Permittivity mapping in the VLF–LF range using a multi-frequency EMI device: first tests in archaeological prospection
In the early years of near-surface geophysical applications, measurements obtained using electromagnetic induction (Slingram or dipole–dipole) instruments were first interpreted in terms of electrical conductivity. Afterwards, studies began considering the importance of magnetic susceptibility. Until now, however, no attention has been paid to possible responses in dielectric permittivity. After reviewing the current state of knowledge regarding the expected theoretical responses in the very low frequency to low frequency (VLF-LF) range (3–300 kHz) and the value range of this property for soils, we propose the use of multi-frequency instruments to define a process allowing for the determination of the apparent conductivity and permittivity in the higher part of the frequency range above 20 kHz. We test this process through a series of experiments at archaeological sites in Greece using the GEM-2 (Geophex Ltd) instrument. In our experiments, the soil permittivity values fall between several hundredths and several thousandths. The results indicate lateral variations different from the other properties with a significant influence of the stone content and of the ionic strength, while a decrease with frequency may also provide further information on soil dielectric behaviours.