Geophysical surveys allow archaeologists to detect archaeological features buried below ground without digging. There are a number of different techniques, each requiring its own piece of equipment. Each technique detects different physical properties of the ground.
Thousands of readings are collected in data-loggers and transferred to a computer for analysis. Maps showing the discoveries below ground can then be printed.
Three main techniques have been employed during the recent investigations of the Chester Amphitheatre and the area around it:
This technique records subtle changes in the local strength of the earth’s natural magnetic field. By collecting readings on a regular grid pattern, a map of hidden features underneath the ground can be made. However, magnetic surveys are heavily distorted by modern structures on the surface, such as iron railings or passing cars.
[Left] This type of survey is carried out using a hand-held device. The operators walk over the ground within a marked grid. Readings are taken automatically, usually at intervals of 25cm.
At Chester, the magnetic survey has been concentrated over the ‘Roman Gardens’ where previous excavations have revealed evidence for a large kiln, in which clay pipes were manufactured up until the 19th century. These structures are usually highly magnetic and it is hoped any similar kilns in the vicinity may be detected by the survey. Magnetic survey has also been used the Dee House Gardens.
This technique measures variations in soil moisture, which often reflect the presence of buried structures, building foundations (less moisture) or ditches and pits (more moisture). By pressing the frame down at precise intervals of 1m on a marked grid, the operator forces the probes into the ground. A small electrical current then passes through the ground.
[Right] The electrodes on Earth Resistance equipment are attached to a portable frame which looks like a Zimmer-frame walking aid.
The earth resistance survey at Chester could only be applied over open areas without tarmac surfacing where the electrodes could make good contact with the soil. This type of survey has been undertaken in the grassed areas of the amphitheatre, Dee House Gardens, Grosvenor Park and The Roman Gardens.
The principles here are much the same as those used by radar at airports to detect flying aircraft, only in this case the equipment sends a short pulse of radio energy into the ground and measures the time taken for any reflections from buried features to return to the surface. This allows the depth of buried structures to be estimated from the length of time it takes for the reflections to reach the surface: the longer the delay, the deeper the structure. Once a large area has been surveyed, it is possible to attempt a 3D reconstruction of an entire site.
At Chester the technique has been used in Dee House Gardens, the County Court car park, on the road between the amphitheatre and St John’s Church, and the car park of the Old Bishop’s Palace.
At Chester the technique has been used in Dee House Gardens, the County Court car park, on the road between the amphitheatre and St John’s Church, and the car park of the Old Bishop’s Palace.