AUV hits Orkney
In the week of large scale remembrances of the Battle of
Jutland that will include amassing parts of the British Naval Fleet, the German
Fleet and possibly parts of the Danish fleet we made our own bit of history by
testing for the first time one of the new generation of survey vehicles the
Gavia AUV in the waters of Orkney. The
Gavia (Teledyn, Iceland) Autonomous Underwater Vehicle is a type of propelled
platform designed to deploy geophysical instruments for survey beneath sea. The vehicle looks like a small torpedo and in
our configuration has onboard a complex navigation system, sidescan-bathymetric
sonar and a high resolution camera. The sonar
is used to map the seafloor and to obtain backscatter images of the features on
the seafloor. The camera can likewise be
used to identify seafloor features and to mosaic complete maps of what is there.
There are two big advantages of using AUVs over conventional surface-based
survey methods. The first is that by
flying an AUV beneath the surface it will not be affected by the noise and
motion of waves and thus be able to “fly” in a steadier manner. The second is that it can be pre-programmed
for the survey and launched from either a boat or the shore. With a forward looking sonar onbaord for
collision avoidance the AUV does all the work while you sit back and have a cup
of coffee – at least that is what should happen in theory!
The AUV (named “Freya”) was brought to the site by a team
from the Scottish Association for Marine Science out of Dunstaffnage (SAMS) and
was part of a small grant awarded to us by the Marine Alliance for Science and
Technology Scotland (MASTS). For John
Howe, Karen Wilson and Colin Abernethy it was a first for not only visiting
Orkney but also for using the AUV on an archaeological project. Time was split between surveying the loch of
Harray, near to the Ring of Brodgar and Ness of Brodgar archaeological sites
and surveying in the Bay of Firth close to areas where we have previously
mapped and reported unusual features on the sea floor.
Context
The Rising Tides project has been investigating the area
around the Ring of Brodgar for a number of years. Most recently we have completed a full
bathymetry and sub-bottom sonar survey of the loch of Stenness to the
west. The purpose of this work was to
reconstruct the palaeo-landscapes and environments when the Ring of Brodgar and
Ness of Brodgar were being built and used. What our work demonstrates is that
the loch was smaller at that time and further that there was a complex history
of sealevel rise associated with the infilling of sediments with a step change
in sediment input coincident with changes in vegetation in the catchment at
about 6ka BP. For more details on this see our publication “The environmentalcontext of the Neolithic monuments on the Brodgar Isthmus, Mainland, Orkney”
Caroline and I have also been experimenting with digital
reconstructions for the area that can be seen on our test site "Ancient Lands"
AUV in the Loch of Harray
In order to match the work in the loch of Stenness we
brought the AUV to the loch of Harray.
This loch is a shallow, freshwater loch to the east of the Brodgar
isthmus. Previous surveying with the
sub-bottom profiler showed it to not only very shallow at the southern end (less
than 4m water depth), but also that the sediments were gas saturated for the
most part. Because of the very shallow
water the AUV was not able to “fly” beneath the water and had to be operated as
a surface vehicle. This, combined with
the fresh water challenged both the vehicle and the operations team to come up
with novel ways of ensuring the AUV did not either crash into the bottom or
into any of the small fishing vessels on site! Unfortunately, the weather also
proved problematic with strong winds.
The vehicle was able to survey a few lines of data but the automated
tracking features did not work correctly and so the vehicle did not manage to
stay on course in order to survey a complete grid of data.
AUV in Bay of Firth
Following the tests in Harray (also useful because it was a
confined loch and so there was no chance of the AUV being lost out to sea!) we
took Freya to the Bay of Firth. Here she
was much more at home back in saltwater and also with a greater depth range to
work in (up to 9m water depth). A survey
grid was programmed in the centre of the bay over areas where we have
previously recorded and dived on unusual stone features. Freya was deployed from a small rib and
perfectly performed her planned mission of bathymetry mapping, sidescan sonar imaging
and photography. The preliminary
results, processed within minutes of return to shore (another huge advantage
over old methods of survey) showed a resolution that we had not seen
before. Fantastic new images of the
seafloor which we will now ponder over before our return to dive on them in
August. Look out for results in a later post.
The AUV opens up a new era for us with archaeological
survey. There are so many sites that
could benefit from this type of survey and many new and exciting discoveries to
be made.
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| The AUV team with Richard and Caroline in Orkney - check out the renewable power sources! |
