After nearly 20 years with Time Team, the total distance surveyed by archaeological geophysicist John Gater and his team is equivalent to the diameter of the moon (3,470km – in case you were wondering). The other statistics are equally impressive: 196 sites surveyed across England, Scotland, Wales, and Northern Ireland, including 49 Scheduled Ancient Monuments (SAMs) and five World Heritage sites, which equates to over three months of continuous data collection. The banter between John and the diggers is a staple ingredient of every Time Team episode, as is the ‘go’ moment when the survey results come in.
The growth of geophysics (affectionately known as ‘geofizz’) as an essential aspect of archaeological practice mirrored the growth of the Time Team programme. In the first few years of the 1990’s when developers and planners viewed early screenings of Time Team, it became obvious to them how geophysics could make their lives easier and save them money. Why go into projects blind, with expensive test trenching and invasive evaluations, when geophysical survey could be done more cheaply, provide more information, and thus allow for more accurate forecasting?
An example of this early adoption is when British Gas employed the technology in advance of their North Sea pipelines, a massive project that John worked on after graduating from Bradford University in 1979. In some cases, the route of the pipeline was adjusted in order to avoid archaeological sites identified by geophysics – it was cheaper to deal with the archaeology in the planning stage and to chart a new route, than to press on without knowing what they might find. Just like imagining our workday life without e-mail, it is hard to imagine doing archaeology today without geophysics. Commercial archaeology enjoyed the fruits of this acceptance and geophysics quickly became a calling card for many professional units.
Putting the fizz in geophysics
It must be said, however, that television is different. Landscape analyses by helicopter, eureka moments in the trenches, banter in the finds tent, and sexy computer graphics all make more exciting viewing than watching someone tramp back and forth across a field carrying what looks like two fluorescent tubes. But it is geophysics that sets the agenda for every Time Team shoot. Not a spade is sunk nor a digger mobilised until the survey results are in; each programme’s success depends on the trenches coming up with the goods. The quick pace of results is what has given the show its sense of jeopardy; but in the case of the geofizz, it is very rare to be proven right or wrong so quickly. In most cases in developer-led archaeology, feedback from excavations to surveyors is scarce – unless something has gone wrong; in the case of Time Team, it happens within hours.
Geophysics is complex science; and geophysics on the programme is complex science done on an extremely tight timescale – but done to such a high standard that Time Team is often sought out for jobs on sensitive and complicated sites. Because of their flexibility and resources, they can complete evaluations that might be beyond the scope of English Heritage, Historic Scotland, Cadw, or the Northern Ireland Environment Agency, and that is undoubtedly part of the reason why so many SAMs have made it onto Time Team’s agenda through the years. This would not be the case, however, if the work was in any way compromised by the format of the show. John Gater’s archive of Time Team geophysical evaluations is a treasure trove of the very best sites in British archaeology, much of which would not exist if the show had not been available to take on the jobs. So, how did it all begin?
The most common techniques in archaeological geophysics are: magnetometry, resistivity survey, and ground-penetrating radar (GPR). Metal detectors are also geophysical sensors but are not capable of generating high-resolution imagery. Although metal detectors are regularly used on Time Team, this work is usually carried out by locals – the geophysics team only use specialised detectors on some sites, such as when searching for ‘lost’ aircraft. While geophysical techniques have not really changed over the years, new instruments have been developed, and increased computing power has enabled more powerful software programmes to be used. GPR has seen most changes, including 3D modelling of buried features, as well as the arrival of Swedish company Mala’s Mira carts, which are pulled behind tractors and can collect data at amazing speed and at incredibly high resolutions. These carts were used by Time Team in 2009 at Cunetio – a Roman Town in Wiltshire, near Mildenhall – and are being employed by the Stonehenge Hidden Landscapes Project, Britain’s largest virtual excavation, which aims to use geophysical techniques to recreate the Stonehenge monument and its surroundings as it was more than 4,000 years ago (CA 246).
