izontal surfaces. Due to dissipation of energy in pulses with distance 

 traveled, as well as the effects of lower angle of incidence, targets farther 

 from the towfish will give weaker returns. However, the newest systems with a 

 time-variable gain circuit can compensate for the signal dissipation to a 

 large degree. 



31. Features are only identifiable on sonographs when they present a 

 contrasting reflectivity to their surroundings. The resulting amount of 

 detail is sensitive to resolution, primarily controlled by frequency, range, 

 and degree of towfish motion in directions other than the straight- line track 

 direction. Discrimination of isolated features is aided by acoustic shadows, 

 although shadowing presents problems in distinguishing nonisolated, irregu- 

 larly shaped targets. For example, individual armor units usually cannot be 

 distinguished from their neighbors due to shadowing and lack of contrast. 

 Figure 3 shows an example sonograph from a qualitative survey of the rubble - 

 mound breakwater at Crescent City, CA. The following noncomprehensive list 

 gives an idea of what can (sometimes) be seen with SSS : 



a. Outline of structure toe (jetties, breakwaters). 



b. Isolated displaced armor units. 



c. Depressions and mounds (scour holes, spoil piles). 



d. Abrupt changes in slopes. 



e. Sunken objects, vehicles, debris. 



f. Bottom material boundaries (e.g. sand patches on clay bottom). 



g. Bottom relief such as sand waves (useful in determining current 

 and transport direction), dredge/anchor/ice marks. 



h. Cables, chains, piles. 



Vertical structures can be imaged with the towfish in the conventional hori- 

 zontal deployment position, or it may be beneficial to rotate the fish on one 

 of its axes to view the structure from a different perspective. Since verti- 

 cal walls are ordinarily used only in coastal environments with very low wave 

 energy, it may also be possible to fix the transducers in a frame or other 

 more stable, controllable configuration (such as the truck-mounted crane 

 deployment mentioned earlier). However, due to the very low angle of 

 incidence for vertical walls and their other reflectivity characteristics, 

 returns can be so intense that discrimination of features is difficult. When 

 they are present in a site being surveyed, if contrast is adjusted to permit 

 discrimination of less reflective features, there can be a ghost image 



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