A GPR survey was conducted in the frigidarium of the Byzantine baths in 2017 almost 50 years after the area had been excavated, backfilled and re-tiled using a black and white mosaic. GPR signals are interpreted using photographs of the excavation provided by courtesy of Archivio di disegno as well as drawings and texts produced by scholars to retrace the multiple functions that the site had until the Late Antiquity. It is shown that some ancient reticulated wall structures can be recognised in the GPR data, but not all! There are also prominent GPR features which cannot be identified. We conclude that the partial excavation of the site and the backfilling operation have further complicated the structure of the ground below the mosaic of the Byzantine baths.

INTRODUCTION

Can we image again structures that were excavated and backfilled in Ostia Antica during the last century using a Ground Penetrating Radar (GPR) survey? To answer this question, Hanna Stöger proposed that our TU Delft Geoscience & Engineering team carry out a GPR survey in the spring of 2017 in the large frigidarium of the Byzantine baths located in Regio IV, Insula iv. The site had been excavated (and backfilled) three times – in 1940, 1958 and 1970-1971 (Stöger pers. comm. 2017). Photographs of the last excavation (Archivio di Designo) and drawings of the site's lay-out (Lorenzatti 1998; Poccardi 2006) were available and we could use them to compare with our GPR data.

THE FRIGIDARIUM TODAY

The frigidarium of the Byzantine baths is currently accessible via two entrances: at its north-west corner from Via del Tempio Rotondo, along a corridor and stairs; and at its north-east corner through an opening between the remains of the Domus di Giove Fulminatore and a large natatio. At its southern end the frigidarium is characterised by two cold baths with apses (Fig. 3.1, labels 1 and 2). The floor of the frigidarium is partly tiled using white and black mosaics (Fig. 3.1, label 3). Vegetation has invaded the walls fringing the frigidarium and colonised the adjacent floor. Grass, mowed shortly before our survey, covers the parts of the floor where the mosaic no longer exists.