• Resistivity meters are connected to metal probes, mounted on a rigid frame, which are inserted into the ground. Electric currents flow through the earth and archaeological features can then be mapped, when they are of higher or lower resistivity than their surroundings: for example, organic deposits in a ditch might conduct electricity more easily than surrounding soils, whereas a stone wall could impede the flow of electricity.
• Fluxgate gradiometers (a specific type of magnetometer) generally use paired sensors (sometimes several pairs) to measure the gradient of the magnetic field (the difference between the sensors). Materials with a ferrous (iron) component have unique magnetic properties and can cause variations in the Earth’s magnetic field that are detectable with magnetometers. The sensors ‘react’ very strongly to iron and steel, brick, burned soil, and many types of rock, so archaeological features composed of these materials are readily detectable.
• Ground-penetrating radar (GPR) is the perhaps the best-known method. The radar signal is directed into the ground, and objects and stratigraphy (layering) will cause reflections that are picked up by a receiver. The travel time of the reflected signal indicates the depth. Data can be plotted as profiles, or as a map indicating specific depths.
Time-slices at Athelney
One of John Top 10 sites is Athelney, in Somerset, filmed for the first-ever episode of Time Team. Ironically, there was no digging at all at Athelney, which is a Scheduled Ancient Monument (SAM) with links to Alfred the Great and a later abbey complex. English Heritage would not grant permission for the team to excavate, so the programme was based around fieldwalking and geophysics. They had immediate results from the resistance survey and obtained a remarkably clear plan of the abbey remains. What proved most amazing, however, was not just the clarity of the results, but the fact that they were able to print them on a portable printer in the back of the survey vehicle. For the archaeologists on site, and for the viewers of the programme, this was an enlightening moment – and something of a relief for John and the crew, since the Ancient Monuments Laboratory (AML) had failed to find anything at the site in previous investigations.
With this stroke of luck, they had effectively shown the viewing public what to expect from Time Team, and created an essential role for geophysics in the programme’s format – as well as found distinct archaeological evidence on an important site where previous attempts had failed. In many ways, the results of this survey set the course for the next 18 years on Time Team.
Gater says that he often leaves shoots wanting to go back and do more work on the sites, especially as survey techniques become more advanced. Athelney was just such a site, and here he got lucky: the team was granted permission to excavate in 2002, for the 100th episode. The most exciting aspect of the shoot was the chance to use more sophisticated equipment; radar had taken huge leaps forward, but had been used mostly in urban contexts – which, given the complexity and depth of urban stratigraphy, are not perhaps best suited to the technology. The return to Athelney was an opportunity to test new ways of viewing radar data.
Prior to this time, most radar surveys tended to concentrate on collecting radargrams (vertical slices of data that provided depth information, which conventional resistance and magnetic surveys did not really provide). At Athelney, they were able to use new software to provide plan images at differing depths below the ground. Starting at the top, differing plots can being shown at chosen depths below the surface: so-called ‘time-slices’. This worked extremely well at Athelney, but one disappointing result was that the depth of the archaeology proved to be far less than anyone expected, and the natural geology was close to the surface. So, a few outlines of the wall foundations and footings showed in the radar data, the surviving archaeology proved to be quite ephemeral. Nonetheless, it was a heartening example of how advancing techniques in geophysics can provide ever more information about sites that have already been investigated.
Initially, the main advantage of GPR was the ability to view a vertical section through the ground by means of a radargram, but experience has shown that it is far easier to view this data as a plan. By collecting many parallel lines of data and merging the radargrams together into a block, like a loaf of sliced bread, it is then possible to produce a series of ‘time slices’, by slicing the loaf horizontally, instead of vertically. The individual time slices represent a layer below the ground surface, the depth and thickness of which can be varied but is typically 10cm.
