FISHERY BULLETIN: VOL. 85, NO. 4 



Figure 1. — Schematic diagram of a sidescan sonar vehicle being towed over the seafloor 

 (upper) and resulting sonograph (lower) with images of trawl door tracks, tilefish burrow, 

 gravel, and a boulder. 



sidescan vehicle was towed 10-15 m above the 

 seafloor at speeds of 3-7 km/hour and was set to 

 scan 100 m or 150 m to each side of the towed 

 vehicle. At these speeds and scanning range, 

 0.6-1.4 km^ of seafloor could be mapped per 

 hour. 



The sidescan sonograph signatures that char- 

 acterize different habitat types are largely deter- 

 mined by two conditions, topography and fine- 

 scale roughness (in particular, differences in 

 sediment texture). The signals received from 

 tilefish burrows (Fig. 2) and boulders (Fig. 3) pro- 

 vide good examples of differences in strength of 

 the recorded signal due to topographic effects. A 

 strong signal (dark) is received from the side of 

 the feature facing the transducer while a weak 

 signal or shadow (light) is received from the side 

 sloping away from the transducer. Thus, boulders 

 have the strong return nearest the transducer fol- 



lowed by a shadow (Fig. 3), while burrows appear 

 as a shadow preceding the strong return (Fig. 2). 

 Gravel gives a much stronger signal than silt be- 

 cause of the many small facets facing the trans- 

 ducers. Textural differences usually can be distin- 

 guished from topographic differences because 

 there is no shadow associated with them (Fig. 4). 

 Although the sonograph is a map view of the 

 seafloor, there are two distortions that must be 

 compensated for when interpreting these images. 

 The first is the across-track, slant-range distor- 

 tion which results from distances being measured 

 from the sidescan vehicle that is positioned above 

 the bottom and not the zero line on the seafloor 

 below the towed vehicle (Fig. 1). For this reason, 

 the point on the seafloor directly below the fish is 

 plotted away from the actual zero line by the dis- 

 tance the fish is off the bottom (distance h on 

 Figure 1). The second geometric distortion is in 



726 



