Archaeological geophysics and trenching seasons

Archaeological geophysics timing is decided by two things you do not control: what is growing on the field and how wet the ground is. Magnetometry over stubble in September is straightforward. The same survey over a standing wheat crop in June is either impossible or unreliable, and the same field under standing water in January will produce an earth resistance dataset nobody can interpret.

That is why archaeology sits so awkwardly in a development programme. The desk-based assessment can be written any week of the year. The field evaluation that follows it has windows, and those windows are set by the farming calendar rather than by your planning committee dates.

Why geophysical survey depends on ground conditions

Each technique measures a different physical property, so each fails in a different way.

Magnetometry detects small variations in the magnetic field caused by burning, by the accumulation of topsoil in ditches, and by iron. It works in most conditions provided the surveyor can walk a consistent line at a consistent sensor height. What defeats it is vegetation tall enough to prevent that, and modern iron in the plough soil.

Earth resistance survey passes a current through the soil and measures resistance, which depends on moisture. It needs a contrast between the moisture held in a feature and the moisture in the surrounding soil. Saturated ground gives no contrast because everything conducts. Baked ground in a long drought gives none because nothing does. The usable periods sit between those extremes, typically from late spring into early autumn.

Ground penetrating radar needs relatively dry, low clay conditions and a surface the antenna can be dragged across. It is the technique of choice over hard standing and short grass, and the least useful on a heavy clay site in winter.

So the first question to ask the geophysics contractor is not when they are available. It is which technique the ground and the archaeological question require, because that determines the window.

Crop cover, stubble and standing water

For the surveyor, a field is in one of about five states, and they are not equally useful.

1. Bare ploughed ground or stubble: best for magnetometry, good for resistance if moisture allows.
2. Short grass or a young cereal crop under roughly 0.3 metres: usable for most techniques.
3. Tall standing crop: unusable for resistance, unreliable for magnetometry, and the farmer will object anyway.
4. Oilseed rape or maize at height: effectively a closed site.
5. Waterlogged or frozen ground: resistance and radar both unusable.

The practical consequence is that the September and October window, after harvest and before the ground gets too wet, is the one everybody wants. It is also short, and contractors book it early. If a scheme needs geophysics in that window, instruct in spring.

Recent manure or sewage sludge spreading is worth asking about specifically. A dressing of ferrous-rich material across a field makes magnetometry noisy for a season, and the farmer knows what went on and when. Nobody thinks to ask.

Frozen and saturated ground

Frozen ground blocks resistance survey because the probes will not enter and the measurement is meaningless if they do. It also makes trial trenching slow and the recording poor.

Saturated ground is the bigger programme risk because it persists. On a clay site, a wet December and January can put the field out of use until March, and the decision to wait is usually right. An evaluation carried out in the wrong conditions produces a report that a county archaeologist can reasonably question, and repeating it costs more than the delay would have.

The same logic applies to ground investigation for geotechnical purposes, which shares the access constraints, and our note on winter ground investigation sets out where the two programmes can share a mobilisation.

Trenching, reinstatement and crop compensation

Trial trenching is a machine operation with a spoil heap, so it needs firm ground for the excavator and the dumper, space to store topsoil and subsoil separately, and dry enough conditions to see and record what is in the trench section.

Where trenching has to happen through a growing crop, the occupier is entitled to be compensated for the crop lost and for the disturbance, and on tenanted land the tenant rather than the landowner holds that interest. Settle the figure and the reinstatement specification in writing before the machine arrives. Reinstatement means subsoil back first, compaction relieved, topsoil replaced to the original depth, and the trench line rolled, and it is the part that generates most of the ill will when it is done badly.

Allow for the possibility that trenching finds something. A programme that assumes evaluation ends when the trenches are backfilled has no float for the mitigation that follows.

Working around the farming calendar

The farming year gives a pattern that repeats, and building the programme around it removes most of the friction.

Late summer and early autumn, after harvest, is the primary window for both geophysics and trenching. Spring, once the ground has dried and before the crop is established, is the secondary window and it is narrower. Midsummer is available only on grass or with crop compensation agreed. Midwinter is possible on free-draining soils and risky elsewhere.

Ground nesting birds add a constraint that catches people out. Where access requires hedgerow removal or where the field itself supports breeding birds, work between March and August needs an ecological check first. Bird nests in use are protected under the Wildlife and Countryside Act 1981, and a suitably qualified ecologist should confirm the position rather than a site foreman.

Illustrative example: a 14 hectare arable site where the desk-based assessment is written in February, geophysics is instructed in March for delivery in late August after harvest, targeted trenching follows in September and October, and the evaluation report reaches the county archaeologist in November for a January committee. Dates here are illustrative.

Lead times for a written scheme of investigation

Nothing happens on site until a written scheme of investigation has been agreed with the authority's archaeological adviser, which in most of England is a county council archaeology service or a city archaeologist.

That approval is a real programme item. The scheme sets out the research aims, the trench positions and percentage sample, the recording standards, the specialist arrangements and the reporting and archive deposition. Expect to allow several weeks between submitting a scheme and getting it agreed, plus contractor mobilisation on top. Where the trench layout is being designed off geophysics results, the scheme cannot be finalised until the survey report is in.

The professional framework here is set by the CIfA standards and guidance, and councils expect a suitably qualified author. PlanScope's Archaeological Field Evaluation report is a starting point for that specialist rather than a substitute, and professional review is strongly advised.

Sequencing evaluation before determination

Where a site has archaeological potential, the expectation in national policy is that the potential is understood before the decision is made rather than deferred wholesale to a condition. A desk-based assessment comes first, and PlanScope's Archaeological Desk-Based Assessment report covers that stage, drawing on the Historic Environment Record.

Pre-determination evaluation is what the authority will ask for on a sensitive site, and the season means that request can add months if it arrives late. Two moves reduce that risk. Commission the desk-based assessment at the point the site is acquired or optioned, not at the point the application is drafted. And ask the archaeological adviser at pre-application stage whether they will want evaluation before determination, so the field window can be booked rather than chased.

Where scheduled monuments are involved, scheduled monument consent under the Ancient Monuments and Archaeological Areas Act 1979 is a separate consent from planning permission and has its own timescale.

Budgeting across the boundary between two accounting years is the other recurring problem on multi-season programmes, and our note on planning a survey budget across two financial years deals with that.

Enter your postcode on PlanScope to see which reports a site with archaeological potential is likely to need. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.