Big results at Binchester
Another site that tops the list as one of Time Team’s best is Binchester Roman fort, (Vinovia) which picked up where Athelney had left off, in terms of using radar. The site at Binchester had much deeper stratigraphy than Athelney, which meant that John could get the kind of results he had been looking for – as well as something entirely unexpected.Binchester is on the line of Dere Street in County Durham and, with the primary defences enclosing an area in excess of 3 hectares, is one of the UK’s bigger Roman forts. Whilst evidence of a defended vicus, lying on the south-eastern side of the fort, had been discovered through antiquarian excavations, their extent was only made plain by a previous geophysical investigation in 2004. As part of a Time Team episode in April 2007, GSB expanded upon these earlier results, revealing, for the first time, the defences of the earlier fort, offset slightly to the north.
They found much more than they had expected, however, with the discovery of a previously unknown extensive settlement on the north-western side of the fort, which stretched out north along Dere Street, adjacent to the earlier fort defences. But the fort had even more secrets to share: perhaps the most exciting discovery was what appeared to be small, square stone structures abutting the outside edge of the latter fort’s vicus ditch; the suspicion was that this could be either temples or a mausoleum. A small ground penetrating radar dataset was collected to add some detail to the magnetic data, and the results were remarkable: two mausoleum structures were revealed, side by side, with the possibility of a third lying next to them, disappearing out of corner of the grid. This was a breakthrough dataset, with far and away the clearest time-slice images ever seen on Time Team.
From the larger of the structures, it was possible to make out the boundary wall, the small shrine building and areas of demolition material, as well as a curious small square feature with a hole in the middle – identified as a ‘raised bed’ in which a symbolic specimen tree or shrub could have been planted. The team’s excavation matched up with the GPR data almost perfectly – a real home-run for the show, as well as a big step forward for geofizz in terms of accuracy in the data collection, allowing for nearly surgical placement of the trenches, as well as the sophistication of the results. John Gater was pleased; but even as this step forward was happening, newer techniques were already becoming available that would move data modelling into a new dimension. He wanted a crack at it, and it was just a matter of time before the Time Team found their site.
Radar revelations at Caerwent
One of about 15 civitas capitals in the UK, Caerwent in Monmouthshire, was a step into the third dimension for Time Team geophysics. Possibly the most complete Roman town in the UK, the site is well-known and has fantastically preserved remains, with walls upstanding to 4m in some places. Very little geophysics had been undertaken within the walls at Caerwent, however, so this was a golden opportunity to explore. The survey area was quite large, with two insulae (blocks defined by internal roads) earmarked for investigation, each at about 20m2. Initial GPR survey covered about a hectare (100m2). This would prove to be the first time such a large-scale radar survey was carried out in a green-field context.
The 2008 Time Team investigation concentrated on these two insulae, which were relative ‘blanks’, having had little antiquarian excavation carried out within them. The initial magnetic survey of Insula I, in the north-west corner of the town, revealed a mass of ‘noise’ (magnetic disturbance) in the northern third of the area, which was typical of building debris. It was not possible to identify wall lines, so the area was resurveyed with the relatively new, higher resolution Foerster magnetometer, which indicated some intact wall lines – but the picture was still far from clear. Detailed interpretation was difficult; even resistance survey showed little more than a broad spread of debris, rather than any structural detail. The team’s last punt was to throw the radar unit at it, and see what turned up.
The results were a revelation – a clear series of time-slices, showing a multi-phase villa building, which came complete with a clear apsidal end in the earliest level. Given the lack of clarity in the data recorded by the other techniques, the level of detail and ease of interpretation was astounding. It quickly became clear to John and the rest of the team that this would be an ideal dataset to use for the next step of visualisation: creating a 3D data model. The basic premise of this method is to stack all the time-slices in a cube, and then strip away all the ‘quietest’ areas within – i.e. those parts of the ground which are made up of just soil, and/or relatively loose demolition material, leaving only the strongest reflections from the in-situ walls and foundations. Unlike 3D graphical reconstructions, these models are not interpretations of what might have been on the site, but instead are an almost exact model of what is actually left on site – and all without a spade hitting the turf. The resultant model at Caerwent gave an extremely clear image of the wall lines and how they sat within the ground; and it was possible to spin the data round, enlarge, and tilt it to be viewed from any angle, or to more closely examine any detail (animations and models are available to view on the websites cited on p.27).
