SURVEY RESULTS

2005 / 91  RAF Bentley Priory, Middlesex

1.	Survey Area

1.1	Two areas were investigated using both 100MHz and 225MHz antenna arrays on PulseEKKO 100 and Pulse EKKO 1000 GPR units, respectively. The primary investigation was over and around a series of 18th – 19th century brick cisterns, adjacent to Building 268. The second survey area was immediately north of the Officers’ Mess over the purported 18th century music room. This survey was subdivided into two areas, one over the driveway and one over the grassed area to the east of this. 100MHz data were collected at 0.25m stations along traverses 1.0m apart, whilst 225MHz data were collected at 0.1m stations along traverses 0.5m apart. Figure 1 details the survey locations at a scale of 1:1250.

1.2	The survey grid was set out by GSB Prospection Ltd and tied in to adjacent buildings using tapes. Detailed tie-in information has been lodged with the client.

2.	Display

2.1	A series of summary time-slices and associated interpretations have been included in Figures 2 – 9 at a scale of 1:500. A specific filtered 225MHz time-slice at a scale of 1:250 makes up Figure 10 whilst selected radargrams from each of the areas, containing anomalies of interest, are included in Figures 11 – 14. All printed figures are replicated on the accompanying archive CD. The CD also contains a complete set of the time-sliced data in Figures A1 – A12.

2.2	Numbers in parentheses in the text of the report refer to specific GPR anomalies which have been highlighted on the relevant data plots and interpretation diagrams.

3.	General Considerations - Complicating factors

3.1	Conditions for surveying were generally good, with the two locations being predominantly level and covered with short grass, gravel or tarmac. 

3.2	While depths have been indicated on the GPR diagrams, these have to be viewed with caution. The conversion from time to depth depends on the velocity of the electromagnetic signal through the ground. Given the nature of the site, this may vary markedly over relatively small distances and, as a result, any depth conversion is only an approximation. Velocities of 0.06-0.07m/ns have been used for the time to depth conversions following velocity analysis using graphical methods involving the fitting of curves to point source reflections.

3.3	Where there is a strong electromagnetic contrast, the GPR signal can be inter-reflected or reverberated, producing a delay in the reflection of the signal. This is termed 'ringing'. This happens, to some extent, with all reflections and results in a greater apparent depth than actually exists. As a result, it is often not possible to detect the base of features; only the tops of buried features/deposits are detected with certainty (Annan 1996).

4.	Results of Survey – Cisterns

NOTE: Interpretation diagrams and the accompanying discussion in Sections 4 and 5, below, are based on the full time-slice sets (included on the archive CD attached to this report). The bound reports include only summary time-slices and as such some of the highlighted anomalies may not be obvious. For this reason it is advised that the archive section is also consulted whilst reading this section.

4.1	This survey has highlighted the inherent pay-off between resolution and depth of penetration associated with GPR systems. The 225MHz survey has provided reasonable detail of the top 1.5m but precious little below this (perhaps a facet of the clay content of the soils) whilst the 100Mhz array has achieved penetration to beyond 3.0m, but the images lack clarity. Neither array has been able to identify individual cisterns in the area of the HART excavations (Wittrick, 2001) rather there is simply a zone of high amplitude anomalies (1). A higher frequency antenna may have achieved better resolution of this zone but the penetration depth would have been quite unsatisfactory.

4.2	The anomalous zone of high amplitude response (1) over the cisterns is visible in both the 100MHz and 225MHz data from a depth of approximately 0.4 – 0.5m. It is clear there are no other anomalies of this character within the fenced area, suggesting that this complex of cisterns is certainly not significantly larger than expected. The high amplitude region spreads slightly to the north with depth, and this is most noticeable in the 100MHz data.

4.3	The responses over the cisterns peter out (other than some residual ‘ringing’) at around 1.6m in the 225Mhz data but obviously this is not the limit of the structures, rather it is the limit of the energy penetration. In the 100MHz data set, meaningful responses have been recorded to a depth of 2.5 – 3.0m and looking at the radargrams [Figure 11] the limit of penetration is defined by what appears to be a buried surface (2). It may be that a layer of clay, or other impermeable course, was laid under the cisterns in order to help keep water within them.

4.4	There is little in the time-slices or radargrams to suggest an obvious outflow, or take-off point for the water. The cistern complex appears to be relatively isolated, certainly in the 225MHz data, but it is less clear in the lower resolution lower frequency survey. One possibility is a run-off from the southeast corner of the fenced area towards that corner of the survey grid. There are hints of such a feature in the time slices (3) however there is a depth discrepancy between the two data sets and as such these may be separate features. In fact the 225MHz anomaly could be associated with the possible service (4). This is only noticeable in the filtered data [Figure 10] but it seems a reasonable assumption given that this runs towards an old lamp stand at the edge of the survey area. The 100MHz feature is associated with some large responses in the radargrams [Figure 11] perhaps denoting a culvert but this interpretation remains tentative, and it certainly is not as clear in the 225MHz radargrams [Figure 12].

