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

VERO József* EXPLORATION OF ARCHAEOLOGICAL SITES IN WESTERN HUNGARY BY THE GEOMAGNETIC METHOD Abstract — Magnetic measurements can detect fired or burnt clay (kilns, hearths, ceramics); buried pits and gulleys can also be found; depending on local conditions walls and other rock structures can be explored. Magnetic measurements were carried out on th area of a Roman villa in Zsebeháza. Representing the anomalies of the measurements on a map the author draws conclusions from the archaeological objects. The possibility of exploring archaeological sites by tne geomagnetic method is due to the fact that some of tne material of interest t<J archaeology has a magnetization differring from that of the surroundings. It is, however, to be added to this fact that the same refers to a number of other materials, including those of human origin, as well as natural ones which have no significance from an archaeological point of view. The presence of such material represents the most important setback of the method. In the following the methodology used by us is briefly described together with data on the number of points measured per day. A summary is then given on the anomalies from different sources, and lastly an example if presented in detail on a geomagnetic measurement for an archaeological purpose. Measurements are made if possible of the grid points of a quadratic net. The distance between the grid points is generally 1 m, sometimes it may be 2 m in one or in both directions, in certain cases a densification to 05 m may be necessary. The most simple method is to lay out along the two East—West sides of a rectangle (normally 15 to 50 m long) non-magnetic cables bearing marks at every second meter, and a third similar cable is moved perpendicularly to the first two each time by 3 m (1 m to the right and left can be estimated). During the measurement the observer returns after each row to the base to enable the ehmination of the time variation of the geomagnetic field. In our opinion, this is essential for acceptable results. Two people are needed for the measurement: the observer and the recorder; witiiin one normal working day they can measure 2000 to 2500 points, meaning just as much as m

2 -s. The efficiency of the method can be increased if the sensor, which is generally on a pole or on the back of the observer, is lowered near to the surface, some 15 to 20 cm above it. Outlying, anomalous measurements should be repeated. If an anomaly lies at the edge of the area, the measurements should be extended until the anomaly vanishes, in order to get closed isolines; if necessary, densification is made within the measured area. It is advantageous if workers are at the site during the measurements to dig out directly the sources of anomalies, if this is permissible. In such a case it is possible to check continuously to determine whether the anomalous body remains in the soil or whether the soil that has been dug out contains some magnetic source. On the other hand, it is disadvantageous to carry out measurements in a partly excavated area as in such a case both the excavated part and the soil deposit are excluded from the exploration. The simplest method to represent the measurement results is a similar square grid as used in the measurement where the grid points carry the values of the local anomalies (they have been corrected on the basis of the values measured at the base). The map for * Geodetical and Geophysical Research Institute of the Hungarian Academy of Sciences H-9401 Sopron, P.O.B. 5.