Archaeological evaluation or a geophysical survey

Geophysical survey and trial trench evaluation answer different questions, and on most sites the answer to archaeological evaluation or geophysical survey is both, in that order. Geophysics maps anomalies across a large area quickly and without breaking ground. Trenching tells you what those anomalies actually are, how deep they sit, how well they survive and roughly when they date from. One narrows the search. The other produces the evidence an officer can make a decision on.

Both are carried out under a written scheme of investigation agreed in advance with the authority's archaeological advisor, and that document matters more than either technique.

What a geophysical survey detects

Geophysics measures physical contrasts in the ground and plots them. It does not find archaeology directly. It finds places where the soil differs from its surroundings in a property the instrument can read, and an archaeologist then interprets those patterns.

The dominant technique on open land is magnetometry, usually with a fluxgate gradiometer. It detects variation in magnetic susceptibility, which picks up ditches, pits and other cut features backfilled with topsoil, along with kilns, hearths and burnt material. Coverage of several hectares a day is achievable on clear ground.

What it cannot do is date anything, and it cannot distinguish a medieval field boundary from an Iron Age enclosure ditch except by the shape of the pattern. Nor does it see features with no magnetic contrast, which includes many stone structures, graves cut into similar material and anything beneath deep alluvium or made ground.

An Archaeological Desk-Based Assessment comes first in any case, because it establishes what is already known from the historic environment record and determines whether survey is needed at all.

Techniques matched to geology

Choosing the wrong technique wastes the whole fee, and the choice turns on geology and ground conditions.

Two practical points follow. On a clay site, a magnetometer survey that returns little may be telling you about the clay rather than about the archaeology, and the report has to say so. And on urban or brownfield land, geophysics is often skipped altogether in favour of boreholes, test pits and a deposit model, because made ground defeats all three techniques.

Survey conditions matter. Standing crops, waterlogging, livestock and modern surface debris all reduce coverage, and the field report should record what was surveyed and what was not.

What trial trenching establishes

Trial trenching is intrusive and gives you facts rather than anomalies. An Archaeological Field Evaluation by trenching establishes presence or absence, the depth and thickness of deposits, the state of survival, the stratigraphic sequence, and a date range from finds and samples.

That is what a decision maker needs. An authority can condition a scheme on the basis of a known resource. It cannot sensibly condition on a magnetic anomaly of unknown character.

Trenching also produces the information that drives cost later. Whether the archaeology is a thin ploughed-out scatter or a deeply stratified sequence with waterlogged organic preservation changes the mitigation strategy from a watching brief to a full excavation, and that difference is the single largest variable in the project's archaeology budget. The factors behind it are set out in more detail in what drives archaeological evaluation costs.

Sampling percentages and layout

Evaluation trenching samples a proportion of the site, and the proportion is negotiated rather than fixed by any national rule. Figures in the low single digits as a percentage of the application area are typical, with higher sampling on sensitive sites and lower on large rural schemes where only part of the area will be disturbed.

Layout matters as much as percentage. Three approaches get combined.

1. Targeted trenches placed on geophysical anomalies, to characterise and date what the survey found.
2. Blank-testing trenches placed where the survey showed nothing, to test whether the absence of anomalies means the absence of archaeology.
3. A grid or staggered array across areas that could not be surveyed, so coverage is not biased towards the surveyable ground.

Argue the percentage against the development's actual impact rather than against the red line. A scheme that disturbs only the footprint of a building and an access has a different impact from one that strips the whole site, and the sampling strategy should reflect that. The reasoning and the usual counter-arguments are covered in trial trench sampling percentages and how to justify them.

Written schemes of investigation

Neither technique proceeds without a written scheme of investigation approved by the archaeological advisor, usually the county archaeologist or the authority's own specialist, and in London often by the relevant advisory service.

A scheme that gets approved first time sets out the aims and objectives, the methodology including instrument, traverse spacing and data processing for geophysics or machine, trench dimensions and hand excavation protocols for trenching, the sampling strategy with its justification, staffing and who will supervise, the approach to finds, environmental samples and human remains if encountered, monitoring arrangements and notice periods for site visits, reporting timescales, and the archive and publication arrangements including the online record.

Write it to the relevant professional standards and guidance published by the Chartered Institute for Archaeologists, because that is the benchmark the advisor applies. Councils expect a suitably qualified archaeological contractor to prepare and deliver it, and professional review is strongly advised on any draft. Where the work is being done to discharge a condition rather than before determination, the sequence differs, and discharging an archaeological condition through the written scheme sets out that route.

Which the county archaeologist asks for

The pattern is fairly predictable once you know what the advisor is weighing.

Geophysics alone is asked for where the desk-based assessment shows moderate potential, the site is open farmland on suitable geology, and the advisor wants to know whether there is anything there before deciding how far to go. On a large rural site this is the proportionate first step.

Trenching is asked for where the potential is high, where geophysics has found something that needs characterising, where the geology defeats geophysics, or where the site is small enough that survey adds little. It is also the normal requirement where the advisor wants a pre-determination answer rather than a condition, which happens on sites near scheduled monuments, in archaeological priority areas and where the scheme would cause total loss of any remains present.

Both are asked for on most greenfield majors, run as a sequence with the trenching informed by the survey.

Ask the advisor directly at pre-application stage what they want and why. They will tell you, and a scheme designed around the answer avoids the far more expensive outcome of a refusal for inadequate information about the heritage resource.

Programme and cost comparison

The programme difference is the thing to plan around.

A geophysical survey of a moderate rural site is days of fieldwork plus a short reporting period, and it can be done in most weather and most of the year subject to crop and ground conditions. Trial trenching is weeks rather than days once you account for utility searches, machine and operator availability, environmental constraints such as nesting birds in hedgerows, spoil storage, reinstatement and the post-excavation assessment of finds and samples.

Costs follow the same shape. Geophysics is priced largely by area surveyed. Trenching is priced by trench metreage, machine time, staff days and the post-excavation work, and the last of those is open-ended because it depends on what is found. A trenching programme on a site with waterlogged deposits can cost several times the original quotation through specialist analysis alone, which is why the contract should deal with contingency explicitly.

So the sequence saves money rather than spending it. Survey first, use the results to target and to reduce the trenching, and keep the written scheme flexible enough that the advisor can agree a revised layout without a fresh approval round.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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