This was the first time Time Team had employed 3D modelling on a site, and it was a resounding success. Cadw was delighted with what was discovered, and have funded a further survey based on the results. According to Rick Turner, Inspector of Ancient Monuments, Cadw, ‘The Time Team dig has left Cadw with a fund of new information that will certainly add to what we know already about the history of this fascinating Roman town.’
Time Team has had a huge impact on the public’s awareness and knowledge of archaeology; without the show, it is unlikely that most people would have heard the term ‘geofizz’; or have any reason to be interested in this kind of information. Whilst John is aware of this positive exposure for the discipline, he is also wary on some levels, saying: ‘I worry sometimes that we’ve made geophysics look too easy. It’s not easy, and you don’t typically get such instant results. I’m really encouraged that so many amateur groups have become involved and have acquired equipment through the HLF to do their own projects, and I have spent a lot of my time on weekends throughout my career working with these groups. But what worries me is that some of them have been doing commercial work, quite possibly in cases where there isn’t sufficient knowledge or training to interpret the results correctly. That’s really challenging for geophysics on the whole, because we can’t afford to get a reputation for getting things wrong. I would just urge caution when stepping into commercial work – you only get one chance with a developer.’
An ‘own goal’ at Ribchester
As John says, you cannot be right all the time, and some of the most amusing moments on Time Team have come from geophysics interpretations that went somewhat awry. A perfect example is the shoot at Ribchester, in the Ribble valley.
The Roman fort at Ribchester – Bremetennacum Veteranorum – was established during the late 1st century, and was strategically located to ensure that the River Ribble and the main Roman roads passing through Ribchester were carefully guarded. The fort’s remains are located under modern playing fields, and Time Team was engaged to find evidence of the Roman roads expected to criss-cross under the fields.
John and the surveying team scanned a couple of squares, and found ditches intersecting each other at right angles, which they assumed were Roman. Excitement was mounting, and the survey was expanded. More straight lines were found, with some that curved distinctly into an arc and joined up. This was a totally unexpected result, and Mick Aston enthusiastically proposed that they must have found the apsidal end of a previously unknown bathhouse or church. But John’s shock grew as he looked again at the results, and the pattern in the ground became even clearer….
Repeated linings of the football pitch over the course of many years had significantly changed the moisture content of the soil under the boundary lines, which had been picked up by the highly-sensitive resistance survey. In the years since, John has frequently used these plots as a tool in lectures to illustrate the problems of sampling and how important it is to always keep the big picture in mind – especially when looking at small plots.
And herein is the same friction that exists in all areas of archaeology, between the need to make archaeology accessible, enjoyable, and welcoming for the public but to also acknowledge and accept that professional expertise is necessary, especially with the implementation of PPS5. It must be clear to developers that archaeology is not a ‘contaminant’ to be removed by the lowest bidder, but rather a built-in, essential aspect of the construction process. As John Gater says: ‘We’re not here to delay the process – we’re here to participate.’
So what is next? Channel 4 has renewed Time Team for another two seasons, which means John’s constant crash course in reading geophysics plots – of being proven right, or wrong, almost instantly – will carry on. But what is left to do for the team who found Edward II’s round table, under the Queen’s private lawn at Windsor Palace? John says that the researchers are coming up with ever more challenging sites and environments, such as in fields of crops, sites under rivers, and on difficult terrain or in wooded areas, all of which cause the geofizz team to heave a huge sigh when the project design comes through. There are also a few ‘dream sites’ he would like to have the opportunity to survey; he started the geophysics on Orkney’s prehistoric landscape – where Nick Card and Orkney College have had such amazing success over the past few years (CA 241, 248) – and he would love to work on similar landscape projects in Cornwall. What about surveying the moon? They have already nearly walked themselves there, and it would be an incredible achievement, especially in just three days…
Dr John Gater
Dr John Gater, Director
GSB Prospection Ltd
Tel. +44 (0) 1274835016
Gaffney, C. and Gater, J Revealing the Buried Past: Geophysics for Archaeologists.Tempus, ISBN 0752425560.
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