4.5	In the shallow 225MHz slices, the area surrounding the cisterns shows relatively low amplitude responses (5) which in itself may potentially be of interest. Whilst the first slices have undoubtedly been affected by the gravel spread across the fenced area, zone (5) appears to be different; the orientation is altered from that of the fence. Two slight, low amplitude trends (6) run out to the northwest under the electricity sub-station, but their nature do not imply service routes. It is possible that this relates to some form of altered drainage in order to gather ground/spring water around the cisterns, assuming that they relied on water percolating through the steening (Wittrick, 2001).

4.6	Aside from these trends it is difficult to establish how water was brought to or taken from the chambers. If culverts existed, it is assumed that they would have been detected, given the obvious responses from other, more modern, services such as (7) – (10) and potentially, trend (11). The first are assumed to be modern, as (7) and (8) respect the line of the present access road running past the site and in turn (9) and (10) appear to then relate to these.
4.7	The remaining significant anomalies are those immediately northwest of the fencing (12) and another three (13) – (15) in the north of the survey area. The high amplitude zone (12) can be subdivided into three parts, each with a different depth extent. The shallowest part (12a), is not easily identifiable in the individual radargrams, especially towards its southern end, thus interpretation is difficult. At the top end its character may suggest a pipe or culvert. However, at this north end it is coincident with the potential service (9) and as such may be related to this. Interpretation of the next deepest section (12b) is again difficult; in some radargrams [Line 0018 Figure 11, Line 37 Figure 12] the feature appears to be a discrete object or pipe section (given the shape of response and ‘ringing’, characteristic of metallic features) whilst in others it shows as responses more typical of disturbed ground [Line 39 Figure 12]. The greatest depth extent is shown by (12c), where the relatively broad hyperbolic responses and strong ‘ringing’ suggest sizeable targets. However, as with (12b), the response is very close to the sub-station and a metal access cover. It may be that this is a drainage/service feature given the proximity of the service anomaly (10), but a historic interpretation cannot be dismissed.

4.8	A similar ambiguity in interpretation exists with both anomalies (14) and (15). They both display a reasonable depth extent but are adjacent to the electricity sub-station and the western side of (14) is adjacent to an access cover. Anomaly (14) is also at one end of potential service trend (11), whilst (15) could conceivably be an effect from footings at the corner of the concrete pad upon which the sub-station is built. Again, a historical origin has not been ruled out.

4.9	Anomaly (15), despite not being obvious in the 100MHz time-slices, has been highlighted as it coincides with a slight depression in the ground surface. In the 225MHz radargrams [Line 54 Figure 12] it manifests itself as a shallow surface feature, an increase in response depth and a distinct, but confined (it is only obvious on one line) hyperbolic response. What makes this response interesting is the nature of the ‘ringing’ which would suggest this is a metallic object rather than a large isolated stone or piece of rubble. While an exact interpretation is speculation, the response is consistent with a metal cap or a similar size ferrous object. This hyperbolic feature is also clear in the 100MHz radargrams [Line 0028 Figure 11] and does not appear to relate to any potential service features. 

5.	Results of Survey – Officers’ Mess

NOTE: The western 3.0m – 4.0m of Area B is approximately 1.0m higher than the ground to the east, which it runs down to via a very short slope. This obviously affects the relative depths to features.

5.1	It was hoped to identify any responses that may relate to John Soane’s 18th Century music room, shown on historic plans. It is not clear from records whether this was actually built and subsequently demolished, or if it never got past the drawing stage. The theoretical position of the room is known to be coincident with a section of the Officers’ Mess which juts out. Unfortunately, part of the area in front of this building is covered by a privet hedge, precluding survey.

5.2	Should remains associated with this room be present, responses would be expected along the eastern side of the survey Area A (over the driveway/car park) and along the extreme western edge of Area B (lawn section). The fact that such responses (16) do exist is, however, still not proof that the music room was built.

5.3	The difficulty lies in that the ground looks to have been artificially built up in front of this part of the Officers’ Mess to provide a level drive and perhaps also in preparation for the music room’s construction. If this is the case, the area adjacent to where the ground drops away again may be constructed with far coarser aggregate in order to give this area extra strength.

