GNGTS 2021 - Atti del 39° Convegno Nazionale

443 GNGTS 2021 S essione 3.2 Survey The GPR survey was carried out with a RIS MF Hi-Mod system designed by IDS and equipped with a multifrequency antenna (200-600 MHz). The GPR technique utilizes propagating electromagnetic waves which are sent by a transmitting antenna into the ground and recorded back by a receiver antenna after being reflected from underground bodies with different electromagnetic properties compared to the surrounding medium. Measuring the traveltime of the electromagnetic waves ( t ) and defining, with a preliminary calibration process, the average velocity of the medium ( v m ) (Davis and Annan, 1989; Goodman D. and Piro S., 2013), it is possible to evaluate the depth of the reflector ( h ) with the equation: h = v m ∙ t 2 rmed inside the church and 9 outside it, for approximately re divided in 8 longitudinal to the major axis of the church f 0.8 m between each profile was applied. Because of the e the measurement area, a complete and regular grid was good coverage of the investigated area. RED 3D software from IDS. A processing step of filtering rove the quality of the records (Fig. 3a). The “C-scans” epth-slices for different depth ranges, which represent the rface (IDS Ingegneria Dei Sistemi S.p.a., 2009). terpretation of the scans has highlighted that the 600 MHz MHz antenna for this kind of investigations, achieving a olution and an acceptable depth of penetration of about 2.5 y in the subsurface of San Sebastiano church a series of etry. The most interesting feature is a barrel vault crypt with a square section and side of around 2.8-3.0 m. The 1.5 m below the superficial church floor. Figure 3a shows a radargram of TPV_9 profile. In the scan is visible and well s geometry, with the typical barrel vault. before the GPR survey, a first crypt with remains of human structure of the church. Therefore, it is reasonable that the rly used as mass grave of burial. maller reflectors with different shapes and dimensions, kely structural associated; these are characterized by high ong dielectric contrast and are possibly caused by small es, buried walls or old foundations. likely lithological associated; these are lower amplitude al or with a slight slope, possibly related with subsurface avities, lithological variations between fill material and the fill material. ucture it was noticed a high dampness of the subsurface, nt particle size of the fill material. In the external area no ied, except for a small isolated feature of generic nature A total of 20 GPR scans were performed inside the church and 9 outside it, for approximately 250 m of acquisition. The inside scans are divided in 8 longitudinal to the major axis of the church and 12 transverse (Fig. 2). A spacing of 0.8 m between each profile was applied. Because of the presence of a scaffolding structure inside the measurement area, a complete and regular grid was not realized. However, it was achieved a good coverage of the investigated area. The radar data was processed with GRED 3D software from IDS. A processing step of filtering was pplied o the radargrams, to improve the quality of the records (Fig. 3a). The “C-scans” function has all wed to extract planar depth-slices for d fferent depth ranges, which represent the signal amplitude distribution in the subsurface (IDS Ingegneria Dei Sistemi S.p.a., 2009). Fig. 3 - a) Transversal section (TPV_9), showing the reflections related to the crypt identified beneath this area; b) planim try of the church with the identifi d objects by GPR survey.