Detailed magnetic survey known as
magnetometry is an effective and efficient geophysical survey technique
used to define areas of past human activity by mapping spatial
variations and contrast in the magnetic properties of soil, subsoil and
bedrock. The technique can be carried out over areas of grass, crop and
open soil. Archaeological Surveys use a multi-sensor cart based system
combined with RTK GNSS for precise positioning.
The image to the right represents magnetometer data collected at
Truckle Hill Roman Villa in Wiltshire. The black lines represent
magnetically enhanced soil and imply the presence of cut ditch-like
features whilst the narrow white lines indicate material of low
magnetic susceptibility; in this case the oolitic limestone footings of
the villa complex.
Recent excavation work confirmed the large ditch surrounding the
villa, probably dating to the 3rd century. The ditch in the centre of
the image appears as a possible 'cross-dyke' and is prehistoric
although it is mirrored to the north by parallel Roman ditches. Late
Iron Age ditches are visible at the bottom of the image.
Weakly magnetic iron minerals are always present in soil and magnetic enhancement is associated with localised zones of higher magnetic susceptibility and permanently magnetised thermoremnant material.
Magnetic susceptibility relates to the induced magnetism of a material when in the presence of a magnetic field. Within soils this can effectively be considered as a permanent magnetism due to the presence of the Earth's magnetic field. Thermoremnant magnetism is permanent and acquired by iron minerals that, after heating to a specific temperature known as the Curie Point, are effectively demagnetised followed by re-magnetisation by the Earth's magnetic field on cooling.
Silting and deliberate infilling of ditches and pits with magnetically enhanced soil creates a relative contrast against the much lower levels of magnetism within the subsoil into which the feature is cut. Systematic mapping of magnetic anomalies will produce linear and discrete areas of enhancement allowing assessment and characterisation of subsurface features. Material such as subsoil and non-magnetic bedrock used to create former earthworks and walls may be mapped as areas of lower enhancement compared to surrounding soils.
By plotting data collected in the field as a greyscale, a plan of
magnetic anomalies can be created. Archaeological features are then
characterised by their pattern and form.