5.4	It is difficult to determine whether these responses are purely a facet of the landscaping or indeed in-filled cellars/building footings. There are some suggestions of linearity to the responses east of the hedging, but it is only a narrow strip and very close to the break of slope thus it is not clear just how reliable these are. Strong parabolic responses [Figures 13 & 14] in these extremes of the two survey areas do not seem to be consistent between traverses which again could indicate rubble material rather than coherent structure.

5.5	The same is true of the responses in the north western ‘arm’ of Area B. There is some linearity to anomalies but the radargrams [Lines 0051, 0053, 127 & 129 Figure 14] are again slightly ambiguous. Perhaps invasive work might help resolve these issues.

5.6	Possibly of interest is the distribution of reflected energy (17) across the lawn area, further east than the music room’s assumed position. Most obvious in the 225Mhz data (but also discernible in the lower frequency set), there is a distinct rectilinear distribution to the responses coming away from those detected on top of, and down, the slope toward the west of Area B. It is possible that this represents the remains of additional structures or perhaps garden features; there is a great deal of disturbance in the southern half of the radargrams, with a more even stratigraphy to the north. Since the field work was undertaken a 1939 Air Ministry Plan has been found that suggests that two east-west aligned brick foundation wooden huts were present in this region (N. Hammond, pers.comm.); it is possible that these temporary structures have influenced the results at this point. It should be noted that, in the shallow 225MHz slices, the linear spread of energy (18), although not reaching it, may be related to the small metal cover (M).

5.7	North of (17) an area of increased response (19) is produced, in part, by at least one buried surface [Lines 101 & 109 Figure 14]. Part of this surface has produced a very strong reflection and caused ‘ringing’ down into the deepest 225MHz slices. It may represent a feature such as an old section of pipe running parallel with the traverses, but this interpretation is cautious.

5.8	The responses in the deepest slices of the 100MHz data set are very deep and as such it is unclear whether these are related to the shallower features interpreted as possible historical remains. Some of the responses highlighted could be associated with the access cover and concrete pad which are obvious in the lower frequency data set (20) [Line 32 Figure 14].

6.	Conclusions

6.1	The main assembly of brick cisterns at the RAF Bentley site has been identified using both the 100MHz and 225MHz GPR systems. The vertical extent seems to have been delimited by a potential buried surface, perhaps an impermeable layer to seal the base of the chambers. It has not been possible to separate the individual structures given their densely packed nature and resolving limits of the GPR frequencies employed. A higher frequency system would not have been an option due to the limitations of effective penetration depth, possibly owing to the clay content of the soils. 

6.2	The lack of other responses on the same scale, in terms of magnitude and extent, has probably ruled out the existence of a further system of wells, at least with similar dimensions to those previously excavated. A small number of larger anomalies have been identified but generally the interpretation is somewhat ambiguous owing to the possible association with service features, a number of which have been identified.

6.3	One isolated anomaly, coincident with a slight depression in the ground certainly carries some significance. If this were found to be of interest in relation to the existing well structures, then extra importance may be added to the aforementioned larger anomalies.

6.3	There are few features obviously associated with the supply of water to, or extraction from, the system. Given the obvious anomalies produced by what have been interpreted as modern features, it seems reasonable to assume that if there were historic culverts/lift points present, they would have been detected.

6.4	Next to the Officers’ Mess the survey areas have shown up high amplitude anomalies in the assumed location of the purported John Soane music room. The difficulty in interpreting this has been that the responses could merely be a facet of the landscaping. These responses may merely be masonry rubble or some such used to shore-up this side of the raised ground; especially if a building were intended to stand upon it. There is some hint of rectilinear features along the west of the lawn area but being on a slope any interpretation is tentative.

6.5	The distribution of reflected energy over the centre of the lawn area appears to be of a rectilinear nature in both data sets and it is possible that this relates to some unrecorded structure or earlier garden features.

6.6	The survey, certainly over the cisterns, has highlighted the difficulty of finding a compromise between feature resolution and penetration depth; a problem inherent to GPR surveys.

Project Co-ordinator:	J Adcock
Project Assistants:	M Brolly and E Wood

Date of Survey:		12th – 15th December 2005
Date of Report:		20th January 2006

References:

Annan, A.P. 1996	Ground penetrating radar (workshop notes). Sensors & Software Inc., Canada.

Wittrick 2001	Bentley Priory: Brief Report on the Construction and Significance of a Group of Buried Brick Cisterns Uncovered During Works at this Site. Historical Analysis & Research Team (HART) Reports and Papers 51. English Heritage.

SSEW 1983	Soils of England and Wales. Sheet 6, South East England. Soil Survey of England and Wales.

