Archaeological geophysics: matching technique to ground
Geophysics detects contrast, not archaeology. Geology, land use and the target decide which technique will work, and a negative result never proves a site is empty.
Choosing an archaeological geophysical survey technique is a question about the ground rather than about the archaeology. Magnetometry, earth resistance and ground penetrating radar each detect a different physical contrast, and each is defeated by a different set of site conditions. Commission the wrong one and you get a clean set of plots showing nothing, which tells the county archaeologist very little and usually leads to trial trenching anyway.
The published reference for survey design in England is the Historic England guidance on geophysical survey in archaeological field evaluation, and the work itself should be carried out to the Chartered Institute for Archaeologists standard.
What geophysics can and cannot detect
Geophysics detects contrast. A ditch filled with darker, more magnetic soil against a chalk subsoil shows clearly. The same ditch cut into a magnetically similar deposit does not show at all, however important it is.
That leads to the caveat that belongs in every report and every planning statement that relies on one. A negative survey result is not evidence that nothing is there. It is evidence that nothing was detected with that technique, in those ground conditions, at that sample interval. County archaeologists read these reports every week and they treat an overconfident negative with proper scepticism.
What geophysics does well is target the next stage. Used before trial trenching it turns a blind sample into an informed one, which usually costs less overall and is far more useful to a decision maker. The relationship between the two stages sits at the heart of an archaeological field evaluation.
Magnetometry and where it works
Fluxgate gradiometry, or caesium magnetometry on larger surveys, measures small variations in the earth's magnetic field caused by the differing magnetic susceptibility of soils and by thermoremanent features. It is fast, it covers large areas economically, and it is the default first choice for open agricultural land.
It works best on chalk, limestone and gravel geologies, where the contrast between feature fills and the natural subsoil is strong. It finds ditches, pits, kilns, hearths and burnt structures particularly well, and it finds stone walls poorly, because masonry often has little magnetic contrast.
It is defeated or degraded by:
- igneous and some highly magnetic geologies, where background variation swamps the archaeology
- deep alluvium or colluvium masking earlier deposits
- modern ferrous clutter, meaning fences, buried services, made ground, pipelines and vehicle tracks
- ridge and furrow or heavy ploughing that has homogenised the plough soil
Check the geology before commissioning. The British Geological Survey mapping is the quickest way to know whether a magnetometer survey is likely to be worth the money, and a desk-based assessment should have looked at it already, as our note on the archaeological desk-based assessment sets out.
Earth resistance and its conditions
Earth resistance measures the resistance of the soil to an electrical current, which varies mainly with moisture content. Walls, rubble spreads, roads and compacted surfaces read as high resistance, and ditches and pits holding moisture read as low.
It is the better technique for masonry, so it suits Roman buildings, monastic remains, deserted settlements and anything where stone structures are expected. Its drawbacks are speed, since traditional twin probe survey is slow compared with a cart mounted magnetometer, and sensitivity to soil moisture at the time of survey. Waterlogged ground gives poor contrast, and so does ground parched in a long dry summer, though drought sometimes produces striking results.
Seasonality therefore matters to the programme. Where a resistance survey is planned, allow flexibility in the date rather than fixing it against a submission deadline.
Ground penetrating radar on hard surfaces
Radar transmits a pulse and times the reflections, giving depth information that the other two techniques cannot provide. It works through tarmac and concrete, which makes it the technique for urban sites, car parks, churchyards, crypts, floors and vaults.
Its enemy is clay. Conductive, clay rich soils attenuate the signal quickly, sometimes to a useless depth, and high moisture content does the same. On a chalk or sand site with a hard surface, radar can produce excellent results in depth slices. On a wet clay site it may see almost nothing below the make-up layers.
Radar also generates far more data than the other methods and needs more processing and more interpretative judgement, so the cost per hectare is higher and the reporting takes longer.
