The presence of buried archaeological
features can often be detected by magnetometer survey. Small local anomalies in
the earth's magnetic field result from the contrasting levels of magnetic
susceptibility which exist between infilled 'cut' features/structures and the
local substrate or bedrock. This effect is principally due to the varying iron
content in the soil and rock forming minerals. Certain features, notably fired
clay structures such as kilns, hearths and furnaces acquire their own magnetic
identity and display thermoremanent
magnetism which is induced by the geomagnetic field in the
relatively iron rich clays on cooling from high firing temperatures.
The majority of archaeological
magnetometer surveys are undertaken with fluxgate gradiometers which measure
the magnetic gradient between two sensors arranged on a vertical separation,
normally 0.5 or 1m apart, with the lower sensor being carried approx 25cm above
the ground.
Magnetometers are also used in a broader
prospecting role to initially locate areas of 'magnetic activity' and also to
help determine the extent of anomalies associated with previously identified
features. The presence of more subtle anomalies however can only revealed by
detailed coverage. In prospecting sites which extend over many hectares or
where only selective detailed magnetometer survey may be required target areas
can be often be chosen by reference to a topsoil magnetic
susceptibility map combined with the results of other archaeological
assessments.
Examples
The following images show the
results of gradiometer surveys. The unit of measurement of the vertical
magnetic gradient is the nanotesla (nT) = 10-9 tesla (T) . Most have
been carried out with the instrument set to a sensitivity of 0.1 nT. Given a
reasonable response, resolution of features of less than 1m in width can be
expected. The 1m x 0.25m configuration is the one we most widely use for area evaluations.
Under optimal conditions, and using configurations of two or more gradiometers,
coverage of several hectares per day can be achieved. A traverse separation of
0.5m may be deployed to locate smaller features or to provide additional
clarity especially where survey is confined within narrow corridors (e.g..
proposed pipelines). The obvious merits of close traverse
survey (0.5m or less) need to be carefully weighed against consideration of
time and cost. For the purposes of assessment knowing the location, geometry
and extent of buried archaeological features usually provides sufficient
information and more extensive coverage may be considered more important than
high resolution.
The plots in this section have
been prepared as inverted grey shades, with positive values represented as the
dark and negative as the light element. This representation is preferred by
many archaeologists as it gives the impression of infilled intrusive features
not dissimilar to patterns observed during excavation. The gradiometer data can
be imaged in various ways including grey scale, stacked trace, dot density,
contour, 'wire frame' and 'false relief' plots; each presentation allowing the
data to be visualized with emphasis on a particular aspect. For location
purposes a plot which displays an accurate plan of the buried features is
important. A further level of information which give a representation of the
dynamic range of the recorded signal is also required, grey scale images, as
shown here, offer a compromise in displaying both plan and relatively subtle
variations in anomaly strength on the same plot. Stacked ( X-Y)
trace plots are used to both display the raw data and aid interpretation;
other types of presentation are prepared where useful, with colour plots where
appropriate.
Below: A Neolithic and
later hill top enclosure, Raddon Hill,
The example above shows a detailed gradiometer survey of a 5 hectare hilltop. The survey objective was to provide a context for a series of prehistoric features recorded by excavation during the construction of an access road to a reservoir site (the new road runs through the centre of the area from left to right). Gradiometer survey proved the features to form part of a complex series of enclosure ditches and associated pits. Selective excavation demonstrated that the site originated as a Neolithic causewayed enclosure.
Below: A Roman
military installation at Ide, overlooking
Survey area: 150 x
150 m. Geology: Devonian sandstone.
.
The location and plan of former stone
structures can frequently be retrieved by magnetometer survey (two examples are
shown above). In many cases the stonework will have been thoroughly robbed for
subsequent use leaving only a rubble- or earth-filled trench where once the
wall footings stood. If this trench contains material of sufficient magnetic
contrast to produce a local magnetic anomaly the original ground plan of the
building will be apparent on a detailed gradiometer plot. Normally the anomaly
generated by the 'robbing trench' will be positive where topsoil or other
material of relatively higher magnetic susceptibility infills the trench.
Occasionally infilling (either deliberate, or from alluvial silts) with
material of relatively low magnetic susceptibility will cause the trench to
produce a negative magnetic anomaly. In the second example above, although the
walls of the buildings are no longer extant their removal has not continued
down to the base of the footings, and sufficient stone (in this case limestone)
of relatively low magnetic susceptibility survives to generate a pattern of
local negative anomalies. The thermoremanent properties of fired clay also make
brick walls and buildings a suitable target for magnetic survey. Structural
elements built of of fired clay products are also responsive, notably the
hypocausts of Roman buildings, together with fired clay flooring tiles and
concentrations of fallen clay roof tiles. Specific internal features such as
hearths can often be located, and where fire may have destroyed timber
buildings which had clay plastered hurdlework walls or panels (wattle and daub
construction) a good local magnetic response may be anticipated. It is also
possible for certain stone to acquire thermoremanence on burning. In many areas
much early building utilized soft sedimentary rocks, which tend to be of
relatively low magnetic susceptibility. However, in igneous zones more magnetic
rock may be used, some stone, notably basalt, is particularly thermoremanent.
Below.
Enclosures, trackways, pits, quarry pits and structural features. Left:
Romano-British village complex near
Composite views of extensive
archaeological features can often be retrieved cost effectively by magnetometer
survey. Both of the sites above were partially known from aerial photography
and fieldwalking. In these examples the results of one day's survey (left) and
two days' survey (right) have defined enclosures, trackways, pits and suggested
the location of several structures.
Below: Iron
Age hut circles and ditch underlying a well preserved medieval ridge and furrow
landscape, subsequently sealed by wind blown sand,
Medieval documentary sources record a
major inundation of windblown sand at this coastal site. There are no surviving
surface indications today of the buried ridge and furrow cultivation
(represented graphically on the gradiometer plot by the broad light and dark
banding) which excavation has shown to be in an excellent state of
preservation. The presence of prehistoric roundhouses was also confirmed.
Situated close to a group of nationally important Iron Age burials, which were
excavated in the early years of the twentieth century, this survey has provided
the first evidence for contemporary prehistoric settlement.
Below: A gradiometer plot showing an enigmatic riverside complex of
Romano-British date overlain by medieval ridge and furrow,
In some situations natural
formations such as palaeochannels of river systems can be seen on magnetometer
plots. The example above clearly shows the edge of a former channel (dotted
line) against which an enclosure, demonstrated by excavation to be of
Romano-British date, has been appended. Biological activity can also alter the
magnetic properties of the deposits within man-made features: the fills of
certain fenland drains, for example, can become highly visible to magnetometer
survey due to processes associated with organic decomposition.
