Magnetic signal prospecting using multiparameter measurements: the case study of the Gallic Site of Levroux

Abstract

The'magnetic signal'that combines both the induced (J i ) and the remanent (J r ) magnetization is widely used in archaeological and pedological prospecting. Magnetic prospecting recording the lateral variations of the total magnetization is the most frequently used measurement before in-phase magnetic susceptibility (K ph ) and magnetic viscosity (K qu ) mapping. The work presented here brings together three types of prospecting technique: magnetic field survey and electromagnetic measurements with both frequency and time domain devices that measure magnetic susceptibility andviscosityrespectively.The site studied, the Gallic town of Levroux (Indre,France), isparticularly interesting because it includes features such as pits and ditches dug into the calcareous substratum partly filled with topsoil and with residues of different metallurgical and fire activities.The field results indicated anomalies with different types of characterization: (i) many compact features filled with magnetic, electrically conductive and minimally viscous materials; and (ii) elongated anomalies characterized by lower magnetic properties and electric conductivity but relatively higher magnetic viscosity than those of the compact features. In addition to the location of the features, the combination of the information brought by the different types of measurements allows us to evaluate the possible erosion of their upper parts by ploughing, to assess their depth (never deeper than1.30 m) and to precise the nature of the feature's fill.

Mapping of quadrature magnetic susceptibility/magnetic viscosity of soils by using multi-frequency EMI

“…Nevertheless, this error adds to the possible different gains between the in-phase and quadrature measurement channels, and prevents any precise quantitative determination of the ratio between the magnetic susceptibility and the magnetic viscosity, which can open the way to study the in-field variations of the grain size distributions. In all published magnetic viscosity measurements in the field , Pétronille et al 2010, the maps of this ratio were informative and from the archaeological point of view supported assumptions about the functions of the detected features (metallurgy, pottery, domestic waste). In paleosoil studies too, when this ratio is recalculated from the frequency dependence of the in-phase magnetic susceptibility, this ratio is more discriminant for layer identification than the susceptibility itself (Thiesson 2007).…”

“…This decrease of the in-phase susceptibility and the value of the quadrature susceptibility correspond to the same parameter called magnetic viscosity. The viscosity is also measured in the field using TDEM instruments (Colani andAitken 1966, Thiesson et al 2007) and all the field results, (see for example Pétronille et al 2010), correspond to a slope of the logarithmic decrease close to − 1 for the ∂BðtÞ ∂t signal measured in the receiver coil in accordance with the above formula. The in-phase soil susceptibility range of values is also quite large, from 1000 · 10 −5 SI in volcanic areas to 10 · 10 − 5 SI, but values close to 1000 · 10 −5 SI are rare.…”

Electromagnetic induction in subsurface metal targets: Cluster analysis using local point-pattern spatial statistics

“…Magnetometry is widely used in archaeological prospection (Kvamme, 2006;Perttula et al, 2008;Conyers and Leckebusch, 2010) and UXO mapping and detection Doll et al, 2008), sometimes in conjunction with EMI Pétronille et al, 2010). The G-858 magnetometer is a passive device that measures the intensity of the sum of the background geomagnetic field and the much smaller magnetic field due to any nearby ferrous objects.…”

Slingram EMI prospection: Are vertical orientated devices a suitable solution in archaeological and pedological prospection?

“…This technique may also be used in magnetic susceptibility studies. It has a high potential: either in light weight systems easy to use in difficult field conditions or coupled with magnetic prospecting (Benech et al, 2002;Pétronille et al, 2010;Simpson et al, 2009b).…”