**4. Conclusions**

In this study, the integrated GPR and UT inspection was conducted on the floor of a historical church. The performed investigations focused on the detection of air gaps and other anomalies located under the stone tiles and the tombstones. The research was supported by laboratory models illustrating the behavior of electromagnetic and ultrasonic waves. The numerical models were prepared to illustrate the propagation of electromagnetic waves in the medium containing inclusions such as air gaps, brick walls, pipes and brick rubble. The results obtained provided useful information about the possibilities and limitations of the GPR and UT methods.

The possibility of an efficient application of the GPR technique for detecting small concentrated inclusions was confirmed. The table tennis ball was identified in the concrete slab (both in the numerical and experimental results); the pipe was detected in the area of the trial pit; some bricks and concentrated inclusions were observed in the ground under the floor. The larger surface and volume elements (stone tiles, tombstones) were successfully imaged too. The GPR method was also able to show the air gaps as lines (single or double, depending on the air gap thickness). However, the exact imaging of the air voids was possible only for thick layers; in the case of thin air gaps, it was difficult to state whether the lines denoted the air gap or a boundary between two media.

The UT measurements allowed an efficient detecting of the air gaps, independent of their thickness. This resulted from the phenomenon of total reflection of elastic waves at the boundary of the analyzed element. The location of stone tiles and tombstones could be also clearly visible. On the other hand, because of wave reflection, the UT inspection did not allow to detect anything below the air gap. What is more, ultrasonic waves did not identify concentrated inclusions, such as the table tennis ball, the pipe and the ground inhomogeneities, which could be caused by the limitations of the UT antenna used.

The tomographic snapshots provided the overall image of the examined area at a specific depth. This type of imaging could be useful when the exact location of underfloor inclusions in relation to the entire scanned area is the object of interest. However, when the detailed profile of a part of considered structure is important, an analysis of B-scans is more appropriate. For the presented study, a good compliance of both ways of imaging was observed.

To increase the applicability and practicality of non-destructive inspection techniques, the use of integrated GPR and UT methods was recommended. The methods complemented each other, allowing an exclusion of their limitations. The UT technique was efficient at visualizing air gaps of different thicknesses, however, it was not suitable for imaging small inclusions and anything below the air voids. What is also important, the UT measurements were time-consuming. The GPR method successfully detected concentrated inclusions with different sizes; it also allowed inspecting a large area with a relatively low time cost. However, the GPR measurements did not allow detecting air voids.

To summarize, the integrated inspection combining the GPR and UT techniques appeared to be effective for non-destructive diagnostics of underfloor structures in cultural heritage buildings. The proposed approach can be useful for the detection of anomalies laying under the floor, such as air gaps, bricks and pipes, which can appear in historical objects. The complementarity of both methods enables a precise analysis of the tested structure.

**Author Contributions:** Conceptualization and Methodology, M.R., E.W. and M.Z.; Experimental Investigations, M.R., E.W. and M.Z.; FDTD Calculations, M.R.; Formal Analysis, M.R., E.W. and M.Z.; Visualization, M.R. and E.W.; Writing—Original Draft Preparation, M.R., E.W. and M.Z.; Writing—Review and Editing, M.R., E.W. and M.Z.; Supervision, M.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors would like to thank Michał Osek OP, the Prior of St. Nicholas' Church, for enabling in situ surveys and providing valuable historical information. The support of Jacek Lachowicz during the GPR surveys is gratefully acknowledged. The authors would like also to thank Piotr Samól for providing a plane view of the church (the plane view was used in the background of Figure 6).

**Conflicts of Interest:** The authors declare no conflict of interest.
