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ABINGDON ARCHAEOLOGICAL GEOPHYSICS

 

 

 

 

OXFORDSHIRE

Here we have the same area with a magnetic susceptibility survey on the left with a (slightly altered) magnetometry survey on the right. The lowest susceptibility readings are in the bottom right hand corner with the deep blue in the top part of the area being the higher readings.

magnetic susceptibility survey compared with magnetometry

The magnetometry is in 30 metre square grids with lines 1m apart and 4 readings per metre. The susceptibility has readings every 12.5 metres.

Whilst the magnetometry shows some anomalies, which are presumably ploughed out barrows and linear features, the susceptibility also seems to identify the area of the barrows.

The problem is that barrows, being burial features, should not have much burning and the susceptibility sample interval was not close enough to be much influenced by the filled ditches. Theoretically therefore susceptibility should not have identified the barrow area.

The susceptibility could be locating areas where more household waste was put on the fields as part of the manuring process and the soil in the area of lower readings could be partially alluvial with different particle size and other characteristics than the rest of the field.

What could have happened could be that the susceptibility readings obtained with a Bartington MS2 were influenced by the dampness of the soil. The south-east corner was low lying and the barrows were located on the higher parts of the fields. C. Benech and E. Marmet refer to this possibility in their article "Optimum Depth of Investigation and Conductivity Response Rejection of Different Electromagnetic Devices Measuring Apparent Magnetic Susceptibility." Archaeological Prospection 6,31-45 (1999). This appears however to be contradicted by Maier, Scholger and Schon 'The influence of soil moisture on magnetic susceptibility measurements' Journal of Applied Geophysics, June 2006 as they consider the moisture influence on field measurements to be minimal.

Further trials are proposed during dry soil conditions as the first susceptibility survey was carried out when the soil was damp in November .