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. 1 A: Effect of a magnetic dipole from theoretical calculations (Breiner, 1973) B: The magnetic effect of the kiln of a medieval house measuredin the field (Pattantyus-A., 1986). future (Scollar, 1970). A field data logger would make the measurements faster and more efficient and it would also be suitable for displaying the results. Direct current methods Geoelectric methods can be applied when the resistivity of the objects differs from that of the environment. The application of direct current measurements is advisable when looking for stone walls, building remnants, cavities, i. e. objects having a higher resistivity than their environment. In an environment of high resistivity, however, the zones of lower resistivity can also be detected, which may be targets of prospecting too (e. g. mine galleries filled in with debris or water). The relatively small changes in resistivity due to pits, graves and ditches are generally difficult to distinguish from the noise caused mainly by geological changes. Our resistivity measurements were performed with a low power digital, portable instrument (manufactured by ELGI, Budapest). The instrument (see Photo 1) is fed by NiCd cells, it operates by direct current, and it calculates the apparent resistivity from the weighted average of four pulses ftwo positive and two negative), therefore it is very accurate. Jn archaeological prospecting the dipole-dipole array is mainly applied. The electrode spacing is 1 or 2 m, depending on the depth of investigation. Using 30 prearranged electrodes, resistivity can be measured along a profile at a maximum of 27 points, providing information about the resistivity distribution from several depths along the profile. Figure 2 shows the electrode array and the reference points. For representing the measured data the most simple method, proposed by Hallof (Edwards, 1977), is applied. By placing the MN receiver dipole farther from the AB transmitter dipole, the depth of investigation can be increased as shown in the figure. Using the same dipole distance, resistivity profiling related to a given depth can be performed; while keeping the reference point constant, vertical sounding, referring to one point, is possible by moving the dipoles symmetrically. From these data, conclusions may be drawn on the quality and depth of the layers.