Archaeological excavation is by its very nature a destructive activity. In order to properly understand and record the archaeology it may necessary to actually remove any physical traces of the archaeology leaving behind only written records, drawn plans and photographs. Commercial archaeology is in a lot of cases an attempt to rescue or record archaeology before it’s destroyed by future building work. Part of the problem is that while historic records can often give an idea of what might be found within a development area, the actual nature of potential archaeological remains actually require some intrusive investigation…… or do they?

This is where the science (or some might argue the art) of geophysical surveying can allow a non-intrusive view into the past. If the location of archaeological remains can be identified without sticking random holes in the ground it can allow a much more targeted, and potentially less destructive, approach to be taken. This can also save a lot of time and money within the construction process. A geophysical survey can also allow the bigger picture of a site to be revealed.

There are a number of different geophysical techniques used within the archaeological world, unfortunately none are perfect for all conditions and locations. There are a few main techniques that are widely used

• Resistivity. Resistivity involves an electrical current being fed into the ground and the resistance to this current being recorded. The usual approach being a two pronged machine placed into the ground at regular intervals across the required area with readings taken at each location. High resistance readings may suggest walls or rubble fills, whilst low resistance readings can indicate ditches or drains.
• GPR (Ground Penetrating Radar). GPR uses radio pulses transmitted vertically downwards and the reflection of these pulses from buried layers and structures to build up a picture of ground below. GPR has the ability to provide a three-dimensional view of a buried site.
• Magnetometry. Magnetometry relies on the ability of the magnetometer to measure very small magnetic fields associated with archaeological remains. These magnetic fields are either the result of thermoremanence or magnetic susceptibility. Thermorenanence occurs when weakly magnetic material is heated up and then cools. The material can then gain a permanent magnetisation associated with the direction of the earth’s magnetic field while it cools. The magnetic susceptibility of a material is related to the magnetism induced when the material is placed within a magnetic field. Since the earth’s magnetic field is always present, the magnetic susceptibility of buried material can be measured using the magnetometer.

There are other geophysical techniques used in the search for buried archaeology, seismic, microgravity, induced polarisation and metal detecting to name a few. However the three main techniques are resistivity, GPR and magnetometry.

Within commercial archaeology the most widely used method is magnetometry. This is mainly due to the speed at which large areas can be covered and the impressive results that can be obtained. As with all methods however the site conditions will dictate how suitable the technique is. Within built-up urban locations magnetometry will be next to useless due to the interference of external magnetic fields from buildings, cars, modern services or modern rubbish. All these and more can mask the small magnetic fields generated by buried archaeology. In this situation GPR may well be a far better choice.
I’ve now been working for Allen Archaeology for 5 and half years and whilst these days I am allowed out to occasionally dig, my main duties are as a Geophysical Project Officer. This involves both the physical part- actual surveying, and the office based part- processing data and writing reports. I look upon the latter as a necessary evil which allows me to spend time on the much more enjoyable (mostly) former. Fortunately the majority of the sites we survey are not completely waterlogged, and despite the reputation the British weather has for rain, I do seem to manage to stay fairly dry. Except for my feet when I don’t realise my (non-metallic) boots have holes in them.

Geophysical surveying large sites can be very hard work. There is a lot of walking involved, both in setting out grids and in actually surveying. My legs and feet have suffered somewhat over the last few years; blisters are a not uncommon occurrence. However as an aid to fitness nothing beats walking 20-25km a day across fields. The exciting part is of course when I get to see the downloaded data for the first time. On many occasions there can be a little disappointment as all that is revealed is a former field boundary or in some cases nothing of interest at all. However every so often something far more exciting is revealed. I get to be the first person to see Romano-British field systems, forgotten medieval settlements, ploughed out ridge and furrow cultivation, or even outstanding modern drainage systems.

The practical side of the geophysical survey is that it can be done fairly swiftly, covering 2-3 hectares a day (all depending on site conditions of course). Then specific areas can be targeted for excavation if necessary. This can potentially allow a ‘key-hole surgery’ approach to the archaeological remains, limiting the destruction whilst maximising the information gathered through digging. Plus it can be very satisfying when an excavation reveals my geophysics results to be completely accurate.