42
Synthetic aperture sonar (SAS) is
a form of sidescan sonar that
uses advanced processing of the
sonar data to produce very high-
resolution acoustic images of the
seafloor.
Originally developed for
defence-related applications,
SAS is increasingly being used
for commercial applications such
as offshore energy,
communications, and marine
archeology.
Recently, Drs. John Jamieson and
Caroline Gini, marine geologists
at Memorial University of
Newfoundland in St. John’s,
Canada, teamed up with Kraken
Robotics, an offshore technology
company also based in St. John’s
that specialises in the
development of SAS, to explore
how SAS can be used to map the
geology of the seafloor at a level
of detail that rivals visual surveys
(e.g., using ROVs, towed
cameras, or AUVs) but at a spatial
scale of larger bathymetric
surveys.
SAS data are collected using
sensors mounted on towed
vehicles, AUVs, and ROVs, as well
as surface vehicles for shallow
surveys.
The resolution and survey swatch
can vary, depending on the
frequency of the sonar, vehicle
altitude, beam angles, and
survey speed.
However, a typical SAS survey,
flown at an altitude of 15 m
above the seafloor, can produce
imagery with a horizontal
INTERFEROMETRI
Researchers
are pushing the
boundaries of
what synthetic
aperture sonar
can do,
revealing the
ocean floor in
unprecedented
detail.
resolution better than 3 cm up to ~200 m from either
side of the survey vehicle.
Sidescan sonar resolution is primarily limited by the
length, or aperture, of its transducer. A longer
aperture will yield higher resolution data. In the case
of SAS, a synthetic aperture is created by stacking
the signal as the survey vehicle advances along-track
to simulate a longer transducer.
With traditional sidescan sonar data, image
resolution decreases with distance from the survey
vehicle. However, with SAS, objects at the far range
of the survey swath will be imaged with more pings
than objects in the close range, and image resolution
therefore remains constant and independent of
lateral distance from the vehicle.
Using interferometry, bathymetric data, with a
resolution of ~25 cm, can also be simultaneously
generated from the SAS data (called interferometric
synthetic aperture sonar, or InSAS), resulting in co-
registered 3D depth data and 2D imagery.
Kraken Robotics MINSAS sensor, mounted on a
Kraken Katfish towed vehicle. Image: Caroline Gini,
Memorial University of Newfoundland
SIDESCAN VS SAS