of the intensity of the returning acoustical pulse. The stronger the 

 returning pulse the darker the image. A number of factors affect the inten- 

 sity of the returning signal, including acoustic reflectivity of the target, 

 slope of the target face, contrast between the target and surrounding 

 material, and the number of reflecting surfaces. 



Target material and 

 orientation influences 



14. The acoustic reflectivity of the target is a function of the 

 acoustic impedance of the material (material density times the speed of sound 

 through the material). Consequently, steel has a higher reflectivity than 

 does stone or concrete, which has a higher reflectivity than wood. The 

 coarser the sediment, the higher the reflectivity. Therefore, gravel reflects 

 more acoustic energy than sand, which reflects more than silt or clay. The 

 air/water interface is also a very strong reflector of sound; consequently, 

 air entrained in the water column may block the acoustic signal from more 

 distant objects and severely degrade the quality of the sonograph. Propeller 

 wash and wave entrained air may be particularly troublesome if the resultant 

 acoustic turbidity is between the towfish and the target. 



15. The slope also affects the strength of the returning signal. As 

 the slope of the target face becomes more perpendicular to the incoming sound 

 wave, the strength of the reflected signal increases. Solitary projections, 

 such as an armor stone sitting on the bottom, will produce a strong dark image 

 on the trackline side of the sonograph and will cast a shadow away from the 

 center line of the record (Figure 4). The acoustic shadow zone (Figure 3), 

 where no signal is reflected, shows up as a white area on the sonograph. This 

 relationship between acoustic shadow and target is reversed for a depression 

 or hole. The sound is not reflected by the hole but reflects off the far side 

 of the hole. Consequently, the shadow is closer to the trackline, and the 

 object's reflection is farther from the trackline (Figure 4). 



Vessel speed effects 



16. Distortion parallel to the trackline of the towfish occurs due to 

 varying boat speeds. At the speeds recommended for inspection, 3 knots" or 

 less, distortion parallel to the line of travel is negligible. Distortion 



A table of factors for converting non-SI units of measurement to SI 

 (metric) units is presented on page 3. 



13 



