Geophysics - Finding Objects in the Ground

In archaeology, geophysics is a non-destructive way to see beneath the soil. Geophysics is the study of the ground and it uses the Earth’s own magnetism and electricity to show what is below the ground without having to dig a hole. This is sometimes called geophysical survey or remote sensing.

There are several different methods of remote sensing; the most commonly used ones are resistivity, magnetometry and ground penetrating radar, although sometimes archaeologists will use other techniques too.

Resistivity

Resistivity is the cheapest and easiest type of geophysical survey to use. A resistivity meter is a box on a frame with wires leading down to a pair of metal spikes that are stuck into the ground. This is the type of survey that you often see on programmes like Time Team.

A resistivity survey

It works by passing an electric current through the ground using the spikes. The box measures the resistance of the ground to the electrical current passing through it (this is where this method gets its name from).

In other words, the box measures how hard it is for the electricity to get through the soil. The amount of resistance is affected by how much water there is in the soil. It works on the idea that electricity passes through wet ground more easily than it does through dry ground. So, if ground is wet, resistance to electricity will be low, but if the ground is dry, the resistance will be high.

Now, archaeological features (which are made by people) like walls and ditches under the ground will affect how wet the ground is. For example, a ditch or pit will collect ground water and the soil will stay wetter for longer than the soil around it. That, in turn, will create a patch of low resistance.

The opposite is also true; if you have a wall under the ground, then there is less soil there to store moisture. That patch of soil will be dryer and the dryness will cause high resistance. Measurements of the ground’s resistance to the electricity are taken at regular intervals on a grid. These readings can then be turned in to a picture using a computer. These pictures show where archaeological features are likely to be.

Resistivity survey

If you look carefully you can see a circle on the pictures. This circle could be what is left of the wall of a building hidden under the ground.

Magnetometry

The second most common type of geophysical survey is magnetometry. Magnetometers measure the magnetic field of the earth and of archaeological features. Archaeological features can sometimes have their own magnetic field. For example, iron objects do and areas where there has been burning do too. Ditches also show up through this type of survey. This is because they fill up with soil and some of that soil will be made up of magnetic bits.

A magnetometer survey is better at picking up ditches than a resistivity meter, but not as good at picking up walls.

A magnetometer in action A magnetometer is a much larger version of a metal detector. The machine does not have to go into the ground or even touch it. It simply measures the magnetic field.

Measurements of the ground’s magnetic field are taken at regular intervals on a grid. These readings can then be turned in to a picture by using a computer. These pictures show where archaeological features like ditches are likely to be.

If you look closely, you can see the outline of a Roman Villa at Shapwick, Somerset.

Shapwick Roman villa uncovered!

Ground Penetrating Radar

The third, and by far the most expensive type of geophysical survey, is Ground Penetrating Radar (GPR).

GPR in action

GPR works a bit like resistivity or magnetometry. This machine has an antenna that sends out radio waves into the ground. These waves then bounce off objects that are in the soil and back to the antenna of the GPR.

Bigger features and objects create stronger bounces. The longer the time it takes for the bounce back to the antenna, the deeper under the ground the object.

A really strong bounce happens when the electromagnetic pulse hits something really hard and thick, like a wall or rock. The readings from the bouncing pulses can then be turned in to a picture by using a computer. GPR can build up a picture of an archaeological feature at different depths under the ground, helping to build up a 3-D picture of the feature. Resistivity and magnetometry show us a flat, 2D picture.

GPR survey results

© The British Museum 2012 | Credits