14.05.08
Imaging the subsurface
Reynolds Geo-Sciences Ltd (RGSL) is the leading and largest independent geophysical consultancy in the UK. The team of highly qualified and experienced geophysicists are used to dealing with projects small and large and are currently working on managing geophysical projects for Crossrail and the Thames Tideway Project in London. RGSL has also been involved with the London Gateway Project and with DLR, as well as with other recent major infrastructure investigations in Dublin, Merseyside and Cumbria, for example, and a variety of tunnel investigations as well as other projects internationally.
Managing or developing a site not knowing what might lie in the ground is like playing Russian roulette – sometimes you win, sometimes you lose – big time! Such an approach does not make business sense. But how can you identify potential problems and minimise site time without having to dig up the entire place?
One answer is to use geophysical surveys to scan the ground to identify anomalies that can then be investigated using intrusive methods. This has the overall benefit of covering as much of a site as possible, typically >85% by area. As long as there is a contrast in the physico-chemical properties of the targets being sought relative to the background, and the targets have a reasonable physical size, it is likely that the geophysical methods will detect the features required.
Modern geophysical equipment is capable of acquiring a great deal of data very quickly, typically anywhere from 1-4 ha per day, dependent upon technique and spatial sampling used. Methods are deployed typically over the ground surface, some just by carrying the equipment and without it having to be in contact with the ground. Other techniques require small sensors to be placed on the ground or dragged along the surface, such as in ground penetrating radar, which can be used for a wide variety of applications including mapping badger setts in embankments and investigating the inside lining of tunnels in addition to railway ballast condition surveys. Electro-magnetic methods can be used to identify conductivity anomalies (metal pipes, ash, culverts, cellars, voids, mineshafts etc.) and magnetic methods materials that disturb the Earth’s magnetic field (ferrous metal, non-metallic materials such as ash, bricks, clinker, etc.). Micro-gravity is very good for detecting shallow voids (caves, mine workings, solution features). Seismic methods can be used to image bedrock at significant depth and provide information about the ease of excavating material. Electrical resistivity methods are excellent at imaging through closed landfills and mapping leachate plumes, which can also be monitored to see how they change with time, and through landslides.
Where anomalies are identified, an experienced interpreter should be able to give an indication as to what is causing the anomaly, how big the feature is and possibly its depth. Computer modelling can give a better idea and also an indication as to the limitations of the detectability. Some larger anomalies may mask the presence of smaller more subtle features.
Using geophysical methods reduces the risk of missing important and potentially costly in-ground features, natural or man-made. High-resolution seismic surveys are used routinely to map geological structures (faults), depth to competent rock and infilled buried channels that might affect the design and construction of tunnels and underground stations. Failure to recognise old landslides may result in potential mass slope failure onto key infrastructure. Buried mine workings can lead to subsidence and ground collapse. Significant voids behind tunnel linings may result in deteriorating assets. Can you afford not to use geophysical methods?
In the UK there are some a dozen or so geophysical contractors ranging from one man bands to more established and more professional outfits with significant staff numbers and their own equipment. Choosing between contractors, let alone deciding on what geophysical techniques should be used and how the survey should be designed, can be a daunting task to the inexperienced. Independent engineering geophysical advisers (EGA), in the form of geophysical consultants, have been recommended in the guidelines issued by the Geological Society/CIRIA in 2002. These organisations, which should be independent of any contractors, should be able to provide independent advice to clients and help design the survey and supervise it. Experience over many years indicates that there is a far higher probability of the survey achieving its objectives and the client being satisfied with the outcome when there is a partnership between the client, the EGA geophysical consultant and the geophysical contractor. Leaving contractors to design and undertake a survey on their own can be a recipe for problems. Similarly, an inexperienced client who chooses to design their own survey will more often than not make big mistakes that will jeopardise the entire survey and waste money as a result. In the worst case, it could lead to people being killed. For example, had it not been for one geophysical survey, a property developer could have excavated foundations using a mechanical digger not knowing that there was a 133 kV underground cable present. As a result, a very careful hand excavation was made exposing the cable safely. Even the utility company had not known it was still there and live!
Geophysical surveys improve the technical reliability of ground investigations when used in conjunction with appropriate intrusive methods, reduce technical and legal risks and can ultimately save significant amounts of time and money.
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