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

Magnetic measurements Magnetic research is based on the fact that - above natural or artifical objects having an inhomogeneous magnetic composition - the normal, regular magnetic field of the earth becomes detectably disturbed. The size and the shape of the disturbance depend on the relative magnetic susceptibility of the materials under the surface and on the shape and spatial position of the objects of different susceptibility. The biggest magnetic anomalies are caused by ferromagnetic materials, i. e. iron ores or various objects made of iron. However, these seldom occur as archaeological targets in such large quantities or so close to the surface as to be able to detect them reliably. In archaeological prospecting the ferromagnetic effect is generally present as a disturbance that has to be eliminated (e. g. iron objects of modern times left close to the surface). Baked objects which occur frequently and sometimes in large quantities at archaeological sites often cause an easily measurable magnetic effect since while cooling down after baking they gain permanent magnetization (thermo-remanent magnetization). Thus brick, tile, pottery, remnants of kilns can generally be detected by means of magnetic measurements. A measurable magnetic anomaly may also be caused by the change in soil susceptibility, which can be observed at dwelling-, clay- and refuse pits. This change, however, generally falls in the order of magnitude of geological noise, i. e. magnetic methods can be utilized for the detection of such structures only in magnetically undisturbed, geologically homogeneous areas. Although, according to the literature, almost all archaeological sites can be investigated successfully by the magnetic method there are still only a very few results obtained in Hungary in this respect. One reason for this may be that while prospecting archaeological sites we can rarely apply an instrument continuously recording the diurnal variation of the magnetic field simultaneously with the magnetic measurements. This would in fact be necessary because the diurnal variations of unknown magnitude of the magnetic field may distort the results and may cause false-anomalie s (Weymouth and Lessard, 1986), although this effect may also be eliminated by other methods (e. g. measurement at base-stations or gradient measurements). On the other hand a lot of scrap-iron can be found on the surface or at shallow depths, the magnetic influence of which distorts the results mainly at small sensor heights. At higher sensor heights however, only magnetic structures of larger extent cause interprétable changes. In spite of this, a few successful magnetic surveys have been performed in Hungary for industrial-archaeological purposes (Verő, 1981). The aim of these, however, was not the magnetic survey of large area but the examination of the magnetic effect and detectability of various objects (e.g. iron smelting furnaces) in various environments. The measurements were performed with a Geometries G-816 type protonprecession magnetometer with sensor heights of 0.5 and 1 m, generally in a measuring grid of 1 m. In some cases, sufficient information could be provided by a survey carried out with a sensor placed in a backpack (Holló and Verő, 1984). If the dimensions of the investigated objects are not too large compared with the distance of the sensor, we can represent its magnetic field as the field of a magnetic dipole placed in the middle of it. Figure 1 compares the theoretical effects of a medieval house measured in the field. We obtained the approximately ideal anomaly shape in Fig. 1 B after highpass filtering the gradient map calculated from the measurements with two sensor heights, since the measurements were performed in a volcanic environment and with very high surface noise. In order to be able to perform the measurements with two different sensor heights simultaneously, a two-channel differential magnetometer would be expedient in the