Ground conditions that defeat all of them
Some sites are simply not suitable for geophysics, and recognising that early saves a wasted survey:
| Condition | Effect |
|---|---|
| Deep made ground or demolition rubble | Masks or mimics archaeological features across all techniques |
| Heavy contamination with ferrous debris | Magnetometry unusable, resistance and radar degraded |
| Standing buildings and dense obstructions | Coverage becomes patchy and unrepresentative |
| Deep alluvium over the horizon of interest | Features lie below the depth the technique reaches |
| Steep or heavily vegetated ground | Traverses cannot be kept regular, and data quality drops |
Where the site falls into one of these, say so in the evaluation strategy and go straight to intrusive work. A council is more persuaded by a reasoned explanation than by a survey that produced nothing.
Survey design, spacing and reporting
Resolution is a design decision with cost implications. The Historic England guidance sets out appropriate traverse separations and sample intervals for each technique, with closer spacing where small features are the target, and a survey carried out at wider spacing to save money can miss narrow ditches and graves.
A report worth relying on contains: the technique and instrument used, the traverse and sample intervals, the grid registration and how it was georeferenced, the raw and processed data plots with the processing steps listed, an interpretation drawing distinguishing archaeological, geological, agricultural and modern responses, and a statement of limitations naming the ground conditions on the day.
Interpretation is where value is added or lost. A plot with every anomaly labelled "possible archaeology" is not an interpretation. Annotated confidence levels, and a clear distinction between anomalies of probable archaeological origin and those probably geological, is what allows the next stage to be designed.
Using the results to target trenches
Targeted trenching tests the interpretation: does the anomaly correspond to a feature, what period is it, and how well does it survive. Blank area trenching tests the negative: is the apparently empty part of the site actually empty.
Both are needed. An evaluation that only trenches the anomalies cannot tell the decision maker anything about the rest of the site, and county archaeologists routinely ask for a proportion of trenches in blank areas for exactly that reason. The sample percentage expected varies between authorities and with the scale of the scheme, so agree the strategy in the written scheme of investigation before anyone mobilises.
Where the results change the picture, they can change the scheme. Geophysics that reveals an enclosure complex in one field can move a development footprint at a stage when moving it still costs little, which is the strongest commercial argument for doing the survey early rather than treating it as a condition response. What happens after the field work, including analysis and archive, is covered in our note on post-excavation assessment, archive and publication.
This is general guidance, not legal advice; survey design and interpretation should come from a suitably qualified archaeological specialist, and the strategy agreed with the local planning authority's archaeological adviser. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Which geophysical technique should be used first?
On open agricultural land over chalk, limestone or gravel, magnetometry is the usual first choice because it is fast, economical over large areas and good at detecting ditches, pits and burnt features. Where masonry is expected, earth resistance performs better, and on hard surfaced urban sites radar is generally the only viable option.
Does a negative survey mean the site is archaeologically empty?
No. It means nothing was detected with that technique, in those ground conditions, at that sample interval. Features can lack contrast with the surrounding deposits, lie below the depth reached, or be masked by made ground. Reports and planning statements that treat a negative result as proof of absence are routinely challenged.
Why does geology matter so much?
Because each technique relies on a physical contrast that the geology either provides or suppresses. Igneous and highly magnetic geologies swamp magnetometry, clay rich soils attenuate radar, and deep alluvium buries the horizon of interest. Check the British Geological Survey mapping before commissioning any survey.
What should a geophysical survey report contain?
The technique and instrument, the traverse and sample intervals, grid registration and georeferencing, raw and processed plots with the processing steps listed, an interpretation drawing that separates archaeological from geological, agricultural and modern responses, and a limitations section describing the ground conditions on the day of survey.
How are the results used to plan trial trenching?
Targeted trenches test whether anomalies correspond to real features and how well they survive, and blank area trenches test whether the apparently empty parts of the site are actually empty. Both are needed, and the sample percentage expected varies by authority, so agree the strategy in the written scheme of investigation first.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.