Imaging beneath the ground using non-invasive techniques

Intrusive sampling of contaminated sites to find buried targets is like trying blindfold to pin the tail on the donkey! With increasing pressure to develop more brownfield sites more cost effectively, failure to locate features that might affect a successful development can be very expensive as well as environmentally harmful or pose a Health and Safety risk.

The use of geophysical remote sensing should be encouraged to improve the effectiveness of traditional ground investigations, help manage risk, and increasingly is being recommended and, in some cases, required by the Environment Agency.

As an instance, a geophysical survey was undertaken and specific anomalies were characterised to anticipate the nature of the materials causing them. With follow-up trial pitting, it was found that it was possible to identify anomalies associated with vitreous iron foundry slag that had leached sulphur onto its surface, which represented a significant hazard had site operatives come into contact with it. As a consequence, appropriate measures were taken to deal with the hazard safely.

Desk studies, when undertaken properly, are fine for revealing what is known officially about a site, but what often causes problems are those aspects that were illicit or for which records have been lost! Features that lie within the ground may be unknown physical structures or may pose a significant hazard due to toxicity, flammability, etc.

It is important to locate such targets in order to minimise the health and safety risk to site workers, as well as to obtain a more robust ground model for the benefit of the development and/or remediation design. For example, at a site of an old steel rolling mill that had been long demolished, a geophysical survey revealed an area with significantly elevated ground conductivities that did not correspond with any features identified from desk studies.

Even during shallow excavations, there was doubt expressed by some that there really was nothing there. However, the geophysical interpretation suggested a conductive target at between 3.5 and 5 m depth. On deeper excavation into the reworked estuarine sands that covered the site, the digger revealed sands heavily contaminated with hydrocarbons, with tins of paint, rags, etc.

It was later discovered that this had been the site of an old pond and workers had disposed of tins of old sump oil, etc., by throwing them into the pond. A comprehensive desk study and trial pitting had missed this important environmental hazard but was found, mapped and its depth below ground determined from the geophysical survey.

Geophysical surveys provide a means of obtaining information that not only increases knowledge of a site but also improves an understanding of the processes that relate the components of a site together. For example, it is possible using geophysics to map the capping layer of a closed landfill, levels of unsaturated and saturated waste, determine the landfill boundaries, identify zones with different material characteristics, and identify areas of leachate migration from the base/edges of the landfill.

However, if it is possible, for instance, to relate the leachate levels to hydro-geological ground water flow directions, indicate how a plume might be related to a defective capping layer, or might be permitting leachate migration through faults in the bedrock, then one can begin to understand how to interact with this system more holistically.

Modern geophysical methods are very sophisticated and can now acquire data at rates of up to ten measurements a second, each of which can be tied into a specific location with an accuracy of 0.05 m using differential Ground Positioning Systems.

It is possible to survey a 0.5 ha site within a day (depending upon the technique used), but importantly with a ground sampling of typically better than 2 m by 1 m. This means that some 85-90% of a site can be surveyed compared with a typical intrusive sampling regime of less than 1 % (based upon a 25 m by 25 m grid). Importantly, the chances of finding a target measuring 5 m by 5 m using direct means are less than 10% whereas using geophysical methods, the chances exceed 90% as long as the target has material properties that contrast with those surrounding it. This is exemplified by comparing the ground models shown in Figure 1. The top profile was obtained after three sets of intrusive investigations (boreholes, trial pits and ground strip). Following a geophysical survey, further locations were selected from interpretation and analysis of the anomalies identified and additional trial pits excavated. The lower ground model was derived from this more comprehensive and integrated site investigation.

Current advice from CIRIA/Geological Society is to engage an independent Engineering Geophysics Adviser (EGA) who can specify the type of survey and compile a technical specification that best meets the requirements of the client. The EGA can advise clients on the choice of geophysical contractors, supervise their fieldwork to ensure best data quality as well as provide additional information that aids the interpretation.

The EGA can then undertake objective higher-level analysis, computer modelling as necessary and integrate a wide variety of information to produce a more holistic and understandable interpretation. Surveys that are unsuccessful have often been poorly specified, badly undertaken and, particularly commonly, inadequately interpreted and reported.

A ‘spot the anomaly’ interpretation is another form of pin the tail on the donkey, only this time the donkey picture is a computer-generated colour graphic! Interpretation of geophysical data requires considerable expertise and experience and must be given sufficient emphasis for the real benefits of the survey to be realised.

Case histories

HOUSING DEVELOPMENT, SOUTH WALES

Several landfills had previously existed on a site containing a mixture of wastes (PFA, building rubble, and contaminated industrial waste). Steel drums were known to have been buried within the site around the 1950s but their locations were unknown and had not been discovered through subsequent site investigations. The earthworks had covered over the remains of the landfills. At this late stage in the development of the site, it was suggested that the locations of steel drums should be determined so that they could be dealt with.

Most of the 13 ha site was surveyed using a magnetic gradiometer mounted on a non-magnetic trailer towed by a quad-bike with dGPS position fixing. Trial pits were excavated at the locations of particular anomalies, to reveal steel drums (Figure 2) thus verifying the modelling of the data. The geophysical techniques had been able to identify potential targets very rapidly that would otherwise have been missed using intrusive investigations alone.

HOUSING DEVELOPMENT, NORTHWEST LONDON

Three successive sets of intrusive investigations had been undertaken on a former industrial site in northwest London comprising boreholes, trial pits and a ground strip of the top 300 mm prior to construction. The Environment Agency ordered site work to stop due to the fear of contamination of an adjacent canal.

At the EA’s insistence a geophysical survey was undertaken that revealed the location of culverts draining towards the canal, old metal gas pipes, buried concrete slabs and drains, as well as several underground cellars and basements (Figure 3), one of which was heavily contaminated with hydrocarbons and oily water. It also posed a health and safety hazard to site workers and could have delayed the development considerably.

The survey provided the answers to the Environment Agency that permitted the developer to continue his project safely and without further potential environmental damage, saving him substantial amounts of money.

Suggested further reading:

Reynolds, J.M. 1997. An introduction to applied and environmental geophysics. John Wiley & Sons Ltd, Chichester & London, 796 pp.

Reynolds, J.M. 2002. The role of geophysics in the investigation of an acid tar lagoon, Llwyneinion, North Wales, UK. First Break, 20(10):630-636.

Reynolds, J.M. 2004b. Environmental geophysics investigations in urban areas. First Break, 22(9):63-69.

Author details:

Professor John Reynolds is the author of ‘An introduction to applied and environmental geophysics’ published by John Wiley & Sons Ltd in 1997 with a second edition in preparation. He has over 30 years experience in applied geophysics research, education and consultancy. He is currently Managing Director of Reynolds Geo-Sciences Ltd, an independent geophysical and geological consultancy company he founded in 1994. He was appointed an Honorary Professor at the Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, in June 2005.

Reynolds Geo-Sciences Ltd was the Wales Small Business Champion 2003.

Reynolds Geo-Sciences Ltd

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