**Magdalena Rucka 1,\*, Erwin Wojtczak <sup>1</sup> and Monika Zieli ´nska <sup>2</sup>**


Received: 21 May 2020; Accepted: 1 June 2020; Published: 3 June 2020

**Abstract:** The paper presents the results of integrated ground penetrating radar (GPR) and ultrasonic testing (UT) measurements conducted on a historical floor in St. Nicholas' Church, Gda ´nsk, Poland. The described inspection was the first stage of the technical state assessment of the building. The aim of the study was the detection of underfloor air gaps, which were observed in a few trial pits. The condition of the ground under the floor was determined by localizing other inclusions such as rubble, human remains, brick walls and pipes. To identify the phenomenon of electromagnetic and ultrasonic wave propagation within the air gap, laboratory tests 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. The conducted research was supported by the numerical models of electromagnetic wave propagation. The obtained results showed that the integration of the GPR and UT methods provided an effective imaging of the floor and the area under it. Ultrasonic testing was proved to be a good technique for identifying air voids, while the GPR method allowed detecting concentrated anomalies and determining the degree of ground homogeneity under the floor.

**Keywords:** non-destructive testing; historical floor; integrated diagnostics; ground penetrating radar; ultrasonic testing; in situ surveys; finite-difference time-domain modeling
