Remote sensing in archaeology

Indiana Jones look-a-likes spend months painstakingly slogging on a skeleton armed with toothbrushes. Admittedly, in the past, archeology was indeed somewhat artisanal. But times have changed. Archeology is catching up with high-tech. Innovative research methods arise from various disciplines that expand the collection of archaeological techniques.

A small plane floats in Limburg airspace with a special mission:tracking down the defense line around the Genneperhuis. Special cameras take pictures of the landscape. “Nothing special,” you might think, “Google does that too.” But these cameras photograph what we cannot see with the naked eye:near-infrared light emitted by trees and plants. When the planes are safely back on the ground, the photos are analyzed using the computer. The winding patterns of the line can be discerned in the colorful landscape photos. Archaeological traces, found without the use of a shovel.

Preliminary archaeological investigation

It has always sparked the imagination:how do archaeologists know exactly where to dig? Normally, an excavation is preceded by an extensive 'desk research' into archives and historical documents. After this desk research, people go into the field to carry out random soil drilling. Archaeologists place the drill in the ground at fixed distances to confirm or disprove suspected finds. The disadvantage is that drilling research is fairly labor intensive, and archaeological traces are sometimes overlooked. With 'remote sensing' (literally 'observing at a distance') this preliminary research can possibly be tackled a lot more efficiently.

Multispectral aerial photography

But how can you make archaeological traces under the ground visible on the basis of aerial photos? That's how it works:trees and plants emit an invisible light that we call 'near infrared'. Suppose the soil is very dry, or too wet, this will affect the nature of the near-infrared light emitted by the crops growing there. Even if the plant roots are not given sufficient space to grow, because they are obstructed by archaeological traces in the ground, for example, this has an influence on the near-infrared light that the plants emit. In this way, patterns can be observed in the vegetation, allowing us to discover archaeological traces at an early stage of the research.

“Multispectral aerial photography does not replace archaeologists' exploratory drilling research. It is intended to make archaeological preliminary research more efficient,” emphasizes Rick Ghauharali of VB Ecoflight, a company specialized in remote-sensing techniques. "Especially when large areas have to be surveyed, multispectral aerial photography can help to plan targeted drilling research, and thus also exclude large areas from research."

However, there is also a caveat to this method. What if the vegetation in question is not deeply rooted, but the archaeological remains are deep? “Then you don't see any discoloration on the multispectral photos,” says Rick. “But every situation requires a specific approach. When you know the nature of the archeology you are looking for, you also know which remote sensing technique is best for you.”

LIDAR and radar

More technical blessings can be expected from above. The same aircraft used for multispectral aerial photography can also be equipped with LIDAR. This technique, also called 'laser altimetry', uses laser beams. This laser emits pulses to the earth's surface. If you very accurately measure the time between sending and receiving the pulse, you can calculate the distance between the aircraft and the earth's surface. Taking into account the height of the aircraft, one can calculate the spatial shapes of the earth's surface in this way. This is possible with an accuracy of 15 centimeters.

This technique can be applied by archaeologists, for example, to detect creek patterns and old waterways. This is purely about the difference in height in the country, a difference in height that is often not visible when you are in the field.

However, LIDAR also has a drawback; for example, the laser pulses only partially pass through the leaves of trees. The leaves weaken the signal. We call this phenomenon 'laser shadow'. You can't really see what's under the tree because of this. “Radar techniques offer a solution for this,” says Rick. "Radar works in much the same way, only with radio waves. These radio waves can "see through trees". However, this technique is not yet fully established in the Netherlands."

Geophysics

Remote sensing is not necessarily limited to observing from great heights. “Although that obviously depends on your definition of remote sensing,” Rick smiles. “There are certain techniques in geophysics that, in my opinion, also fall under remote sensing, but they don't happen from the air, but just on the ground. Such as, for example, 'Ground Penetrating Radar' (GPR), 'soil resistivity measurements' and 'radioactivity measurements' (the latter two both measure the composition of the soil). Just like the aforementioned techniques from the air, you do not directly measure the phenomenon in which you are interested. You first do ~~from a small distance~~ measurements and can only interpret the phenomenon after analyzing your data.”

The soil archive

Remote sensing gives us a glimpse into the soil without disturbing the soil archive. The disadvantage of excavating is that it can only be done once. As soon as we put the shovel in the ground we destroy layers of ancient archaeological traces. Not only because we turn up the ground, but also because the spores come into contact with sunlight, oxygen and bacteria. The government's policy is therefore to leave archeology untouched in the ground as much as possible (conservation in situ ). This is the best method of conservation, until technology provides us with even better methods of excavating in the future.