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12th International Conference on Ground Penetrating Radar, June 16-19, 2008, Birmingham, UK
The Villiers Street Chapel Burial Vaults: a Case Study in GPR Detection
Erica Utsi1 and Frigga Kruse2 1Utsi Electronics Ltd, Sarek, Newton Rd, Harston, Cambridge, CB22 7NZ, U.K.
[email protected]. 2Ian Farmer Associates, Unit 1, Bamburgh Court, Team Valley Trading Estate, Gateshead, NE11 0TX, U.K.
Abstract - This paper presents the case study of the Vil-liers Street Chapel Burial Vaults which were located using Ground Penetrating Radar (GPR). In addition to pre-senting the positive survey results, the paper considers whether the smaller features which remained undetect-ed in this survey might have been detectable had com-mercial constraints not been a consideration & con-cludes from comparative data that this was not the case. The limiting factors relate to the vault construction, in particular the use of multiple layers of brick.
Keywords - Archaeology; Burial Vaults; Brick Investigation; Frequency Selection.
I. INTRODUCTION It is normal practice in the UK to make an assessment of potential archaeological remains whenever major urban re-development work is carried out. This is done in order to mitigate the impact of the new construction on the remains of the past wherever possible (UK PPG6, 1990). Ground Penetrating Radar (GPR) is a useful method of archaeological assessment even in potentially complex environments, particularly where the timing of the assessment means that manmade surfaces such as concrete or tarmac are still in place. The development site situated at 12 Villiers Street, Sunderland, Tyne and Wear, England was rumoured to be underlain by tun-nels, potentially disruptive to the sinking of boreholes, while ne-cessitating thorough archaeological investigation. Documentary research revealed that the site had been the location of a non-conformist chapel completed in 1818, and subsequently enlarged in 1829 to hold a congregation of 1000. Concurrent with the chapel’s enlargement, subterranean burial vaults, said to comprise 99 chambers, were constructed immediately to the north of the building. Further complexity was added in 1849 when a Sunday school was built partially over the top of the catacombs. The religious use of the buildings ceased by 1933. In the late 1970s the buildings were partially demolished and the site rede-veloped to form a garage with internal and external workshops and an area of outdoor parking (Figure 1). The later use of the site meant that the position of existing walls did not necessarily match that of the previous buildings. An ini-tial appraisal of the building suggested that the current Villiers St frontage was probably that of the Chapel.
A GPR survey was commissioned to locate the burial chambers, to find the former entrance, and to discern as much detail as pos-sible prior to accessing the catacombs. Since the current ground surface both inside and outside of the garage consisted of con-crete, GPR was the only suitable method of non-intrusive geo-physical investigation.
II. THE IMPORTANCE OF THE VILLIERS ST VAULTS ‘The early 19th century was marked by a great wave of chapel building, as various nonconformist denominations achieved social respectability’. However, the association of a non-conformist chapel with such substantial burial vaults is unusual (Kruse, 2007). A search for other regional or national examples has proved fruitless. It was evident that secular vaults such as the ones at Spitalfields, London, were usually placed in church basements, whilst family or individual vaults occurred on much smaller scales (Reeve et al 1993; Heslop et al, 1999). By the time that burial activity ceased in 1854, the Villiers St vaults housed 409 inter-ments. The Villiers St vaults are therefore unique in their loca-tion, extent and non-conformist association.
