This example illustrates the use of resistivity sounding in the site investigation for a proposed road improvement scheme. Along the route show in figure 7.3
It was necessary to locate the upper surface of the bedrock,which might be at a depth of 10 m or more. The principal solid formation is carboniferous limestone ,this being overlain by glacial drift, which comprises glacial till in the east and glacial sand and gravel in the west. In order to provide information on the thickness and nature of the glacial drift. 18 offset wenner soundings were measured . A maximum electrode spacing of 64 m was used where possible,although this was reduced to 32 m where access was a problem. where the proposed route ran along the existing road, the soundings were located 5 m to 10 m to either side of the road boundary in order to reduce any effects of wire fences and services , to an acceptable level. The sounding interpretation were checked on a computer and adjusted to give geological consistency.
The final results were presented as a geoelectrical section , part of which is shown in figure 7.4 . As there is a strong resistivity contrast between the limestone and the overlying glacial material,