Geophysics for Archaeology: how low can you go?

This article looks at the depth below the ground surface that archaeological features can be detected using magnetometry and ground penetrating radar (GPR).

This image shows cart-based magnetometry equipment used for archaeological surveys. The vertical ‘poles’ are sensors which detect small variations in magnetism.

This image shows cart-based magnetometry equipment used for archaeological surveys. The vertical ‘poles’ are sensors which detect small variations in magnetism.

What equipment is used?

Magnetometer…

The main workhorse of geophysical surveys is the magnetometer. These can be cart-based (pictured above) which are tracked by GPS or handheld (pictured below) in which the surveyors walk at a constant pace within GPS established survey grids.

Hand-held magnetometer.

Hand-held magnetometer.

The vertical poles contain small sensors known as fluxgates (hence the name fluxgate magnetometers) which are typically set 1.0m apart; one at the top and the second at the bottom of the tube. They measure the effects that buried archaeology has upon the local magnetic field. The equipment has been designed to detect buried archaeological features typically down to a depth of 0.75m. This is because on most greenfield sites, buried features are normally located in the top 0.5m, except where alluvial (river flood deposits) might be present.

The sensors are very sensitive – a pit may produce a magnetic anomaly as small as 1-2 units compared to the earth’s magnetism of 48,000 units. Thus, archaeological features which have stronger magnetic content can be detected to greater depths. For example, pottery kilns can result in magnetic fields measuring a few 100 units in strength and along with tile and brick kilns can be detected at greater depths.  

Ferrous objects have the strongest magnetic fields because of their metallic contents. So, a buried oil drum or UXO (with a large ferrous component) can be detected at depths of say 1m to 2m. An old Ford Cortina car may even be detected beneath 5m to 10m... although why you would bury one at these depths would be questionable! Magnetometry can, however, be useful to detect crashed aeroplanes.

It should be noted that it is difficult to gauge the actual depth of a feature based on a magnetic survey alone; in order to get the best estimate of depth, GPR can be employed to investigate a magnetic anomaly, as shown in the example below.

1. This image shows a strong magnetic anomaly (in red).

1. This image shows a strong magnetic anomaly (in red).

2. Next, the same magnetometry data are presented as an XY trace (showing the full strength of the response). The red line shows the location of the radar transect depicted to the right.

2. Next, the same magnetometry data are presented as an XY trace (showing the full strength of the response). The red line shows the location of the radar transect depicted to the right.

3. This image shows a radar transect – a vertical slice through the ground giving an indication of depth..

3. This image shows a radar transect – a vertical slice through the ground giving an indication of depth..

4. The excavation of the feature and a camera being inserted through a small hole which opened at the surface.

4. The excavation of the feature and a camera being inserted through a small hole which opened at the surface.

5. This photograph shows the detected feature as an intact medieval tile kiln. The floor is at a depth of some 2m below the ground.

5. This photograph shows the detected feature as an intact medieval tile kiln. The floor is at a depth of some 2m below the ground.

Ground Penetrating Radar (GPR)…

The second workhorse of geophysical surveys is Ground Penetrating Radar (GPR) equipment. Again, this equipment can be either pushed across the site or towed using vehicles.

GPR equipment being towed.

GPR equipment being towed.

GPR equipment being manually pushed.

GPR equipment being manually pushed.

GPR works by pulsing electromagnetic waves into the ground. It identifies the depth and density of buried objects by measuring the strength and time delay of returning signals.

With various applications to be used, including greenfield and brownfield sites, there are no real 'hard and fast rules'. The choice of the best equipment is determined by the surveyor, dependant upon what the client hopes to find, as well as the size of the survey area.

Typically, SUMO uses High-Density Ground Penetrating Radar equipment to collect as much detail as possible. This is used on sites such as Scheduled Monuments and research projects.

When looking for very shallow features (such as rebar) or very deep features (such as Victorian drainage pipes) we use a single channel GPR system with an appropriate antenna. Mid-range features would see us use a High Density or Dual Frequency antenna. We research as much as we can about the site, its history and what we are looking for to aid with this decision.

This is an example of 3D data collected by Ground Penetrating Radar (GPR) equipment. It was an ideal technique for this site in which the aim was to trace the outline of a Roman Villa.

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