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An Integrated Approach for Ground and Drone-Borne Magnetic Surveys and their Interpretation in Archaeological Prospection

Bruno Gavazzi, Hugo Reiller et Marc Munschy
p. 165-168

Résumé

– Availability of fast and precise ground and drone-borne magnetic surveys.

– Extraction of information on the sources through potential field transforms.

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Texte intégral

1Magnetic surveying is nowadays a popular tool for the detection and mapping of archaeological remains (e.g. Gaffney, 2008; Fassbinder, 2017). The EAC Guidelines for the use of geophysics in archaeology (Schmidt et al., 2015) give an overview of the practical uses of magnetic methods. Magnetometry is often the faster and cheaper of the geophysical methods, and is frequently used for prospecting before or in complement of other geophysical methods and archaeological excavations. Most of the time, measurements are made with fluxgate vertical-component gradiometers, which have the great advantage of being light, robust, relatively cheap, mostly unaffected by time-induced variations (i.e. noise), and sensitive to very near-surface heterogeneities (i.e. the first meter). Less commonly, scalar magnetometers can be used (based on alkali vapor optically-pumped or proton precession principles). They are heavier, more fragile and more expansive than fluxgate magnetometers. They are impacted by time-induced variations but are more sensitive to deeper sources and smaller magnetization contrasts than gradiometers. Two scalar magnetometers can be placed vertically to build a scalar gradiometer. An instrument of this kind has an output quite similar to the one of fluxgate gradiometers. Magnetic compensation (i.e. correction of the effect of the magnetization of the equipment on the data) is not performed with the described instruments. Instead, the sensors are deported as far as possible of the magnetized components of the equipment. Finally, the interpretation is usually made directly on maps of the pseudo-vertical gradient. The use of quantitative methods such as potential field transforms, direct modeling or inverse problem to obtain additional information on the sources is not very common. This is possibly due to the fact that the output of the device is not strictly the gradient (a flux), but an approximation (difference between two sensors). Such approximation is usually considered accurate enough for producing a pseudo-gradient map, but can be hazardous when computing the data in the spectral domain (hence using potential field transforms). Magnetic surveys are also quite common in the field of resources exploration and geological studies, usually using scalar magnetometers mounted on aircraft (Nabighian et al., 2005). A magnetic compensation of the equipment is performed with heavy active compensation systems and the time-induced variations are corrected post-flight. The interpretation of the data is usually performed using tools from potential field theory to gain information on the sources (precise horizontal position, depth, magnetization). Here we propose an innovative approach aiming to integrate different aspects of the two communities, in order to built new tools to study archaeological objects, from the excavation to the regional scale. Our approach is based on the use of three-component fluxgate magnetometers. They are based on the same physical principle as the fluxgate gradiometer, i.e. the measurement of a directional component of the magnetic field. Instead of the measurement of a difference of vertical components between two points (the concept of the gradiometer), three orthogonal components are measured simultaneously at the same point. The aim is to compute the magnetic intensity (i.e. like the output of scalar magnetometers) in order to be able to use the potential field theory and perform a passive magnetic compensation of the equipment. This passive compensation ability from a simple procedure in the field was published in 2007 for the detection of unexploded ordnance with a precision of two nT (Munschy et al., 2007). Mono- and multi- magnetometer systems have been developed for ground, drone and aircraft surveys (Gavazzi et al., 2016). Such devices have been used in ground surveys for archaeological prospection with increased resolution (0.3 nT), in large scale surveys over dozens of hectares (e.g. Gavazzi et al., 2017) to high-resolution surveys of subtle structures like post holes (e.g. Wassong & Gavazzi, 2020). Overall, the use of this kind of equipment with potential field interpretative tools showed a high versatility for archaeological, environmental and geological applications at different scales, with surveys conducted with profile spacing and height-to-ground ranging from 0.1 to 120 m and acquisition frequency ranging between 25 and 500 Hz (Gavazzi et al., 2019).

2Through this presentation we aim to illustrate our latest advances in metrology and potential field theory through different results such as a comparison between measured and computed vertical gradient (Fig. 1), a first drone-borne survey achieving ground survey accuracy (Fig. 2), and a new revised depth estimator introducing a form factor to the 2D analytic signal.

Figure 1. Comparison between: (A) vertical gradient computed from TMI data acquired 1 m above the ground; (B) vertical gradient measured 0.3 m above the ground (Gavazzi et al., 2017).

Figure 1. Comparison between: (A) vertical gradient computed from TMI data acquired 1 m above the ground; (B) vertical gradient measured 0.3 m above the ground (Gavazzi et al., 2017).

Figure 2. Comparison between the results of a standard gradiometer survey and a drone borne survey on the Roman site of Oedenbourg (France). (A) Pseudo-gradient measured approximately 0,3 m above the ground with a cart-mounted gradiometer array (modified from Reddé et al., 2005); the colorbar is not provided in the original document but data range from 10 nT (white) to -10 nT (black). (B) Total magnetic anomaly measured 1 m above the ground with a drone. (C) Vertical gradient computed from the total magnetic anomaly (B). The results show that the same observations can be done on the drone data after potential field transformation as on the gradiometer data but at a much faster rate of acquisition (x 4). The differences of resolution between the maps is explained by different spacing of the profiles (1 m for the drone survey, 0.5 m for the gradiometer survey).

