Project background

Air-raid shelters are a relatively neglected component of the numerous archaeological studies of conflict that have arisen. Any remaining shelters in the urban environment are therefore a limited resource and a significant reminder of a crucial time in European history. However, they also pose a risk to development. A Housing Trust in London identified that a redevelopment site may contain air-raid shelters. The exact location and presence of the shelter structures needed to be determined to aid further ground engineering works and to classify buried obstructions that could impact the development of the site. The geophysical investigation was undertaken by RSK Geophysics as part of wider ground investigations by RSK Geosciences. The geophysical techniques employed were ground-penetrating radar (GPR), an electrolocation survey with cover lifting exercise to identify buried utilities, a time domain electromagnetic (TDEM) survey and a microgravity survey. Magnetic surveys are commonly used in archaeological site investigations, but, because of the noisy urban nature of this site, they were not used in this case. Microgravity surveys are rarely used in archaeological investigations but are commonly used in geotechnical site investigations, for example, when looking for near-surface voids from buried structures. The geophysical data were further constrained by a targeted intrusive investigation.

Geophysical survey results

Electromagnetic methods are often used in archaeological investigations, as data are collected rapidly. Man-made features such as buried foundations, reinforced structures and services of metal construction will show a high contrast in conductivity measurements compared to the surrounding ground and will generally manifest as regular geometric anomalies that can be traced within the data. The electromagnetic data at this site (an open grassed area with no evidence of structures) showed a large, distinct rectangular anomaly 15 m x 4 m in size orientated SSW–NNE. The feature was indicative of an in situ reinforced structure. GPR surveys have become very commonly used in archaeology, as they can detect buried objects up to 5 m below ground level in ideal conditions. At this site, reflection features indicative of the top of the roof of the buried structure were detected just below the ground surface.

Intrusive verification

Based on the geophysics result, four targeted trial pits were dug to locate the shelter walls and 150 mm diameter drill cores were made into the hidden roof structure. The combined dataset using the geophysics and targeted intrusive locations helped to precisely constrain the location and depth of the buried air-raid shelter to within centimetre-scale accuracy. Non-invasive, surface geophysical methods could be successfully used to locate not only shelters but also other buried man-made obstructions such as tunnels, basements and mineshafts in a rapid, safe and cost-effective manner.

RSK Geosciences project - Fig 1 - geophysical survey of hidden air-raid shelter

Figure 1: Plan view TDEM data showing a large anomaly in the location of the buried shelter. Cores and trenches targeted on the feature subsequently confirmed the depth and location of the structure.