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2. Methods and innovations

Potential and Challenges of UAV-Borne Magnetic Measurements for Archaeological Prospection

Volkmar Schmidt et Joris Coolen
p. 207-209

Résumé

– Unmanned aerial vehicles (UAV) can be used for airborne magnetic prospection.

– Comparison of UAV-borne and ground measurements shows good agreement.

– Measurements at different altitudes show a decline of the anomalies.

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Texte intégral

Introduction

1Magnetic surveying is an established method in archaeological prospection, and it has been continuously improved in the last few decades thanks to technical progress in measurement equipment. Modern systems can map magnetic field variations with high accuracy and spatial resolution. One of the latest developments in magnetic surveying is the use of unmanned aerial vehicles (UAV) as measurement platforms. UAV-borne magnetic measurements have proven to be a valid method to map magnetic fields. They enable data acquisition in harsh terrain and unsafe areas, like volcanological studies (Gailler et al., 2021) or detection of unexploded ordnance (Gavazzi et al., 2016). However, the potential of UAV-borne magnetic surveys for archaeological prospection has hardly been explored so far (but see Schmidt et al., 2020). Archaeomagnetic prospection requires an accurate, fine-spaced sampling of the magnetic field close to the ground. These requirements are challenging for UAV-borne measurements. We have developed and tested a UAV magnetic system at an archaeological site and compared the results with ground-based measurements. We report the accuracy of the UAV-borne data and the altitude-dependency of magnetic anomalies.

Description of the UAV measurement system

2The system consists of a multi-rotor drone (payload 5 kg) and a sensor unit (Fig. 1). The drone is equipped with an RTK-GNSS-receiver and a distance meter. The sensor unit (MagArrow from Geometrics) includes a miniature caesium total field magnetometer with sampling rate of 1000 Hz, GNSS-receiver, and an inertial measurement unit. The magnetometer is suspended from the UAV using ropes of 2.5 m length to minimize the magnetic interference of the UAV. The flight time with payload is restricted to 20 minutes due to battery capacity. The UAV flies autonomously on a predefined route, which is programmed using the software MissionPlanner. Position and sensor data of both the UAV and the sensor unit are downloaded using WiFi or a serial connection after landing. The data are then merged and processed using a MATLAB-based software developed in-house.

Figure 1. The UAV magnetic measurement system: the sensor unit is suspended from the drone using ropes of approx. 2.5 m length.

Figure 1. The UAV magnetic measurement system: the sensor unit is suspended from the drone using ropes of approx. 2.5 m length.

Test measurements and comparison to ground measurements

3Test measurements were conducted at the site of a ditch and bank enclosure in Kneblinghausen (North Rhine-Westphalia, Germany). Based on a typical shape and alleged clavicula gates, the structure has been interpreted as a Roman military camp (Cichy, 2008; Rudnick, 2014). Excavations revealed that the enclosure was built at the site of a Late Iron Age settlement but have so far not produced any absolute dating evidence for the camp. Two-thirds of the enclosure is forested today, while the eastern part is used as farmland. Remnants of the embankment are preserved in the forest and are well-visible in the high-resolution digital terrain model. On farmland, both ditch and bank have been levelled, but a magnetometry survey carried out in 2007 showed the multiphase ditch as a weak linear anomaly. The magnetic map also shows several smaller anomalies that might be caused by pits or ovens, as well as numerous small-scale dipolar anomalies caused by iron objects and magnetic debris in the topsoil. The presence of various archaeological features in the open as well as forested area, and the availability of high-resolution data from ground-based surveys for comparison made this a suitable test site for the UAV measurements.

4Ground-based data was acquired in 2020 using a motorised system with an array of eight fluxgate gradiometers (Förster FEREX CON 650), towed by an ATV. The probes were spaced 25 cm apart; the sampling rate was 50 Hz, which amounts to an in-line sampling interval of approximately 11 cm. An RTK-GNSS receiver was used for positioning and georeferencing. The data were logged on a tablet computer using LBI ArchPro’s LoggerVis software (Sandici et al., 2013), which also served for navigation. The data were processed with the software ApMag, developed by Alois Hinterleitner at ZAMG Vienna and LBI ArchPro.

5The UAV survey was flown at a speed of 2 m/s using a line spacing of 1 m. The area was mapped several times at different sensor altitudes between 0.5 m and 6.5 m above the ground. The magnetic anomaly map at an altitude of 0.5 m agrees well with the ground data; the linear anomaly of the ditch and numerous anomalies from small objects are clearly imaged (Fig. 2). The intensity of the anomalies declines quickly with increasing altitude, but most anomalies are also imaged at an altitude of 1.3 m. The linear anomaly is even visible at an altitude of 3.1 m, due to the slower decline of the anomaly generated by an elongated magnetised object. The results show the importance of measurements close to the ground, when weak and small anomalies need to be detected. For detection of large-scale anomalies such as ditches, flights at higher altitudes may suffice.

