Some consider the technology primitive by modern standards;
it consists of a camera, attached to the end of a cable, that
displays a 50-foot section of the bottom at a time and requires
someone to watch it 24/7 for weeks on end. (Patience, anyone?)
To put it in perspective, if you wanted to search a 1-mile by
1-
mile box traveling at about 1 mile per hour (typical survey
speed) with full optical coverage, it would take about 60 hours,
and that doesn’t even account for turning the ship around.
When searching from a large ship, the lanes are typically much
longer than a mile since it takes so long to turn the ship around,
especially when there are several miles of cable behind it. When
searching for the Titanic, the researchers were not looking for
full optical coverage though; they were looking for a debris trail,
so they left nearly 1 mile in between each of their 5- to 8-mile-
long lanes. Searching in the deep ocean has never been easy, but
technology has made some aspects of it very different. Other
aspects remain tediously familiar.
Deep shipwreck hunts are expensive. A ship can cost
$15,000 to $60,000 a day, depending on the capabilities desired,
and it’s just the platform from which the search is launched.
Finding ways to maximize time on the bottom is imperative;
fortunately many technologies facilitate just that. Like Argo,
side-scan sonar is towed behind a ship at a designated altitude
off the bottom; it emits sound waves to create an acoustic
image of what’s around it. Since sound waves travel so
efficiently underwater, side-scan can provide a picture of the
seafloor several hundred meters to either side of the unit, a
much wider swath than the camera sleds of years past.
Side-scan is reasonably priced (models range from $12,000
to $100,000), and it has become the principal tool used by
most wreck hunters around the world. Side-scan’s versatility is
hard to beat: The units are small, they’re useful for both deep
and shallow exploration, and they offer relatively wide bottom
coverage. Multibeam sonar is a variation of side-scan that
emits multiple sound waves and creates a three-dimensional
image of the bottom. It is often hull mounted but is sometimes
towed, and it is significantly more expensive than side-scan.
The next generation of side-scan is called synthetic
aperture sonar (SAS), and it makes traditional side-scan
look like an old standard-definition TV relative to a new
HD model. While side-scan emits one sound pulse followed
by another, SAS emits many sound pulses and then uses
sophisticated processing techniques to interpret and display
them as an image. At first glance, the image from SAS looks
almost identical to side-scan, but then you realize the image
from a single lane is nearly two miles wide. Not only is it so
wide, but even at the very edges of the lane the resolution
is capable of showing the propeller of a small airplane and
allowing the viewer to count the blades. Although the swaths
are not quite as wide in shallower water, the resolution is still
substantially higher than in traditional side-scan.
To cover a wide swath of seafloor with side-scan, you would
start by using low frequency to scan a large area. If you found a
target you would make another pass with a narrower swath but
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Traditionally,
hunting
for
shipwrecks starts
in the library.
Brett Seymour
Brett Seymour