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 Earth’s properties 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 looks like 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. It works by passing an electric current through the ground. The box measures the resistance of the ground to the electrical current passing through it. In other words, how hard it is for the electricity to get through the soil. This is where this method gets it name from. The amount of resistance is affected by how much water there is in the soil. It works on the idea that electric current passes through wet ground more easily than through dry ground. So, if ground is wet, resistance to electricity will be low, but if the ground is dry, the resistance to the electrical current will be high.
Now, archaeology 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 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 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 electrical current 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 walls or ditches are likely to be.
Magnetometry
The second most common type of geophysical survey is magnetometry. Magnetometers measure the magnetic field of the earth and of archaeological features, which sometimes have a magnetic field of their own.
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 at least some of that soil will be made up of magnetic particles.
A magnetometer survey is better at picking up ditches than a resistivity meter, but not as good at picking up walls.
A magnetometer looks like a larger version of a metal detector and like a metal detector, but unlike a resistance meter, the machine does not have to touch or go in to the ground.
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| A colour representation of the Shapwick Villa site, rendered using a magnetometer survey. | A magnetometer in use. |
Ground Penetrating Radar
The third, and by far the most expensive type of archaeological geophysics, is Ground Penetrating Radar, also called GPR. This instrument has an antenna which sends out electromagnetic pulses into the ground. These pulses then bounce off objects that are in the soil and back to the antenna of the GPR.
The stronger the bounce, the bigger the object or feature. 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. This makes the GPR really good for showing up stratigraphy, the layers beneath the soil. GPR gives a 3D picture of what is happening under the ground. Resistivity and magnetometry show us a flat, 2D picture.
An example of an interpreted geophysical survey - Shapwick Roman Villa
