A geophysical survey may be required ahead of evaluation trenching to assist in informing whether important archaeological features are likely to be encountered across a site.
The appointed archaeological contractor must be a specialist in geophysical survey techniques.
The City Archaeologist will determine the area to be surveyed and advice will be taken on the most suitable method of survey depending on site geology and soils. The most common survey type is magnetometry.
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Detailed magnetometer (or other method where appropriate) survey conducted with a continuously recording magnetometer of appropriate sensitivity.
The survey grid, the network of control points used to locate the geophysical survey measurements relative to base mapping and/or position on the Earth’s surface, must be located to survey-grade accuracy (+/- 0.1m). The survey grid must be independently re-locatable on the ground by a third party either by measurement to permanent features and/or by the use of Global Positioning System (GPS) coordinates.
The survey grid will be tied into known Ordnance Survey points with a total station.
The data will be logged in 30 meter grid units with the sample interval set to 0.25 meter and the traverse interval to 1 meter or as discussed and agreed with the City Archaeologist.
The interpretation of magnetometer data must endeavour to distinguish between anthropogenic from other causes of magnetic enhancement on the site.
A clear distinction must be made between interpretation that is scientifically demonstrable and that which is based on informed speculation.
Any reference to negative evidence must be fully explained. Lack of geophysical anomalies cannot be taken to imply a lack of archaeological features.
The report should include:
- a plan of site with scale showing survey grid and 8 figure National Grid Reference
- discussion on geology and the reason for choosing survey type
- reason for survey
- methodology and recording procedures
- discussion of the site and any archaeological/historical information available from the Historic Environment Record (HER)
- image of minimally processed survey data, preferred scale 1:1000
- where appropriate a trace (or X-Y) plot of raw magnetic data
- For very large sites a sample of data might be supplied instead, to support specific interpretation of anomalies on identified greyscale images
- a greyscale plot, or dot density plot. Minimum scale 1:1000
- one or more interpretative plans or diagrams. Minimum scale 1:1000
- summary and discussion of results and interpretation of anomalies, conclusions and any recommendations for further work
- an index to and details of the location of any digital archive, submitted with Archaeology Data Service (ADS)
The long term care of the archive must be provided for. Depending on archival arrangements this may incur a fee.
Our Historic Environment Record (HER) supports the 'Online Access to Index of Archaeological Investigations' (OASIS) project; the archaeological contractor must complete the online OASIS form. We will only accept reports as digital files. The report must be submitted as a Portable Document Format (PDF) files.
Once a report has become a public document by forming part of a planning application, we will place the information on our website.
- The contractor must give at least 7 days notice in writing of the start of works on site to City Archaeologist
- The contractor must produce a written synopsis of the results of the work and submit this to us no later than 2 months after the completion of work on site
- The contractor may be subject to regular monitoring visits by us
- Reasonable access to the site must be given at all times to the City Archaeologist
- All work must be undertaken in a professional manner paying attention to CIFA Standards and Guidance and English Heritage, 2008, Geophysical Survey in Archaeological Field Evaluation