Science Spin November 2009

What lies beneath, becomes clearer

By Sean Duke

The big contribution of geophysics to world might be that it ends altogether the need to dig for information, can for example, pinpoint ancient building foundations, buried bodies, or illegal dumps. In the past it, the gathering of information from underground was totally dependent on digging.

That has changed over the past 20 years or so, and now digging is used more sparingly and generally only after geophysics has been done on a site. In Ireland one of the people that have been at the forefront of the development of the field of geophysics is Dr Paul Gibson, based in the Environmental Geophysics Unit of the Department of Geography at NUIM.

IRELAND

Geophysics has been around for some time, said Dr Gibson, but in Ireland, the field was given a major boost in 1995 when the European Structural Fund provided funding for the first higher diploma in GIS (geographical information system) remote sensing, which later turned into an MSc. offering. GIS is a system of analyzing data relating to a particular location, while remote sensing is the term used to describe the gathering of data on the Earth, remotely, from sensors on airplanes, or satellites.

This GIS remote sensing course at NUIM trains students, among other things, in the use of geophysical techniques in the field. In the past 14 years, about 200 students have completed the course, and these are out working mainly in industry. The course continues to attract undergraduates from many fields, including archaeology, zoology and engineering. One of its great strengths, and one which helps to justify the €2,000 fee to students, is that students get work placement as part of the deal. Often students can use this as a 'foot in the door' to the workplace, and are taken on later on a full-time basis.

The origin of geophysics lies in oil and gas, where a lot of money is spent on finding raw materials before any drilling starts. Drilling is expensive, and any technique that narrows down a potential drill area is valuable. However, it soon became clear that geophysical techniques could be applied to other areas where people wanted to 'look underground'. The main areas today are: geological geophysics where rock structures are investigated; environmental geophysics where researchers can pinpoint the location of illegal dumps; archaeological geophysics which can reveal entire towns, villages or landscapes that lie buried underground; and forensic geophysics where police are trying to find buried bodies.

ARCHAEOLOGY

The use of geophysics has simply transformed the field of archaeology in the past 15 or 20 years. Whereas it used to be routine for archaeologists in Ireland to conduct 'digs' during the summer months, now the use of geophysics, and the superb information it can yield on what lies beneath, means that digging is far less common and justifiable than before.
The problem with digging, of course, it that it destroys, to an extent, the area that is the object of study, and is costly.

Also, the great advantage with geophysics is that it can over a huge area quickly and identifies archaeological 'anomalies' that are quickly apparent to the trained eye. So, for example, an area with no obvious archaeological value, apart from perhaps a ruined castle, can be revealed, with geophysics, to have once been a thriving town, sited on an important road.
It means that even an uninspiring field, with no remaining features, could be revealed as a site of ancient importance.

"Archaeology is a big one for a number of reasons," explained Dr Gibson. "It (geophysics) is non destructive, non-intrusive, (a) very useful reconnaissance tool, cost effective. For archaeology, when you dig you destroy what you are studying. If you are faced with a 200 or 300 acre field you want to know: Where are the sites of interest? Where we should go, and, Can we map the structure underground without having to dig?"

Often, areas which look uninspiring to an archaeologist, suddenly, after the application of geophysics become very interesting indeed. Dr Gibson described an example of this when he found a 2,000 year-old Iron Age hut, underneath a flat, featureless piece of land in Co Carlow. Further investigations revealed 'Burgage Plots', which are dwellings that front onto a main road, but have strips of land going back behind. These plots allowed people to have a bit of valuable street frontage, as well as some land behind their dwelling. There was also a bailey, where animals were kept, roadways, enclosures, and an ancient dump, all located underneath the ground in an area where no trace remains on the surface.

Similarly, as part of working collaboratively with the Discovery Programme - a public body set up to do advanced research in Irish archaeology - Dr Gibson and his student Terence O'Rourke unveiled a treasure trove of information at a site in Co Westmeath, where an old tower house sat alone in the landscape. There would be little justification for archaeologists to dig around the tower, and so geophysics was the best option. This picked up the presence of a military road, now buried, but mentioned in the historical record. The tower house was probably guarding the military road, said Dr Gibson. The road itself dated from the 14th century. An older road, dating from around the 10th century was investigated in Co Offaly and was made up of slabs. The road builders dug down two metres or about six feet, and filled the trench in with loose material before the slabs were put down.

The use of geophysics can often reveal that a landscape feature that was not regarded as highly important is, in fact, of great interest to archaeologists. For example, there is a mound near the village of Tulsk, in Co Roscommon that people were curious about, but little more was known. Dr Gibson, and his team did used resistance techniques to survey the mound and came up with a lot of dark area, which indicates the presence of lots of buried stone. A dig confirmed the stone, and the excavations are now into their fourth year. Spurs, musket balls and coins were also found from the 14th and 15th century, and the theory is that this was the located for a guardroom to watch over the former Lord Lieutenant's house - which is today the building that is home to the Dominican Convent.

Also, near Tulsk, in a place called Carns, a site that was regarded as medieval was shown to be inhabited as far back as the Iron Age. The geophysics showed a series of ever larger circles out from a central circle - something like a dart board. The evidence pointed to an ever increasing settlement, used for thousands of years, and bodies were also found.

