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

the archaeologist contains a simplified version, where 2-5 red and blue signals symbolize pozitive and negative anomalies with a ratio between coverage and the absolute value of the anomaly. Theoretically it is possible to enhance anomalies by mathematical methods, but with our measurements, the quick completion of the map was always preferred to any special anomaly enhancement or smoothing. One can distinguish several types of anomalies on these maps. The parameters for the distinction are the dimension and form of the anomaly, its value, and whether the anomalies are of one sign (e. g. positive ones) or of both signs; and, in the latter case, the ratio of the absolute values. Negative anomalies he mostly to the north of the positive ones, thus the connection between the two remains clear even in disturbed areas. The first group of anomalies is linked with the thickness of the humus layer. Some typical examples are: in the area of the village of Vörs (W of Lake Balaton) the soil was partly sandy, partly humic. The values measured over the humic part were higher by 10—20 nT. At the village of Sarrod, next to an old, rotten tree trunk, we found an anomaly of 10—15 nT. It was rather disturbing as the „archaeological" anomalies there were very similar in shape and value. In Dénesfa and

Zamárdi, linearly extended anomalies may be caused by ancient brook-beds: these anomalies had values of 10 to 30 nT. Extended anomalies due to variation in the humus-content can be ehminated by measurements at two levels (near the surface and on a 2 or 4 m high pole). The extended humus-anomaly remains at the higher level, too, while the archaeological ones are strongly attenuated, i. e. they are better expressed as the difference between the two anomalies. It is a general experience that rocks occurring in Western Hungary have low magnetization so rock fragments or stone walls cannot be detected (e. g. a grave covered by sandstone slabs in Sopron Krautacker and a clay wall in Petőháza could not be identified). Obviously, magnetized rocks do occur, e. g. basalt fragments from road-building have been found several times. The next group contains fired burnt clay objects. In several cases small-scale excavations were able to discover the source of the anomaly immediately after the measurement: in Vörs and in Sarrod they proved to be due to small pottery fragments and hearths below them (anomalies of 10—15 nT). Hearths generally have rather large anomalies, and together with the humic filling, they cause the sometimes significant anomalies of pitdwellings (10—50 nT). Such anomalies,among others, were found at Zamárdi and Röjtökmuzsaly. The size of the anomaly depends on the dimensions of the pit. In Sopron— Krautacker, the brick walls of a lately demolished house could also be clearly traced. Most of our experience is connected with iron smelting furnaces. They are partly Celtic ones (but in their detection geophysics played no part), partly early Medieval, Avar or Hungarian from the epoch of the conquest or somewhat later (9—12th centuries). The remnants of the furnaces are fragments of walls and scattered pieces of slag. In our experience, the anomaly is proportional to the quantity of the brick walls: the more walls remained, the greater the anomaly .Coirespondingly the anomalies lie between 10 (Magyarfalva) and 100-150 nT (Répcevis). The effect of fist-sized brick samples may also be rather different on a sensor in the immediate vicinity : generally it is around 10 nT, but in an exceptional case (a small fragment from Sarrod of yellowish colour and visible crystals) it reached 200 nT. Among fired clays, the material of the Sopron „redwall" should be mentioned; this is, essentially, in situ burnt clay. Measurements were strongly influenced or prevented by the urban environment, only in a large yard behind the one-time Lutheran vicarage were some valid measurements enabled. The anomalies are around or over 100 nT. Fistsized samples caused anomalies of 20—30 nT in the vicinity of the sensor, thus confirming strong heating of the clay.