An Archaeologist's Guide to
Headache-free GPR
How it works
The science behind GPR is complex and has been the source of plenty of headaches for archaeology students. A very basic explanation of the technology works like this:
The antenna of a GPR system shoots radio pulses into the ground. Each pulse travels through the ground as a wave.
Within the ground there are different layers of subsurface materials (soils, rocks and, hopefully, archaeological remains).
Every time this wave comes in contact with a new layer of soil or debris, the velocity of the wave changes. This causes some of the energy of the wave to “bounce” back as a reflected wave.
The reason the velocity changes is because the new material has different electrical and magnetic properties to the layer it was passing through before. The greater this difference is, the greater the change in velocity will be.
A large change in velocity is helpful because the amplitude of the reflected wave will be higher, which makes it easier to detect.
The rest of the energy continues into the ground.
The GPR system records the time it takes for the reflected wave to return (it’s a matter of nanoseconds). This allows for the depth of the material it reflected off to be determined.
The system also records the shape of the reflected wave, which will vary depending on the material it bounced off.In the course of a field survey, a GPR system will send out thousands of radio pulses, producing thousands of reflected waves and depth measurements. This information is automatically entered into a database and plotted on a reflection profile, a map that shows where waves occur and what they look like.
Archaeologists, working with building/landscape scientists, will analyze these waves, and their depths, and determine what is likely to be causing the various reflections.
How deep GPR will detect archaeological materials varies depending on the equipment used (ie- how much power the transmitter is getting) and soil type. Beneath asphalt, archaeologists will likely only be able to detect objects that are a few meters below the surface. In sandy soil, on the other hand, they can find objects that are more than 50 metres deep.
Make it 3-D
Reflected waves, if mapped three dimensionally, can give a rough view of the shape and size of a structure.
Mesagne, on the southeast end of Italy, has an ancient necropolis that dates from Roman Imperial times, before the Romans took over the area (a time when the Messapian language was used).
It can provide a powerful and intuitive means of communicating complex information to non-geophysicists.
A team of researchers, led by Giovanni Leucci, used ground penetrating radar and created 3-D images of what they found. The image revealed underground burials called “ipogei,” used in ancient times, with 3m x 3m x 3m dimensions. They also reveal what appears to be the wall of an ancient road leading up to the necropolis. The chief advantage of mapping something in 3-D, according to the researchers, is that, “(it) can provide a powerful and intuitive means of communicating complex information to non-geophysicists.”Scanning huge areas
In 2004 a multi-channel radar cart (a sophisticated GPR) was used over a period of two days to scan 75 square
kilometres of land at the site of Le Pozze (The Puddles), in the suburbs of Lonato in Northern Italy. The development of increasingly sophisticated GPR systems, like this cart, has been increasing the efficiency of the technology, allowing researchers to investigate more land in less time.Archaeological investigations have confirmed that Le Pozze has been settled since at least the Iron Age. A Roman village existed there and the site was inhabited until the 14th century when it was destroyed.
The GPR survey found wall remains, pavement and buried foundations. It also found the remains of a large Roman Villa with its biggest room being 20 x 20 meters in size, and a public building that is 20 by 10 meters wide. Overall they estimate the Roman village to be about 15,000 square meters in size, with two-thirds of it taken up by the villa and the public building.
But Don’t Try it at Home
Using GPR is something that, ideally, shouldn’t involve the archaeologists working alone. Former English Heritage Conservation Director John Fidler writes in the book Ground Penetrating Radar that researchers should work with both electrical/radar engineers and building/landscape science specialists to collect and analyze GPR information.
People in those disciplines are better prepared for the task of collecting GPR data and understanding what kind of material is being uncovered. Archaeologists need to work as a team with them in order to get the most accurate data and interpretation possible.
Avoid Excavation
GPR is a good alternative when archaeologists want to avoid excavating a site.
For instance, the first emperor of China, Qin Shi Huang, was buried in an enormous necropolis, larger than the Great Pyramid of Khufu. This expansive site includes more than 8,000 terra cotta soldiers.
One section is a mostly unexcavated “underground palace” built to help the emperor rule in the afterlife. A team of Chinese archaeologists, led by Yuan Bingqiang, used Ground Penetrating Radar to identify buried pits that are believed to lie just outside the palace.
These pits likely contain terracotta warriors or officials, which were built for guarding the palace and running the administration in the afterlife.
Chinese researchers have been careful to avoid unnecessary excavation around the palace, so as not to risk damaging artefacts. In this case, GPR allowed them to find the probable location of the terracotta figures without having to tear up a vast amount of the site. In the future, if archaeologists do excavate those areas, they will know where to look.
Avoid jail
GPR is also useful when legal or ethical reasons bar excavation of a site.
For example, in North America GPR has become popular among archaeologists for investigating Native American burials without actually digging them up. In the United States federal laws, such as NAGPRA (Native American Graves Protection and Repatriation Act) ban or severely limit the excavation of these burials.
One notable example is in Wisconsin where GPR was used to find the truth behind what turned out to be an archaeological myth.
The West Prairie Mound Group is made up of thirteen low, cone shaped earth mounds within Fort McCoy (a modern day military base in central Wisconsin). It has long been believed that these were pre-Columbian Native American burial mounds, but archaeologists could not determine this since they were prohibited from excavating.So a team of researchers, from the University of Wisconsin and the Army Reserve, used GPR on two of these mounds to try andfind the actual burials. They discovered that in fact these mounds were natural features and there is no evidence that they were ever used for burial or cultural practices.
Limitations of GPR
GPR does have its drawbacks. In modern urban areas GPR will pick up contemporary equipment, such as pipes and buried equipment, along with archaeological remains. This can make interpreting data very difficult if not impossible.
Water can be a big problem for GPR. As archaeologist Lawrence Conyers writes, if the ground has even a small amount of water it can make detecting archaeological materials difficult.
The reason is that in dry ground, human artefacts, such as metals and stone-work, tend to have very different electrical properties than the soil around them.
However, when the soil is wet, this changes the properties of soil to the point where, electrically, they become similar to human artefacts such as metal.
So if the artefacts are in wet soil, there will be less of a difference, electrically, between the soil and the artefacts. This means that the velocity change of the wave will be reduced and the reflections produced will be weaker and more difficult to detect.
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