Evaluation by exploratory fieldwork is recommended in the relevant Planning Guidance Note, PPG16. This usually takes three forms; fieldwalking of agricultural land earmarked for development, geophysical survey, and test trenching across fields to be build-over. It is not uncommon that a combination of all three are requested by the Planning Authority to inform the Planning Decision Process. To mitigate the damaging effects on the archaeological heritage of green-field development, where previously unknown archaeological sites may await discovery, it is becoming common practice for Local Planning Authorities to ask developers to undertake such fieldwork on green-field sites to be destroyed by substantial developments.
Geophysical survey is a non-intrusive technique used to identify buried features, which are then further investigated by excavation to determine whether or not they are archaeological in origin.
There are several main methods:
Electromagnetic surveying measures changes in the magnetic field. The fill of a pit or ditch for example, will produce an anomaly because of the contrast in the strength of the magnetic field between the magnetically enhanced topsoil used to fill the feature and the surrounding subsoil. Palaeochannels and other geological features, hearths, ovens, burnt material and metal objects will also create anomalies. When these anomalies are mapped using an instrument called a magnetometer, the geophysicist can deduce whether these features are likely to be of archaeological origin.
Resistivity passes an electric current into the ground through a line of probes and measures the resistence to the current at set points across the site, which depends on the conducting abilities of the soil and any features within it. Ditches or pits for example give a low resistence. Features such as wall foundations, rubble, yards, roads and cobbled trackways give a high resistivity. Again, the features can be mapped allowing a preliminary assessment of the site's likely archaeological potential to be drawn up.
Ground Penetrating Radar emits a pulse of energy into the ground and measures the time it takes for its echoed return. Travel times are recorded and converted into depth measurements. Thus interfaces between different strata and features can be detected. The advantage of this technique is that it can provide a 3D view of a buried site. Unlike electromagnetic surveying and resistivity which work best on undisturbed green-field sites, GPR works well on deeply stratified urban sites.
Source - "The use of Geophysical Techniques in Archaeological Evaluations" IFA Paper No. 6 and "Geophysical survey in archaeological field evaluation", Research and Professional Services Guideline No 1, English Heritage.