PART IV: DATA INTERPRETATION 



28. It is beyond the scope of this report to give a detailed guide to 

 sonograph interpretation; however, it is useful to have some introduction to 

 the subject when considering investing in SSS equipment. Experience plays a 

 primary role in interpretation, more an art than a science at present. Simi- 

 larities exist between sonographs and other types of remotely sensed images 

 (for example those obtained from side-looking airborne radar); thus many prin- 

 ciples of interpretation are held in common. An important aid to interpreta- 

 tion is a familiarity with the area or structure to be surveyed; thus 

 knowledge from local fishermen, annotated plans, etc., is very helpful. 



29. As mentioned in the introduction, the high-frequency (ultrasonic) 

 acoustic pulses are emitted in two fan-shaped beams (Figure 2) traveling per- 

 pendicularly outward from the towfish and encountering the bottom and its fea- 

 tures. The instantaneous area being illuminated (the analogy to light rays is 

 useful for visualization) is long in the across -track direction and narrow in 

 the along- track direction. The angle of incidence is almost zero near the 

 fish and increases with distance out to the sides. The sonograph is created 

 so that a strong returning signal (reflected energy directed at the trans- 

 ducer) results in a dark trace on the display, whereas no returning signal 

 leaves a blank trace, where the shade of the image corresponds to the amount 

 of background noise. Thus, there is a similarity to a black and white photo- 

 graphic negative of a scene lit by a pair of spotlights moving forward along 

 the track. 



30. The factors determining the strength of signal returns are 

 (a) acoustic reflectivity of the target; (b) orientation or slope of the 

 reflecting surface with respect to the impinging pulses; and (c) distance sep- 

 arating the target from the transducer. Acoustic reflectivity of a target 

 (including the seabed) is determined by both the material itself and its sur- 

 face texture. Examples of good acoustic reflectors are smooth steel and con- 

 crete, whereas soft, textured objects, such as seaweed beds, would generally 

 tend to absorb rather than reflect. For bottom materials, gravel has the 

 highest, sand has intermediate, and mud has the lowest reflectivity. Target 

 shapes or orientations that tend to reflect back at the transducer result in 

 strong traces. Consequently, surfaces sloping toward the transducer (such as 

 breakwater faces) can be expected to reflect well relative to surrounding hor- 



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