Geophysical survey for archaeology: when it works

An archaeological geophysical survey tells you where to put the trenches. It does not tell you whether archaeology is present, and a report that says "no anomalies of archaeological interest identified" is not a clean bill of health. On the wrong geology, magnetometry can walk straight over a Roman villa and detect nothing at all, which is why a county archaeologist who knows the local ground conditions will sometimes decline to accept a geophysical survey as the whole evaluation.

Used properly the technique earns its fee many times over. It is non-intrusive, it covers whole fields quickly, and it converts a trenching programme from a statistical sample into a targeted exercise. The skill lies in knowing when it will work.

What the techniques detect

Three methods account for nearly all commercial archaeological survey in England.

Magnetometry measures small variations in the earth's magnetic field caused by buried features. It responds well to anything that has been burned, so hearths, kilns, furnaces and fired brick show clearly, and to ditches and pits where topsoil rich in magnetic minerals has accumulated in a cut feature. It is the default for large areas because a two-person team can cover several hectares a day.

Earth resistance measures how easily current passes through the ground, which depends on moisture. Walls, rubble and metalled surfaces resist current and read as high resistance. Ditches and pits holding damp fill read as low. It is slower than magnetometry and better suited to smaller areas and to masonry sites.

Ground penetrating radar sends a pulse into the ground and records reflections, giving depth information the other two cannot. It is the most expensive per hectare, works best on dry sandy or gravel ground, and is defeated by heavy clay. On a complex urban site or where vertical stratification matters, it is often the only option worth having.

Ground conditions that defeat magnetometry

This is the part commercial reports gloss over and the part that decides whether the survey is worth commissioning.

Magnetometry struggles where the contrast between feature and surrounding soil is weak. On some chalk and limestone, and on certain brickearths and alluvial soils, a ditch filled with material similar to its surroundings produces almost no measurable response. Igneous geology produces so much background magnetic noise that archaeological signals are lost in it.

Modern interference is the other defeat. Buried services, fence lines, field gates, scrap metal, pylons, railway lines, former agricultural buildings and the magnetic halo around a modern track all generate responses far stronger than any archaeological feature. A site that has been used for storing farm machinery may be effectively unsurveyable by magnetometry.

Ask the contractor, before commissioning, what the technique is expected to achieve on your specific geology and whether they have local comparative results. A contractor who says the technique may be of limited value on this soil is giving you the most useful answer available, and should be listened to rather than replaced.

Resolution, coverage and transect spacing

The survey specification determines what can be seen, and it is where cost is cut invisibly.

Magnetometer data is normally collected along transects at a stated spacing, with readings taken at a much finer interval along each line. Wider transect spacing covers more ground per day and misses narrow features running parallel to the lines. A narrow gully or a wall foundation can sit between two transects and never register.

Three questions therefore belong in the brief. What transect spacing and sample interval will be used, and does it meet the standard appropriate to the archaeology expected. What proportion of the site will be covered, and is coverage continuous or in sample blocks. And how will the survey handle areas that cannot be walked, such as standing crops, woodland, hardstanding and the margins along hedges, where a surprising amount of archaeology sits.

Partial coverage is not automatically wrong, but the unsurveyed areas have to be declared on the plan and carried forward into the trenching design rather than quietly treated as blank.

Interpreting anomalies honestly

A good report distinguishes what was measured from what it is thought to mean, and uses graded confidence.

The interpretation plan should classify anomalies as archaeological, probable archaeological, uncertain, agricultural, natural or modern, and the accompanying text should say why each judgement was reached. Ridge and furrow, land drains laid in a regular herringbone, plough scarring and geological variation all produce patterns that an inexperienced interpreter reads as archaeology, and the reverse mistake, dismissing a genuine feature as a land drain, is equally common.

Treat the greyscale plot as the primary evidence and the interpretation as opinion. Ask for both, keep both, and expect the county archaeologist to look at the raw plot.

One honest limitation: absence of anomaly is weak evidence of absence of archaeology. Burials, timber structures with no magnetic contrast, truncated features and deposits sealed beneath alluvium can all be invisible. On a site with deep alluvial or colluvial cover, the survey may be reading only the recent ground surface.

Using the results to target trenches

This is the pay-off. A trenching scheme designed from geophysics does three things at once.

It places trenches across identified anomalies to characterise and date them, because a ditch on a plot tells you nothing about period or significance until something is recovered from it. It places trenches in apparently blank areas to test the negative result, which is the step most often omitted and the one that protects you later. And it uses the layout of anomalies to decide where the scheme can afford to put built form.

Set the sequence out in the Archaeological Desk-Based Assessment and the written scheme of investigation, with the geophysics report appended. Where the results indicate significant buried remains, an Archaeological Field Evaluation follows as the intrusive stage. Councils expect a suitably qualified author for both, and professional review of any draft is strongly advised.

When not to commission a survey

Four situations where the money is better spent elsewhere.

On a small site, a householder plot or anything under about half a hectare, where the area is too small for pattern recognition and a single trial trench gives better information for less money. On geology where the contractor advises the technique will not resolve features, in which case go straight to trenching or to radar. On land heavily disturbed by modern activity, where the data will be dominated by interference. And on sites where the desk study already establishes the archaeology, since survey adds precision rather than knowledge and the county archaeologist may want excavation regardless.

Urban sites are a particular case. Made ground, services and cellars usually render magnetometry and resistance useless, and the realistic options are documentary research, radar in open areas, and targeted test pits. A Heritage Statement for an urban site should explain why geophysics was not undertaken rather than leave the omission unaddressed.

Reporting and archiving

The report should name the instrument and its configuration, state the transect spacing and sample interval, give the survey grid tied to the national grid, present raw and processed greyscale plots with the processing steps listed, provide the interpretation plan with the graded categories, and set out the limitations encountered on site.

Beyond that, the data has to be archived. The relevant professional standards expect the digital dataset to be deposited with the local historic environment record and with a recognised national archive, and the survey to be reported into the national record of archaeological investigations. Make deposition a contractual requirement, because a survey that exists only as a PDF on a consultant's server has lost most of its long-term value. Where structures rather than buried deposits are involved, the same logic applies to recording techniques, as our note on laser scanning and photogrammetry before demolition sets out.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Enter your postcode on PlanScope to see which reports your site is likely to need.

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