Geophysical Prospecting

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Study areas are shown in Fig.1 and were investigated by high-resolution GPR during 2012 and 2013 surveys. The main aim of prospection was to verify the presence of archaeological structures still buried in the subsoil at different depths.

Radar sections were recorded by using the SIR 3000 (GSSI) system, equipped with two bistatic antennas having a constant offset and a nominal frequency of 400 and 900 MHz, respectively.

During 2012 and 2013 surveys a total amount of 365 parallel profiles were recorded with different lengths and a constant separation of 0.5m. After usual prearranged tests, and accounting both for the geoenvironmental conditions and the type of supposed structures, the following instrumental setting was adopted: 80 and 115 ns full time scale for the 400 MHz antenna and 65 ns for the 900 MHz; 16 bit dynamic range; 512 sampling for trace. Positioning along each profile, which is necessary to define the surface geometric localization of the antenna and therefore the position of detected anomalies, was assured through an odometer set on the radar carriage. Radar traces were taken with a line scan procedure consisting in a continuous motion of the antennas along a predefined direction (profile). The "time-slice" elaboration and representation technique was applied in order to gather all information from parallel profiles and obtain an overview of the investigated subsoil volume. Time slices were rendered with appropriate time-intervals (4ns and 2ns for the 400MHz and 900 MHz antenna, respectively) and then represented as 2D maps for different depths of investigation. Indeed, results obtained from GPR prospection were referred to several surfaces at depth, for an investigated subsoil thickness of about 3.5 m with the 400 MHz antenna.

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  Aree Investigate. Valle S. Andrea (Fondi). Localizzazione delle aree investigate con i metodi Georadar e Gradiometro Fluxgate.
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  GPR 400 MHz p. 0.80m.
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  GPR 400 MHz p. 1.30m.
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  GPR 900 MHz p. 0.40m.
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  GPR 900 MHz p. 1.20m.

Fig.2 reports the time slice at the estimated depth of 0.80 m from the topographic surface, which were acquired with the 400 MHZ antenna. Georadar anomalies (red-coloured) can be referred to structures located at this depth of investigation; in particular within area A the main anomaly follow the tren dof the ancient route and can be attributed to the road pavement or, alternativley, to a canalization running along the route right side.

Fig.3 reports the time slice at the estimated depth of 1.30 m from the topographic surface, which were acquired again with the 400 MHZ antenna . The signature of the anomaly along the route track is reduced, whereas those due to the presence of tanks in the subsoil clearly appear.

Fig.4 reports the time slice at the estimated depth of 0.40 m from the topographic surface, which were acquired with the 900 MHZ antenna. Georadar anomalies (red-coloured) can be referred to structures located at this depth of investigation; in the area A, a clear correspondence with anomalies detected with the 400 MHz antenna cab be recognized.

Fig.5 reports the time slice at the estimated depth of 0.40 m from the topographic surface, which were acquired with the 900 MHZ antenna. The signature of the anomaly along the route track is reduced, whereas those due to the presence of tanks in the subsoil clearly appear.