Weber et al: Seabed classification for trawlability determined with a multibeam echo sounder 
75 
of the study site, and the most obvious error occurred 
on the north-south transect intersected 153. 9°W in an 
area with high oblique-incidence S b . 
Discussion 
The oblique-incidence S b and the S 6 -slope followed the 
expected trends when separated into trawlable and un- 
trawlable classes and these trends were verified from 
video data collected with the SDC and ROV. Untraw- 
lable areas were expected to have a larger oblique in- 
cidence S b and S fe -slope than trawlable areas on the 
basis of backscatter models (e.g., Fig. 1). The normal- 
incidence S b did not appear to discriminate very well 
between trawlable and untrawlable seafloor and tended 
to have a wider distribution of backscatter values than 
would have been expected on the basis of consideration 
of the oblique-incidence S b and the modeled values 
shown in Figure 1. There are several possible reasons 
for the lack of discrimination with normal-incidence S b , 
including higher-than-expected normal-incidence S b in 
the sands and silts caused by gas or heterogeneities 
within the sediment volume in some trawlable areas 
and higher-than-expected roughness in the areas of 
cobble and rock that caused a larger-than-anticipated 
reduction in the normal-incidence S b for some untraw- 
lable areas. 
Although quite variable throughout the study area, 
the mode of the SI was slightly higher for the untraw- 
lable seafloor than it was for the trawlable seafloor. 
This difference seems plausible when we consider the 
SI to be a metric for how many scatterers are contrib- 
uting to the sonar return within a beam footprint. A SI 
value near 1 suggests that there are a large number of 
scatterers (i.e., the central limit theorem applies, and 
the backscatter amplitude is Rayleigh distributed), as 
might be expected from a sand or silt seafloor. On the 
other hand, a larger SI indicates that there are only a 
few dominant scatterers within the beam footprint, as 
might be expected from a seafloor of cobbles or boul- 
ders. Although the data indicate a trend in the correct 
direction, SI alone has not provided a clear separation 
between trawlable and untrawlable seafloor (e.g., a 
