Image copyright James Hutton Institute

Forensic soil scientist Prof Lorna Dawson is helping detectives solve decades-old murder cases.

As she stepped in to witness box at the trial Christopher Halliwell she knew the evidence she gave could help convict a man who police believed could be a serial killer.

Halliwell was accused of the rape and murder of 20-year-old Becky Godden in Swindon more than a decade ago.

Prof Dawson's job was to try to explain to the jury in a simple and unbiased way the evidence she was giving.

For instance, could soil found on a spade in Halliwell's home be the same earth found at the place where the body was buried?

Image copyright James Hutton Institute Image caption Soil samples being collected at a crime scene by Prof Lorna Dawson

It is delicate, intricate work. So where does she start?

When it came to Christopher Halliwell's trial, Prof Dawson, who works at the James Hutton Institute in Aberdeen, got soil samples from tools and other items taken from Halliwell's house.

She compared them with soil taken by the forensic archaeologists who worked at the site of the grave to see if they were similar.

"There was a piece of silver tape that was found in the gravesite beside Becky's remains - there was the soil on that," she says.

"There was black tape that was found with one of the spades in the garden shed of Mr Halliwell and there were various other items - a fork and a pick axe in the shed at the home of Mr Halliwell.

"There could be little bits of fibre, little bits of DNA, skin, so we look at it very carefully under a macro lens."

Image copyright Wiltshire Police Image caption In the Christopher Halliwell case, Prof Dawson analysed soil found on a spade and black tape found in the killer's house

Soil was recovered from each of the items and then analysed and compared with soil from the grave where Becky Godden was found.

Each bit of soil has its own unique signature.

"You're looking at the colour, you're looking at the structure. The colour reflects the iron oxide," says Prof Dawson.

"The more brown it is also reflects how much organic matter is there. All these things are like multiple layers. As you can imagine you stack up layers of different information."

Advances in science and DNA mean instead of needing at least a thimbleful of soil to make an analysis, Prof Dawson can work with a tiny sample of earth - the equivalent of six grains of sugar.

This is what helped bring the infamous World's End killer, Angus Sinclair, to justice after several decades.

Sinclair was jailed for a minimum of 37 years in 2014 for the murders of Helen Scott and Christine Eadie in Edinburgh 37 years before.

Image copyright James Hutton Institute

In 2004 three Scottish police forces came together to launch Operation Trinity, a review of the 1977 unsolved murders.

"I worked on that at the beginning. When we were able to apply new techniques that could look at small amounts of sample we were able to miniaturise our methodology.

"DNA is one of the great advances that has been made. That's why they can revisit cold cases."

In the World End's case, the biologist worked on the ligature that was round one of the girls necks.

They had developed techniques to look at the position of the DNA on the knot which, along with the other new techniques, enabled the team to determine that Sinclair had tied it.

Prof Dawson was also able to look at trace evidence found on the soles of Helen Scott's feet and match it with soil from the field where she was buried.

She says: "That allowed the prosecution team to show that the alibi that Sinclair came up with in court at the last minute could not possibly have taken place.

"His version of what had happened could not have happened."

There are strict rules about how such evidence is treated. Testing of samples has to be done in complete isolation so there's no contamination.

Prof Dawson says: "The lab has to be completely wiped down, you change all the personal protection, lab coats, gloves and equipment. The sample needs to be sealed and intact. Sample continuity is paramount".

Image caption Prof Dawson has been involved in the convictions of Christopher Halliwell (L) and Angus Sinclair

Image caption Her team is also helping in the search for Ben Needham (L) and Moira Anderson

As the principal soil scientist in the environmental and biochemical sciences group at the James Hutton Institute in Aberdeen, her work ranges from wildlife and environmental crime to civil and criminal law.

She has also been an adviser on TV series "Vera" and to author Ann Cleeves.

Some of her team, which includes botanists and mineralogists, are working on the site on the Greek Island of Kos where British toddler Ben Needham went missing 25 years ago.

Prof Dawson is also keen to point out the work of her team is a world away from popular crime drama CSI - especially when it comes to explaining it in court.

She says: "Everyone out there potentially could be on a jury so it's our duty to make sure they understand, as much as possible, the underlying science.

"So they're not expecting and not being influenced on what we call the CSI effect - that you expect everything to be done instantly with an instant answer."

"It doesn't always happen that way and they've got to understand that there is uncertainty associated with some of the scientific measurements that are made."

Later this year she'll be involved with Police Scotland in trying to find the body of Moira Anderson after almost 60 years.

Eleven-year-old Moira left her home in Coatbridge on a snowy night in February 1957 to buy butter from a local shop and never returned.

The professor and her team have used ground-penetrating radar already and will begin digging to see if they can find her in a few months.

When Christopher Halliwell was convicted, it turned out he was already serving a life sentence for murdering Sian O'Callaghan in 2011.

Police say it's "conceivable there are further victims".

Wiltshire Police are looking at the possibility that Christopher Halliwell could be a serial killer.

Will Lorna Dawson help answer that question?

"I'm happy to be part of that team should there be any necessity in terms of involving soil," she says.