Magnetic Susceptibility

Archaeological Surveys currently measure the magnetic susceptibility of soil samples from most areas where we carry out magnetometry in order to improve our understanding of the magnetometer anomalies across a site.

On occasion we also sample the fill of former ditches, pits and other naturally produced features to assist archaeological interpretation. The samples may help determine industrial activity or natural variations in magnetic susceptibility. Magnetic susceptibility may also be used as a form of reconnaissance survey ahead of more detailed magnetometry. Our samples are measured in-house using a Bartington MS2 meter with MS2B sensor or with the MS2D field coil on site.

Iron minerals within the soil can be altered through biological decay and burning which can enhance the magnetic susceptibility of the soil. Field equipment can be used to measure the magnetic susceptibility of the soil allowing zones to be mapped which may indicate areas of potential archaeological activity. This also allows subsequent targeting of higher resolution survey techniques such as magnetometry or resistivity in order to obtain more detail.

Plot of magnetic susceptibility data (red = relatively high levels of magnetic enhancement)

Targeted detailed magnetometry over an area of relatively high enhancement. The linear magnetic anomalies are responses to the magnetically enhanced fill of archaeological cut features. The strongest magnetic linear anomalies (1-9nT) directly correspond to the highest zones of magnetic enhancement seen in the magnetic susceptibility survey plot.

Magnetic susceptibility reconnaissance surveys are generally carried out at 10 or 20m intervals. This is a cost effective and efficient methodology that allows a rapid assessment of large areas whilst maintaining meaningful data collection. It can also be carried out across small areas at a closer spacing (e.g. 1 - 5m) in order to pinpoint areas of archaeological activity in greater resolution.

The magnetic susceptibility surveys are conducted using an MS2 meter with MS2D field coil manufactured by Bartington Instruments Ltd. The instrument can be used in conjunction with RTK GNSS and GIS software to allow for accurate positioning and display.

Mass specific magnetic susceptibility measurements of soil samples are recorded using the Bartington MS2B sensor. Both high frequency and low frequency measurements are recorded as frequency dependence can be an additional indicator of areas of former habitation due to the presence of very fine magnetic particles related to burning.

Archaeological Surveys also carry out fractional determination of soil magnetic susceptibility. Soil samples are heated to 650°C in controlled conditions in order to determine their maximum magnetic susceptibility, the results are expressed as a percentage of the maximum value achieved by the soil before heating. This percentage value may be a more accurate indicator of anthropogenic activity as natural spatial variation of a soil's iron content can be accounted for. The technique can be useful where soils vary abruptly due to natural processes.