a. A good model for attenuation as a function of relative particle size 

 must be used. Such a model is established (Flammer 1962). 



b. The measured bottom echo will vary as a function of the integrated 

 attenuation. In the case of the SUPERTANK project, it can be 

 assumed that the attenuation is low before a run is started. This infor- 

 mation can be used to constrain the system of coupled equations. 



c. The system can be constrained by imposing physical constraints. The 

 mean concentration, for example, can be required to increase toward 

 the bottom. 



d. A model of particle distribution as a function of depth can be 

 included. 



ft -2 



0.4- 0.6 0.8 

 Distance from transducer (m) 



Figure 8-6. 



Spreading models for the 2.4-MHz system. The near-field 

 model was applied to the data shown in Appendix H 



146 



In practice, steps (a)-(d) require substantial work as well as hardware that 

 can measure all the pertinent parameters. The 2.4-MHz system fails in this 

 regard because of the strongly filtered output. For the data in Appendix H, 

 we have limited the correction to a fixed value of 2.8 db/m (one-way) for the 

 combined effect of particle attenuation and water absorption. 



Velocity measurements. The 2.4-MHz BB-ADCP was calibrated at the 

 tow tank at Memorial University, Newfoundland, after the SUPERTANK 

 project was completed. Tow carriage velocities were adjustable from 1 to 

 200 cm/sec. In the short-range mode (see pre-processing and filtering) used 



Chapter 8 ADCP Measurements at SUPERTANK 



