Geophysical survey: when it earns its place

Geophysical survey earns its place on an open, reasonably large site with sedimentary geology and no made ground, where the result can reduce or focus the trenching that follows. On a small urban plot, on deep made ground, or over igneous geology, it produces a report saying that conditions were unsuitable, and the curator asks for the same trench sample anyway. That is the whole decision, and it is worth making deliberately rather than by habit.

Cost per hectare is a small fraction of trenching, which is why it gets commissioned reflexively. The saving only materialises if the technique can work on that ground.

What magnetometry detects and what it misses

Fluxgate gradiometry, the standard technique on development sites, maps small variations in the magnetic field caused by differences between disturbed and undisturbed ground. It picks up ditches and pits filled with topsoil, hearths, kilns, burnt structures and ferrous objects very well.

It is much weaker on features with little magnetic contrast. Stone walls in a limestone landscape, dry stone structures, timber buildings leaving only postholes, and shallow features truncated by ploughing often do not register. Features buried under a metre of alluvium or colluvium are usually invisible.

So a blank magnetometer survey means the survey found nothing, and that is a different statement from the site being empty. Any report that presents a blank result as evidence of absence is overstating it, and a curator will say so.

Ground conditions that defeat the technique

The technique fails, rather than underperforms, in several situations that can be identified before commissioning:

• igneous and some volcanic geologies, where background magnetism swamps archaeological contrast
• made ground, demolition rubble and modern landfill
• hard standing, which prevents close instrument coverage and often conceals reinforcement
• proximity to services, buried cables, fences, overhead lines and parked vehicles
• heavily magnetic drift deposits, which vary regionally

Check the geology on a published map and check the site history before spending anything. A desk study will usually have covered both, which is why the Archaeological Desk-Based Assessment should always come first.

Resistivity and radar as alternatives

Earth resistance survey measures how readily the ground conducts a current, and it responds to moisture differences. Stone footings read as high resistance and ditches as low, so it complements magnetometry on exactly the sites where magnetometry struggles. It is slower and more expensive per hectare, and results shift with soil moisture, so the season matters.

Ground penetrating radar works on hard standing and inside buildings, gives depth information, and is the technique of choice in urban contexts and for vaults, cellars and floor surfaces. It performs poorly on wet clay, and it produces a large volume of data that needs skilled interpretation.

Electromagnetic and magnetic susceptibility techniques have narrower uses, largely in prospection and in characterising made ground. Match the technique to the question rather than to the price list.

Using the results to target trenches

The value of a survey is that it converts a uniform sampling problem into a targeted one. Anomalies get trenched to establish what they are, and the remaining coverage confirms what is happening between them.

Set the survey brief so that the output is usable for that purpose. Ask for anomalies to be reported with coordinates in the scheme grid, classified by confidence, and supplied as a digital layer rather than only as a figure in a report. A trenching contractor who has to scale positions off a printed plan will introduce errors of several metres, and those errors show up as trenches that miss the feature.

An illustrative example shows the effect on programme. A 14 hectare greenfield site is surveyed by magnetometry over three days. The survey shows a sub-rectangular enclosure with associated linears across roughly 2 hectares in the south west corner and a blank response elsewhere apart from agricultural trends. The trenching strategy that follows puts a dense array over the enclosure, a normal baseline array elsewhere, and the masterplan moves the attenuation basin onto the enclosure so that the remains stay under open space. The excavation that would have followed a design fixed before the survey is avoided entirely. This example is illustrative.

Where geophysics saves money and where it wastes it

It saves money on large greenfield sites where the curator's opening position is a percentage sample across the whole area and the survey can show that the potential is concentrated in one part. It saves money where the result supports avoiding remains through layout changes, because avoidance is almost always cheaper than excavation.

It wastes money where the ground conditions defeat it, where the site is small enough that trenching it directly costs little more, and where the curator has already said the trench sample will not change whatever the survey shows. Ask that question before commissioning, in writing.

Interpreting anomalies honestly

Report interpretation in graded language and keep the grades consistent. Anomalies of probable archaeological origin, anomalies of possible archaeological origin, anomalies of uncertain origin, agricultural trends, and modern or ferrous responses are the categories most reports use.

Resist the temptation to join dots into enclosures. A confident line drawing over an ambiguous dataset reads well and creates an expectation that trenching then has to disprove, which costs money and credibility. Where a response could be a ditch or a geological feature, say both and let the trench decide.

The same discipline applies downstream. Where remains are identified and the scheme would affect them, the assessment of harm follows the ordinary heritage tests, including the substantial harm and justification test where a designated asset is involved.

Report negative areas explicitly as well. A statement that no anomalies of archaeological interest were detected in a defined block, with the survey coverage plotted, is what supports a later argument for reduced trenching over that block. A report that only maps the interesting part gives the curator nothing to agree to.

Reporting standards and what to ask for

Commission against the professional standard for archaeological geophysical survey and the national guidance on the use of geophysics in field evaluation, and say so in the brief. A compliant report includes the survey methodology, instrument make and model, traverse and sample intervals, processing steps applied, unprocessed and processed greyscale plots, an interpretation plan and a statement of confidence.

Processing is where surveys go wrong quietly. Aggressive filtering can remove real weak anomalies along with the noise, so ask for the minimally processed data to be presented alongside the interpretation.

Timing affects quality more than most briefs acknowledge. Magnetometry can be carried out in most conditions, but not over standing crops, waterlogged ground or lying snow, and not usefully on a field that has just been ploughed and left rough. Earth resistance results vary with soil moisture, so a survey in a dry August and the same survey in a wet March will not read the same. Build the survey into the seasonal programme alongside the ecology and the trenching rather than treating it as something that can be slotted in at any point.

Deciding whether to commission it at all

Work through four questions. Does the geology support the technique. Is the ground accessible and clear of made ground, hard standing and services. Is the site large enough that focusing the trenching will save more than the survey costs. And will the curator actually reduce the trench sample on the strength of a result.

If three of those are yes, commission it. If two or more are no, spend the money on trenching and put the reasoning in the Heritage Statement so the decision is on the record rather than looking like an omission.

This is general guidance, not legal advice. Check the requirement with your local planning authority and its archaeological adviser 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. For specialist technical reports, qualified professional input may be required.