resolve the correct phase change. For the SUPERTANK project, this meant 

 transmitting a series of pulses with increasing separation and decreasing unam- 

 biguous velocity range. Under the assumption of quasi-stationary conditions 

 during this burst of "micro-pings," the correct velocity can be resolved even 

 for the last pulse-pair - which has the desired characteristics of low statistical 

 uncertainty. 



Data Analysis 



Analysis and interpretation methods for data presented in Appendix H 



The 2.4-MHz BB-ADCP collected profiles of along-beam velocities as well 

 as profiles of relative acoustic scattering strength. The horizontal velocity u 

 was calculated by combining the Doppler measurements along two opposing 

 beams slanted 30 deg with respect to the vertical. The vertical velocity w and 

 the relative scattering strength EL were obtained directly from the third beam 

 pointing straight down between the two slanted beams. 



Each profile of u, w, and EL was recorded at regularly spaced intervals. 

 The depth intervals and the distance from the transducers to the first depth cell 

 were kept constant through the project and are given in Table 8-3. There is a 

 difference in cell position between the u and w components because the 

 slanted beams are used to calculate u and the vertical beam is used to calculate 

 w. Backscatter strength was sampled four times faster than velocity. 



Table 8-3 



Position of Depth Cells for Different Data Types 





if 



w 



EL 



Position of first depth cell 



26.6 cm 



30.8 cm 



18.2 cm 



Vertical extent of depth cell 



6.9 cm 



8.0 cm 



1.9 cm 



Velocity data. Velocity data were pre-processed to resolve ambiguity 

 errors. Where this pre-processing failed, post-processing was used to retrieve 

 the correct velocity. For single-ping data, the retrieval procedure was suc- 

 cessful for most of the runs. 



Four plots (see Appendix H, contained in Volume II of this report) have 

 been generated for each run. Each plot shows an aspect of the velocity 

 statistics during a 5-min interval. In all plots, u (positive toward the beach) is 

 shown as a solid line and w is shown as a dashed line. Table HI includes the 

 start time and the length of the data segment used to generate the plots. The 

 four velocity plots are: 



148 



Chapter 8 ADCP Measurements at SUPERTANK 



