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Airborne thermal prospecting is based on the principle that there is a fundamental difference between the thermal characteristics of buried remains and the environment in which they are buried. The French ‘Archéodrone’ project aims to combine the flexibility and low cost of using an airborne drone with the accuracy of the registration of a thermal digital camera. This combination allows the use of thermal prospection for archaeological detection at low altitude with high-resolution information, from a microregional scale to the site scale. The first results have allowed us to assess the contribution of this technique for the detection of ancient roads, land plots boundaries, site plans and underground caves. Copyright © 2013 John Wiley & Sons, Ltd.
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Low Altitude Thermal Survey by Means of an
Automated Unmanned Aerial Vehicle for the Detection
of Archaeological Buried Structures
Nicolas Poirier, Florent Hautefeuille, Carine Calastrenc
To cite this version:
Nicolas Poirier, Florent Hautefeuille, Carine Calastrenc. Low Altitude Thermal Survey
by Means of an Automated Unmanned Aerial Vehicle for the Detection of Archaeological
Buried Structures. Archaeological Prospection, Wiley-Blackwell, 2013, 20 (4), pp.303-307.
<10.1002/arp.1454>.<hal-00844459>
HAL Id: hal-00844459
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Archaeological Prospection, 20, 303-307 (2013)
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Low altitude thermal survey by
means of an automated UAV for the
detection of archaeological buried
structures.
Nicolas Poi rier, Florent Haute f euille, Carin e Calastrenc
UMR 5608 TRACES, CNRS - Unive rsité Toul ouse 2 - L e Mirail
Université Toulouse 2 - Le Mirail, Maison de la Recherche, UMR 5608 TRACES, 5 Allée Antonio
Machado, FR-31058 Toulouse cedex 9, nicolas.poirier1@univ-tlse2.fr, +33561502350
Abstract : Airborne thermal prospecting is based on the principle that there is a
fundamental difference between the thermal characteristics of buried remains from the
environment in which they are buried. The French 'Archéodrone' project aims to combine the
flexibility and low cost of using an airborne drone with the accuracy of the registration of a
thermal digital camera. This combination allows the use of thermal prospection for
archaeological detection at low altitude with high-resolution information, from a micro-
regional scale to the site scale. The first results have already allowed us to assess the
contribution of this technique for the detection of ancient roads, parcel boundaries, site plans
and underground caves.
Keywords : UAV, thermal survey, site detection, mapping, aerial archaeology, remote
sensing
State of the art and objectives
Richard Atkinson is usually considered as the first who developed, for the first time in 1946, a
ground-based non-invasive method to get a detailed map of the archaeological site by the
study of the electrical resistivity of soil (Atkinson, 1946); although earlier experiments have
been documented (Bevan 2000) It was followed by the development of techniques such as,
magnetic mapping and Ground Penetrating Radar. The development of air and space-based
techniques, such as multispectral satellite remote-sensing or Airborne Laser Scanning, has
diversified the methods and thus increased the number of tools to visualize archaeological
remains without direct intervention (Devereux et al., 2005; Bewley et al., 2005; Lasaponara et
al., 2010).
Airborne thermal prospecting is one of these non-invasive methods. It is based on the principle
that there is a fundamental difference between the thermal characteristics of buried remains
Archaeological Prospection, 20, 303-307 (2013)
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from the environment in which they are buried (Eppelbaum, 2009). Therefore archaeological
structures tend to react differently to changes in temperature compared to the surrounding
sediment (warming or cooling more or less quickly depending on their nature). For example,
structures built of stone and mortar present a greater thermal inertia than the surrounding
sediment; conversely, hollow structures (ditches and pits) have a lower thermal inertia,
allowing their detection on bare soils (Rowlands and Sarris, 2007). On cultivated soils, the
hydric stress of cereals disturbs their thermal regulation, permitting the detection of remains
before they are visible on conventional pictures as crop marks (Aqdus et al., 2012) as visible on
figure 3 for example.
