Geophysical survey techniques are used to both discover new archaeological sites as well as to help define and interpret known remains. They build up a picture of the past landscape, detecting archaeological features below ground. The results from a geophysical survey complement other survey methods, such as aerial photography and fieldwalking.
We recommend that an appropriate geophysical survey is undertaken on a site before planning an excavation.
See below for information on the different techniques and guidance on how to start planning a geophysical survey.
Geophysical survey is a broad term covering several different methods all used to map contrasts between the physical properties of buried archaeological remains and the surrounding soil.
When choosing which geophysical technique to use several factors need to be considered, including the type of site, the expected nature and burial depth of the archaeology and the area to be covered given the time available. Local site conditions such as vegetation, geology, soils and terrain also influence the decision.
The different techniques are also often best employed in combination to maximise the information obtained.
Earth Resistance ‘resistivity’
Resistivity is a useful survey technique for building remains, such as foundation or walls, particularly masonry buildings. It works by measuring the local soil moisture content over a site with an electric current via electrodes pressed into the ground. Ditches filled with moisture will have low resistance; more solid features which as masonry will have high resistance. The best time to conduct a resistivity survey is spring or autumn when the ground conditions are less likely to be waterlogged or too dry.
Ground Penetrating Radar (GPR)
Ground Penetrating Radar (GPR) is useful to record the depth of buried archaeology and can also operate over hard surfaces such as tarmac and concrete. The GPR sends a short pulse of radio frequency into the ground and records the time and amplitude of any reflections from buried remains.
Planning a geophysical survey
Geophysical survey is a specialist skill, requiring specific equipment and preparation. Equipment hire can also be expensive, so it is best to team up with a local amateur group, university or archaeological unit who may already own or have access to and experience with the equipment.
Before you start, it is important to write a project design and do your research and planning , so you are clear about what you are doing, how you are doing it and why. Remember you must obtain permission from the landowner and tenant. Geophysics on a scheduled monument would also need a licence from Historic England/Secretary of State. The Archaeological Service can help advise on your project.
To help you to decide which geophysical survey technique to use, you will need to consider the size of the site, type of archaeology you are hoping to find, past land use and the underlying geology.
Magnetometry is often the best place to start to get a broad picture of the site and then you can decide whether to use a secondary technique to target particular areas or possible features, for example Earth Resistance surveys will be better at identifying stone structures.
Geophysical Data in Archaeology: Guide to Good Practice, Archaeological Data Service (2009)[View on archaeologydataservice.ac.uk]
Archaeological Geophysics: a Short Guide (18), British Archaeological Jobs Resource (2008)[View on bajr.org(PDF)]
Guidelines for the Use of Geophysics in Archaeology, Historic England (2016)[View on historicengland.org (PDF)]
Step by Step Guide to Conducting a Geophysical Resistivity Survey and a Magnetometry Survey, Jigsaw Cambridgeshire and how to process the data [View on jigsawcambs.org(PDF)]