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

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