Figure 2. Left: Result of ground measurement using fluxgate gradiometers; right: total field anomalies measured by the UAV system at an altitude of 0.5 m. Dynamics: +/-3nT.

Figure 2. Left: Result of ground measurement using fluxgate gradiometers; right: total field anomalies measured by the UAV system at an altitude of 0.5 m. Dynamics: +/-3nT.

Discussion and conclusion

6The data shows that a UAV system is capable of acquiring data of comparable quality to traditional ground-based systems, although it has some drawbacks. Since the orientation of the caesium magnetometer has an influence on the measured value, artefacts can occur when the sensor is intensively oscillating below the UAV. A post-processing routine has to be applied to correct for this effect. Data acquisition speed of the single-sensor UAV system is lower compared to multi-sensor arrays, which are often employed in ground surveys. However, a larger UAV could also carry a multi-sensor array. The greatest advantage of the UAV system is that it is operable in areas that cannot be walked or driven on. This opens up possibilities for surveying areas that were until now inaccessible for magnetic prospection. As the method leaves the area fully untouched, landowner permits will be less problematic and the dependency on agricultural cycles, which in our experience is a major issue when planning ground surveys on a larger scale, is overcome. Mapping the magnetic anomalies in multiple and variable heights can also increase the signal-to-noise ratio (Florio et al., 2019) and may improve depth estimation of magnetic bodies.

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Bibliographie

Cichy, E., 2008. Neues aus dem „Römerlager“ Kneblinghausen. In P. Kracht (ed.), Jahrbuch Westfalen 2009, Aschendorff-Verlag, Münster, 139-141.

Florio, G., Cella, F., Speranza, L., Castaldo, R., Benoit, R.B., Palermo, R., 2019. Multiscale techniques for 3D imaging of magnetic data for archaeo-geophysical investigations in the Middle East: the case of Tell Barri (Syria). Archaeological Prospection, 26: 379-395.

Gailler, L., Labazuy, P., Régis, E., Bontemps, M., Souriot, T., Bacques, G., Carton, B., 2021. Validation of a new UAV magnetic prospecting tool for volcano monitoring and geohazard assessment. Remote Sensing, 13: 894.

Gavazzi, B., Le Maire, P., Munschy, M., Dechamp, A., 2016. Fluxgate vector magnetometers: A multisensor device for ground, UAV, and airborne magnetic surveys. The Leading Edge, 35(9): 795-797.

Rudnick, B., 2014. Kneblinghausen, Stadt Rüthen, Kreis Soest. Römerlager in Westfalen 1. Altertumskommission für Westfalen: Münster.

Sandici, V., Scherzer, D., Hinterleitner, A., Trinks, I., Neubauer, W., 2013. A unified magnetic data acquisition software for motorized geophysical prospection. In W. Neubauer, I. Trinks, R. Salisbury, Ch. Einwögerer (eds.), Archaeological Prospection: Proceedings of the 10th International Conference, Vienna, May 29th–June 2nd, 2013, Austrian Academy of Sciences Press, Vienna, 378-379.

Schmidt, V., Becken, M., Schmalzl, J., 2020. A UAV-borne magnetic survey for archaeological prospection of a Celtic burial site. First Break, 38(8): 61-66.

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Table des illustrations

Titre Figure 1. The UAV magnetic measurement system: the sensor unit is suspended from the drone using ropes of approx. 2.5 m length.
URL http://journals.openedition.org/archeosciences/docannexe/image/9645/img-1.jpg
Fichier image/jpeg, 1,3M
Titre Figure 2. Left: Result of ground measurement using fluxgate gradiometers; right: total field anomalies measured by the UAV system at an altitude of 0.5 m. Dynamics: +/-3nT.
URL http://journals.openedition.org/archeosciences/docannexe/image/9645/img-2.jpg
Fichier image/jpeg, 385k
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Volkmar Schmidt et Joris Coolen, « Potential and Challenges of UAV-Borne Magnetic Measurements for Archaeological Prospection »ArcheoSciences, 45-1 | 2021, 207-209.

Référence électronique

Volkmar Schmidt et Joris Coolen, « Potential and Challenges of UAV-Borne Magnetic Measurements for Archaeological Prospection »ArcheoSciences [En ligne], 45-1 | 2021, mis en ligne le 16 août 2021, consulté le 09 novembre 2025. URL : http://journals.openedition.org/archeosciences/9645 ; DOI : https://doi.org/10.4000/archeosciences.9645

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Auteurs

Volkmar Schmidt

Corresponding author, University of Münster, Institute for Geophysics, Corrensstrasse 24, 48149 Münster, Germany

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Joris Coolen

LWL-Archäologie für Westfalen, An den Speichern 7, 48157 Münster, Germany

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