III. PLANNING THE GPR SURVEY Prior to the GPR survey, relatively little was known about the construction of the Villiers St vaults beyond the fact that they were expected to be made of brick due to their date of construc-tion. No plan of the vault survived and therefore there was no guide to the internal or external construction. The burial register indicated a total of 409 interments and, although the intended potential capacity was known, it was not known whether or not this matched the capacity of the vaults as constructed (Vicker, 2000). Although the precise depth to which the vaults reached
Figure 1: The central garage area of the Villiers St site
12th International Conference on Ground Penetrating Radar, June 16-19, 2008, Birmingham, UK was unknown, there was an expectation that the floor of the vaults would be less than 3m below the current ground surface. The underlying soils, a mixture of glacial sand and gravels, were expected to be suited to GPR survey. The water table was be-lieved to lie deeper than the maximum depth of the vaults. The current ground surface was concrete in all areas but it was sus-pected that some areas of the workshops might contain reinforce-ment bars (rebars). The first option was to use a low frequency antenna, in this case, 400MHz, with a depth capability of up to 5m in the site condi-tions, and for which a transect spacing of 50cm could be consid-ered reasonable in order to generate horizontal time slices from a 3-dimensional data set (Utsi, 2007). The main potential problem with deploying a low frequency antenna was that this was unlikely to generate a short enough wavelength to penetrate reinforced concrete. The alternative was to use a high frequency antenna, for example, 1GHz. Although this frequency was likely to be high enough to penetrate reinforced concrete, the maximum depth of penetration of the electromagnetic waves was unlikely to exceed 2m and there was a risk that the floor of the burial vaults might not be detectable. Using the 1GHz antenna would also require much closer transect spacing, 20cm as a maximum. This would have a direct impact on the time spent on site, the processing time and therefore the funding required. The decision was taken to deploy the 400MHz antenna initially and to aim to complete the survey within a single day on site. The 1GHz antenna was retained on standby in case any areas of rein-forced concrete were discovered.
IV. THE GPR SURVEY Separate surveys were carried out in 3 areas of the site (the main workshop & spray workshop, the central area of the garage, and the parking area outside the buildings) using a Groundvue 3 GPR with a 400MHz transducer pair. An additional external workshop could not be surveyed because it had still to be cleared of recent occupation debris. Care was taken during the survey to relate the survey reference lines from each area to those of the remaining areas so that a single 3-dimensional data block could be formed from the combined data of all survey areas. This was important in order to detect the continuity of features that were buried below more than one section of the site. The time sweep was set to 80ns to ensure the full 3m depth would be surveyed while allowing for the possibility of a slower than anticipated velocity of propagation. Since the target was relative-ly large, the sampling interval along the line of travel of the radar was set to 4.8cm. Although reinforced concrete was discovered in the parking area, this was sufficiently coarse (c. 20cm spacing) that it was not necessary to deploy the high frequency antenna (Figure 2).
V. SURVEY RESULTS The 2-dimensional data clearly illustrates that there are building remains of different types lying beneath the site (Figure 3). The results are typical of archaeological remains from an urban con-text. An initial appraisal of the 2-dimensional data suggested that the more extensive remains lay beneath the car park and only limited remains beneath the main garage workshop. This was originally incorrectly interpreted as the foundations of a
smaller building to the North with the vaults lying beneath the car park.
Time slices extracted from the 3-dimensional data set confirm the partial outline of a large rectangular structure below the car park and a smaller structure beneath the garage and adjoining the vault entrance (Figure 4). Signal strength for the two areas is markedly different being much stronger in the area below the car park and more strongly attenuated in the vicinity of the vault entrance.
Below the NW corner of the main workshop lie a third, smaller group of remains, apparently unconnected with either of the other two main groups (Figure 5).
Re-examination of the 2-dimensional data revealed the existence of arches in the areas directly to the north and south of the smaller rectangle (Figure 6). The areas covered by these arches can also be seen in faint silhouette in Figure 4. Signal strength is very poor in comparison with the returns from the main rectangular structure visible in the upper trace in Figure 3.
Figure 3: Evidence for Archaeological Remains below the Garage (above) and below the Parking Area (below)
Figure 2: Data from the Car Park showing Depth Capability of 400MHz antenna within the area of reinforced concrete
12th International Conference on Ground Penetrating Radar, June 16-19, 2008, Birmingham, UK
VI. COMPARISON OF GPR RESULTS WITH THOSE OF THE ARCHAEOLOGICAL INVESTIGATION On completion of the GPR survey, the time slices were compared to the historical information derived from the documentary search. This process suggested that the large rectangular outline below the car park matched that of the former chapel footings and that these footings must extend beneath the workshop which could not be surveyed. By implication, this confirmed that the existing front-age to the garage must be that of the Sunday School and not, as originally thought, that of the Chapel.
