Promoting awareness of the archaeology and history of north Devon
Promoting awareness of the archaeology and history of north Devon
First Outing with the Resistivity Meter - Trevor Dunkerley (Newsletter No 7 2004)
Thanks to the Tarka Millennium Award Scheme, NDAS now has its own resistivity metre. Trevor Dunkerley and Jim Knights tried it out. Having managed to put together the equipment which had arrived by parcel post, and made sure it was working, it was obviously necessary to try it out.We decided it was best to give it a go on a site which had previously been surveyed, so that we could check our results against something already known.We chose a site at Charles, near Brayford where Exeter Archaeology had already carried out a magnetometer survey. On March 10th 2004, therefore, a very cold, brisk day, I met Jim Knights at Charles where he had already marked out the corners of a 20m square. What would resistivity reveal?
We set about laying out the first of two 20m x 20m grids, inserting the
probes at the prescribed distance from each other and from the grids.
The meter was connected to the probes, and then began a frantic search
through the instruction manual to determine how to start the machine.
We had decided with the first grid to input each meter manually, before daring to venture into auto-mode, but how to start the wretched thing?! Then the penny dropped - just press the ‘start grid’ button! So off we went. Jim carried out the logging, and I ensured he could see the meter marks on the tapes and moved the tapes appropriately. The double beep of the machine at the end of each run assured us we were on the right track.
And so to grid two which had been set out adjacent to the first, By now the temperature on that exposed hillside was starting to plummet. Time for auto-mode - that should speed things up a little. Back to the manual, where we read, “Go to menu, then click with the + button to go into auto-mode”. That didn’t work! After several further attempts we put our heads together and once again the penny dropped - just press the OK button! So off Jim went up and down the grid.
After clearing up and cleaning tapes, pegs and probes, we set about trying to download into the laptop. Computers are great when they work, but for some reason this laptop would not recognise that we had connected the resistance meter into its USB port. Although no longer distracted by the cold - having moved into the warmth of Jim’s home -our patience remained unrewarded: the laptop was being totally unco-operative! I knew this infernal machine would download into my own computer, so I sped home and after a quick connection, had the data snug and safe wherever such data goes to. But why wouldn’t it connect with the laptop? Of course! We’d omitted to install the appropriate software!
And so finally we had the first NDAS resistance meter image - the two grids joined together and looked like, well, nothing much actually - just lots of shades of grey. Now comes the important part: filtering, clipping, and subtly manipulating. Yet more to learn. Still, if nothing else, at least we can say there is lots of resistance in Charles near Brayford.
The NDAS Resistivity Meter - Jim Knights (Newsletter No 8 2004)
Eight sites have now been examined and confidence in use of the resistivity meter is growing. It proved itself at Higher Holworthy where it indicated the site of a building and subsequent test-pitting confirmed it.We are currently surveying earthworks in Bratton Fleming, thought to be an early road through the village. In December we will be examining an interesting earthwork at Dean between Combe Martin and Parracombe. The mechanics of the survey are now carried out with confidence and, dare I say, competence. However the use of the software filters requires more successful experience on known features to gain greater assurance. One of the conditions of our acquiring the machine through the Tarka Millennium Award Scheme (via Trevor Dunkerley) was that we should be prepared to lend it to other groups, provided they insure it. We have been asked by a group at Branscombe if they may borrow it, and I shall shortly be taking it down to South Devon to help with the survey of a Brionze Age site.
A Year with the Resistivity Meter - Jim Knights (Newsletter No 10 2005)
In the spring 2004 newsletter, Trevor Dunkerley reported on the first outing of the N.D.A.S. resistivity meter at Malcolm Faulkner’s Welcombe Farm (Charles). The article gave a flavour of the learning curve we were just about to start on, but I have to say that after many outings with the meter we are still on the curve.We now have records for 13 sites, and have mastered the mechanics of the survey, but there is still a lot to learn about the computer manipulation of the data and its interpretation. I shall not be allowed to forget the tucking mill leat suggested as adjacent to Rack field at Combe Martin, which turned out to be a leaking 5 ins. copper pipe!
The equipment we have is excellent, but the software originally did not meet the standard of more expensive equipment. The latest version of free software shows some promise and is much more user-friendly, but it is only a Beta version and may have bugs. Surveyed grids are relatively easy to down-load, and stitch together. The filtering of the data is perhaps where the skill comes in - or is it trial and error?
We have had some promising results, but until the surveyed sites are dug, we can’t be sure what they represent in the ground. At Higher Holworthy Farm we managed to identify precisely the corner of a lost building which is shown on the tithe map, and to prove it with a test pit. A privy in my garden, and land drains in an adjacent field were also positively tested. At High Bray we located some areas of high resistance, and we later dug test pits to discover the stones producing the high readings.We suspect they were part of a rabbit warren but we would have to dig much bigger trenches to be absolutely sure.
