Environmental geophysics investigations in urban areas

Abstract

Dr John M. Reynolds, managing director and principal geophysicist at Reynolds Geo- Sciences, presents some case histories illustrating how the obstacles of noisy urban sites have been overcome to produce highly useful interpretable data sets, often far exceeding the expectations of the clients involved. Urban areas represent some of the most challenging environments in which to undertake geophysical investigations. Restricted space, the necessity for traffic management, high ambient noise (electrical and vibration), underground utilities, etc. all help to complicate the design and setting out of the geophysical profiles or grids. In many cases, such sites could not be investigated as the acquisition methods, the type of equipment and subsequent data processing software were not readily available. Over the last few years, significant changes in the capabilities of the equipment and software have permitted geophysical surveys to be undertaken in these hitherto inappropriate areas. The first case history at Field Road, Reading, Berkshire, was by far the largest, most complex and technically challenging. It shows how with very careful survey design, particular attention to data acquisition, and specialist data reduction and modelling, a labyrinth of shallow chalk mine galleries were mapped successfully using micro-gravity. The second involved a housing development in northwest London where the intrusive investigations had failed to satisfy the requirements of the Environment Agency, the environmental regulator in England and Wales (Scotland has a separate but similar regulatory body). Furthermore, the client and his engineering advisers were sceptical about the use of geophysics. Despite the technical difficulties of a complex partially developed site, the geophysical investigation demonstrated clearly the benefits of such a survey when integrated with intrusive investigations targeted on the basis of the geophysical survey. The client and his advisers became keen advocates of the use of geophysics once the results were made known. The third example is a complete contrast to the first two in that, despite the site being in a town, it comprised open space. The method used was the tried and tested and now very well established technique using a Geonics EM31 ground conductivity meter. (Detailed descriptions of geophysical methods discussed here have been presented by Reynolds (1997)). It demonstrates that even simple, long established techniques used very simply can produce some very clear and graphic results. The last case history involved the development of a former landfill site in south Wales. The 12.6 ha site was being cleared and earthworks to level the site were well in progress. At this late stage, it was suggested that steel drums might have been buried on the site but their location was unknown. A detailed geophysical investigation on the extremely busy construction site was undertaken using predominantly magnetic gradiometry and drum graves were located successfully and proven by excavation. The additional feature of all these case histories has been the involvement of what has subsequently become known as an ‘Engineering Geophysics Adviser’, an extremely experienced geophysicist (or group of geophysicists) independent of the geophysical contractor. This role has been advocated within the most recent guidelines for the use of engineering (and environmental) geophysics (McDowell et al., 2002).