Once this process was complete, the archaeological team entered the vaults and, taking due care of health & safety measures, in particular for working in a confined space, a detailed assessment was made of the vault contents and condition (Kruse, 2007). This process identified a main vault chamber, close to the floor of the garage complex, accessed by a set of stairs (Figure 7). The position of the main vault chamber and the western side chambers corresponds to the feature visible in the upper trace in Figure 3 and also to the small rectangular feature visible in the 20ns time slice (Figure 4). As expected, the vault was constructed of brick.
Figure 6: The Curves of the Vault Arches to the North (above)
and the South (below) of the Main Chamber.
Figure 5: Time Slice at 27ns showing Building Remains in the NW area of the main Garage Workshop
Figure 4: Time Slice at 20ns showing Building Remains beneath the Garage and the Car Park
12th International Conference on Ground Penetrating Radar, June 16-19, 2008, Birmingham, UK
Figure 7: The main vault chamber, looking westwards from
the stairs. Leading off the main chamber, were 10 side chambers, 5 to the north and 5 to the south. The position of these corresponds to the arches visible in Figure 5 and to the faint outlines visible in the 20ns time slice (Figure 4).
A considerable number of smaller chambers beyond the central area were also identified by the archaeological team, leading to the deduction that the full extent of the vaults formed a cruciform shape (Figure 8). However, no evidence for these smaller side chambers was visible either in the 2-dimensional or in the time slice data. It was not possible to identify the remains in the NW corner. These lie beyond the limits of the vaults and there is no means of underground access into this area.
Figure 8: Plan of the Vaults (Kruse, 2007)
At the same time as the vaults were investigated, an archaeologi-cal evaluation of the car park was completed by means of 2 trial trenches. As would be expected in an urban context, these re-vealed considerable complexity in the occupation sequence. They also served to identify 2 features visible in the time slice se-quence: a near surface brick wall post dating the destruction of the Chapel and the foundations of a substantial limestone wall from the Chapel itself. The latter is visible as a line in the time slice lying in an approximate North/South orientation to the East of the parking area (Figure 4). Since no related human remains were uncovered, no further work was undertaken in this area.
VII. DISCUSSION Although this survey was undoubtedly successful in locating both the remains of the Chapel and the central area of the vault, it would not have been possible to deduce the cruciform shape or the full extent of the vault on the basis of the survey alone. The question therefore arises as to whether this was the best use of resources or whether a better result could have been achieved had the survey been tackled in a different manner.
For any archaeological GPR survey, but particularly those carried out within the context of urban re-development, there are a num-ber of competing and contradictory constraints. On the one hand, the re-development represents an opportunity for closer examina-tion of some of the archaeological remains in the area, on the other there are inevitable limits to the resources of time and fund-ing which can be committed to the project. In the case of the Villiers St site, the GPR survey was funded by the site developer. Using a low frequency antenna reduced the amount of time spent on both the survey itself and the subsequent processing since total area coverage was possible with a relatively large transect spacing (0.5m). Since the low frequency antenna also had sufficient depth penetration capability and the expected target size was also within its range, this appears to be both the commercial and theoretical optimal solution. The results of the survey using the 400MHz antenna demonstrate that the vaults lie close to the ground surface and it is noticeable that it is only the smaller chambers that were not detectable. Most of these chambers were empty but some contained infill dating from the time that their use was discontinued. This raises the possibility that the sampling interval, transect spacing and the frequency range of the antenna used might have been inappropri-ate for the smaller parts of the target.
