4 - Ultra-shallow Shear-wave Reflections Locating Near-surface Buried Structures in the Unexcavated Southern Fringe of the Ancient Ostia, Rome

Summary

The southern boundary of Region IV of ancient Ostia coincides with the southern limit of the excavated area of the ancient city. The perceived expanse of the city is influenced by the extent of the excavation. It is not known whether the unexcavated part lying south of Region IV also contains structures of antiquity which might have important historical significance. We have carried out high-resolution, shallow seismic reflection surveys along two profiles, using shear (transverse) waves. The goal of these pilot surveys was to see whether any indication of ultra-shallow scatterers, indicating the potential location of shallow-buried structures, can be found in the shearwave data. The results show very distinct backscattered shear-wave arrivals from a mysterious tumulus, whose location along Line A was known. It has been possible to interpret with reasonable confidence the location of several conspicuous, shallow scatterers in the two seismic profiles. Use of shear waves and a high-frequency, electromagnetic shear-wave vibrator was crucial to achieving seismic resolution of nearly 25 cm. The amplitude of the scattered energy is helpful to locate the relatively strong scatterers. Our results suggest that the unexcavated areas located south of Region IV most likely contain buried underground structures. 3-D shear-wave seismic reflections together with new seismic-imaging approaches will be promising to illuminate the unknown shallow subsurface of this important archaeological site in a non-invasive manner.

INTRODUCTION

The archaeological site of Ostia, the important harbour city of ancient Rome, presents impressive, well-preserved architectural remains that shed light on the complexity of the Roman urban life of antiquity. Ancient Ostia was situated about 25 km west of Rome, at the mouth of the river Tiber. With time, due to deposition of sediments from the river, the shoreline migrated southwards. Although the earliest human activities in the area date back to 1400-1000 BC, there is a legend that Ostia was founded in the late seventh century BC by the fourth king of Rome, Ancus Marcius. However, the oldest archaeological remains so far discovered date back to the fourth century BC. The vast majority of the excavated buildings are from the first to second centuries AD, which represent the period of most active developments in ancient Ostia during the reign of Trajan, Hadrian and Antoninus Pius.