Geophysical survey in archaeology: what it can find

Geophysical survey detects contrast, not archaeology. A magnetometer picks up differences in magnetic susceptibility between a filled ditch and the soil around it, and if that contrast is weak, the ditch is invisible whether or not it is there. Understanding that one sentence saves developers from the two mistakes that cost money: treating a clear survey as proof of an empty site, and paying for geophysical survey archaeology planning work on ground where the technique was never going to produce a readable result.

The techniques and what each detects

Three methods do most of the work on development sites in England.

Magnetometry, usually with a fluxgate gradiometer, is the standard first pass across open ground. It responds to burning, to accumulations of topsoil-derived fill, and to iron, so it finds ditches, pits, kilns, hearths and industrial features well. It covers ground quickly, which is why it suits sites measured in hectares.

Earth resistance survey measures how readily current passes through the soil, so it responds to moisture differences. Walls, rubble spreads, metalled surfaces and floors show as high resistance, ditches and pits as low. It is slower than magnetometry and more sensitive to recent weather.

Ground-penetrating radar transmits pulses and times the reflections, which is the only one of the three that gives depth information. It works well on hard standing, inside buildings and on dry sand and gravel, and poorly in clay, where signal attenuation limits penetration to shallow depths. Electromagnetic induction and magnetic susceptibility survey have their uses, mostly as complements rather than substitutes.

Ground that defeats a survey

The geology and the recent land use decide the outcome more than the equipment does.

A brownfield site with a demolition phase in living memory is usually a poor candidate for magnetometry, and the honest recommendation is to go straight to targeted trial trenching. Consultants who survey it anyway produce a report full of modern ferrous anomalies and an inconclusive summary, and the county archaeologist asks for trenches regardless.

Where geophysics sits in the sequence

The evaluation sequence in England runs from desk study to non-intrusive survey to intrusive evaluation, each stage narrowing the question. The Archaeological Desk-Based Assessment comes first: Historic Environment Record data, historic maps, aerial photography, geology and previous fieldwork. That tells you what to expect and whether geophysics can plausibly detect it.

The survey then goes into a written scheme of investigation agreed with the council's archaeological adviser, usually the county archaeologist or a Greater London Archaeological Advisory Service adviser. Doing the survey before permission gives you a better negotiating position on conditions. Doing it under a pre-commencement condition means the result lands when the design is fixed and cannot respond to it.

Trial trenching then targets the anomalies and, critically, samples the blank areas as well. Any Archaeological Field Evaluation scheme that only trenches the anomalies will be sent back.

Interpreting anomalies without over-claiming

A competent report classifies responses rather than asserting them. Anomalies are typically described as archaeological, probably archaeological, uncertain, natural or modern, with the reasoning given for each. A report that labels a scatter of dipolar responses as a Romano-British enclosure without qualification is not doing the reader any favours, and the county archaeologist will discount it.

Two categories of archaeology are routinely invisible. Inhumation burials without a grave cut of contrasting fill rarely register. Timber structures, stake holes and stone-free features in similar soil produce almost no contrast. On sites where those are the expected asset type, the survey is close to pointless and the money is better spent on trenches.

Standards matter here because they set what a report must contain. Historic England's guidance on geophysical survey in archaeological field evaluation and the relevant Chartered Institute for Archaeologists standard both require the instrument, traverse interval, sample interval, processing steps and interpretation basis to be stated. Ask for those in the brief.

Why a clear result rarely closes the question

Councils treat geophysics as one line of evidence towards a proportionate assessment of significance, not as a clearance certificate. The historic environment chapter of the current National Planning Policy Framework, published on 17 August 2026, expects the applicant to describe the significance of any heritage asset affected, including archaeological interest, at a level of detail proportionate to the asset and sufficient to understand the impact. A negative geophysical result on ground where the technique is unreliable does not meet that.

So the practical expectation on a greenfield site of any size is a condition requiring trial trenching, or a pre-determination evaluation where the archaeological potential is high. On sites near a scheduled monument, the Ancient Monuments and Archaeological Areas Act 1979 adds separate consent requirements for works within the scheduled area, and those cannot be conditioned away.

Timing and ground conditions

Geophysics is not season-locked in the way ecology surveys are, but conditions matter. Arable sites need to be after harvest and, for magnetometry, ideally on stubble or a light cultivation rather than deep ploughing or a standing crop. Resistance survey needs soil moisture in a usable range, so a long dry spell or saturated ground both degrade results. Frozen ground is unusable, and a survey pushed into January to suit a planning deadline produces data nobody can rely on.

The programming lesson: book the survey window against the farming calendar, not against your design programme. On a tenanted arable site, the accessible window between harvest and drilling can be a fortnight, and missing it can mean a year.

Using the results in a submission

The survey report goes in as a technical appendix, with its interpretation summarised in the heritage section of the Heritage Statement or planning statement. Say plainly what was surveyed, what was found, what the technique could not have found, and what is proposed as a result. Where the outcome is a trenching condition, propose the wording yourself.

One coordination point that gets missed on larger sites. Evaluation trenches, habitat creation areas and attenuation basins compete for the same open ground, and a trench line through a proposed biodiversity parcel creates an argument nobody needs. Our note on archaeology and biodiversity net gain on the same site sets out how to resolve that overlap at layout stage. On sites with wider visual sensitivity, the setting question is separate again, and is dealt with in our piece on World Heritage Site buffer zones and visual impact.

Geophysical survey is worth commissioning where the geology gives you a chance, the expected asset type produces contrast, and the result can still influence the layout. Otherwise, spend the fee on trenches and get an answer.

Budget for both on a greenfield site of more than a hectare or two. The survey narrows the trenching, but it seldom removes it, and a programme that assumes otherwise will slip.

This is general guidance, not legal advice; the scope of evaluation on any site should be 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, and archaeological fieldwork requires a suitably qualified archaeological contractor.