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Multi-Frequency Loop Electromagnetic System Measurement on Shallow Offshore Archaeological Site of Oulos

François-Xavier Simon, Nikos Papadopoulos, Julien Guillemoteau, Dimitris Oikonomou et Kleanthis Simirdanis
p. 215-218

Résumé

– Multi-frequency loop electromagnetic system measurements can be done on shallow offshore sites.

– Multi-frequency sounding is relevant in this environment.

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Texte intégral

The project “Cretan cultural landscapes over time: highlighting the marine and mountainous environment of Mirabello”-DIATOPO is part of the Priority Axis “Strengthening the competitiveness, innovation and entrepreneurship of Crete” of the OP “Crete” 2014-2020 and co-financed by the European Regional Development Fund.

Introduction

1Electromagnetic induction (EMI) equipment with a multi-frequency harmonic source of the Slingram type and small coil spacing, was initially intended for multi-depth electrical conductivity mapping. This type of “vertical” multi-frequency EMI sounding implies that the operating frequencies significantly change the depth of investigation, which, in the case of the small loop loop Slingram systems, requires measurements at large induction numbers. In the frequency range used by these systems (300 Hz-90 kHz), this condition is rarely encountered because near-surface conductivity values are very often less than 0.5 S/m. For this environment, the depth of investigation is then essentially determined by the coil spacing and geometry rather than by the frequency of the transmitter. With this constraint, these devices have nevertheless proved to be interesting for multi-parameter approaches where in some contexts, dielectric permittivity and magnetic viscosity can be estimated (Simon et al., 2015, 2019). In a marine setting, the high conductivity of the water layer allows us to move out of the low induction number domain (Hunkeler et al., 2016). In this context, it is expected that the salt-water layer amplifies the frequency dependence of the vertical sensitivities, thus making the information from a “multi-frequency” sounding usable for the characterization of the underlying environment. In order to determine the feasibility of such an approach in the context of archaeological prospecting, we performed a two-stage study. Initially, we carried out 1D and 3D simulations based on a realistic model. We then performed a field survey using both the electromagnetic and electrical methods (3D ERT) in an area where the bathymetry was accurately determined.

Methods

2We used the GEM2, which provides measurements over a frequency range between 300 Hz and 90 kHz. With this device, it is possible to collect phase and quadrature data on five different frequencies simultaneously. It also has a fairly good mechanical stability. The presence of a ‘bucking coil’ located at a distance of 1.035 m from the transmitter also lets the instrumental drift be regulated. The spacing between the transmitting and receiving coils is 1.66 m, which allows structures to be resolved on a meter scale. The GEM2 is a relatively lightweight and fairly easy to handle device, and can be connected directly to a GNSS system. It was placed on a platform of floating buoys (Fig. 1), Data acquisition was carried out in a favourable meteorological context in order to avoid any pitch and roll effects that could considerably affect the signal quality. All these instrumental parameters were taken into account when modelling the response of the device, considering 1D and 3D distributions of electrical conductivity. The 1D solution is based on the method of integrals that allowed the effect of the different frequencies and the seawater layer to be visualized. The 3D modelling procedure is based on the method of moments (Tabbagh, 1985).

Figure 1. Acquisition of multi-frequency electromagnetic data at the archaeological site of Olous (Greece).

Figure 1. Acquisition of multi-frequency electromagnetic data at the archaeological site of Olous (Greece).

3The first modelling results show that below a layer of saline water it is possible to study the substratum conductivity at variable depths with multi-frequency EMI data. The dynamics of the measured signal is relatively strong for relatively thin water slices but when the water thickness increases, this dynamic is strongly reduced. It is therefore essential to know the thickness and the conductivity of the water layer in order to better characterize the variations of conductivity below the seabed.

4The results of the 3D modelling also show that the sensitivity to conductivity variations is different for the in-phase signal and for the quadrature signal, but that the observed difference is barely dependent on the frequency used (Fig. 2).

Figure 2. 3D-modelling of a wall under the sea surface at a water-depth of 0.3 m.

Figure 2. 3D-modelling of a wall under the sea surface at a water-depth of 0.3 m.

Results

5A test was carried out in August 2020 on the archaeological site of Elounda in Greece. A section of the site had already been investigated with 3D electric prospecting and a precise bathymetric model also exists. The average water depth of this site is 0.3 m. The electromagnetic device was positioned on a floating platform at 0.3 m above the water level in horizontal mode and connected to a GNSS antenna (Fig. 1). The observed results (Fig. 3) show significant variations in phase and quadrature component, indicating a high induction number domain. Measurements made at different frequencies allow maps to be drawn with different electrical conductivity distributions over the different frequencies. These maps also show dissimilar lateral variations which could be associated to the multi-depth response of the substratum. However, one must remain vigilant regarding expected complex interactions (i.e., specific to each frequency) with the thickness of the seawater layer.

