The constant improvement in space technologies is impacting other scientific subjects, starting with archaeology. February 8, 2024, the review Scientific Reports published a study written by Florent Michenot, doctoral student at CentraleSupélec. The latter then focused on the study of several sites in France and Central America, using data acquired by the Sentinel-1 satellite system as part of the European Copernicus program. Under the direction of Laetitia Thirion-Lefevre, Israel Hinostroza and Régis Guinvarc’h, researchers and professors at CentraleSupélec, the team succeeded in identifying structures hidden by the jungle. Meticulous work that the four academics agreed to comment on in an interview with Futura.
Futura: To begin, I think it would be interesting to define the distinction between SAR technology, which you exploit in your study, and LiDAR technology.
Laetitia Thirion-Lefevre : Both with SAR (synthetic aperture radar) and LiDAR (Laser imaging detection and ranging)), we send waves from a device at altitude which refracts on the surfaces below. We measure the time it takes for these waves to propagate. This is achieved with the help of a laser for LiDAR or with radio waves for SAR. LiDAR offers precision of the order of nanometers with LiDAR and a few centimeters for SAR.
Futura: Do you observe any variability in precision between the two technologies?
Florent Michenot : LiDAR performance is much better. The distance between the instrument and the ground plays a role in the quality of the data collected, as does the wave difference between the SAR and the LiDAR. If we decided to integrate Sentinel-1 technology into an aircraft, as is the case with LiDAR and therefore closer to the ground, the precision would still be lower than the data acquired by laser.
Régis Guinvarc’h : A notable difference is also the operating cost. Using an airplane for LiDAR research is very expensive. For the SAR, these are free images, obtained within the framework of the Copernicus programCopernicus program. An agreement between the European Commission and theEuropean Space AgencyEuropean Space Agency (ESA) provides researchers with a database acquired in orbitorbit by the Sentinel-1 dyad.
Futura: The images provided by the ESA are free, but do you incur additional costs to carry out your research?
Florent Michenot : Our technique is very economical, which is notably due to the free images. We pay for the treatment and calculations. We may use a service cloud computingthe one we use the most is the Google EarthGoogle Earth engine. You create an account, enter lines of code, and Google does the calculations for you. For LiDAR, you have to rent the plane, the equipment, pay the pilot…
Laetitia Thirion-Lefevre : LiDAR is very expensive. An airborne mission can quickly cost several tens to hundreds of thousands of euros.
Régis Guinvarc’h : SAR technology also makes it possible to reduce costs by making it possible to carry out “reconnaissance” over large geographical areas. We can then restrict the area and then why not dispatch aircraftaircraft LiDAR and ground teams to obtain more data.
Futura: Do you hope to achieve complementarity with other fields of study, by looking into unexplored or difficult to access areas?
Laetitia Thirion-Lefevre : We must keep in mind that we can never be 100% sure of what is located below the top of the treestrees. The data obtained by SAR allows us to determine if there are structures, without however being able to ensure the condition of the remains. But if we determine that there is a high chance of discovering temples hidden by the jungle, then it makes more sense to deploy LiDAR. Recently, the ESA contacted us to make data available to observe the entirety of South America.
Futura: In the event that you discover an as yet unexplored site, what is the next step?
Régis Guinvarc’h : Depending on the area, we will contact nearby archaeologists. The objective is to set up a project and release funds. Which is obviously one of the most important factors in ensuring the continuity of the work.
Laetitia Thirion-Lefevre : The difficulty is to get scientists to exchange with each other. Even in sometimes relatively similar fields, the different notions and vocabulary can be off-putting. The dialogue can therefore take time.
Futura: Do you think that the rise of these technologies could breathe new life into archaeological sciences?
Laetitia Thirion-Lefevre : We can carry out a gigantic pre-research work, making it possible to reduce the research area for archaeologists with the help of these technologies. But we do not replace the field work that they carry out on the excavation sites.
Régis Guinvarc’h : We must be in a permanent dialogue with archaeologists, already to establish what we must look for. It is the archaeologists who would tell us the elements that we could look for. It is worth remembering that our area of expertise is not archaeology.
Futura: So it’s necessarily an interdisciplinary collaboration?
Laetitia Thirion-Lefevre : If we study the Earth’s surface with Copernicus, we are much more efficient working with environmental specialists, for example! Our plan was to detect gold panning sites using Sentinel-1: we then had to talk with town planners or the military to get an idea of the buildings and structures built by the gold miners.
Israel Hinostroza : We face large variations regarding the electromagnetic signature of the structures detected by Sentinel-1. Collaborations with scientific communities are therefore essential, without which we would not be able to exploit the data.
Futura: How long does it take you to study a site like Lamanai, in northern Belize?
Florent Michenot : When I know where the site is located, it can be very quick. To download the images, it can take approximately an hour. The pre-treatment takes half a day and the treatment takes a second half day. I then have to try to determine if any buildings are detectable, a task made easier by the Google Earth Engine. The complexity is distinguishing between terrain relief and artificial structures.
Futura: Do you plan to travel to the site after a detection?
Laetitia Thirion-Lefevre : We favor going to sites observed by Sentinel-1. This allows us to understand field work. And most importantly, we can determine which elements cause approximations or errors. The radar image cannot be interpreted like an optical image, and we learn a lot from our mistakes.
Régis Guinvarc’h : We work with Natural Resources Canada (NRCan) on Canadian boreal forests. We have already visited the site to analyze the topography and distribution of speciesspecies. We thus establish a repositoryrepository which we use during the following work.
Futura: Your work will not replace the research carried out by archaeologists in the field?
Laetitia Thirion-Lefevre : As I said previously, we will never replace experts on the ground, who can observe and analyze elements in real time. Everyone has their own area of specialty.
Régis Guinvarc’h : It’s easy to obtain maps, formulate hypotheses and establish algorithms, it is much more difficult to validate this. It is critical and time-consuming. The ground truth is obtained by searching the archives in search of archaeological work, or by communicating with the archaeologists present on site. In this way, the Copernicus program and SAR technology are of public utility and truly enable large-scale scientific collaboration between many disciplines!
Interview by Dorian by SchaepmeesterDorian by Schaepmeester
