In May we told you about what geophysics is. Apart from helping to keep me fit with sometimes ridiculous amounts of walking across fields, why else do we use geophysics as part of the archaeological investigations into a site?

Commercial archaeology is often about revealing and recording archaeological remains before new development takes place. Whilst sometimes the nature of the development can be altered or adjusted to allow preservation of the buried features, in many cases we are recording the archaeology before its destruction.

In order to reveal and record archaeological remains we first have to find them. No matter how much we might wish it, not every field has a buried Roman villa, or a medieval settlement just waiting for a keen archaeologist to come and dig it up. In many cases the most exciting hidden treasure within a field is a former field boundary and a few modern field drains.

This is where a geophysical survey can help provide the information necessary to evaluating the archaeological potential of an area and allowing the development of a suitable plan to gain as much information as possible before building work commences. The survey can allow specific pinpointing of trial trenches over the most interesting features. This is much like keyhole surgery, hopefully producing maximum results for minimum effort and cost.

So how effective can this keyhole approach to archaeological remains be? If we take as an example a site in northeast Lincolnshire near Grimsby, we can see how useful this approach can be. The entire site in question was approximately 19 hectares in size, of which 11 hectares was suitable for the geophysical survey. The remaining area was either too overgrown or flooded making it impossible to survey. Fortunately the 11 hectare area covered the entire area proposed for construction.

An assessment of the archaeological and historical background of the area had revealed possible prehistoric to Roman cropmarks to the northwest and west of the site, with medieval activity involving earthworks and find scatters located immediately to the southeast of the site. The geophysical survey revealed a lot lot more.

Suddenly the archaeological potential of the site has increased markedly. Not only can we clearly see the modern system of land drains cutting across the site, but we can also see potential settlement and agricultural enclosures, as well as ridge and furrow cultivation practices. A large dipolar spike, the white circle with a black centre to the right of the centre of the site was also tentatively identified as a potential kiln.

So what happens next? Before geophysics was available, a number of trial trenches would likely have been placed within the field to attempt to reveal any archaeological remains. As you can see from the results however, there are large areas of the field where the geophysics has not revealed any archaeological features. Random trial trenches would be just as likely to miss the interesting areas as to hit them, and an incomplete picture of the archaeology would be more than likely obtained.

Now that a geophysical survey has been done, the trial trenches can be located to test the results. This is useful for two reasons, the first is that we can target archaeological features to gain a better understanding of what’s going on, and hopefully retrieve sufficient finds to allow us to comfortably and confidently date the features. The second reason is more for the benefit of the geophysicist. The results from the trial trenching can be used to validate the geophysical results. I was fortunate to be involved with the trial trenching and it was very satisfying to see how well the archaeological features within the trenches matched up with the geophysical results.

The yellow features within the trenches relate to archaeological features, whilst the black features represent furrows. Finds recovered from the features allowed us to date them from late Iron Age to the Roman period, with the furrows indicative of medieval farming practice. The large dipolar spike was revealed as hearth or kiln, although its precise date or function unfortunately remains unknown.

The results from the trial trenching correlated very well with the geophysical survey, and this allowed us to gain a much greater understanding into the archaeological potential within the site.

So why bother with geophysics? Hopefully I’ve managed to give you a window into just how useful it can be. I certainly think it’s worth all the walking I do, and although I don’t find something exciting every time I go out, I do feel I’m helping us to understand more about our buried past.

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.