Geophysics

Geophysics is used in archaeology as a non-invasive technique to help locate features not visible on the grounds surface and find sites of previous human activity.  This can identify targets of interest that can be excavated to find out what they are. 

As part of the twenty pence project a group called RheeSearch have carried out a geophysical survey.  The techniques they used were magnetometry and resistivity.  These techniques are summarised below:

Magnetometry

The technique called Magnetometry is used to measure changes in magnetism in the soil.  It can be used to detect changes in magnetism from buried features and be used to give an indication or for detection of archaeology beneath the surface of the soil.

Soil can be affected by the activities of man in the past.  One of the things that changes the magnetic properties of items is from burning.  If the soil or material from an area of burning is moved around by later activity, for example ploughing or infilling of ditches, this activity can be traced by the variations in the soil magnetism against the background reading of the earth’s magnetic field.

The magnetic properties of soil can also be altered by the action of microorganisms and in particular by decomposition.  So for example a ditch that has been open for a period and later filled will show up because of the differences caused by microorganisms.  This microbial action only rarely allows the detection of graves because they involve putting the same soil back into the hole very quickly after it was dug out.  This is too short for any action by microorganisms to change the magnetivity.  However cremations will be detected due to the burning.

Resistivity

Resistivity is performed by passing an electrical current through the ground at regular intervals.  Any changes in resistance with regards to the surrounding are detected and recorded.  For example a filled in ditch may have a different moisture content which will give a different resistance to undisturbed soil.  Organic materials will also change the resistivity in the soil.  Stone foundations may impede the flow of the current.

Twenty Pence Geophysics

Below are the first geophysics images from the Twenty Pence road project. Aerial photos show no visible features on the ground, unlike Bullocks Haste next door, but one can clearly see some archaeology present - even if at this stage we do not know the date of it!

A big thank you to Rheesearch for carrying out the geophysics and getting results back to us on the same day.

Click on any of the images to view a larger picture.