TDEM magnetic viscosity prospecting using a Slingram coil configuration

Abstract

ABSTRACTSoil magnetic properties are significantly modified by pedogenesis and human activity; consequently 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 measurements 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 perpendicular 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 sensitivity 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.