Archaeological Project Services

Geophysical Survey :: Introduction

Geophysical survey represents a cost-effective approach to archaeological evaluation allowing an assessment of the potential of the site to be quickly made and any subsequent intrusive investigations to be more effectively targeted.
 
In many circumstances this can be an effective first step in evaluation. However, these techniques work best on green-field sites and responses do vary with background geology. For an assessment of the suitability of the technique in your case, please call or contact one of our team of project managers.

Range of recent Geophysical surveys
Range of recent Geophysical surveys

 
Magnetic survey carried out using a dual sensor Grad601-2 Magnetic Gradiometer

APS has carried out geophysical survey on sites ranging from small plots of c. 1ha up to larger scale development of over 50ha ranging from housing developments in Cambridgeshire and Lincolnshire to quarry sites in North Yorkshire and Kent, from pipeline routes in Leicestershire to historic parkland survey in Berkshire.

Magnetic survey is carried out using a dual sensor Grad601-2 Magnetic Gradiometer manufactured by Bartington Instruments Ltd. This consists of two high stability fluxgate gradiometers suspended on a single frame.


Each gradiometer has a 1m separation between the sensing elements so enhancing the response to weak anomalies. Although the changes in the magnetic field resulting from differing features in the soil are usually weak, variations as small as 0.2 nanoTesla (nT) in the overall field strength of 49,000nT can be accurately detected using this instrumentation.

The mapping of magnetic anomalies in a systematic manner provides an indication of the type of material present beneath the surface. Strong magnetic anomalies are generated by buried iron-based objects or by highly-fired structures such as kilns or hearths. Pits and ditches can also be seen well where their fills contain more topsoil which is generally richer in magnetic iron oxides and provides a contrast with the natural subsoil. However, these anomalies are more subtle and response can vary depending on the nature of the underlying deposits. Wall foundations can show as negative anomalies where the stone is less magnetic than the surrounding soil, or as stronger positive and negative anomalies if of brick, but are not always responsive to the technique. Resistivity is the preferred technique in these circumstances, and can also be offered, but the survey process is slower and this is not as useful for large area prospection.

Processing is performed using specialist ArcheoSurveyor software. This can emphasise various aspects contained within the data which are often not easily seen in the raw data. Basic processing of the magnetic data involves 'flattening' the background levels with respect to adjacent traverses and adjacent grids. 'Despiking' is also performed to remove the anomalies resulting from small iron objects often found on agricultural land. Once the basic processing has flattened the background it is then possible to carry out further processing which may include low pass filtering to reduce 'noise' in the data and hence emphasise the archaeological or man-made anomalies.

 

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Survey in progress at Ilam Hall

Survey at Willow Tree Farm