*3.4. Experimental Surveys for Floor*

Figure 17 presents two examples of the GPR B-scans acquired in the area of the south aisle. The A-19 scan (Figure 17a) clearly reveals the location of the tiles based on the reflections from the tile grouts. The bottom faces of the tiles were identified at different depths, thus they had different thicknesses, varying between 0.04 and 0.09 m. The tombstone with a thickness of approximately 0.16 m could be observed in the center of the scan. An additional reflection denoting the tombstone adornment was also visible at the top of the scan. The ground under the tombstone had many concentrated inclusions, probably being the rubble or the human remains from the original crypts (cf. Figure 14c,d). Aside from this area, inhomogeneities were not observed. It was difficult to state whether there were any brick walls under the edges of the tombstone because the hyperbolas denoting the concentrated inclusions blurred the image. The A-117 scan (Figure 17b) shows the tiles with

different thicknesses between 0.03 and 0.08 m. The tombstone was also visible; however, its shape was far more regular compared with the one from the A-19 scan. The ground below the tombstone had no strong inhomogeneities, thus it could be stated that not all the tombstones were laid in the area of the original crypts. Other interesting reflections were the hyperbolas located vertically one above another at the distances of 1.3, 3.4 and 4.2 m. They indicated the presence of three brick walls (cf. Figure 14b,d), that might be the remainders of the original supporting structures of the crypts.

**Figure 17.** GPR B-scans for the south aisle (survey A): (**a**) A-19; (**b**) A-117.

The next two GPR B-scans from the south aisle are presented together with the corresponding results of the UT scanning. In the A-121 GPR B-scan (Figure 18a), the tiles with different thicknesses (in the range of 0.02–0.04 m) were visible, and also a tombstone with the thickness of about 0.12 m was identified. The ground below the tombstone contained some concentrated inclusions (probably rubble or human remains). The brick wall was present at a distance of about 3.4 m. The UT scan (Figure 18b) clearly revealed the tile pattern, and also the tombstone was observed. The presence of the multiple reflections under each tile allowed stating that there were many air gaps below them, which agreed with the fact that the floor of the south aisle experienced significant settlement. The reflections denoting the air gaps generally did not appear below the tombstone, probably because of its weight. What is also worth noting is that no additional reflections from the ground heterogeneities could be identified. The UT scanning did not detect concentrated inclusions, which were already observed for the pipe in the area of the trial pit and the table tennis ball in the concrete slab. The GPR B-scan for the trace A-129 is presented in Figure 19a. The tiles with different thicknesses (varying between 0.02 and 0.06 m) are clearly detectable. Two tombstones with a thickness of approximately 0.12 m and irregular shape of the bottom face were also present. The ground under both tombstones had many concentrated inclusions unlike aside from this area. The brick wall could be visible at the distance of about 4.3 m; however, the intensity of the reflections from the bricks was not as strong as in the A-121 scan. In the UT scan (Figure 19b), the tiles and the tombstones could be localized. There were no significant reflections under the tombstones, thus the air gaps were not likely to be there, unlike under

the most of the tiles, where multiple reflections appeared. This observation corresponded with the settlement of the floor in the area of the south aisle.

**Figure 18.** GPR and UT B-scans for trace A-121 (south aisle): (**a**) GPR scan; (**b**) UT scan.

**Figure 19.** GPR and UT B-scans for trace A-129 (south aisle): (**a**) GPR scan; (**b**) UT scan.

Figure 20 shows the examples of the GPR B-scans for the north aisle. The B-5 scan (Figure 20a) shows the regular pattern of tiles with a constant thickness of 0.03 m. Several evenly spaced hyperbolas were visible at two levels (the depth of 0.11 and 0.18 m) and the distance of 0.0–2.1 m. These reflections represented the reinforcement of the staircase located under the floor. The ground below the tiles aside

of the staircase seemed to be devoid of significant heterogeneity. In the B-35 scan (Figure 20b), the tiles had different thicknesses in the range of 0.02–0.06 m. The tombstone had a thickness varying between 0.10 and 0.15 m and the skew shape of the bottom face. The ground below the tombstone had some concentrated inclusions. The B-143 scan (Figure 20c) presents the tiles with the thicknesses between 0.02 and 0.05 m. Two tombstones were also visible, the left one with the thickness of about 0.09 m and the right with the thickness of approximately 0.16 m. The ground under the thinner tombstone did not have inhomogeneities, unlike the ground under the second one, where multiple reflections from the concentrated inclusions occurred. This observation confirms the fact that some of the tombstones were placed aside from the area of the original crypts.
