Magnetometry
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.
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.
Archaeological Surveys use the latest gradiometer manufactured by Bartington Instruments to carry out magnetometry surveys.