Our final #BSW17 post come from Erica Carrick (EMC Radar Consulting) and Rachael Hall (National Trust). Erica describes how Ground Penetrating Radar is being used to assess the internal structure of the Marble Hall at Kedleston Hall, Derby.
For some time, the National Trust has been concerned about possible distortion in the floor of the beautiful Marble Hall at Kedleston. Walking across the floor gives the impression that the floor may be deflecting in certain areas. EMC Radar Consulting was commissioned by the National Trust to carry out a GPR survey of the floor to examine its structure and confirm the findings of a laser survey carried out previously.
GPR works by transmitting radio waves into the ground. Every time that these electromagnetic pulses meet a change of materials, part of the signal is returned to the receiver antenna. As the radar crosses the floor, a vertical pattern of the boundaries between the different materials below the surface is built up. The radar cannot identify the materials but it does distinguish where one type of material ends and another begins so we are able to look at the internal structure of the floor.
This is essentially the same technology as is used in other types of radar but the wavelength of the radio waves is much shorter than, for example, aerial radar. Since the floor itself is a relatively thin structure, it is essential to use a high frequency radar, in this case a 4GHz system. High frequency radars have very short wavelengths. This means that they cannot be used for deep investigations since the number of wavelengths transmitted is limited (independently of the frequency). However, since radars measure in terms of their wavelength, they have the advantage of being able to detect and measure on a much finer basis than a typical radar that might be used on an archaeological site in open ground.
The two radar traces shown are vertical images from the West side of the Marble Hall (below) and from the centre of the Hall (above). Both traces show the layers of marble, supported by brick with a sand/lime mix below and the evidence of wooden floorboards beneath that. There is a big difference between the two results. The lower trace shows even banding across the whole floor, suggesting that there is relatively little deflection along this line. The upper trace, however, shows that the sand/lime layer increases and then decreases across the line of the floor, suggesting that at least part of this section has moved.
The data from the survey is being combined into a 3-dimensional set from which we will also be able to extract horizontal views across the entire floor.
Find out more about Kedleston Hall, National Trust