This detection method has been used in archeology since the 1970s (Bellerby et al., 1990; Ben-
Dor et al., 2001; Berlin et al., 1977; Lundén, 1985; Prakash et al., 1995; Tabbagh, 1977). But the
use of aircraft or satellite imagery made the method difficult to access due to its cost and lack
of organizational flexibility: flights must be scheduled several weeks in advance and the
relatively high flight altitude is only able to detect large structures (abandoned roads and
parcel boundaries).
The parallel development of Unmanned Aerial Vehicles (UAV) and miniaturization of sensors
and data transmission suggests a technological breakthrough in terms of archaeological
prospection (Bendea et al., 2007; Chiabrando et al., 2011; Eisenbeiss & Zhang, 2006; Eisenbeiss
et al., 2005; Everaerts, 2008; Hendrickx et al., 2011; Verhoeven, 2009). However, the recent
development of UAVs has so far benefited archaeological survey by the addition of
conventional digital cameras, to achieve vertical or oblique aerial photographs, mainly at the
scale of a site or excavation.
The French 'Archéodrone' project, developed at the University of Toulouse 2 - Le Mirail
(France), aims to combine the flexibility and relative low cost of using an airborne drone with
the accuracy of the registration of a thermal digital camera. Indeed, even if the price of
acquiring the equipment is quite high (several dozens of thousand Euros), the possibility to
repeat as many flights as needed makes the methodology affordable compared to the price of
a single plane or helicopter flight. This combination allows the use of thermal survey for
archaeological detection at low altitude, resulting in a higher resolution of information, from a
micro-regional scale to the site scale, and the possibility to repeat several flights above the
same area in different air and soil conditions.
Equipment used
The UAV used is an octoroctor helicopter (Figure 1) with electric propulsion and a wingspan of
about 80 cm, with a three-dimensional inertial stabilization system and a GPS for its
geolocation. It is radio controlled and allows a payload of 3 kg. It is accompanied by a
telemetry system to display real-time flight data: battery consumption, flight time, altitude,
distance from the point of takeoff, and GPS status. The same system is used to display a map
and manage the GPS' points of interest (waypoints).
Archaeological Prospection, 20, 303-307 (2013)
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The sensor used is a FLIR T620 thermal digital camera with high sensitivity for the detection of
temperature changes of about 0.04 °C, on images of 640 x 480 pixels. A radio transmission
system captures the camera images in real-time and can track the flight on a LCD screen with
return video.
The whole equipment was acquired for about 70,000 €. This is a large investment for a
research team, but we consider this as a low cost method given that this material will be
usable for several years, several times a day and several days in a row if needed during a
survey campaign. This should be compared to expensive "one-shot" data acquisition such as
airborne Lidar or thermography.
Intervention process
The intervention process first consists in the definition of flight plans ahead of field data
acquisition. For this, the Mikrokopter Tool software allows extraction of geographical views at
the plot scale from web portals like Google Earth or Virtual Earth. It is then possible to
generate flight plans by creating waypoints to be followed by the UAV in automated flight
mode guided by GPS. These waypoints can be automatically generated, including regular grids
which the operator defines the equidistance and altitude. This option is particularly useful
when it comes to cover an entire plot of land (Figure 2).
In the field, the operator can choose to perform manual takeoff and landing operations, while
the flight itself can be automated following the waypoints through GPS guidance. When the
UAV is stopped on each waypoint, the operator starts data capture, controlling in real-time
quality and recording thanks to the return video system.
Post-processing and first results
In the laboratory, the captured images can be processed to improve the recognition of thermal
anomalies (stretching and compression of the histogram of the measured values, application
of different ranges of colors, etc). It is possible to restrict the range of representation of the
color composition to reduced amplitude of a single degree Celsius, using the FLIR Tools
software.
The calibration process of the intervention protocol is still in progress, but the first results have
already allowed us to assess the contribution of this technique for the detection of ancient
roads and parcel boundaries (Figure 3). But the experiment in progress promises a high
potential for detection at the site level of smaller structures, thanks to the low altitude of the
flights. For example, the first tests performed over the ancient roman town of Saint-Bertrand-
de-Comminges (South of France) revealed evidence of correlation between ancient urban
plans (streets network, buildings) and linear thermal anomalies (Figure 4).
