to get good definition of the toe, a pair of fishing boats arrive. After 

 carefully explaining the situation to their captains over the VHF radio, they 

 agree to give way, allowing a straight survey line. A third pass is made fly- 

 ing the fish at 30 ft (above the bottom) . This position allows the towfish to 

 look down the slope of the breakwater to help distinguish breaks in slope of 

 the armor layer due to settling or loss of units. Based on review of imagery 

 from the earlier passes, several locations showing obvious inconsistencies are 

 noted and the above -water sections are photographed during the third run for 

 position reference. A decision is made to have divers sent to more closely 

 inspect these spots as soon as possible. 



Limitations and Problems : Recommended Practices 



20. For the most part, the desirable aspects of equipment and operating 

 procedures have been highlighted in the prior sections; however, some of the 

 reasoning must be explained. The state-of-the-art digital auto-correcting 

 system has important advantages for coastal engineering applications. First, 

 the need for operator experience has been greatly reduced. Second, there is 

 much greater consistency in the quality of the imagery generated from one sur- 

 vey to the next, since the internal electronics now control much of the knob 

 tweaking. Third, removal of speed variation- and slant range-induced distor- 

 tion aids greatly in interpretation of the imagery. 



21. Speed variation correction consists of matching the advance speed 

 of the display (speed at which the chart paper passes under the printing mech- 

 anism) to the actual over- the-bottom speed of the towfish. The use of posi- 

 tion-fixing equipment is highly recommended. With currently available 

 equipment and careful attention to the towfish position relative to the 

 onboard antenna, pinpointing positions of features can be done with relative 

 accuracy. Also, over-the-bottom speed of the towfish can be supplied to the 

 speed-correcting unit as an alternative to the towed speed log data. Slant- 

 range correction consists of using the towfish altitude to convert the actual 

 straight- line distance between the transducer and the target to the corre- 

 sponding horizontal distance between a point on the seabed directly below the 

 tow fish and the target (Figure 2) . Although these corrections are helpful in 

 interpreting the sonograph, there are several distortions not compensated for 

 by the currently available systems or introduced by the correction process. 



11 



