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

Prospecting - VERŐ József: Exploration of archaeological sites in Western Hungary by the geomagnetic method

Anomalies listed hitherto are characterized by mostly positive anomalies, a shifted negative anomaly is lacking or it is much weaker. If it is a negative anomaly all the same, then it may be due (in the case of furnaces) to anomalous slags. The effect of slag samples seldom reaches 10 nt, but sometimes higher values have also been found. Thus, in Dénesfa values of 10—20 nT occur and, especially in Zamárdi, slag samples had nearby anomalies of 30—50 nT. The positive main anomaly was accompanied in Dénesfa by a negative secondary one; in Zamárdi the anomalies were rather complicated, nevertheless, negative anomalies did occur. The low magnetization of slags is surprising as in Bohemia slags are connected with anomalies of several hundred, or even a few thousand nT. The difference is most likely due to a difference in technology. Anomalies of both signs with about equal positive and negative values are mostly due to iron objects. In agricultural areas, a great variety of iron objects occur, beginning with wire piece? and öxshoes, followed by parts, of tractors and tins, to iron fences and pipelines. Measurements are possible only at certain distances from greater iron objects such as fences, tanks, reinforced concrete structures, etc, to be determined individually; but at a distance of say 10 m, no high quality measurements can be expected. A rather small, 20 to 30 cm long wire may cause an anomaly of 200 to 300 nT, a tin 20 to 100 nT. Sometimes nails are detected, too, if they lie just at the measurement point. There are anomalies with the typical properties of iron objects where the anomalous body could not be identified. As an example, at Sopron-Krautacker the excavation did not reach adequate depths (we found an elongated double anomaly of about 1000 nT; the maps of public utilities did not indicate any pipeline or similar structure, so the source remained unknown). Sometimes the iron objects themselves may be of archaeological interest, too; thus in the Röjtökmuzsaly area, the „pits" corresponded to anomalies over 100 nT (see the map - Fig. 1) of both signs. These anomalies were partly due to the waste of Roman time smithies. In Vörs, an anomaly was caused by small iron object a few cm

3 in volume that occurred in great quantities together with Roman pottery fragments at depths of 20 to 60 cm. At a nearly site a greater, heavy rock fragment caused an anomaly of about 1000 nT. Another kind of disturbance is due to electric networks with alternating (or direct) current: in their environment measurements are impossible due to the effect of the quickly changing fields. This is why measurements are impossible in cities, even in less densely built outskirts. In an urban environment, it may even be impossible to measure the effect of a sample near the sensor. As a summary, it is to be said that the main condition for geomagnetic measurements with archaeological purpose is that the site should be free of recent human activities. Even fillings and earth masses supposed as being „clean" contain so much iron waste or fired clay that the result of the measurements becomes dubious. The same refers in an even higher degree to different kinds of electric cables, or pipelines, where partly the ac current, partly the iron content of the pipes interfere with the archaeological anomalies. Finally, we should shortly summarize the results of a geomagnetic measurement with archaeological purpose from 1987. The example was chosen to show typical anomalies due to sources different from iron smelting furnaces. In the Zsebeháza area of the village of Röjtökmuzsaly (Győr-Sopron country) the remains of a Roman villa are known

. A large slag occurrence became known from the vicinity of this villa, and the aim of the measurement was to identify the source of these slags. Measurements were rarried out on an area of 20X25 m with the-described method, but the boundaries were several times crossed to enable the construction of closed isolines. The sensor was near to the surface, measurements at two levels were not necessary. The selected base at the bottom right comer of the map was slightly anomalous therefore the isolines on the map are not symmetrical to zero. Time variations of the geomagnetic field