leighandgill

Archaeology in East Oxford

Geophysics Weekend

We have just spent the weekend – well the days, at least! – attending a geophysical survey weekend at Iffley’s Rose Hill Primary School. Olaf Bayer, our project officer, has negotiated permission to do a survey of the playing field at the back of the school, and decided to use the opportunity to familiarize more people with the kit involved.

Location of Rose Hill Primary School

He intends to start a programme of doing surveys on as many of the open spaces as he can get access to, and this weekend was a start on building up a team to carry out the work. We turned up on Saturday morning, being really thankful for the fact that it wasn’t raining (after all, we do have a hosepipe ban), though up on the top of the hill, with allotments to one side (North West) and a recreation ground to the East, the wind cut right through us – and in the morning it was bitter.

The kit all laid out, gradiometer out of its box, signing-in sheet ready for action, with our wellies, lunch etc.

Olaf started off, after the obligatory signing-in and confirmation of having read the Health & Safety info (easy to take the mickey about, but not so funny when things go wrong), by explaining about the preparation needed before we started doing any geophysics and the equipment we would we using. Initially we got the piece of kit – the gradiometer – out of its box to settle down to ambient temperature, and then he started to talk about the need for laying out a grid. This is laid out in squares of 30 metres a side over the area to be surveyed – the size of the squares being a compromise between large (less faffing about surveying but bigger gaps fitting the squares into an irregular shape) and smaller (more surveying, more turning around and toing-and-froing during the survey but fewer gaps around the edges of the survey). We needed to lay it out accurately in the sense of accurate squares (obviously) but also in terms of position as we want to extend the survey into the recreation ground to the East of the Primary School – it’s separated by a tall, painted steel fence – and we need to be able to marry up the future survey with the one we’re doing this weekend.

The area we could survey in and a rough idea of where the grid was laid out

The idea was to use the projects survey-grade GPS (it’s like a sat-nav but works to centimetre accuracy, instead of the 3-5 metre accuracy of your normal off-the -shelf device – you pay a fortune for the precision, though) to fix the points where the squares meet. Not all of them, just along two lines at right angles to form a cross, and then use tapes and ranging rods to fix the other points from that base-line. The GPS has a neat facility where you can input the co-ordinates in the comfort of your own home or office, and then it’s just a matter of selecting the right co-ordinates and up pops a screen like this:-

Screen on the GPS device

The two arrows on the right of the screen show how far – in metres –  that you have to move the device to get to the exact position that you had earlier keyed in, get the numbers down to the sort of precision you are interested in and there you are; change to a different Point ID and do the next position. The trouble was the numbers kept on jumping around, even when we held the device totally still! At first we thought it was because we were right next to a big sycamore – survey-grade GPS relies on a direct line-of-sight link to the various satellites – its major draw back; but when we had done four positions in a line, we noticed that they were definitely not in a line! Much head-scratching and re-checking ensued. Olaf has used this sort of kit a lot, and has never come across this sort of problem, but he said he has never used this particular make and model before. I think it’s down to the perennial problem with modern technology, the more functionality you pack into one device, the more details you have to get spot-on or you get problems down the line; and I think we have all had experience with struggling to wrest information from manuals. So we decided to cut our losses and go back to the old way of doing it – tapes, right-angled triangles and using an optical square and Olaf would sort out the device at a later date.

It was the first time any of us had come across an optical square; a little hand-held optical device for locating you on a straight line between two points, and then working out a right angle from that base-line (you can get some more details about the principles involved here). But first we had to establish straight lines which we did by sighting along the ranging rods and using tapes to work out the 30 metre grid points. We then used a combination of the optical square and good old Pythagorus (3-4-5 triangles, and a new one Olaf told us – if the two sides are 10 each, the hypotenuse is 14.14) to lay out the complete grid. If it looks like there are a lot of gaps from the diagram above, it was because there was a lot of rough ground around the edge of the playing ground proper, as well as a small pavilion and some vegetable beds planted out (I don’t think we would have been flavour of the month for stomping over them in our wellies).

After a break for lunch and, more importantly, something warm to drink, we started on actual surveying. First we had to divest ourselves of anything ferrous; the gradiometer is incredibly sensitive. It works by detecting tiny variations in magnetism so anything magnetic near it throws out its data; so no zips, eyelets, metal buttons, watches, mobiles, steel-toecaps, etc.. It basically means wellies, track-suit bottoms and tee-shirts – though Olaf had some waterproofs he has customized by cutting out all the zips and other metal work and then sewing in Velcro. The sensitivity is why we had to take it out of its case to settle down and why it has to be calibrated – the tube which you can see in the photo has a magnetometer at either end.

Calibrating the gradiometer

By subtracting the reading from the one at the bottom of the tube from the reading from the top one, you “edit out” the effect of the Earth’s magnetic field and are left with any magnetism caused but stuff buried in the ground; archaeology hopefully but just as probably services, old nails or any old iron. To do the calibration Olaf had to wander around a lot to find a relatively “clean” area – a place with as little magnetic noise as possible; you can set the device so it beeps when the magnetic field changes so you aim for as quiet an area as possible. Then you go through a routine of facing the points of the compass and turning the device upside-down (basically you are following the instructions  on the screen that you can just see being peered at above). Once it is calibrated we were almost ready to get stuck in – just one more thing before we started; we all had to checked to make sure no stray ferrous items had crept in by accident!

Queuing up to be checked for unknown metal bits

Then we could start. The basic procedure is that one has to walk up and down in lines in the 30 metre square, a metre apart at a time until the whole square has been covered. To make this easier, we use a “trapeze”; two poles with holes a metre apart through which are threaded cords 30 metres long – you can see where the name trapeze comes from – and the mid-point of each pole is marked with tape. Then, on opposite sides of the square we stretch measuring tapes which have marks along their lengths at every other metre. You lay the trapeze down lined up with marks on the tape, the operator walks up one cord then back down the other one, the trapeze is moved up to the next mark, operator walks up the cord and so-on. The cords have marks every metre and the gradiometer beeps every metre so that when you are the operator, you aim to get the beep happening when the tube is directly over one of the marks; you set the device with a speed you are comfortable with for that to occur – fast for Olaf and embarrassingly slowly for the rest of us.

Gill taking readings – you can see the trapeze (the blue cords) with the spacing pole at my feet in the distance. Thanks to Olaf for the photo.

You would think that just walking up and down a laid-down cord at a steady pace would dead easy, wouldn’t you? Just goes to prove that you have to try something before you can pass judgement. It really does take a bit of getting used to – quite a bit of deleted and repeated lines, and later on in the day Olaf stepping in finish in a reasonable time! We did the southern most three squares (North is at the top in both maps, sorry Jane, as usual I’ve forgotten to put in the North arrow) on Saturday and finished off the rest on Sunday. At the end of the day on Sunday, Olaf got out his laptop and did an initial pass through the data we had collected – a fairly obvious services pipe at the North end of the field and some rather symmetrical oddities dotted around – but at first glance, no henges! Early days, though, and a valuable learning exercise for all involved – a big thank-you to Olaf for being so patient with us, especially as he is more used to the speed at which the pressures of commercial archaeology force one to. I know Gill and I are both looking forward to putting our experience to use in the future.

Leigh

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