Answers.Body: <p>If you do not wanted to run an acoustic model like bellhop, I have the feeling that 20 log(r) is good enough for ranges out to 1 km.</p>
  <p>I know there will be bottom interaction. But most likely you have to deal with the Lloyd-mirror effect (surface reflection).</p>
  <p>You always can assume that the direct arrival of the sound is following closely the spherical spreading law. The interference pattern is then influenced by bottom properties.</p>
  
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Answers.Body: <p>Hmmm …. the most accurate models are normal mode models if you know the maximum number of modes to use. The normal modes are also well suited for low frequencies where the number of modes can be few. I would go with a model like krakenc along with the field program that comes with it. If the depth varies along range then you will also have to account mode-coupling. You could use these steps to write up an accurate prediction model. For high frequencies you may want to check ray models. There are models and further models. It also depends what accuracy you want and your background in knowing to use or build models. Check the ocean acoustics library website for a lot of free to use modeling tools. Also check out the book on Computational Ocean acoustics by Jensen et.al.</p>
  
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Answers.Body: <p>RAMGEO and Kraken would be good models for low-frequency sounds in shallow water environments.</p>
  
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Answers.Body: <p>I agree with WMXZ that spherical spreading is a pretty good model. If you think about it the right way, it really defines a reference that all other physics depart from.</p>
  <p>Each arrival tends to follow spherical spreading unless there is loss (scattering or absorption). The underwater environment can be pretty benign, and these losses stay low. In general, as you move out in range you get more arrivals (multipath). These add together at the receiver to produce the signal level, and these multiples mean the loss is generally less than spherical spreading predictions (eventually approaching cylindrical spreading in the modal limit). That being said, if you have a way to separate out these arrivals (time delay or angle), each individual arrival can still follow spherical spreading closely.</p>
  
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Answers.Body: <p>I agree with Walter here: when range is no more than approx 10 x water depth and there is no hefty thermocline ect, then 20log(R) is likely not too far off. Try see fig 3 in this paper: <a href="https://marinebioacoustics.files.wordpress.com/2018/05/hermannsen_et_al_2014.pdf" rel="noreferrer">https://marinebioacoustics.files.wordpress.com/2018/05/hermannsen_et_al_2014.pdf</a></p>
  <p>Evidently, you can try run a couple of models to see if they suggest something substantially different than a simple spherical model for the range in question, but the main contributor to variation in SL for a given vessel is the changing aspect to a directional source (the vessel) more so than errors in TL estimation for shorter ranges like your situation.</p>
  <p><strong>Reference</strong></p>
  <p>Hermannsen, L., Beedholm, K., Tougaard, J., &amp; Madsen, P. T. (2014). High frequency components of ship noise in shallow water with a discussion of implications for harbor porpoises (Phocoena phocoena). The Journal of the Acoustical Society of America, 136(4), 1640-1653.</p>
  
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Answers.Text: No likely well longer than that, but 10log(R) is also not really realized in most situations at long ranges and you will need a proper sound propagation model that takes SVP profile and bottom composition into account.
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Answers.Text: Would a transition from spherical to cylindrical be necessary after that roughly 10x water depth distance?
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Body: <p>I am trying to estimate source level on some ships that passed by my hydrophone at a distance greater than 100 meters. A few ships passed 60-80 meters away and I used an assumption of spherical spreading to estimate transmission loss - but most of my ships passed further away, some up to 1 km away. The water depth at the hydrophone is 70 meters, and gets as deep as ~150 meters at the range of the furthest ship passing.</p>
  <p>I'm not sure what the best approach for estimating transmission loss would be for these further away ships. I don't want to assume spherical spreading for these ships further than 100 meters away, since it's shallow water, I think the 20log(r) equation will not be as accurate as it was for estimating those ships that passed by within 100 meters.</p>
  <p>Is there a better equation or model to use to estimate transmission loss in shallow water at distances of several hundred meters away?</p>
  
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Comments.Text: Not sure about 20log(r) -  is this the 1980s's?  If you're going to go simplistic, look to Marsh and Shulkin (1962).  I also don't agree that BELLHOP is the best approach for LFN - mostly HFN (Sonar).  GEORAM is probably the better choice.  Even at short distances, bottom effects and sound speed profile will matter...                                                                                                     
  The commercial software is okay, as long as it's properly calibrated for site specific events, Erik K.
Comments.UserDisplayName: ejk
Tags: <source-level><propagation><transmission-loss>
Title: What type of propagation model should I use for a shallow water environment when the source is over 100 meters away?
ThreadText: # What type of propagation model should I use for a shallow water environment when the source is over 100 meters away?
  
