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In the realm of King Sil

Three-dimensional seismic modelling is being used to investigate the structure of Silbury Hill, a prehistoric burial mound in Wiltshire.Dan Simpson reports.

Silbury Hill, near Marlborough in Wiltshire, is the largest artificial prehistoric mound in western Europe. Built in the late Neolithic age, 4500 years ago, the mound is 39m high and covers an area of nearly 22,000m 2.It was traditionally thought to be the tomb of the legendary King Sil, who was supposed to have been buried inside the hill with his horse and armour.

During 1776-7 the Duke of Northumberland mined a vertical shaft from the top of the hill to its base in search of artefacts. In 1849 two more tunnels were dug horizontally through the base of the hill to its centre and between 1968 and 1969 cavities were excavated beneath the centre of the hill.

Sadly, all these explorations ended in disappointment.

It was initially thought that the vertical shaft had been backfilled. At the end of May 2000, it became clear this was not the case, as the top of the hill became unstable and its surface gave way, leaving a hole in the summit about 1.8m square and 10m deep. Heavy rainfall led to further collapses in December 2000 which expanded the hole to 7.2m by 5.5m and 3.8m deep.

Because of its chalk-based flora and fauna, Silbury Hill has been designated a site of special scientific interest. This means any damage to the hill has to be kept to a minimum and the use of hazardous substances is banned.

Short-term stabilisation and infilling of the shaft was carried out by contractor Skanska Construction. The hole was backfilled with polystyrene and capped with more than 40t of chalk fill, brought to the site by helicopter in about 60 bags, each weighing 700kg.

Skanska Construction firm 3D Tomographics carried out a seismic survey for site owner and manager English Heritage to obtain information about the interior of the hill.

The survey will allow the company to build up a 3D model showing the location of any cavities or excavations and will determine the extent of the repairs and stabilisation required.

Four small diameter boreholes were sunk to a depth of 50m, ending in the undisturbed chalk bedrock beneath the hill.'The main aim of the project is to look into the hill to determine where the cavities are. It is all about long term stabilisation, ' says 3D Tomographics project manager Mark Kirkbride.

Gloucester based site investigation contractor Geotechnical Engineering sank the boreholes using one of its in-house developed Pioneer rigs. The rigs can be switched from dynamic sampling to rotary drilling in less than a minute, allowing continual sampling even when there is a dramatic change in ground conditions.

The 2.7t rig had to be winched up the steep hillside on a trailer, the same method that was adopted to take drilling supplies to the summit.

'The rig was perfectly suited for the site as it is compact enough for the small working area on top of the hill, ' explains Geotechnical Engineering managing director Andrew Milne.

Dynamic sampling was used to obtain 100% core recovery through the soft, uncompacted chalk down to 30m. Here the rig was switched over to rotary drilling to core down through the competent chalk bedrock. Milne says drilling rates varied between 15m and 20m day. Cores were boxed up and taken to a coldstore where English Heritage is logging them for archaeological information.

The structure of the hill is being surveyed using seismic signals released from a source lowered down the boreholes and picked up with receivers in adjacent boreholes. This will provide geophysical information about the core of the hill, which is the main area of concern.

A surface seismic survey will also be carried out, with geophones placed in the top of the hill and the sources in the boreholes as before. This provides geophysical data from the hill core to its surface, allowing a complete model of the hill to be built up.

'There is nothing new about the survey technique, it is the software that interprets the data that is new, ' says Kirkbride.

The software used to interpret and analyse the seismic data from the survey has been developed by US company NSA Engineering. It converts the raw data into 3D images that can be easily understood. The colour of the images depends on the seismic velocity of the surveyed rock: cooler colours represent low velocity seismic waves, ie cavities or low density material, and warmer colours represent higher velocities - strong, dense material.

The 3D model can be picked up and rotated or sliced in any direction and different colours or seismic velocities can be removed from the model to highlight the lowest or highest velocities.

Investigation work started on the site in August and was completed by the beginning of October.

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