Geophysical Survey I
In determining the grid for the initial round of geophysical surveys the project’s specialist team headed by Lawrence Shaw and Paul Cheetham have used a Differential Global Positioning System (DGPS). The use of a Leica VIVA GNSS Smart Net Differential GPS was employed to create the grids used for the survey. A 20 x 20 meter grid was created in ArcGIS 10 prior to the survey and uploaded onto the GPS (Figure 2). The stakeout function was then used on the GPS system to intern the pre-produced grid, which was located within 2cm accuracy. This grid was used for both the resistance and magnetics surveys.
Figure 1: Map showing the 20×20 metre grid used for the survey. © Crown Copyright/database right 2013. An Ordnance Survey/EDINA supplied service.
This survey saw the use of a GeoScan Research RM15 to undertake a resistivity survey. The machine was calibrated at 1 metre spacing to take 1 metre samples along 1 metre traverses within 20 x 20 metre grids. The traverse recording method was zig-zag and the grids were walked from south to north. Resistivity is an active survey technique which uses an electric current that passed through the ground, measuring the strength at which it passes from one probe to another. The measurement of Ohms identifies features beneath the ground affecting the strength that the current can pass through them. In particular, resistivity is very useful when looking to identify any ditches and bank such as those already associated with the Arbor Low Avenue.
The Bartington Grad 601-2 Fluxgate Gradiometer was also used to survey the site. This twin probe setup was calibrated to take 0.125 metre sampling along 1 metre traverses in a parallel method within 20 x 20 metre grids. Traverses were walked in the same directions as the resistivity surveys (South to North). This passive form of survey measures how the strength of a magnetic field varies across an area. By measuring the nanoteslas (nT) between two points, the fluxgate gradiometer can create an image of any magnetic disturbances found throughout a study area. Features such as ditches, pits and hearths should be represented as positive readings due to their altered magnetic levels.
Post Processing Geoplot and ArcGIS were the two software programs used to process the three datasets gathered during the survey
Conditions Weather conditions varied from sun to heavy rain during the different surveys. However this does not seem to have had an effect on the results seen.
Figure 2: Raw resistivity data including the scheduled area of the Arbor Low avenue. © Crown Copyright/database right 2013. An Ordnance Survey/EDINA supplied service.
Figure 3: Raw magnetometry data. © Crown Copyright/database right 2013. An Ordnance Survey/EDINA supplied service. Processed Data
Figure 4. Processed resistivity data. © Crown Copyright/database right 2013. An Ordnance Survey/EDINA supplied service.
Figure 5: Labelled resistivity anomalies. © Crown Copyright/database right 2013. An Ordnance Survey/EDINA supplied service.
Within the results seen in the resistivity survey there are 8 anomalies identified as being of interest in Fig 5. The first of these is the identification of the previously known avenue bank and ditch found within the scheduled area, A1 and A2. Here the bank of the avenue is visible as a high reading running east west and to the north of the avenue ditch which bands the south side of the bank. Both features measure around 3.5 meters in width and were visible on the ground whilst surveying. From Figure 5 it is also visible that the scheduling of this area misses a small section of the ditch. The high low anomaly seen to the north of anomaly A2 is a result of a shadowing effect created by the bank. To the south of the avenue ditch, there I also a number of small, high resistivity anomalies that run parallel with the ditch, anomalies E1, E2, E3 and E4. Whilst undertaking the survey, it was noted that these areas were very stony under the resistivity probes and it is possible that this could be up cast associated with initial cutting of the avenue ditch. Alternatively, it would appear that previous use of the field saw ploughing run from north east to south west, and these anomalies could be stones dragged across for the avenue bank.
