Plate 1. Distribution of boreholes, Phase 1
The boreholes were drilled by Hymas GeoEnvironment Ltd, under the supervision of staff from On-Site Archaeology. The cores were examined, described and sieved to recover any dating evidence. Each borehole was surveyed to locate it against the Ordnance Survey grid system. The data can therefore be used to specify the depth below current ground level at each borehole location where each historical period will be encountered.
This type of investigation is far less destructive than excavation of the deposits, and results in a broad understanding of the archaeology of a large area, rather than a detailed understanding of a small trench.
The relative heights of the deposits encountered have been used to produce a three dimensional model of the ground beneath the development area using G-Sys GDMS software. This allows us to visualise those deposits, and also 'fills in the gaps' between each borehole through a process called 'interpolation'. By calculating the difference between neighboring boreholes, and joining, for example, a medieval deposit in one borehole to the medieval deposits in neighboring boreholes, a surface can be created representing the surface of the area during the medieval period.
The Borehole work is being undertaken in several phases. During Phase 1 of the work, 110 boreholes were drilled at intervals across the site (Plate 1), though access to properties constrained their position in some cases.
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Plate 1. Distribution of boreholes, Phase 1 |
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Often this kind of survey uses the data from engineering boreholes, and produces fairly limited results (the core samples are often removed from site by the engineers to be analysed in their laboratories for contaminants etc.). In this case, however, the boreholes are being undertaken purely for archaeological reasons, and all the core samples are being sieved to recover dating evidence. The quality of information retrieval is therefore extremely high.
The samples, once extracted, were examined and each different deposit within them recorded (colour, texture, depth below ground level etc.) and sieved. Any artefacts were recovered and dated, and the state of those artefacts also recorded. A very abraded roman pottery fragment might indicate that it had been ...
Finally, the locations of each borehole were recorded using a Total Station, which records their position based on the Ordnance Survey grid and also their height above sea level. The height above sea level of each individual deposit can then be calculated. Plates 2-4 show fieldwork in progress.
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Plate 2. Borehole drilling rig |
Plate 3. Archaeological examination of borehole core |
Plate 4. Core samples awaiting sieving |
The information retrieved was subsequently entered into an Access 2000 database. This allowed the correlation of undated deposits with those nearby which had been dated based on the colour and texture of the deposit, and on the inclusions present. In this way those undated deposits can be assigned a probable date.
The data was then used to create the three-dimensional model of the buried deposits using the G-Sys software package. This creates three dimensional meshes, contour plots and graduated bitmap images of each layer, in this case modern, post medieval, medieval, roman and 'natural'. Figures 1 and 2 illustrate the 3D mesh, while 3 and 4 illustrate the contour plot and graduated bitmap image for the surface of medieval deposits.
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Fig. 1. 3D Mesh |
Fig. 2. Rendered 3D Mesh |
Fig. 3. Contour |
Fig. 4. Bitmap |
Once bitmap images had been created for each period to be included in the model, these were combined to create a single multi-period model. This can be viewed in plan or in 3D, and when the cursor is moved over the model, the depths of each different period at the cursor location are shown on screen.
Phase 1 results showed that there was insufficient data to create an accurate model, though some of the underlying topography was begining to become apparent. The lack of data is clearly evident in Figure 3, the contour plot, which shows that contours have been interpolated across huge distances. This had been expected, and the model was therefore used to establish exactly where further information would be required to refine the results. On this basis a number of new boreholes, Phase 2 of the work, are being drilled within the development area (May 2002). This page of the web site will be updated once the results from Phase 2 have been processed.
The following tables provide access to extracts from the original Hymas borehole logs. These can also be accessed directly from Plate 1, by clicking on a borehole.
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2
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3
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4
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5
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6
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7
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8
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9
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10
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11
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12
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401
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501
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801
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901
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1001
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1101
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1201
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