Geophysical survey in archaeological evaluation
Thanks to Time Team, large swathes of the British public are now aware of the term geophys and what it entails, even if they do not entirely understand the science behind how it works.
Geophys is basically shorthand for geophysical survey one of the most common forms of non-intrusive and non-destructive archaeological evaluation techniques employed in commercial archaeology today.
Mapping of anomalies
Geophysical survey is intended to enable archaeologists to determine whether the data they have collected, signifies the possible presence of buried archaeological features within the area subject to survey. Basically, geophysical survey creates maps of subsurface anomalies which could represent archaeological features such as pits, ditches or structural remains such as wall foundations or surfaces.
In commercial archaeology this would generally be within a site proposed for development of one kind or another.
Geophysical survey within a planning-led and / or developer-led environment is generally required by Local Planning Authority Archaeologists when proposed developments are at application stage.
The requirement is applied at this stage so that the presence or absence of potential archaeological remains within a site can be determined and in turn allow the LPA Archaeologist to make an informed decision on what kind of further archaeological investigation will be required.
It is worth highlighting at this point that whilst geophysical survey will detect anomalies which suggest the presence of buried archaeological remains it does not establish the date or necessarily the significance of those remains neither does it always distinguish between what might be a naturally occurring buried feature and what might be an archaeological feature.
This has to be corroborated by intrusive archaeological investigation, most commonly trial trenching.
Geophysical survey is also a relatively inexpensive and rapid method for those involved in development to establish the presence of potential archaeological remains within a proposed site at an early enough stage for them to assess the risk involved and to form a strategy for managing the time and cost implications as the development moves forward.
Adapted from geological contexts
The geophysical methods used in archaeology have been largely adapted from those used in geological contexts, for example mineral exploration.
There are many forms of geophysical technique but there are three which are principally used in archaeology.
Magnetometry measures the magnetic field strength at particular locations within a site.
Electrical resistance measures variations in electrical resistance which are determined by the amount of moisture in an archaeological feature.
Ground Penetrating Radar (GPR)
GPR works by sending electromagnetic pulses into the ground and then measuring any reflected signals from sub-surface archaeological features.
Not infallible, but…
Whilst often very effective at detecting the presence of buried archaeological remains during archaeological evaluation none of the techniques of geophysical survey described previously can be regarded as infallible.
Although geophysical survey is generally speaking a relatively rapid and cost-effective method of evaluating a development site for the presence of buried archaeological remains, it should not be regarded as foolproof.
Before commissioning a geophysical survey as part of a planning application, or in any other circumstance for that matter, it is worth the developer taking some expert advice on whether it is likely to be an effective method of evaluating the particular site concerned.
Taking this advice first could save money being wasted on a geophysical survey which will not be effective on a site where a different method of archaeological evaluation could be more appropriate to achieving the desired result.
Topics for developers’ guide
- desk-based research
- aerial photography and lidar
- cartographic analysis
- review of geotechnical data and deposit modelling
- and more…