108 GEOPHYSICAL AND REMOTE-SENSINGSURVEYS
and metals are better reflectors than finer grained sedi-
ments and will therefore be recorded as darker elements
on the sonar record. Target shape, including sea-floor gra-
dient, also influences reflectivity and backscattering.
Arguably the most important phenomena on sidescan
records for archaeological purposes are acoustic shadows,
which provide a three-dimensional quality to what is
essentially a two-dimensional survey. Acoustic shadows
occur alongside objects that stand proud of, or are par-
tially buried in, the sea-floor. In sidescan sonar data,
shadows can often indicate more about the shape and
nature of a target than the acoustic returns from the tar-
get itself (figure 13.2).
The majority of sidescan investigations follow a pre-
determined survey pattern. Search patterns convention-
ally comprise a series of parallel survey lines with the lane
spacing determined by survey requirements. The lane
spacing must be less than the swath width of the sonar,
thereby allowing for overlap between lines, ensuring 100
per cent coverage of the area. Although data has conven-
tionally been displayed as a paper-trace, modern systems
allow real-time data to be displayed through a computer
and in conventional monochrome or almost any variety
of colour mixtures the operator chooses. Software can then
be used to identify the coordinates of any feature (geo-
rectification) and, by measuring the length of the shadow
and the height of the tow-fish, its projection above the sea-
bed can be calculated. Geo-rectified sidescan images can
be joined together in a mosaic to provide an image of large
areas at the original resolution, thereby decreasing the trade-
off between data resolution and survey coverage.Sub-Bottom Profiling
While sidescan sonar and multibeam swath bathy-
metry are the most effective techniques for finding and
delineating archaeological objects exposed on the sea-bed,Figure 13.2 Sidescan sonar image of the SS Storaa(1943) showing shadow detail complementing the backscatter infor-
mation from the wreck. (Courtesy of ADUS, University of St Andrews)