*2.2. Data Acquisition and Equipment*

Two NDT techniques were used for inspecting the floor: ground penetrating radar and ultrasonic testing. In both approaches, a pulse-echo test system was used which comprises both the transmitting (T) and receiving (R) antennas. During the pulse-echo measurements (Figure 5), the antenna is moved along the tested surface and a single time signal (so-called an A-scan) is recorded for a specific position. The assembling of the A-scans gives an image called a B-scan or echogram. As the antenna approaches an element that differs in electrical (in the ground penetrating radar method) or mechanical (in the ultrasound method) properties from the surrounding medium, the time the wave returns to the receiving antenna changes. A reflection is then created on the echogram reflecting the disturbing element. A reflection from any point inclusion (e.g., circular air gap, reinforcing bar) is represented in the B-scan as a hyperbola, while longitudinal inclusions are represented as line patterns.

**Figure 5.** Scheme of measurements performed in pulse-echo mode: (**a**) collection of signals; (**b**) sketch of obtained echogram.

The Aladdin system (IDS GeoRadar, Pisa, Italy) equipped with a 2 GHz antenna was used to perform the GPR measurements. For each trace, the registered time range was 32 ns and the number of captured samples was 1024. The step distance between particular A-scans was 1 cm. The GPR data were registered in K2 FastWave and then processed in GRED HD by the following operations: automatic time zero correction, bandpass filtration in a frequency range of 500–3000 MHz and smoothed gain. Ultrasonic testing was performed using the pulse analyzer Pundit PL-200 (Proceq SA, Schwerzenbach, Switzerland) equipped with a 50 kHz antenna. For each A-scan, 1000 samples were captured with a time step of 1 μs, and the step distance between particular traces was 1 cm.

The GPR inspection of the floor was conducted in both aisles (Figure 6). During the measurements, 147 profiles were traced in the south aisle (A-1 to A-147) and 150 profiles in the north aisle (B-1 to B-150). The distance between particular profiles was 21.5 cm. Some inaccessible areas were omitted during the GPR scanning, so the shape of the scanned area was irregular. Next, four scans were acquired along the edges of the trial pit (denoted as C-1, C-2, C-3 and C-4 in Figure 7a). Finally, one profile was traced on the concrete slabs. Four measurements were made, each for the different thicknesses of the air gap, i.e., D-1 (1 mm), D-2 (4 mm), D-3 (20 mm) and D-4 (50 mm). Additionally, along selected traces, UT measurements were carried out (see Figure 8). The summary of the conducted GPR and UT surveys is given in Table 1.

**Figure 6.** Plane view of the church with ground penetrating radar (GPR) traces in the south (traces A-1 to A-147) and north (traces B-1 to B-150) aisles.

**Figure 7.** GPR traces registered along (**a**) edges of the trial pit (traces C-1 to C-4) and (**b**) the circular slabs with air gaps of varying thickness (traces D-1 to D-4).

**Figure 8.** Ultrasonic measurements: (**a**,**b**) along traces A-121 and A-129 in the south aisle; (**c**) along edges of the trial pit; (**d**) along circular slabs.


**Table 1.** Scheme of GPR and ultrasonic testing (UT) investigations.
