**1. Introduction**

Non-destructive testing (NDT) is commonly used for assessing the condition of components of engineering structures. It is a quick and efficient approach, the main advantage of which is the ability to examine a structure in a non-invasive way, without damaging or changing the composition or shape of the inspected object. NDT covers a variety of techniques based on a wide range of physical phenomena, including propagation of elastic waves, being the basis for ultrasonic testing (UT), and electromagnetic waves used in the ground penetrating radar (GPR) method. Non-destructive testing can be applied on a selected part of a structure or for a comprehensive inspection of large-size objects, like bridges [1–3], water gates [4], retaining walls [5] or archaeological sites [6–8].

Non-destructive testing is particularly suitable in the case of historical objects. Such an approach is more and more often applied in cultural heritage buildings due to the necessity to preserve such structures in an untouched condition for future generations. The exact structure of historical objects dated several centuries back is usually unknown, since the technical documentation is incomplete or entirely gone. An efficient method to collect information in such situations is in situ inspection, often supported by numerical analyses that provide design guidelines and recommendations for planned reconstruction, strengthening and restoration works [9,10]. Non-invasive testing conducted within the flooring area allows detecting crypts, tombs and hidden rooms, as well as evaluating the technical condition of the floors and ceilings. GPR was successfully used for the diagnostics and condition assessment of different historical objects [11–17]. Despite the advances in non-destructive testing technology, there is no one technique suitable for every situation. Researches are often carried out using several methods to ensure that the obtained results are correct. Drahor et al. [18] compared the results obtained using the GPR and electrical resistivity tomography (ERT) while searching for cracks and damages that could occur in the church floor. Perez-Gracia et al. [19] compiled two methods, GPR and the capacitively coupled resistivity method, to obtain 2D images of the shallow subsurface under and around the Cathedral of Mallorca. The ground penetrating radar technique and laser scanning technique were combined by Tapete et al. [20] in order to interpret the displacements influencing the condition of the archeological monuments. Moropoulou et al. [21] presented the implementation of the integrated non-destructive methods, such as digital image processing, infrared thermography, ground penetrating radar, ultrasonic testing and fibre-optic microscopy for the inspection of historical objects. They detected air voids, delamination, moisture, material wear and degradation in the evaluation of the effectiveness of interventions and the assessment of the compliance of the repair method used. Faella et al. [22] took comprehensive measurements of a church in Bethlehem and estimated the condition of the floors, walls and columns, employing GPR and ultrasonic methods as well as thermography.

Combining the GPR and UT methods is commonly used in imaging, monitoring and analyzing the condition of engineering structures. GPR is particularly useful to conduct surveys on large areas because it allows handling a large amount of data in a reasonable amount of time. In previous studies, the GPR method has shown high efficiency in the imaging of reinforcement bars [23,24], cracks [25], defects in the form of air voids, delamination and moisture [26–28] or systems applied for concrete strengthening [29,30]. On the other hand, the UT techniques were proved to work well in the detection of defects such as notches [31–33], micro- and macro-cracks [34], small air gaps [35] or minor scratches [36], as well as in the evaluation of plate-like structures [37,38] and adhesive materials connections [39,40]. What is more, ultrasounds can be applied for inspecting conductive materials, unlike electromagnetic waves used in the GPR method. If both methods are applied appropriately, they may be considered as complementary. The assessment of the examined element becomes more reliable by implementing these two measurement techniques. The literature shows that both methods ensure a unique way of imaging the studied surface, enabling thus to resolve multiple research problems. Guadagnuolo [41] juxtaposed the GPR and UT techniques while examining the walls and floors of a historical church. Binda et al. [42] implemented the UT and GPR research to verify the damages and possible preservation works of the walls and piers due to the restoration of a damaged cathedral. Furthermore, the parameters of the mortar used as a possible means of repairing a damaged wall were controlled by performing ultrasonic tests. Perez-Gracia et al. [43] combined the UT and GPR techniques in the assessment of the geometry and physical properties of historical columns. The above-mentioned papers have presented many successful applications of combined GPR and UT methods; however, a thorough comparison of these methods by analyzing measurement data recorded along the same traces is rather limited.

The paper presents the results of the integrated ultrasonic testing and ground penetrating radar inspection conducted in St. Nicholas' Church in Gda ´nsk, Poland. The aim of the study was to present the practical aspects of the application of both techniques in detecting and imaging the underfloor inclusions, such as air voids, brick walls, pipes, rubble and human remains. Experimental measurements of the floor were conducted in the area of both (south and north) aisles, and also around a trial pit. Preliminary investigations were conducted on physical models consisting of two concrete slabs stacked on top of each other and gradually moved apart to simulate a slot of varying thickness, in order to better understand the phenomena of electromagnetic and ultrasonic wave propagation within the air voids and concentrated inclusions. In addition, the numerical simulations of electromagnetic waves were performed to support the interpretation of the GPR results. The analysis of the results obtained allowed concluding that GPR was suitable for the imaging of concentrated inclusions, whereas UT enabled detecting air voids. The presented research revealed the possibilities and limitations of both methods, indicating their complementarity in the context of non-destructive diagnostics of historical buildings.
