Mapping the Past

How do archaeologists interpret geophysical survey data?
You may have seen the moment on Time Team, where John Gater lays the results of ‘geofizz’ on the bonnet of the Land Rover and delivers an interpretation of what it all might mean. This is not a ‘dark art’ but actually represents a systematic process of analysing the data in detail, comparing and contrasting other types of archaeological data, before presenting the results.

Explaining the geophysical survey results at Birrens Roman Fort

How do we present the data?
Mostly, archaeologists use greyscales to present the results of geophysical survey. This takes all the individual readings collected both in magnetic and resistance survey, and presents it on a grid. We often put this on a map to help us analyse what features ‘on the ground’ areas of interest in the data might relate to.

For resistance surveys a greyscale shows areas of high resistance as black – this may relate to buried stonework – such as walls, paths and roads. Areas of low resistance are shown as white – this may indicate cut features filled with wetter soil – such as ditches.

For magnetic surveys areas of positive magnetic response are depicted as black – this may relate to areas of disturbed soils. This may be caused by factors such as burning – so it’s a very good way to spot kilns, hearths and ovens. Cut features, such as pits and ditches which are backfilled with magnetically enhanced soil will also show up as black on the greyscale. Areas of negative magnetic response are shown as white.

How do we interpret the data?
Looking at greyscales often identifies areas of interest – anomalies of high or low resistance, or positive or negative magnetic enhancement. The next step involves thinking about what they might represent. Geophysical survey does not give us dates, and it is important to remember it shows us everything that has been detected – from modern pipelines to archaeological features.

The process of interpretation relies upon comparison with other archaeological data. This might be looking at historic maps (such as those found at http://maps.nls.uk) or exploring aerial photographs to look for archaeological features – including those which have been ploughed level and only survive as cropmarks. It is also important, if possible, to record what survives above ground, and an earthwork survey often goes hand-in-hand with geophysics.

Sometimes it is possible to excavate the feature to try and find out what it might be, including trying to obtain a date for it. This process can be very useful in helping us to interpret features in the future. Using the results of excavations over areas which have been identified originally in the geophysical survey, it has been possible to build up a database of what some geophysical ‘signatures’ might represent.

However, it is important to remember that the interpretation of geophysical survey results only gives us suggestions of what these anomalies might represent. Sometimes, it is only possible to confirm what you have found by putting a spade in the ground.

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