Heading north-westwards, outside of the scheduled area, is the continuation of the avenue ditch, anomalies B and B1. However, these are segmented by an area where the ditch appears to be lost, F. Although there may be some faint sign of the ditch continuing between these two sections, there is nothing substantial. Similarly, there is no obvious evidence of the bank continuing past the area of scheduling other than in one or two discrete locations. Upon identifying that the avenue ditch continues to run up to the north western wall of the field, it was decided to undertake a single survey grid to other side of the wall to confirm if the ditch continued through to the other side. Unfortunately no clear features were visible other than a number of high resistivity readings, anomalies G1 and G2. These are also seen in the magnetometry survey as anomaly K. Whilst undertaking the survey in this area, it was identified that the ground had been heavily disturbed by possible quarry pits and this may be the reason the feature does not appear to continue through into the next field.
Also visible in the survey are the three linear anomalies seen running from the north west to south east of the eastern end of the survey, anomalies D1, D2 and D3. Although only partially visible, these negative anomalies measuring around one meter in diameter, were also recorded in the magnetometry survey running from the fence line in the north, toward the avenue ditch (anomalies D1 to D5). It is unclear whether these anomalies continue past the avenue ditch; however their dimensions and readings suggest they are geological rather than archaeological. The anomaly marked C in the interpretation shows a low resistance areas seen in the survey that appears to run from north the south along the north west wall of the field. This is also picked up in the magnetometry survey (anomaly C) and was noted on the ground whilst survey was undertaken as being associated with previous farming that had taken place in the field. Anomaly H is a large low resistance feature measuring around five meters in diameter. Unfortunately the whole of this feature was not surveyed but the reading suggests this may be an area of interest.
Figure 6: Magnetometry with labelled interpretations. © Crown Copyright/database right 2013. An Ordnance Survey/EDINA supplied service.
As seen in both the resistivity survey and illustrated in Fig 6, the magnetometry survey identified the previously known avenue bank and ditch seen within the scheduled area, anomalies A1 and A2. Similarly, the magnetometry survey also revealed the continuation of the avenue ditch through the middle of the field, anomalies B1. However, unlike the resistivity, the magnetometry shows the continuation of the avenue ditch through to the field wall on the western side, anomalies B3 and B5. As well as this there would appear to be possible evidence of part of the avenue bank along a section of the ditch, recorded as anomaly B4. However, as seen in the resistivity, no evidence of the bank and ditch continuing into the neighbouring field can be seen, even after surveying a larger area.
The anomalies marked as J do show a high and low magnetic linear feature running in a similar direction to that of the avenue. However, it is unclear whether they are related. Anomalies E and H, identify an area of high reading anomalies of varying shapes and sizes. It would appear that the area has seen some activity taking place across it and this may be evidence pits. Similarly, the anomalies seen at I show varying shape size and levels of magnetic response, which are different to those seen elsewhere in the survey. This could also therefore, indicate an area of archaeological activity within the survey area.
To the Northwest corner of the field, anomalies F1 and F2 identify a high reading anomaly that runs from the North to the Southwest before turning at a right angle towards the North West. This feature also appears to continue into the neighbouring field, anomaly F3. Judging from the size and shape of this anomaly, it may be possible that this is a ditch. In the top Northwest corner of the field there is a large irregular shaped, high magnetic anomaly, anomaly G. This was visible on the ground as a large, sub circular mound with a depression in the centre and appeared to be filled with rubble. It is likely that this is associated with quarrying work that was undertaken in the area.
Through the use of two different survey methods, this survey has shown that the avenue bank and ditch associated with Arbor Low henge, does continue for at least another 100 meters to the west of the point it was previously thought to have finishing. Although it is not seen to be continuing in the neighbouring field, it would seem strange that the ditch and bank respect the boundaries of a later field wall, so it would therefore be appropriate to assume that the avenue does continue on past this field. Due to more modern quarrying activity interfering with the monument in this area, it is unclear as to where the avenue runs past this point and further work will be required to identify where the avenue runs from this point on and where it terminates. As well as identifying the continuation of the avenue, the surveys have also identified a number of anomalies and features that may be associated with the pre-historic activity taking place in this area, of which further investigation may be required through excavation.