FORENSICS

The use of geophysics to find graves of murdered people is something that the Gardai have become interested in, in recent years. It can be used to find graves, as magnetic surveys can reveal the presence of metal objects such as belt buckles, ear rings, and keys that remain on the body. There are also fluids released after death, and these can be detected too, but it is a battle to find the trace of these fluids before all that remains of the body is the bones. The aim is to find 'anomalies' or features underground that stand out, and experts can, thus, pinpoint potential dig areas for the Gardai.

In a recent case of a missing person Dr Gibson was called in to use geophysics to try and locate a body in a forested area. This body, sadly was not found, despite the presence of the army, who deployed metal detecting equipment normally used to trace landmines, and two Garda 'cadaver' dogs. There was some excitement when the geophysics pointed to an anomaly, and this was confirmed by the two dogs. But it turned out to be buried metal wires, with paint, and the aroma of paint, apparently, can confuse the dogs, as it smells like the smell given off by buried bodies. The investigation, however, did manage to confirm that the body in question was not in this area, and that at least allows the Gardai to rule it out.

The real benefit of geophysics to a murder investigation, where a body is missing, is that areas that might be suspect, which could comprise several hundred acres, can be covered, and anomalies found or not found, without digging. To find a body, a magnetic contrast must be established between the body, with a metal object or two attached to it, and the background. In the absence of metal objects, resistivity can be used to pick up the fluids emitted by a decomposing body. Studies have been done on buried pigs to watch how long they take to decay, and what is the pattern of fluid emission. However, without metal objects on the body, it is a race against time, as resistivity is not effective to find just bones.

ENVIRONMENT

The problem of large-scale illegal dumping in Ireland has been an issue for a number of years. One of the difficulties for those charged with prosecuting the offenders is that an area that was used for dumping, even large amounts of material, can be covered over very effectively so that it is very hard to detect a dump by just looking at the surface features.

One of the problems here is the money involved. In story published online by the BBC as far back as 2004, it was determined that an average waste contractor in the Republic was being paid Stg2,500 to remove a 20-tonne load of waste. After paying the driver, and the landowner to permit the illegal dumping, that yielded a profit of Stg£2,200 - that was just for one lorry-load, and on some illegal dumps, eyewitnesses reported multiple lorry loads arriving each day. With, perhaps four lorry-loads per day arriving at a site, that would yield profit to the waste operator of €44,000 over five days.

Today, however, the environmental enforcement network - made up of people in state agencies and the police on both sides of the border - can fight the illegal dumpers using geophysics. The presence of domestic waste underground, when resistivity is used, will yield massively low resistance values. This made it possible for Dr Gibson to pinpoint hidden landfills in Co Monaghan, for instance, beneath 'perfect' looking fields. As well as the waste, it was possible to pick up the trace of waste fluids, or leachates, spilling out from the dump into the surrounding area.

FUTURE

There are not a number of specialized geophysics companies operating in Ireland, and even in these difficult times, their future looks bright. The are qualified graduates constantly emerging from courses at NUIM and UCC, and these graduates can find their way into the Gardai, EPA, the Archaeological Survey of Ireland, join existing companies or set up their own.

Dr Paul Gibson is proud of the student body that have emerged from his MSC at NUIM, which came to Maynooth from many different counties, and now represent a impressive network of contacts that present students can tap into.

In terms of the general picture, Dr Gibson would like to see archaeological assessments conducted as part of environmental impact studies, but he is realistic about what is possible. The county councils, he said, make owners pay for archaeological studies that are done on their lands, following the reporting of finds, so this is a major disincentive for people to report. It could, for example, delay the building of a house, and cost thousands to do the surveying work. For that reason, he believes that geophysics will only be called in when developments are proposed that lie near, or beside areas of known archaeological value. But, that approach is not ideal, he said, as "there is a lot of unknown stuff out there."

GEOPHYSICAL TECHNIQUES

-Ground penetrating radar. An electromagnetic pulse is sent out towards a target area, and information is reflected from that area back to a receiver. The reflected pulse can be interpreted to reveal buried objects, or structures of interest. A machine can be run over a large area, and information gathered in real-time about where items are buried and how deep.
- Electrical resistivity. A direct electrical current is inserted into the ground, and the resulting voltages that are created in the ground are measured. The voltage information can be used to determine rock types, or where an aquifer is located. Also if there are areas of high resistance that might indicate the presence of walls or buildings as it is very hard to send an electric current through the walls. It is possible to determine where walls are located, and what depth they will be found at.
-Magnetics. The idea here is to measure disturbances in the Earth's natural magnetic field. These disturbances are usually caused by rocks laden with iron that are, therefore, magnetic. This could also be used to try and find a body, since it could pick up 'anomalies' produced by the presence of man-made, iron containing objects like belt buckles or coins.
-Electro-magnetics. Here the geophysicist in the field generates an electro-magnetic field above ground, which creates a magnetic field underground. By measuring the magnetic field produces, sub-surface properties can be determined. This might, for example, be used to detect leachate from a dump, or salt water in groundwater, or the location of an aquifer.
-VLF. This acronym refers to the measurement of disturbances in the magnetic field of VLF, or very low frequency, radio waves. The nature of the disturbances can provide information on the location of geological faults, and mineral deposits.