Magnetic Viscosity Of Baked Clays And The Possibility Of Its Use In The Location Of Buried Ceramic Objects

Abstract

Electromagnetic (EM) methods have been used in archaeology since about 1960. The study of soil electrical conductivity plays a significant part in archaeological surveying, as in the case for any superficial exploration. That is why one of the most useful prospection instrument still in use today is the inductive soil conductivity meter: surveys can be carried out almost as fast as the operator can walk, making this among the fastest ground geophysical methods now available. Metal detectors. which work on a similar principle. are frequently used to search for ancient metallic objects not yet completely oxidized and to sort out modern cultural pollution from magnetic anomalies deriving from older sources such as hearths (Wynn, 1986). I_Jse of Slingram EM systems with carefully designed spatial and frequency parameters now permits archaeologists to obtain both electrical conductivity and magnetic susceptibility data at the same time without having to establish physical contact with the ground (Tabbagh, 1986). As EM techniques were becoming more and more sophisticated, it has been found that secondary magnetic field is affected not only by conductivity and susceptibility but by magnetic viscosity of earth’s upper layers as well. C. Colani and M.J. Aitken studying archaeological sites in England revealed that magnetic viscosity of soil influences substantially EM data obtained with pulse metal detector ( 1966). The effects of soil magnetic viscosity on transient electromagnetic methods (TEM) were also observed and reported by Buselli ( 1982). In 1985 the soil magnetic viscosity has been found to produce TEM anomalies measured at some sites in Western Pri bai kalje (Vakhromeev and Kozhevni kov, 1988). Being measured in the time domain magnetic viscosity manifests itself in the slowly decaying transients (Colani and Aitken, 1966: Buselli, 1982: Vakhromeev and Kozhevnikov, 1988). In frequency domain magnetic viscosity gives rise to the imaginary part of magnetic susceptibility and to the dependence of its real part on frequency (Tabbagh, 1986: Kozhevnikov and Snopkov, 1990). In either case the interpretation of EM data without regard for magnetic viscosity results in soil conductivities too high when compared to actual. If an object to be investigated is well conductive. magnetic viscosity effects are small as compared with those produced by eddy currents. However, when studying resistive media magnetic viscosity effects become dominant. It has been found that for the most part the magnetic viscosity of soils is caused by relaxation of superparamagnetic (SPM) particles (Buselli, 1982). Recall that superparamagnetlsm is a property exhibited by single domain (SD) particles of ferro - and ferrimagnetic materials. In contrast to large particles which consist of many domains, in the small one a multidomain structure is impossible and thus the whole particle is a single domain. In SD particles the response to an applied field is a rotation of the magnetization rather than domain wall movement (Parkinson. 1983 ).