Figure 2. Comparison between the results of a standard gradiometer survey and a drone borne survey on the Roman site of Oedenbourg (France). (A) Pseudo-gradient measured approximately 0,3 m above the ground with a cart-mounted gradiometer array (modified from Reddé et al., 2005); the colorbar is not provided in the original document but data range from 10 nT (white) to -10 nT (black). (B) Total magnetic anomaly measured 1 m above the ground with a drone. (C) Vertical gradient computed from the total magnetic anomaly (B). The results show that the same observations can be done on the drone data after potential field transformation as on the gradiometer data but at a much faster rate of acquisition (x 4). The differences of resolution between the maps is explained by different spacing of the profiles (1 m for the drone survey, 0.5 m for the gradiometer survey).
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Bibliographie

Fassbinder, J., 2017. Magnetometry for archaeology. In A.S. Gilbert (ed). Encyclopedia of Geoarchaeology. Encyclopedia of Earth Sciences Series. Springer, Dordrecht.

Gaffney, C., 2008. Detecting trends in the prediction of the buried past: a review of geophysical techniques in archaeology. Archaeometry, 50(2): 313-336.

Gavazzi, B., Alkhatib-Alkontar, R., Munschy, M., Colin, F., Duvette, C., 2017. On the use of fluxgate 3-axis magnetometers in archaeology: Application with a multi-sensor device on the site of qasr ‘allam in the western desert of Egypt. Archaeological Prospection, 24(1): 59-73.

Gavazzi, B., Le Maire, P., Mercier de Lépinay, J., Calou, P., Munschy, M., 2019. Fluxgate three-component magnetometers for cost-effective ground, UAV and airborne magnetic surveys for industrial and academic geoscience applications and comparison with current industrial standards through case studies. Geomechanics for Energy and the Environment, 20: 100117.

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.

Munschy, M., Boulanger, D., Ulrich, P, Bouiflane, M., 2007. Magnetic mapping for the detection and characterization of uxo: Use of multi-sensor fluxgate 3-axis magnetometers and methods of interpretation. Journal of Applied Geophysics, 61(3-4): 168-183.

Nabighian, M.N., Grauch, V.J.S., Hansen, R.O., LaFehr, T.R., Li, Y., Peirce, J.W., Phillips, J.D, Ruder, M.E., 2005. The historical development of the magnetic method in exploration. Geophysics, 70(6): 33ND–61ND.

Reddé, M., Nuber, H.U., Jacomet, S., Schibler, J., Schucany, C., Schwarz, P.-A., Seitz, G., Ginella, F., Joly, M., Plouin, S., Hüster Plogmann, H., Petit, C., Popovitch, L., Schlumbaum, A., Vandorpe, P., Viroulet, B., Wick, L., Wolf, J.-J., Gissinger, B., Ollive, V., Pellissier, J., 2005. Oedenburg. Une agglomération d’époque romaine sur le Rhin supérieur: Fouilles françaises, allemandes et suisses à Biesheim-Kunheim (Haut-Rhin). Gallia, 62: 215-277.

Schmidt, A., Linford, P., Linford, N., David, A., Gaffney, C., Sarris, A., Fassbinder, J., 2015. EAC Guidelines for the Use of Geophysics in Archaeology. Questions to Ask and Points to Consider. Europae Archaeologia Consilium (EAC).

Wassong, R., Gavazzi, B., 2020. Apport des prospections magnétiques haute résolution à la compréhension d’un habitat protohistorique : l’exemple du site de hauteur fortifié du maimont. Archimède. Archéologie et histoire ancienne, 7: 283-293.

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Table des illustrations

Titre Figure 1. Comparison between: (A) vertical gradient computed from TMI data acquired 1 m above the ground; (B) vertical gradient measured 0.3 m above the ground (Gavazzi et al., 2017).
URL http://journals.openedition.org/archeosciences/docannexe/image/9325/img-1.jpg
Fichier image/jpeg, 767k
Titre Figure 2. Comparison between the results of a standard gradiometer survey and a drone borne survey on the Roman site of Oedenbourg (France). (A) Pseudo-gradient measured approximately 0,3 m above the ground with a cart-mounted gradiometer array (modified from Reddé et al., 2005); the colorbar is not provided in the original document but data range from 10 nT (white) to -10 nT (black). (B) Total magnetic anomaly measured 1 m above the ground with a drone. (C) Vertical gradient computed from the total magnetic anomaly (B). The results show that the same observations can be done on the drone data after potential field transformation as on the gradiometer data but at a much faster rate of acquisition (x 4). The differences of resolution between the maps is explained by different spacing of the profiles (1 m for the drone survey, 0.5 m for the gradiometer survey).
URL http://journals.openedition.org/archeosciences/docannexe/image/9325/img-2.jpg
Fichier image/jpeg, 2,1M
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Bruno Gavazzi, Hugo Reiller et Marc Munschy, « An Integrated Approach for Ground and Drone-Borne Magnetic Surveys and their Interpretation in Archaeological Prospection »ArcheoSciences, 45-1 | 2021, 165-168.

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Bruno Gavazzi, Hugo Reiller et Marc Munschy, « An Integrated Approach for Ground and Drone-Borne Magnetic Surveys and their Interpretation in Archaeological Prospection »ArcheoSciences [En ligne], 45-1 | 2021, mis en ligne le 16 août 2021, consulté le 27 mai 2024. URL : http://journals.openedition.org/archeosciences/9325 ; DOI : https://doi.org/10.4000/archeosciences.9325

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Auteurs

Bruno Gavazzi

Corresponding author, Institut Terre et Environnement de Strasbourg (ITES), UMR 7063, Université de Strasbourg/EOST, CNRS, Strasbourg, France

Hugo Reiller

Institut Terre et Environnement de Strasbourg (ITES), UMR 7063, Université de Strasbourg/EOST, CNRS, Strasbourg, France

Marc Munschy

Institut Terre et Environnement de Strasbourg (ITES), UMR 7063, Université de Strasbourg/EOxST, CNRS, Strasbourg, France

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