Two surveys with ground-penetrating radar (GPR) were carried out in June and December 2000 at the Roman Ships archaeological site (Pisa, Italy). Both surveys were undertaken at selected locations, placed on the plan of excavation (5 m from the ground surface), using a GSSI Sir System2 incorporating 35, 100, 200 and 500 MHz centre-frequency antennae. The main purpose of the two surveys was to test the value of radar in respect of penetration depth and, therefore, to reconstruct the geological stratigraphy, given the general not too favourable site conditions. The results showed that most of the GPR data acquired with the 35 MHz antenna were contaminated by strong reflections caused by above-ground objects near the survey lines. In fact, the archaeological area is protected on every side by iron barriers, around 6 m high, in order to guarantee the stability of the walls and to contain the present shallow groundwater. Therefore, it is very important to recognize the reflections through air (surface scattering) and not to confuse them with the reflections from underground geological features. For this purpose higher frequency antennae, less prone to surface scattering problems, although allowing shallower penetration depths, also have been used. Their higher resolution permitted the identification of some anomalies probably related to shallow stratigraphical boundaries, as well as other anomalies probably attributable to archaeological features. Moreover, the numerous measurements carried out in the second survey to estimate the velocity of propagation of the electromagnetic waves in the ground, contributed to a better interpretation of the geology of the site.
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