M. Járó - L. Költő szerk.: Archaeometrical research in Hungary (Budapest, 1988)

Prospecting - CSATHÓ Beáta, PATTANTYUS-Á. Miklós: Possibilities and experiences of geophysical prospecting at archaeological sites in Hungary

Fig. 4 Theoretical scheme of the SLLNGRAM method real component (in phase with the transmitter current) and the imaginary component (sliifted by 90°) of the magnetic field are measured separately. The real component is sensitive mainly to the change in susceptibility thus it can be used primarily for the detection of metal objects. In the case of homogeneous soil and standard method of operation (horizontal coils, 1 m instrument height) the imaginary component is nearly directly proportional to the conductivity of the soil (McNeill, 1980) in the resistivity range (10-1000 Hm) important for us. The instrument has been calibrated so that for homogeneous soil its characteristic conductivity can be read off directly. In the case of inhomogeneous areas the so-called apparent conductivity values reflect the structure of the upper part of the soil. The depth range to be surveyed is 0-1.5 m with the EM—

38 instrument (r: dipole distance = 1 m, f = 13.2 kHz) and 0-6 m with the EM-31 (r = 3.66 m, f = 9.6 kHz). By means of the EM—

31 instrument of ELGI, purchased in 1986, several successful archaeological measurements were carried out (see in this volume: Balácapuszta, Jászdózsa). As in archaeological investigations the objects to be detected have small horizontal and vertical dimensions, it is practical to utilize the characteristic feature of the method, that by changing the plane of the coils or the height of the instrument above the surface, the distribution of eddy currents generated in the soil and thus the role of objects located at different depths in developing the measured apparent conductivity can be varied (Fig. 5).