© 2007 European Association of Geoscientists & Engineers 363 ABSTRACT Soil magnetic properties are significantly modified by pedogenesis and human activity; conse- quently their study is an important source of information about soil structure and history. In addition to measurements of magnetic susceptibility, now commonly practised, magnetic viscosity measure- ments can be undertaken as a routine survey method if well-matched instruments that are easy to use in the field and that offer a sufficient depth of investigation, exist. A theoretical study, supported by field tests of a prototype, demonstrates that a Slingram perpen- dicular coil configuration with a one-metre inter-coil spacing allows the required one-metre depth of investigation to be reached for both 1D and 3D models. The sensitivity of the instrument response to the viscosity is also very high over the whole measurement range (10µs to 1ms), while the sen- sitivity to electrical conductivity is reduced to a minimum. A first experiment at the Roman site of Vieil-Évreux illustrates how viscosity measurements can complement magnetic field and susceptibility measurements for a more accurate identification of an archaeological feature. TDEM magnetic viscosity prospecting using a Slingram coil configuration Julien Thiesson * , Alain Tabbagh and Sébastien Flageul UMR 7619 Sisyphe, UPMC-Paris 6, 4 Place Jussieu, 75252 Paris Cedex 5, France Received July 2006, revision accepted May 2007 however, a critical parameter when searching for features located beneath the superficial layer of soils, and the conclusion of the first series of studies was that a device with a metric depth of investigation does not yet exist. The assessment of the depth of investigation is the first required step in the definition of a new apparatus that fits archaeological and pedological requirements in terms of soil structure analysis over a significant depth. Several pulse induction meters or TDEM metal detectors are today commercially available, but they are all of the coincident- or concentric-loop geometry type, whether old (DECCO, Littlemore Scientific Engineering Ltd) or newer (EM61 and EM63 Geonics Ltd, TS6 Protovale Ltd). Although they have been built as metal detectors, they can be calibrated to express measurements in terms of quadrature susceptibility K q , which indicates the magnetic viscosity in the frequency domain (Dabas 1989; Tabbagh and Dabas 1996). However, as for fre- quency-domain devices (Benech and Marmet 1999), coincident and concentric loops both correspond to a lower depth of inves- tigation. The aim of the present study, making use of both numerical modelling and field trials, is to define a new TDEM device that permits the measurement of soil K q with an investiga- tion depth of one metre. MODELLING Theoretical simulations permit the definition of optimal appara- tus characteristics faster and more efficiently than experiments alone. In the quasi-static approximation, these characteristics are only geometric and must fit the following requirements: INTRODUCTION The first study showing the main characteristics of the magnetic susceptibility of soils (K) was published by Le Borgne (1955), while the magnetic surveys carried out by Aitken et al. (1958) first described the magnetic susceptibility of archaeological earth features and suggested that the magnetic susceptibility was directly enhanced by human activities and settlements. Later, the development of electromagnetic devices allowed absolute meas- urements of K, while magnetic prospecting only revealed lateral changes. It is now commonly accepted (Thompson and Oldfield 1986) from numerous experiments that K is a good tracer for pedogenesis and changes in soils due to human activity. Among other possibilities, wide mesh prospecting can be used to locate archaeological sites (Marmet 2000). Nevertheless, magnetic susceptibility K, which corresponds to a physical reaction to the application of a magnetic field, must be considered as a complex quantity, i.e. K= K ph – iK q , where the quadrature out-of-phase component K q represents a time-delayed reaction. It could be interesting to measure this component, and in fact, the decayed loss or gain of induced magnetization, known as magnetic viscosity, has been observed for 40 years. Measurements in both time-domain electromagnetism (TDEM) and frequency-domain electromagnetism (Colani and Aitken 1966; Mullins 1974; Tabbagh 1974; Tite and Mullins 1971) have been performed and published. The depth of investigation is, * julien.thiesson@ccr.jussieu.fr Near Surface Geophysics, 2007, 363-374