  (Asked by: username_0 on 2022-07-05)
  
  I am trying to estimate source level on some ships that passed by my hydrophone at a distance greater than 100 meters. A few ships passed 60-80 meters away and I used an assumption of spherical spreading to estimate transmission loss - but most of my ships passed further away, some up to 1 km away. The water depth at the hydrophone is 70 meters, and gets as deep as ~150 meters at the range of the furthest ship passing.
  
  I'm not sure what the best approach for estimating transmission loss would be for these further away ships. I don't want to assume spherical spreading for these ships further than 100 meters away, since it's shallow water, I think the 20log(r) equation will not be as accurate as it was for estimating those ships that passed by within 100 meters.
  
  Is there a better equation or model to use to estimate transmission loss in shallow water at distances of several hundred meters away?
  
  **Tags:** source-level, propagation, transmission-loss
  
  > **Comments:**
  > **username_1**: Not sure about 20log(r) -  is this the 1980s's?  If you're going to go simplistic, look to Marsh and Shulkin (1962).  I also don't agree that BELLHOP is the best approach for LFN - mostly HFN (Sonar).  GEORAM is probably the better choice.  Even at short distances, bottom effects and sound speed profile will matter...                                                                                                      The commercial software is okay, as long as it's properly calibrated for site specific events, Erik K.  
  
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  ## 5 answers
  
  ### Answer by username_6 on 2022-07-05. Score: 14 | Accepted answer
  
  I agree with Walter here: when range is no more than approx 10 x water depth and there is no hefty thermocline ect, then 20log(R) is likely not too far off. Try see fig 3 in this paper: <https://marinebioacoustics.files.wordpress.com/2018/05/hermannsen_et_al_2014.pdf>
  
  Evidently, you can try run a couple of models to see if they suggest something substantially different than a simple spherical model for the range in question, but the main contributor to variation in SL for a given vessel is the changing aspect to a directional source (the vessel) more so than errors in TL estimation for shorter ranges like your situation.
  
  **Reference**
  
  Hermannsen, L., Beedholm, K., Tougaard, J., & Madsen, P. T. (2014). High frequency components of ship noise in shallow water with a discussion of implications for harbor porpoises (Phocoena phocoena). The Journal of the Acoustical Society of America, 136(4), 1640-1653.
  
  > **Comments:**
  > **username_7**: Would a transition from spherical to cylindrical be necessary after that roughly 10x water depth distance?  
  > **username_6**: No likely well longer than that, but 10log(R) is also not really realized in most situations at long ranges and you will need a proper sound propagation model that takes SVP profile and bottom composition into account.  
  
  ---
  
  ### Answer by username_2 on 2022-07-05. Score: 7
  
  If you do not wanted to run an acoustic model like bellhop, I have the feeling that 20 log(r) is good enough for ranges out to 1 km.
  
  I know there will be bottom interaction. But most likely you have to deal with the Lloyd-mirror effect (surface reflection).
  
  You always can assume that the direct arrival of the sound is following closely the spherical spreading law. The interference pattern is then influenced by bottom properties.
  
  
  ---
  
  ### Answer by username_3 on 2022-07-08. Score: 4
  
  Hmmm …. the most accurate models are normal mode models if you know the maximum number of modes to use. The normal modes are also well suited for low frequencies where the number of modes can be few. I would go with a model like krakenc along with the field program that comes with it. If the depth varies along range then you will also have to account mode-coupling. You could use these steps to write up an accurate prediction model. For high frequencies you may want to check ray models. There are models and further models. It also depends what accuracy you want and your background in knowing to use or build models. Check the ocean acoustics library website for a lot of free to use modeling tools. Also check out the book on Computational Ocean acoustics by Jensen et.al.
  
  
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  ### Answer by username_4 on 2022-07-11. Score: 2
  
  RAMGEO and Kraken would be good models for low-frequency sounds in shallow water environments.
  
  
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  ### Answer by username_5 on 2022-07-06. Score: 2
  
  I agree with username_2 that spherical spreading is a pretty good model. If you think about it the right way, it really defines a reference that all other physics depart from.
  
  Each arrival tends to follow spherical spreading unless there is loss (scattering or absorption). The underwater environment can be pretty benign, and these losses stay low. In general, as you move out in range you get more arrivals (multipath). These add together at the receiver to produce the signal level, and these multiples mean the loss is generally less than spherical spreading predictions (eventually approaching cylindrical spreading in the modal limit). That being said, if you have a way to separate out these arrivals (time delay or angle), each individual arrival can still follow spherical spreading closely.
  
  
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