiments to have controlled conditions.
  
  
  ---
  
  ### Answer by username_2 on 2022-07-13. Score: 2
  
  As OP points out, target strengths are meant to be used to get a 'broad' idea of object reflectivity in the far field. However, when the net is so close to the animal, the situation begins to move towards a near-field type scenario.
  
  While looking for ways to measure target strength measurement I found van Ruiten 1989. Here the author proposes a method used to detect objects that are large in comparison to the sensor/detector (e.g. underwater pipes).
  
  Instead of using the classical single number target strength, the author proposes using a 'reflectivity matrix' where the matrix entries have the reflection coefficients across different frequencies and angles.
  
  While the problem of cetacean click directionalities remains, hopefully the reflectivity matrix will provide some kind of framework to describe object detection without having to resort to simplified target strengths or detailed numerical simulations?
  
  References
  
  * van Ruiten, C.J.M., 1989, Near field target strength measurements, in *Underwater acoustic data processing*, pp177-182, Kluwer Academic Publishers
  
  
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