The potential is also important regarding the detection of underground cavities whose
presence is manifested on the surface by different temperature from that of the atmosphere
and the surface of the soil. We already experienced this detection process of underground
Archaeological Prospection, 20, 303-307 (2013)
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cavities on the two prehistoric sites of Kroomdraï and Sterkfontein in the south-African Cradle
of Humankind (Figure 5).
The coupling of the thermal camera and the drone is still new. Ongoing tests aim to define an
intervention protocol to determine what is the optimal flight altitude, time, and ground cover
for the detection of buried remains based on a small corpus of sites periodically overflown in
various flight conditions. The flexibility of the machine makes it possible to take the decision to
fly on the day, and repeat it several times a day over the same plot if necessary. It is already
obvious that the low altitude of flights permits a gain in resolution to detect archaeological
buried remains.
Acknowledgements
The purchase of the equipment was funded by the European Union through the FEDER Presage
35827 project.
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Figure 1 : Drone Octorotor with FLIR T620 thermal camera onboard
Figure 2 : Definition of a flight plan by equidistant waypoints automatically generated at a plot
scale
Archaeological Prospection, 20, 303-307 (2013)
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Figure 3 : On the left hand, the thermal image taken 100 m above the ground shows the cold
mark (in dark) of a disused path to a farm, now integrated in a plot of land (Odars, France), and
invisible on a conventional image (right hand). The temperature scale on the left hand image
indicates that the soil is 2°C cooler where was the path, probably because of the accumulation of
wetness.
Figure 4 : On the left hand, the thermal image taken 100 m above the ground shows warm marks
(in light).The circles show where these thermal anomalies are related to buried roman walls, which
are invisible on the conventional image of the right hand (Saint-Bertrand-de-Comminges,
France).
Figure 5 : On the left hand, the thermal image taken 50 m above the ground reveals warm marks
(in light) indicating warm air outbreaks from underground caves (Sterkfontein cave, South Africa).
The circles highlight the warmest anomalies interpreted as paths to hidden cavities. The
conventional image (on the right hand) allows no distinction between the different holes.
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An essential principle of geophysical methods application at archaeological sites is difference of physical characteristics between the ancient remains and surrounding medium. Majority of archaeological objects – industrial, agricultural and weapon targets, cultural and worship pieces, many remains of ancient constructions, etc. – have contrast (comparing with the host media) thermal properties. Thus, temperature near-surface measurements (in small boreholes at the depth from several tens of centimeters up to several meters) can contain useful and sometimes unique information about the ancient objects occurring at some depths below the points of observations. At the same time, near-surface temperature survey is rarely carried out at archaeological sites. It was caused by a few reasons, most important from which is the noise induced by seasonal temperature variations propagating with some delay from the earth surface to the points of observations. Other disturbing factor is terrain relief effect significantly distorting the observed temperature field. Finally, analysis of temperature anomalies during the long time was limited by the absence of efficient quantitative procedures for temperature field examination. The developed interpretation scheme includes: (a) elimination of seasonal temperature variations by the use of linear filtering with utilization of repeated temperature observations and data of meteorological stations in the vicinity of the area under study, (b) calculation of terrain relief influence by a correlation technique, which facilitates the identification of anomalies associated with concealed geological features, (3) effective interpretation of temperature anomalies observed under complex environments. The last item is based on the essential similarities between the thermal and magnetic fields make it possible to apply to thermal prospecting improved modifications of characteristic points and tangents methods developed for magnetic prospecting. These methods are applicable to complicated environments: inclined relief, arbitrary magnetization (polarization), and an unknown level of the normal field. In order to classify the intensity of a thermal anomaly, it is suggested to use a “temperature moment”, equivalent to the “magnetic moment” used in the magnetic prospecting. The interpretation results were successfully tested both on models and in real situations.
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