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Detecting Hidden Crypts

Using GPR (Ground Penetrating Radar), it is possible to conduct a non-destructive (NDT) survey of an area and detect the presence of underground voids, crypts, vaults, sinkholes and subsidence.

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Crypts and vaults

In many churches there are areas below the floor which are reserved for burials and the storage of remains. These areas can take many forms, (such as crypts or plague pits), but when surveyed using GPR they all share the similar characteristics of a void, this means that they can be detected and to some extent imaged using Ground Penetrating Radar.

Understanding GPR capabilities

GPR works by transmitting a signal into the ground and listening for reflections, reflections are caused whenever the GPR signal encounters a change in the electrical properties of the material it is travelling through. The GPR signal is degraded and energy is progressively lost the deeper it travels until all the energy is gone.

The change between the ground material or stone of the church floor and the top of a crypt or void is significant and will create a strong reflector, which is visible on the GPR as a layer. This means that GPR can be used to image to extents and top of the crypt/vault. It may be possible to derive some additional information about the depth of the void and its shape however this is more difficult, much of the GPR energy will have been lost in the initial strong interface.

GPR survey to detect the depth and extents of voids

Due to its small footprint and good penetration capabilities a GSSI UtilityScan was used for this survey. The 400MHz antenna was used to collect GPR data for as much of the (heavily obstructed) church interior as possible, this data had to be carefully reconstructed and accurately referenced inside the floor plan.

Using GPR to detect crypts and vaults

Crypts and vaults can present problems for contractors as they can be lost or forgotten spaces below the surface. When remedial works need to be carried out, these hidden spaces may be discovered resulting in serious issues for construction plans. In this example, the contractor needed firm ground on which to install a lift shaft, during a drilling operation to determine the floor thickness, an unexpected void was discovered.

When a light and camera were lowered into the void, it became evident that the space contained the disarticulated human remains of many individuals. These individuals (further research suggested) were either the exhumed remains of individuals moved when the church was rebuilt on the burial ground of the previous church, or from the 1854 Cholera epidemic as similar crypts have been discovered in the area. It was also clear that the void space continued in different directions and had to be investigated before further work could continue.

Results from a GPR survey to detect the depth and extents of voids

The GPR data showed the top of the void which we were able to map in cross section and in plan view. By making multiple cross sections showing the top of the void and exporting that information to CAD. We were able to reproduce the shape of the vaulted surface of the void, the extents of the underground voids (within the surveyed area) and the positions of supporting walls and doorways.

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