Discovery and investigation of a Byzantine basilica in ancient Miletos: improving geophysical data processing and interpretation through information gained from excavations

The ancient city of Milet is situated in the Southwest of Turkey, about 10km away from the Mediterranean Sea. Since 1993 more than 130 ha have been geophysically investigated using geomagnetics. At places of special interest additional measurements using geoelectrics and electromagnetic induction have been performed. These methods measure the electrical resistivity of the subsurface that can give additional information about building materials, for example.

One of the discoveries through geomagnetic measurements is a Byzantine basilica located to the North of Kalabak Tepe. Its walls are visible on the geomagnetic map as anomalies indicating lower magnetization. Additional geoelectrical measurements have been conducted on several profiles revealing places of higher resistivity that are due to building stones. But the rectangular structure in the southern part of the geomagnetic map could not be explained by these results only. Thus several small excavations were carried out that confirmed the existence of the walls visible in geomagnetics. With the aid of these findings the geophysical data could be newly interpreted.

For the first inversion of the apparent resistivity values standard parameters were used giving a relatively smooth image of the subsurface. Due to the findings of archaeological excavations the geoelectrical data processing was revised and a robust inversion approach was used instead. This results in a relatively blocky appearance of the anomalies resembling well the shape of the walls.

The floor plan of a Byzantine basilica or cemetery church found with the help of geomagnetics was further clarified by geoelectrical measurements. Especially small excavations gave additional information about the sharp-edged shape of the subsurface structures that could be used to improve the inversion of geoelectrical profiles. Because resistivity anomalies were better constrained, the interpretation became clearer. With the improved data processing it was possible to distinguish between different filling materials and thus temporal building phases. A square complex south of the basilica was built first, followed by the cemetery church about 100 years later.