ABINGDON ARCHAEOLOGICAL GEOPHYSICS

Geophysical Survey Requirements 

These are necessary to ensure that surveys can be carried out safely and with a minimum of delay. Please advise us at the outset if you have any difficulty with meeting any of these requirements.  

1          The grass/crop should be less than 6 inches (15 centimetres) high for the magnetometery, although up to 18" may be acceptable for crops such as young wheat which will not obstruct the magnetometer probes from passing between the stalks. Working in such crops will cause some damage to the crop (see 5B below). For resistivity the same also applies although higher grass can be acceptable but this will delay progress and can lead to inferior results. Thistles and nettles must be cut down before work can take place, or work can be delayed until they die in the winter.

2          There should be as few trees or other obstructions as possible as these will not only leave gaps in the data but going round them will delay progress. Trees also remove moisture from soil and in the summer this will lead to areas of high resistivity which only indicate were the trees are rather than where the archaeology is. 

3          There should be no animals in the field. Sheep may is acceptable in some cases but cattle have a desire to eat all tapes and cables which effectively prevents work from happening 

            Dog mess is also a problem to be avoided as no one wishes to clear the mess off the tapes and equipment.

            The  applicant should advise us of any hazards which may exist such as the presence of Blandford flies or of the necessity to avoid certain areas in case they are occupied by newts on other things or plants which need to be protected and could be damaged by being walked over as part of the geophysical survey. 

4   You should advise us when obtaining details of whether we can do the survey if any of the following apply to the proposed site:-      

           A  The site should be reasonably level as the magnetometer equipment is fairly heavy and walking down steep slopes with this can be dangerous. Fields which have recently been ploughed or are filled with potatoes or similar crops also cannot be surveyed as the undulations and crop cause too much disturbance to enable good data to be collected.

          B   As magnetometers are very sensitive, the presence of wire fences can swamp any archaeological data for a distance of 15 metres on each side if a 1.5metre high chain link fence. Similarly iron rubbish and pipes in an area can produce very high readings which can obscure data for, typically, 2 metres from the object. Lorries on nearby roads can also be detected by the apparatus.

           C  Resistivity equipment is sensitive to currents in the earth which arise through leakage from cables and the reverse current under electricity power lines. Surveys can be carried out in such locations but the filtering makes the survey take 4 times longer to do and will thus increase the cost.

5          The person or organisation applying to have the survey carried out must first have obtained the following consents:- 

    A         the landowners permission. 

    B          the farmers permission as farmers can require substantial compensation if their crops are damaged. 

    C         in the case of Scheduled Ancient Monuments the relevant permission from English Heritage to carry out work. This is known as a section 42 licence. 

    D         any other consents which are needed such as the consent of any developer or other similarly interested person.

6           If they wish to have the survey located on a map, the applicant should provide maps either digitally or on paper which can be used to locate the survey area. These should be at a scale of 1 to 1000 or 1 to 500 and the applicant should ensure that any copyright issues are dealt with.

7          It would be advisable for the applicant to have laid out a grid or, at minimum, a base line in the area to be surveyed and to have this linked to the Ordnance Survey map of the area. This would then prevent time delays in the geophysical surveying operations. This should be discussed with us before commencing as it may be best to use an existing site grid or grids can be organised north - south to coincide with the Ordnance Survey grid or to align with existing field boundaries. If you are looking for a known feature it is often best to have the grid at approximately 30 to 40degrees to the alignment of the expected walls or ditches, as this lessens problems which can occur in the data processing stage where features can be lost by the zero mean traverse and edge matching processes. We can, if needed, use our differential GPS equipment to locate survey areas to approximately 0.3 metres in the UK although interference near airports prevents it being useful in such areas.

8          If the organisation requiring the survey wishes to make people available to move tapes in order to speed up the progress of the survey and to minimise the number of geophysical surveyors required (and expense) then this must be on the basis that those persons are covered by the organisation's insurance arrangements as we do not currently have insurance for other people.

9          As the purpose is to ascertain whether archaeology exists in an area we propose to make the results of the survey available to the local Sites and Monuments Officer 3 months from the date of the survey. If the applicant requires any greater time delay this should be agreed with us at the outset.

10        Reports will be in Microsoft Word document format with the illustrations as png pictures. Please establish at the outset whether we can supply other formats you may require.

11        It should be understood that some sites show with magnetometery, some with resistivity, some with both and some just don't show with either method. Whilst we shall carry out as good a survey as ground conditions permit with our method and equipment we can do nothing to change the ground conditions and therefore cannot guarantee the results which can be obtained.