No 3: Geophysical Survey (Magnetometry)

With its important role in modern archaeology, this is the second in a series of brief tutorials into the mechanisms involved in geophysical survey.

Although coming from a radio engineering and electronics background, I am not going to bamboozle you with the boring science,  or big up the process as some sort of black art.

The most popular method of geophysical survey is the magnetometer survey, this is principally because it is  three times quicker to carry out than the next most popular resistivity.

A magnetometer is simply an instrument that can see magnetic fields, there are several different types, but for our introduction we will  just call them collectively magnetometers.

For a magnetometer to work we need a magnetic field. The earth is effectively  a giant lump of iron, with a magnetic field that runs through the planet and surrounds it.   We may think that apart from the  classic school experiment  of putting a magnet under a piece of paper  and sprinkling iron filings to see the magnetic field, our connection with magnetism is confined to those little magnets that keep the kitchen cupboard shut, or fridge magnets.  In reality the earths magnetic field is instrumental to  every part of our lives. Instead of giving them their collective name of the electro magnetic spectrum , i’ll  use terms such as electricity, light, radio, tv, mobile phones, x rays, microwaves, infra red, UV to name but a few. When the sun throws off solar flares, this interferes with the earths magnetic field and can shut down our power and communications networks, and return us temporarily to the stone age.

So magnetism is important, and it is an important tool for archaeology as well.  A magnetometer for use in archaeology  in its simpliest form is a vertical tube with two magnetic sensors.  The sensors are affected by magnetic materials in close proximity, hence the first rule of magnetometer surveying is to make sure you don’t have any metal that is magnetic on/in  your clothing before you start. If you have a replacement hip or shrapnel in your body, then amateur resistivity is probably a better choice. .  One sensor is mounted close to the bottom  of the tube to read the magnetic field that surrounds it, including the ground below it, the other sensor is mounted higher up the tube away from ground magnetic anomalies to record the background  magnetism and act as the reference.   During a survey both sensors take readings and the bottom sensor reading is referenced against the higher sensor as a + or – value.

Any given piece of land will have a background magnetic signature based on the geology, an ironstone outcrop underlying the site, will have a greater impact on a survey and sometimes make it impossible to get useful results, compared to  non magnetic  limestone for example.

What exactly is the magnetometer seeing ?

The minute somebody sticks a spade in the ground they are affecting the background magnetic field.  If you dig a ditch in clay, chalk or any other material,  then the ditch left to fill naturally will have a different fill to the natural. This fill could include the burnt residue from burning off stubble, it could include fire ash and charcoal from domestic or industrial processes, it could include the scale that flies off iron as it is being worked by a blacksmith, in fact anything that is burnt or fired ( pottery) will have a different magnetic signature than the natural. Unfortunately it can also include the shoes of  giant shire horses, lumps of tractor and other  detritus of modern agriculture. Ditches are the bread and butter of magnetometer surveys, if one is present it will likely to be picked up.

Of course it is not always that easy,  surveying the land of a former scrapyard over a henge monument is likely to be more useful in locating a buried  Austin Maxi than part of a  ritual  landscape.

The rest of the process is basic fieldwork. Layout a grid of a preferred size, subdivide each grid square into 1m strips, and traverse the strips taking a reading every meter along each strip, which in a 30m x 30 m grid square would result in 900 readings taken in the square.

Those 900 readings are entered into a piece of software which in the most simpliest of terms gives several colour or greyscale values for ranges from the  lowest readings through the background readings to the highest readings. These individual colours or greyscale values, when the 900 readings are laid out in the order they were taken will highlight anomalies. For instance if a red colour is given to the highest magnetic field  readings, we might get a red line if the feature is a ditch full of fire ash, If a yellow colour is given to the lowest magnetic readings, if   we might expect to get a yellow line.

Of course these things are subject to all sorts of caveats, geophysical survey can highlight archaeology in a given geology to an astonishing degree, in a different geology  with the same archaeology it can be as useful as a chocolate poker.

The apparent absence of archaeology in a geophysical  survey,  does not necessarily mean there is an absence of archaeology on the site. Geophysical survey should be used as part of a fieldwork investigation, it should  not be the only fieldwork investigation, and where it is at all practicable, it should be used in conjunction with other geophysical techniques.

in a nutshell you should if possible carry out both resistivity and magnetometry. Resistivity is good for identifying walls and structures, Magnetometry is good for identifying ditches and pits, bring both results together, superimpose them  on the aerial photographic evidence, and you should have a fair representation of what is below the ground.

Note ‘should have’, no guarantees in this game.

Here endeth the second  lesson.