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How To Save Time and Money With Geophysical Surveys…

Updated: Jan 31

It is a well recognised fact that trenching can be both an expensive and a ‘hit-and-miss’ method to assess the potential archaeology on a site.

We therefore asked SUMO's resident geophysical survey expert Dr John Gater, how clients can save their time and money using geophysical surveys?

Geophysical Survey

So, tell us how geophysical surveys can work in conjunction with trenching?

Normally, geophysical surveys are part of a staged approach to assess both the extent and nature of any archaeological remains on a site. This would usually be before development. The survey work is often carried out alongside (or after) a Desk-Based Assessment (DBA). This looks at existing site records, old photographs, museum archives, past excavations and fieldwalking exercises.

The DBA and the geophysics surveys can then provide an assessment of the potential archaeology present, before test trenching. Trenching is an expensive exercise. Especially if carried out 'blind' and digging sample trenches over a site without first referencing known features or geophysical anomalies, is arguably, an inefficient way for clients to proceed. If archaeology is then found in the random test trenches, it is often necessary to investigate further, incurring yet more costs.

At SUMO, we believe that it is far quicker and cost-effective to complete the geophysical survey work before test trenching commences. SUMO target anomalies identified using the geophysics data, thereby focusing the test trenching. With this approach, a 2% sample may be sufficient, opposed to the more normal 5% sample of trenching, thereby saving significant costs.

The images below are an example of excavation based on geophysical survey data:

Resistance survey showing a Roman villa

Mosaic – resistance survey showing a Roman villa. The red circle shows the area where the trial trench was located. The villa has two wings - in the top right of the data image the rooms and corridors are visible and the indications are that the floors are simply beaten earth between stone / flint walls. By contrast, the wing on the left of the plot shows ‘solid’ floors – and these proved to be intact mosaics, albeit damaged by recent ploughing.

Magnetic data plot

Kiln – the magnetic data plot shows a small Romano-British settlement, comprising individual land parcels (for workshops, houses and shops) separated by tracks and roadways. The stronger magnetic anomalies (‘blobs’) indicate large rubbish tips, quarry pits and small industrial features. A classic ‘kiln-like’ anomaly was selected for excavation as it was hoped any associated pottery would provide a date for the settlement; it was exciting to see a small intact kiln structure surviving below the turf.

Can you explain how geophysics can save clients both time and money?

Most of the time, clients can save time and money by trenching a smaller targeted % sample.

Occasionally, archaeological units argue that when topsoil is stripped (as part of the development work), any archaeology found can be dealt with at that time. But, this approach means that if a ‘major’ archaeological discovery emerges at this late stage, then extra resources will have to deal with that archaeology. This would cost yet more time and money.

Dr John Gater

If a geophysics survey is carried out in advance of the site work, then the extent and nature of the potential archaeology can be understood. Contingencies can be put in place to maintain efficiency and avoid unnecessary additional costs. For example, potential archaeology identified by the geophysics survey can be dealt with in advance of any site work by selected excavation or by redesigning the site plans, so that core areas of archaeology can be left undisturbed and preserved.

The fact that geophysical surveys are much cheaper than excavation, means that they can save developers large sums of money ahead of site works.

How has geophysics advanced to allow for time and cost savings?

In the past, geophysical surveys used sample areas only. But as technology advanced, it became the norm for complete site surveys to be undertaken. Looking to the future, we may see surveys return to a more selective sample of areas. This could be with further evaluation of key areas of interest being undertaken with a second geophysical technique to further identify any potential archaeology. For example, a Ground Penetrating Radar (GPR) survey could follow the completion of a magnetic survey. The magnetometry data can define key interest areas such as building outlines, following which the GPR could be used to provide detailed plans and depth information about buried structures. See example below:

Magnetometry data

Above: Example of magnetometry data. This survey technique was used to locate areas of interest at Brancaster Roman Fort.

Ground Penetrating Radar (GPR) data

Above: Example of Ground Penetrating Radar (GPR) data collected across Brancaster Roman Fort. The areas which were surveyed using GPR were informed by the magnetic survey data (pictured above).

GPR data from Brancaster Roman Fort

Above: This image shows the GPR data from Brancaster Roman Fort overlaid onto the magnetometry data. The correlation between the selected GPR survey area and the larger magnetic survey data can be clearly seen here.

So remember, if you are planning to develop a greenfield site and want to avoid unnecessary trenching work, with potential site delays and increased costs associated with areas of potential archaeology, then contact us today to discuss your survey requirements further:

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