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. 3 Effect of a buried wall in the case of DC resistivity profiling A: Mathematical modelling B. Measurement in the field prospecting of a Roman fortress, Kapospula-Alsóhetény (Csathó, J 983), a. theoretical curve b. smoothed curve c. surface d: curve measured in the field e: filtered curve Electromagnetic methods One of the serious disadvantages of DC resistivity measurements is that for current input and voltage measurement the electrodes have to be placed into the ground. If the geoelectric parameters of the surveyed area are determined by inducing an electromagnetic field changing in time, no galvanic contact is required between the measuring system and the soil. In this way geoelectric measurements can be carried out on areas where the placing of electrodes is impossible or very difficult (e. g. roads, rocky land). Besides, the surveying will speed up and continuous detection of the measured parameters becomes possible too. It has to be emphasized, however, that it is not practical to replace DC resistivity measurements with electromagnetics in each case. The evaluation of DC measurements is fast and simple, the method is more sensitive to objects of high resistivity (e. g. stone walls, cavities). The results of electromagnetic measurements can be significantly distorted by various electromagnetic noise sources (e. g. metal objects, long distance power lines). According to the characteristics of the surveyed area and of the task it has to be determined which method is the most practical. Moreover, in certain cases the simultaneous use of both methods is the best. SLINGRAM measurements, which can be carried out quickly and interpreted simply, have been used in ore prospecting for a long time. The magnetic field of the sinusoidal alternating current of the transmitter (Hp = primary field) induces eddy currents in the surrounding conducting materials the magnetic field of which (Us - secondary field) is superimposed on the field of the transmitter (Ht = total field). The resultant signal is detected as induced voltage in the receiver coil (Fig. 4). Earlier, in archaeological investigations, the so-called zero coupling systems (EM-15, SCM, SH-3) were used (Tabbagh, 1984). In these instruments the primary field component, perpendicular to the plane of the receiver, is zero. These measurements were used to determine the change of magnetic susceptibility of the soil and to localize metal objects if present (Mullins, 1974 and Tabbagh, 1974). In the soil conductivity measuring instruments (EM-31 , EM-38, GEONICS) widely used recently in archaeological investigations (Fröhlich and Lancaster, 1986) the transmitter and the receiver coils are in the same plane (the so-called maximum coupled system). The
