Weber et al: Seabed classification for trawlability determined with a multibeam echo sounder 
69 
0 
rough rock 
Figure 1 
A prediction of the angle-dependent seafloor backscatter strength, S b (dB), ac- 
cording to APL [1994], for the beam configuration used for the Simrad ME70 
multibeam echo sounder at Snakehead Bank in the Gulf of Alaska during a 
cruise of the NOAA Ship Oscar Dyson in October 2009. The areas over which the 
oblique-incidence S b and the slope of the angle-dependent backscatter within 10° 
of normal incidence (S^-slope) were calculated are shown. Normal-incidence S b 
was calculated at 0° incidence angle. 
ture of the seafloor backscatter 
strength, S b . For example, the 
normal-incidence (i.e., 0° inci- 
dence angle) S b that would typi- 
cally be expected for both cobble 
and fine sand are predicted to be 
very similar but are appreciably 
different at increased incidence 
angles (Fig. 1). Angle-dependent 
metrics that describe the back- 
scatter from the seafloor have 
been extracted from MBES data 
in previous studies to determine 
the nature of seafloor sediments 
(e.g., Fonseca and Mayer, 2007). 
Seafloor backscatter collected 
with an MBES, as are the pre- 
dictions shown in Figure 1, are 
often treated as the ensemble 
average of a large number of 
random realizations of scattered 
acoustic intensity. Higher order 
statistics that describe the scat- 
tered intensity may also provide 
information that can be used to 
characterize the seafloor. Often, 
the amplitude of the backscat- 
ter echoes is expected to follow 
a Rayleigh distribution, with the 
underlying assumption that there are a large number 
of contributors to the backscatter from the seafloor at 
any instant in time (Jackson and Richardson, 2007). 
Abraham and Lyons (2002) have linked heavy-tailed, 
non-Rayleigh distributions of backscatter to a model 
with a relatively small number of objects on the sea- 
floor that have high levels of backscatter strength. In 
other words, the details of the probability density func- 
tion that describe the amplitude of the acoustic echoes 
are likely to be related to the size and density of the 
scattering objects and their relative role in the overall 
scattering response. Measures that indicate non-Ray- 
leigh backscatter may give an indication of distributed 
cobble or rock that would render a seafloor untrawlable. 
In this study, we examined the angle-dependent na- 
ture of S b , as well as measures of non-Rayleigh dis- 
tribution of the backscatter and the seafloor rugos- 
ity (roughness) derived from bathymetric soundings, 
in an attempt to discriminate between trawlable and 
untrawlable seafloors. The data were collected with 
a Simrad 1 ME70 MBES (Kongsberg AS, Horten, Nor- 
way) at a study area on Snakehead Bank in the Gulf 
of Alaska, -100 km south of Kodiak Island (Fig. 2). To 
test the efficacy of the acoustic measures as classifiers 
of the seafloor as either trawlable or untrawlable, we 
compared metrics derived from a MBES with observa- 
1 Mention of trade names or commercial companies is for 
identification purposes only and does not imply endorsement 
by the National Marine Fisheries Service, NOAA. 
tions collected with a stereo drop camera (SDC) system 
(Williams et ah, 2010) along with cameras mounted on 
a remotely operated vehicle (ROV) (Rooper et ah, 2012). 
The results of this comparison were then extracted to 
the entire multibeam data set that was collected with 
the Simrad ME70 during our Snakehead Bank surveys. 
Methods 
MBES data were collected with a Simrad ME70 MBES 
mounted on the hull of the NOAA ship Oscar Dyson. 
The Simrad ME70 was developed specifically for fish- 
eries applications (Trenkel et ah, 2008), although it 
also has been used for bathymetric mapping (e.g., Cut- 
ter et ah, 2010). The Simrad ME70 is configurable in 
terms of 1) the number of beams generated, 2) acoustic 
frequency for each beam, and 3) direction and open- 
ing angle of the beams. For our surveys at Snakehead 
Bank, the Simrad ME70 was configured to generate 31 
beams at frequencies ranging from 73 to 117 kHz and 
at beam opening angles that ranged from 2.8° to 11.0°. 
The 31 beams were steered to 0° in the alongship di- 
rection and from -66° to +66° in the athwartship direc- 
tion, with the lowest frequencies steered to the high- 
est beam steering angles to mimimize the possibility 
of ambiguities associated with grating lobes (angular 
regions within a beam pattern of a transducer array 
that have equal sensitivity to the main angular region, 
or lobe, and cause ambiguities in the determination of 