In order to determine whether or not density of sampling was responsible for the lack of detection of the small side chambers, a sample area of the main workshop was re-surveyed using two 1GHz antennas simultaneously on a multi-channel Groundvue 3. The transect spacing was reduced to 0.2m, the depth of investiga-tion to 40ns and the sampling interval reduced to 0.5cm. The area chosen covered the central vault chamber, and the principal side chambers and small chambers to the north of it.
VIII. RESULTS OF THE 1GHZ SURVEY Since the vaults had been explored before the 1GHz survey was carried out, it was possible to relate the position of the vault to the results of the survey. As expected for a higher frequency and higher density survey, the 2-dimensional results provide higher data resolution. They confirm the depth findings of the 400MHz
12th International Conference on Ground Penetrating Radar, June 16-19, 2008, Birmingham, UK antenna and are consistent with the known position of the vault (Figure 9).
The time slices also illustrate the higher target resolution. As an example, some near surface features are defined at c. 3ns (Figure 10). However, with increasing depth, the primary signal returns are from the main chamber of the vault (Figure 11). Increasing the depth beyond this main chamber does not give any positional information for either the main side chambers or the smaller chambers beyond them.
IX. ADDITIONAL FACTORS
The use of brick in the construction of the vaults is one possible
reason why the radar failed to detect the smaller chambers. As a material brick is not considered to be particularly lossy (Daniel, 2004) and there are other examples of GPR detection of buried brick built structures (e.g. Linford, 2000) There are, however, two possible factors which might increase the signal attenuation. The first of these is the chemical composition of the bricks them-selves and the second is the combined effect of scattering losses at each interface between layers of bricks. The chemical composition of the bricks used in the vaults was not tested as, being of archaeological importance, preservation of the structure takes priority. It is possible that this variety of brick is more lossy than might otherwise be expected. The evidence from a comparison of the data obtained from the main chamber of the vault (and below the garage) with that from below the parking area certainly suggests that this might be the case (cf. Figures 3 & 6). The internal construction of the vaults is such that the central chamber is also the closest to the current ground surface and the one with the fewest bricks in its roof. Additional layers of brick are used in the construction of the principal side chambers.
The smaller side chambers have an even greater depth of brick layers (Figure 12). The implication is that signal losses from scat-tering effects at each surface are likely to be significant. It is pos-sible therefore that the smaller side chambers remained undetected due to signal losses caused by a combination of the chemical composition of the bricks used in construction and the type of construction used. Increasing the resources available to the GPR survey would not therefore have produced a corresponding im-provement in target definition in this case.
X. CONCLUSIONS GPR was the only suitable method of non-intrusive survey for this important archaeological investigation. The survey data have accurately defined the position and extent of the central area of the vault, within the constraints of the resources available. The density of survey coverage was primarily led by the need to locate accurately the position of the vaults but was also influenced by commercial factors. In this instance, the commercial consider-
Figure 10: 3ns Time Slice showing Near Surface Construc-tional Features in the NE corner
Figure 9: 2-Dimensional Profile showing good correlation with the known vault position [b=End of Vault; c=End of
Central Chamber]
Figure 11: 14ns Time Slice showing primarily the Main Vault Chamber
Figure 12: View of the Brick Construction of the Side Chambers of the Vaults, showing the increasing thickness
of brick roof in a principal side chamber and in the smaller chambers beyond.
12th International Conference on Ground Penetrating Radar, June 16-19, 2008, Birmingham, UK ations have not diminished the level of information which could be derived from the GPR survey. The results of the commercially driven survey are at least as good, if not better, than the subse-quent trial. The main limiting factor for both surveys appears to lie in the construction of the vaults and possibly the chemical content of the bricks used.
ACKNOWLEDGMENTS The authors would like to thank Atlas Homes for commissioning the survey, Ian Farmer Associates for permission to use the mate-rial from the archaeological assessment and the GPR survey. Figure 7, the vault plan, was drawn by Matt Town of North Pen-nines Archaeology. The photographs in Figures 7 & 12 were taken by Chris Tubman of Ian Farmer Associates.
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