At the Holworthy dig this year, we did identify the stone bank in the north corner of trench 1, but the postholes did not show in our survey, even though they turned out to be more or less precisely where the draughtsman had speculated.
Ten members have kindly worked with me during the last eighteen months (at Holworthy, Holwell Castle, Whitechapel Manor, Mine Tenement as well as Welcombe Farm) and are confident of setting up grids and logging resistivity readings. It is a time-consuming task, and once you start, it is wasteful not to complete an area. So together we have suffered rain, wind and sunburn this year: our sites always seem to be exposed to all the elements.
The most promising unproved results have come from where we started, at Welcombe Farm. Here we have the bonus of working with existing professionally obtained magnetometer results and it has been interesting to combine the two technologies.
In Figure 1 the left hand pair of images shows the magnetometer results obtained by Oxford Archaeotechnics (survey commissioned by Hanson Aggregates plc), who concluded in their report that the site included “a probable defended settlement site comprising a square enclosure some 30m. in width approached by a ditched trackway and defended by at least three concentric outer ditches. Their relationship suggests a composite ground plan possibly of a single phase of occupation. Insufficient detail was recovered from the interior to indicate the geometry of any internal structures”.
The right-hand pair of images in Figure 1 shows, to the south, the results of our resistivity survey, and to the north our survey superimposed on the northern section of the previous magnetometer survey. The presence of 20th century metalwork placed constraints on the original magnetometer survey, but did not affect our resistivity survey, so we were able to extend the southern part of the survey area. Therefore our result shows more of the northern ditch of the “square enclosure”. It also shows more features within the enclosure and some additional areas to the north and west, these areas being outside the brief of Oxford Archaeotechnics. Both surveys show a light line cutting diagonally across the site which is recognised to represent a 20th century pipe.
The features from both surveys have been digitally traced and placed on
top of each other in Figure 2 (Combined Geophysics Surveys). Many of the
features correspond remarkably well, considering that our survey made
no allowance for the sloping land, whereas the Oxford Archaeotechnics
survey was laid out using G.P.S.. The features within the enclosure are
intriguing but we can only speculate on what they represent. It appears
that there may be at least one roundhouse within a rectilinear inner enclosure
accessed by way of an incurved entrance. This looks Iron Age in style,
though in this area it could well be of the Romano-British period. There
is at present no evidence for the date of this site, or even if it represents
a single period.
We do intend to return in the New Year, when additional land will be released for survey. Eventually this site will have to be dug, since most of it is due to disappear in quarrying. Until then, we can try to squeeze more information from it, but only speculate on what it means.
A New Survey Technique? Brian Hummerston (Newsletter No 10 2005)
During the last year we have been contacted by Brian Hummerston, who lives at Woolacombe, to ask whether we have any use for groundpenetrating radar as a survey technique. It came as a surprise, because this is not something that is usually available to amateur groups. Basically the technique involves shooting a radio pulse into the ground which then bounces off solids and produces a trace in the same way as radar “sees” aircraft. It is usually used for detecting underground hollows and cavities such as cellars, mine shafts, etc., but can also be used to detect the interfaces between solids or between disturbed and undisturbed ground and can also detect buried metal objects. The equipment can ordinarily penetrate up to 18 metres into the ground and in the right conditions can detect caves or cavities at up to 40 metres depth. It will provide a “real time “ read-out on a laptop, so the user can walk around and “see” beneath the ground; and in addition there is a magnetometer facility which gives an audio-signal when an anomaly is detected.
On the weekend of November 12th and 13th there was a chance to see what could be done with the technology, when Brian accepted an invitation to bring his equipment to the Church of St Peter ad Vincula in Combe Martin. There was a theory that the chantry chapel in the north-east corner of the church might conceal a crypt beneath its floor. The equipment which Brian brought was surprisingly compact, since when ground-penetrating radar first came on the scene it involved a machine like a small road-roller.
Obviously miniaturising technology has been at work. Brian and his partner Lis set up their laptop on the grand piano, and proceeded to scan the floor of the chapel with something like the wand that they use at airports to detect concealed weapons on your person.
The results were available remarkably quickly. Within ten minutes of completing the scan, Brian was able to show on the computer screen that there did in fact appear to be a void beneath the floor. It did not, however, look like a crypt, but more like a pit. Whether there will ever be any chance to check this result against physical evidence is highly debatable!
Impressed with what we had seen, we invited Brian to come to the Holworthy Farm site and see what the equipment could tell us there about the unexcavated parts of the enclosure.We met there on the morning of 15th November in an icy north wind for a brief demonstration and again on the 16th in beautiful November sunshine for a more thorough survey side by side with a more familiar resistivity survey carried out by Jim Knights. At the moment of writing, the results are still being processed, but when we compared the results of the two surveys on site it did appear as if Brian might have detected a pit just inside the southwestern edge of the enclosure and just beyond the limit of the 2005 excavation.
Brian is still at an early stage in learning to use the equipment and to interpret the results, but we shall keep in touch because this looks like a useful extra technique to have available.