112 GEOPHYSICAL AND REMOTE-SENSINGSURVEYS
anchors, chains, cables and ordnance with large magnetic
signatures can effectively mask archaeological anomalies.
A more widespread but localized difficulty is background
magnetism from geological formations, particularly
where these are in tight folds that reach close to the sur-
face a number of times in a survey area. Various techniques
are available to reduce this problem, including multiple
magnetometers set in a fixed array, which allows differ-
entiation between very large masses such as ferruginous
rocks (containing iron) or nearby steel wrecks and
smaller signatures of potential archaeological significance.
Integrated Surveys
Integrated systems are often used in mapping the under-
water cultural resource. Integrated survey systems may com-
prise a combination of two or more of the following:
bathymetric survey equipment, sidescan sonars, sub-
bottom profilers and magnetometers. This integrated
approach can lead to the acquisition of a large but effec-
tive data set. Generally, data is acquired in digital format
to facilitate offline processing and spatial integration
(figure 13.5 and plate 13.12).Submersibles: ROVs and AUVs
Remotely operated vehicles (ROVs) and autonomous
underwater vehicles (AUVs) are examples of integrated
systems. They can perform many of the tasks of a diver,
including visual searches and photography, but are not lim-
ited in the same way by depth and time spent under pres-
sure. These systems can host a suite of acoustic and video
imaging data-collection devices with some underwater
video systems incorporating image-intensifiers capable
of showing greater detail on a surface monitor than the
diver can see while under water.
ROVs are connected to a support ship via a cable and
controlled from within the ship by an ROV pilot.
Submersible technology is a field of rapid development
but at the time of writing, deep-water ROVs can operate
in depths of up to 6000 m (19,500 ft) while cheaper,
shallow-water systems can operate in depths of around2134Figure 13.5 Geophysics equipment: 1) magnetometer, 2) sidescan sonar, 3) sub-bottom profiler, 4) ROV. (Based on an
original drawing by Mark Redknap)