Induced polarization 3D tomography of an archaeological direct reduction slag heap

Abstract

Both environmentalists and archaeologists are interested in the detection and quantification of the remains of mine slag heaps from ancient smelters: possible pollution of the slag heaps and information concerning the history of the smelter's mine. Although magnetic surveys can be used to detect subsurface slag accumulations, it is not possible to derive the total amount of material from such surveys or to accurately delineate the heaps through the use of this method. Conversely, ‘Induced Polarization’ (IP) surveying allows a relevant and robust assessment of the volume of slag concentrations to be determined. In the present study, we follow up on results obtained in a previous study, carried out at the 14th Castel‐Minier mine (Ariège, France) site, where we have shown that the chargeability is proportional to the slag concentration in the ground and have used this property to perform 3D tomography of the heap. Based on a previous investigation using spectral induced polarization along a simple profile showing that the phase peak related to the slags is situated at approximately 1 Hz, in the present paper we extend our prospection to 3D by using a Terrameter SAS1000 and measuring temporal chargeability. The set of three‐dimensional data is recorded by means of classical transect measurements along parallel close profiles and is then interpreted using the RES3DINV code. Finally, archaeological and auger soundings confirmed the assessments derived from this geophysical investigation. We also briefly discuss the source of the IP signal, which we suspect to be induced by magnetite particles embedded in the slags.

Induced polarization response of porous media with metallic particles — Part 7: Detection and quantification of buried slag heaps

“…The phase φ (in rad) is defined as tan φ ¼ σ 0 0 ∕σ 0 . Buried metals are not easily observed by resistivity tomography alone (Florsch et al, 2011(Florsch et al, , 2012 because metallic targets can behave as insulators at low frequency (see . However, dispersed metallic bodies have a strong ability to polarize in an electrical field (Marshall and Madden, 1959;Wynn et al, 1985;Ostrander and Zonge, 1978;Misra et al, 2016aMisra et al, , 2016b.…”

“…Such slag deposits cannot be easily detected using the electrical resistivity distribution determined by the means of galvanometric or electromagnetic (EM) methods. Although the magnetic method can localize slag heaps due to the magnetic properties of the metallic particles (essentially associated with the presence of magnetite), it cannot quantify the metallic content of the slag because the magnetic field lines do not conserve the magnetic flux (Florsch et al, 2011(Florsch et al, , 2012. Among the arsenal of geophysical methods that we have, the induced polarization (IP) method is probably the most meaningful to fulfill our goals (Florsch et al, 2011(Florsch et al, , 2012(Florsch et al, , 2017.…”

“…In other words, it can map a property called chargeability, reflecting the ability of a material to reversibly store electrical charges under the action of a primary electrical field or electrical current. The iron slag can be considered itself as a porous medium with dispersed metallic grains and notable polarization responses have been reported for such materials (Ullrich et al, 2008;Florsch et al, 2011Florsch et al, , 2012Günther and Martin, 2016). This phenomenon makes IP a suitable and appealing method to detect buried slag heaps.…”

“…Their approach is however empirical, whereas we propose a more mechanistic approach supported by a high number of experimental data sets, as reported in our previous papers. Florsch et al (2011Florsch et al ( , 2012 show the potential of the IP method for the detection and the quantification of the slag heaps; they quantify the slag content rather than the metallic content of the slag. That said, the metallic contents are far more meaningful than the slag content in terms of determining the total quantity of metal still present in the slag heap.…”

Induced Polarization of Rocks Containing Metallic Particles: Evidence of Passivation Effect

“…This is why IP is commonly used in exploration for those deposits where the ore bodies contain disseminated conductive or semiconductive minerals (e.g., Nelson & Van Voorhis, ; Pelton et al, ). In addition, IP can be used for evaluating integrity of permeable reactive barriers made of granular iron (e.g., Slater & Binley, ), for three‐dimensional imaging of slag heaps (e.g., Florsch et al, ) and mining tailings (Placencia‐Gomez et al, , ; Placencia‐Gomez & Slater, ). In these applications, IP often aims to assess the volumetric content of disseminated conductive particles (which we henceforth refer to as”metallic particles”), and can be applied to assess the chemical state of their surface.…”