Phase were recently called in to locate three gasometers at a former gas works site in Scotland. The majority of the gasworks had been demolished in the 1970s and the site had then been levelled. Unfortunately accurate records of the position of the gasometers and other buildings were not kept. The site was now a main town centre car park and the local authority who owned the site wanted to know the locations of the buried gasometers so that they could safely manage the future use of the site.
The fact that the area was now a busy public car park meant that it was not practical to carry out major excavations to locate the buried features. A limited borehole survey had not located the structures but had identified contaminated ground, further emphasising the need for an alternative solution to intrusive work.
Phase Site Investigations Ltd recommended carrying out a combined magnetic and electromagnetic (EM) survey. Each of these techniques can identify certain types of buried structures and combining them increases the range of features than can be found. The client also required a utility tracing survey to accurately map and record the position of buried services so any future intrusive work could avoid them. A combination of radio frequency location and ground penetrating radar (GPR) was used. As well as identifying non-metallic apparatus the GPR data was also be used to provide depth information for anomalies identified by EM and magnetic survey.
The non-intrusive survey successfully identified three gasometers and several associated buildings and the routes of the utility apparatus were accurately located. The survey was tied-in to topographic features on site and the results of the survey were plotted against a digital base plan. The level of detail and information obtained by the survey could only be matched by intrusive means if the entire site had been stripped, which even if it had been a practical option would have been prohibitively expensive.