Geophysics, spectacles and sea-mist: excerpts from John Funnell's field diary
Weather:- Cool and sunny in the morning, warmer later
The Brighton and Hove Archaeological Society (BHAS) group met up with John Cook and Jon Sygrave from Archaeology South-East (ASE) at the entrance to Brighton race course. We moved into the race course complex and while sitting in the area next to the winners enclosure John Cook gave us an introduction to geophysics, and later, more specifically to magnetometry. The importance of not having any metalwork on your person was stressed. My spectacles proved to be ok, although it was later found that another member's hip replacement did affect the machine. After questions and answers we moved out into the field.
We watched as the machines were set up and put together, with the important points being emphasised. We were also shown how to zero in the machines, although the racecourse proved not to be a very ‘good’ area for a magnetometry survey, as there was too much metal work about. We had a go at zeroing in the machine and we were tested for metal content. The east/west/north and south positioning was practised.
We then set out the grids. We were divided into two groups, my group surveyed along the actual race track, as the scheduled ancient monument does cross it at several locations. The other group focused on the north/east section of the monument.
Off setting and working out the calculations was a problem, as we were not all that good at maths, but we did manage to survey six grids on that afternoon.
Weather:- Still cool but sunny at times
This morning we set up the equipment carefully watched by both the Johns from ASE. We practised the zeroing in again and managed to complete nine partial grids down to the Manor Hill Road. The other team focused on the north east quadrant again, but later in the day moved over to the south/east section. We moved south of Manor Hill road and completed another nine gids along the race track. We used a base line along the west side of the race track and this made surveying so much simpler not having so many off-sets. John Cook started setting out and marking the grids for us in the south west section, but we could not start as the machine had reached its memory capacity.
Weather:- Much cooler, with sea mists wafting in from time to time.
We had to park in Monument Way today as the race course had a meeting and needed the car park, but it was no problem and was nearer where we were working. The first task of the day was to watch as the data from the previous surveying was downloaded onto a lap top. We then all moved back to our sections. The grid points were marked with blue ink so easy to find, but the surveying proved to consist of quite a number of irregular square and none of a complete nature, so lots more calculations. Today it was a little harder as the sueveying was over the very steep ramparts of the monuments, and keeping up a steady pace was quite tricky. My knee was playing up a little so Jon and Ed filled in for me a little. The terrain being contoured and irregular made the regular pattern of pacing quite a task, but our lads managed it. The cool mist was quite nice as times, as you can work up quite a sweat running up and down the lines.
At the offices of Archaeology South-East. Cook had set up two of his computers and started showing us how to transfer the data from the magnetometer machine onto the computers. He showed how top set the machine to the correct settings and indicated as the machine figures changed as the information was moved across. The computers now had the data, but had not as yet processed it, it was still raw data.
John Cook went into great detail about the downloading process and also the data processing. He explained the various stages of tweaking and adjusting the data, and how care should be taken to avoid deleting objects or features that may have been obscur3ed by magnetic debris. We were shown the various pieces of software used including Geoplot and TerraSurveyor. The magnetometer is a Fluxgate Gradiometer, and the ‘gradiometer’ is important. The importance of magnetics fields and objects was mentioned and the measurement of nano Tesla readings. A nano reading is one billionth of a tesla which are indicated by the symbols nT.
From the imported data you could then clip and strip the images or use a number of other techniques, but it is imported to always keep the original raw data.
Archaeological features may not be recorded as they may be too deep, if over a metre.
If linear features are known running north/south angle the survey at 45 degrees so that these lines are not regarded as debris.
The grid size of 30 metres square is a good size for walking pace. Try to walk south to north on all lines if possible, but zig-zagging is the general way of doing things as this is much quicker.
Magnetrmometry will be good at metal and glass working sites, where heating has taken place and alos kilns and furnaces should produce good results.
Bipolar images are likely to be modern pipe lines or utilities- black/white strong images, and this could obliterate archaeological features.
Watch out for warning signs about gas and electric pipes.
Dipole – Large blobs, could be metal work such as car parts or other iron or metal objects, but equally it could be a furnace, only way to know is to dig it. IT could alos be clay pipes (Therma), but drains will produce patterns on the results.
Positive linear is highly likely to be archaeology, it will have just onbe colour reading and not black/white.
Football stripes on a field could etch into the sun soil and produce results
Ridge and furrow fields could produce results
Metal fences will produce serious anomalies.
Linear features could be agricultural, plough marks
Magnetic debris is a spread of activity and may not produce good results
Negative linear anomalies could be compacted ground or buried earthworks.
Amorphus areas are likely to be geology.
We had a look at the results form Whitehawk and it is a very complex picture, just happy that I don’t have to choose the digging locations. We had a revision session and then some had a hands on go at transferring the data, and always important o keep the raw data. We examined several book sbaout geophysics and John Cook answered any questions put to him about the activity.
De- spike – various options
De-spke and edit if required- various fields can be used to adjust the images.
Transfer Geoplot into Autocad.
It was a very enjoyable few days in the field and learning how to transfer the data, just in case the Society decides to purchase a machine. It has inspired us to organise and get a geophysics team going, even if it is resistivity with our own RM15 until we feel confident to transfer to magnetometry.
Blog post by John Funnell.
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