Magnetic susceptibility survey at Castle Combe

 

Magnetic Susceptibility

Magnetic susceptibility is an effective technique often used as a form of reconnaissance survey ahead of more detailed magnetometry. We can also take samples and lab based measurements to provide detailed magnetic characteristics for the fill of former ditches, pits and other naturally produced features. The samples may help determine industrial activity or natural variations in magnetic susceptibility.

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.

 

Magnetic susceptibility survey results

 

 

Map of Magnetic Susceptibility survey (red = relatively high levels of magnetic enhancement)

 

Magnetometry results

 

 

Targeted detailed magnetic survey over 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.

 

 

Magnetic susceptibility surveys are generally carried out at 10 or 20m intervals during a reconnaissance survey. 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 a Differential Global Positioning System (dGPS) and GIS software to allow for accurate positioning and display.

Mass specific magnetic susceptibility measurements are recorded using the Bartington MS2B sensor. These measurements are carried out in controlled conditions using soil samples and can be used to support reconnaissance survey results.

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 in a soil's iron content can be accounted for.