SITE SUMMARY SHEET

2005 / 91  RAF Bentley Priory, Middlesex

NGR: TQ 155 933 (approximate centre)

Location, topography and geology

Two areas were surveyed within the grounds of RAF Bentley Priory, near Stanmore in North London. One lay over a series of 18th - 19th century brick cisterns west of Building 268, the other was over a purported music room attached to the north side of the present Officers’ Mess. Both sites were generally level and under a combination of grass, gravel and tarmac. The soils at the site have simply been classified as urban, but other soils in the locality are largely clays, and during previous excavations this has been found in conjunction with a gravely matrix (SSEW, 1983).

Archaeology

Bentley Priory was a cell of Augustian Canons founded in 1171. From the mid 16th century onward, the site saw a succession of private owners and in the late 18th Century the priory buildings were demolished and the present house (currently the Officers’ Mess) built. Further private ownership continued until 1926 when the Air Ministry took over the site. The first phase of cisterns is thought to date to the turn of the 18th century, with a second phase some 70 years later. Further improvement and enlargement took place in the early 19th century. The John Sloane-designed music room would have dated to the end of the 18th century (Wittrick, 2001).

Aims of Survey

The survey over the cisterns was an attempt to confirm the extent of the known brick chambers and identify any potential further examples, should they exist. It was hoped that associated features relating to possible inlets/outlets may also be detected. At the Officers’ Mess the aim was to determine whether a music room, shown on the 18th century architect’s drawings, was ever built. The work has been carried out for Oxford Archaeology on behalf of Defence Estates.

Summary of Results *

The survey has successfully identified the extent of the main cistern assembly both laterally and, apparently, vertically with a buried surface beneath the main responses, suggesting an impermeable sealing layer. It has not been possible to separate the individual chambers but it looks unlikely that a further array of cisterns exists in the immediate vicinity. A means of bringing in or extracting water has not been found, but the obvious presence of apparently modern service-like features suggests that if historic culverts/take-off points were present, they would have been detected.

A small number of large anomalies, away from the main group of cisterns, have been highlighted though interpretation has been tempered by the majority being closely associated with the aforementioned modern services.

At the Officers’ Mess, the search for the John Sloane designed music room had mixed results. High amplitude anomalies were encountered at the site of the purported structure, but interpretation is complicated by the possibility of the landscaping features. The anomalies may be from masonry rubble, or such like, used to strengthen the edge of the levelled area which forms the drive/parking bays. In the centre of the lawn area surveyed, a relatively strong rectilinear distribution of reflected energy may suggest the presence of structural remains or perhaps previous garden features.

* It is essential that this summary is read in conjunction with the detailed results of the survey.

List of Figures

REPORT SECTION:

Figure 1	Location Diagram	1:1250

Figure 2	Cisterns – 100MHz: Summary Time-slices	1:500
Figure 3	Cisterns – 100MHz: Summary Interpretation	1:500
Figure 4	Cisterns – 225MHz: Summary Time-slices	1:500
Figure 5	Cisterns – 225MHz: Summary Interpretation	1:500

Figure 6	Officers’ Mess – 100MHz: Summary Time-slices	1:500
Figure 7	Officers’ Mess – 100MHz: Summary Interpretation	1:500
Figure 8	Officers’ Mess – 225MHz: Summary Time-slices	1:500
Figure 9	Officers’ Mess – 225MHz: Summary Interpretation	1:500

Figure 10	Cisterns: Selected 225MHz Filtered Time-slice	1:250

Figure 11	Cisterns – 100MHz: Selected Radargrams	nts
Figure 12	Cisterns – 225MHz: Selected Radargrams	nts

Figure 13	Officers’ Mess A – 100/225MHz: Selected Radargrams	nts
Figure 14	Officers’ Mess B – 100/225MHz: Selected Radargrams	nts

ARCHIVE SECTION (on accompanying CD)

Figure A1	Cisterns – 100MHz: Time-slices	see plots for scale
Figure A2	Cisterns – 100MHz: Time-slices (cont.)	see plots for scale
Figure A3	Cisterns – 225MHz: Time-slices	see plots for scale
Figure A4	Cisterns – 225MHz: Time-slices (cont.)	see plots for scale
Figure A5	Officers’ Mess A – 100MHz: Time-slices	see plots for scale
Figure A6	Officers’ Mess A – 100MHz: Time-slices (cont.)	see plots for scale
Figure A7	Officers’ Mess A – 225MHz: Time-slices	see plots for scale
Figure A8	Officers’ Mess A – 225MHz: Time-slices (cont.)	see plots for scale
Figure A9	Officers’ Mess B – 100MHz: Time-slices	see plots for scale
Figure A10	Officers’ Mess B – 100MHz: Time-slices (cont.)	see plots for scale
Figure A11	Officers’ Mess B – 225MHz: Time-slices	see plots for scale
Figure A12	Officers’ Mess B – 225MHz: Time-slices (cont.)	see plots for scale