Figure 3. In-phase and out-of-phase mapping of electromagnetic data at 90 kHz, 24 kHz, and 6 kHz.

Figure 3. In-phase and out-of-phase mapping of electromagnetic data at 90 kHz, 24 kHz, and 6 kHz.

Conclusion

6Experimental results confirm the potential of the electromagnetic method in a shallow marine environment. However, further modelling is necessary to separate the substratum response from the (preponderant) sea layer response. According to preliminary sensitivity tests, a small loop-loop multi-frequency Slingram EMI system may allow multi-depth investigations. The continuation of this work will focus on the 1D inversion of the collected GEM2 data including the bathymetry. Additional surveys are also planned with a denser 3D spatial sampling allowing the interpretation of the 3D anomalies, and limiting the lateral aliasing typically observed if the distance between points/profiles is too large.

7If it proves capable of detecting a heterogeneous distribution of conductivity in a marine environment, the EMI method would allow data acquisition that is much less constraining to implement than in the case of the electrical method. Inversion of EMI data is therefore essential, if one wishes to obtain understandable results.

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Bibliographie

Hunkeler, P.A., Hendricks, S., Hoppmann, M., Farquharson, C.G., Kalscheuer, T., Grab, M., Kaufmann, M.S., Rabenstein, L., Gerdes, R., 2016. Improved 1D inversions for sea ice thickness and conductivity from electromagnetic induction data: Inclusion of nonlinearities caused by passive bucking. Geophysics, 81(1): WA45-WA58.

Simrdanis, K., Papadopoulos, N., Cantoro, G., 2016. Shallow Off-Shore archaeological Prospection with 3-D Electrical Resistivity Tomography: The case of Olous (Modern Elounda), Greece. Remote Sensing, 8: 897.

Simon, F.X., Sarris, A., Thiesson, J., Tabbagh, A., 2015. Mapping of quadrature magnetic susceptibility/magnetic viscosity of soils by using multi-frequency EMI. Journal of Applied Geophysics, 120: 36-47.

Simon, F.-X., Tabbagh, A., Sarris, A., Donati, J., 2019. Permittivity mapping in the VLF-VF range using a multi-frequency EMI device: first tests in archaeological prospection. Near Surface Geophysics, 17 (1): 27-41.

Tabbagh, A., 1985. The response of a three-dimensional magnetic and conductive body in shallow depth prospecting. Geophysical Journal International, 81(1): 215-230.

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Table des illustrations

Titre Figure 1. Acquisition of multi-frequency electromagnetic data at the archaeological site of Olous (Greece).
URL http://journals.openedition.org/archeosciences/docannexe/image/9690/img-1.jpg
Fichier image/jpeg, 854k
Titre Figure 2. 3D-modelling of a wall under the sea surface at a water-depth of 0.3 m.
URL http://journals.openedition.org/archeosciences/docannexe/image/9690/img-2.jpg
Fichier image/jpeg, 553k
Titre Figure 3. In-phase and out-of-phase mapping of electromagnetic data at 90 kHz, 24 kHz, and 6 kHz.
URL http://journals.openedition.org/archeosciences/docannexe/image/9690/img-3.jpg
Fichier image/jpeg, 3,6M
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Pour citer cet article

Référence papier

François-Xavier Simon, Nikos Papadopoulos, Julien Guillemoteau, Dimitris Oikonomou et Kleanthis Simirdanis, « Multi-Frequency Loop Electromagnetic System Measurement on Shallow Offshore Archaeological Site of Oulos »ArcheoSciences, 45-1 | 2021, 215-218.

Référence électronique

François-Xavier Simon, Nikos Papadopoulos, Julien Guillemoteau, Dimitris Oikonomou et Kleanthis Simirdanis, « Multi-Frequency Loop Electromagnetic System Measurement on Shallow Offshore Archaeological Site of Oulos »ArcheoSciences [En ligne], 45-1 | 2021, mis en ligne le 16 août 2021, consulté le 29 janvier 2023. URL : http://journals.openedition.org/archeosciences/9690 ; DOI : https://doi.org/10.4000/archeosciences.9690

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Auteurs

François-Xavier Simon

Inrap, Paris, France, Laboratoire Chrono-Environnement, UMR 6249, UBFC-CNRS, Besançon, France

Articles du même auteur

Nikos Papadopoulos

Lab of GeoSat ReSeArch, IMS FORTH, Rethymnon, Greece

Articles du même auteur

Julien Guillemoteau

Institut de Géosciences, University of Potsdam, Germany

Dimitris Oikonomou

Lab of GeoSat ReSeArch, IMS FORTH, Rethymnon, Greece

Kleanthis Simirdanis

Lab of GeoSat ReSeArch, IMS FORTH, Rethymnon, Greece

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