1887

Abstract

The ruins of the Roman-age town, Aquincum, can be found in the northern part of today's Budapest. Geophysical exploration for the Aquincum Museum started in October 1997 on a test area opposite to the museum. On one side, a main road and a railway with high voltage cable, on the other sides living houses border the area. Due to the artificial noises, the area was a good site to test how the different geophysical methods work in such environment. Magnetic method, direct current resistivity mapping and ground penetrating radar were tested. Aim of the work was mapping the ancient Roman town, which is presently covered by soil. An area of 3200 m2 was surveyed in a grid with spacing of 1 m, using proton precession magnetometer of GEOMETRICS G856 (sensitivity 1 nT). The magnetic field varies hundreds of nT in a few seconds period due to the electromagnetic noise of the city (Fig. 1). The amplitude of the magnetic anomalies caused by the archaeological objects is a few tens of nT. Due to the high frequency noise, interpolation of the data of a base station cannot be used to correct the measured data. This problem was solved by connecting the base station instrument and the measuring instrument by a cable and using an outer trigger to start the measurement. The measured data were corrected for the variation in time, the low-frequency, space-domain components were removed by a high-pass filter, the high-frequency, space-domain noises were smoothed by an optimum filter, the data were reduced to the pole, and finally, the magnetic field was downward continued. The synchronous measurement between the base and measuring instruments highly reduced the noise. To further reduce the noise, 5 measurements were made in each point in a smaller area of 300 m2. The mean of the measured values was taken. The repeated measurements resulted in a significant improvement of the quality and reliability of the magnetic map. The result of the repeated measurements is shown in the insert in Fig. 3. The resistivities of the limestone walls and the covering soil differ significantly. This initiated the idea of testing geoelectric methods in the area. DC measurements were made in an area of 50x40 meter using dipole-dipole electrode configuration. The length of the source and potential dipoles was 1 meter. The distance between the centres of dipoles was 2 meters. This electrode configuration gives information from a depth of about 1 meter. To speed up the profiling a "double-fork" electrode system has been constructed. The whole configuration was moved by one meter in the consecutive measurements. East-west as well as south-north profiles were measured. About 4500 data points were available for processing. An apparent resistivity map was computed from the combination of the two direction profiling (Fig. 3).

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/content/papers/10.3997/2214-4609.201406373
1999-09-06
2020-07-14
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