‘You see but you do not observe’

So said Sherlock Holmes, frequently, to Dr Watson. To the forensic geologist, this is a useful tool – though criminals are becoming more forensically aware thanks to programmes like CSI, they can only attempt to remove evidence they can actually observe. But to the forensic geologist, visible evidence is only the beginning.

A team of scientists from the Geological Society’s Forensic Geoscience Group enthralled an audience at the British Science Festival yesterday, as they took them through some of the techniques they use to help police solve crimes, persue criminals and recover buried objects.

While they may be applying it in an unusual context, geology is still central to their work. ‘We haven’t invented anything new’ explains FGG Chair, Dr Laurance Donnelly. ‘The techniques we use are used around the world in mining or mineral exploration. We’re just applying them to a forensic context.’

It makes sense to involve people with expert knowledge of the ground and its processes in the investigation of crime. Dr Donnelly’s own specialism is in search – helping the police to recover bodies or objects using a variety of techniques. Changes in soil composition, leaching of fluids into the soil, even thermal imagining can all lead to discoveries of objects long buried.

On the microscopic scale, Dr Lorna Dawson of the James Hutton Institute described how she uses the complexity of soil particles to assist police.

‘Soil is not just ‘mud’ she explains. ‘It is a complex mix of organic and inorganic materials, easily transferable to objects or persons such as on shoes, car tyres or spades. Soil forensic scientists use a range of techniques, including chemical analysis and mineralogy along with new approaches such as DNA sampling to analyse soil samples collected as part of criminal investigations.’

Following further talks from Dr Duncan Pirrie, Dr Jennifer McKinley and Dr Alastair Ruffell, the audience were invited to participate in the investigation of a genuine crime, using the evidence forensic geologists had available to them. Young people as well as adults were rapt by the mystery, and continued their questions long after the scheduled end of the session.

‘It’s not surprising people are so fascinated by forensic geology’ says Dr Donnelly. ‘These are two topics – geology and forensics – which have always been popular with the public. It’s great to see youngsters here getting involved with the applications of science.’

Laurance Donnelly with the BBC's Richard Hollingham at the Xchange

Dr Donnelly later entertained the XChange audience – an evening event featuring the best of the day’s speakers and topics – and was met with the most frequently asked question of the day: ‘How do you get to be a forensic geologist?’ The answer: ‘study geology, and see where it takes you.’

• Hear Laurance Donnelly delighting the X-change crowd with tales of being a forensic geologist on theX-change podcast

• Read more at the Guardian’s Festival blog and in the Irish Times