Chartered Institute for Archaeologists 2023

P R O F E S S I O N A L A R C H A E O L O G Y | A G U I D E F O R C L I E N T S 2 0 2 3 3 5 CASE STUDIES | MANAGING ARCHAEOLOGY: Evaluation techniques that reduce uncertainty THE VALUE OF GEOPHYSICS IN THE PLANNING PROCESS Multi-channel GPR survey in progress at Queen Anne’s house in Greenwich, London (NHLE 1002060). The survey was undertaken using an Impulse Radar Raptor array, which contains eight transmitter and receiver antennae spaced 8cm apart, with a central frequency of 450 MHz. Credit: Wessex Archaeology Greyscale plot and interpretation of multi-channel GPR survey from Queen Anne’s house in Greenwich, illustrating the location of the observation towers of King Henry’s tiltyard. Digital data reproduced from Ordnance Survey data. Today, geophysical survey plays a major role in developer-funded archaeology. It is now regularly deployed over vast areas, with preliminary results normally available shortly after completion. This allows an initial assessment of the potential archaeological impact of a development scheme and facilitates a proactive planning approach that can maximise available resources and time. Surveys can be undertaken pre- planning or ahead of land purchases to inform development design and potentially reroute schemes if significant remains are encountered. Effective interpretation of these datasets helps to focus resources in subsequent phases of investigation, either through the targeted application of complementary geophysical survey methods or by informing the location of intrusive evaluation or mitigation strategies. This can reduce costs for the client and provide enhanced detail of any archaeological remains that may be preserved in situ. For example, at the development site shown in the greyscale plot of a magnetic gradiometer survey (see greyscale image on page 34), an extensive and complex array of enclosures were discovered, with those in the east of the site forming a ladder settlement. These were dated to the Iron-Age and Romano- British periods in subsequent evaluation trenching. The clarity and detail provided by the survey meant that the design of the development could be adjusted, leaving the focus of the settlement outside of the impact of the scheme. The most widely used geophysical method in the UK is magnetic (fluxgate) gradiometer survey. This is because it responds well to the broadest range of archaeological features, is effective in most rural environments and can cover large areas quickly. Although results can be poor on some geologies and where there are extensive superficial deposits (for example alluvium), deeper geophysical methods, such as lower-frequency ground-penetrating radar (GPR), electrical resistivity tomography (ERT) and electromagnetic induction (EMI) can delineate landforms and subsurface variation, which in turn can be related to archaeological potential. The application of appropriate methods in different landscape settings can therefore be a powerful tool in managing the impact of developments on the historic environment.