Investigation of the lower block rows in the King’s Chamber of the Great Pyramid using ultrasonic testing with shear wave arrays

Abstract This study investigates the internal geometry of the granite block rows in the King’s Chamber (KC) of the Great Pyramid of Giza using Non-destructive testing (NDT) with ultrasonic testing (UST) using shear wave echo arrays. As part of the ScanPyramids (SP) project measurement campaigns in 2022, the lower two block rows of all four walls were examined using ultrasonic shear-wave echo arrays (PD8000, Screening Eagle Technologies). The measurement strategy initially included rapid overview acquisitions using an 8-channel UST device to identify potential areas of interest. High-resolution, detailed measurements were then performed at these locations, particularly along the north wall (NW) and in the area of block labeled NW9, using a 16-channel configuration built from two UST devices. The data acquired during these campaigns were processed using a Fourier Transform Synthetic Aperture Focusing Technique (FT-SAFT) algorithm based on the general Synthetic Aperture Focusing Technique (SAFT), creating two-dimensional (2D) cross-sectional images that reveal internal reflectors. A reliable determination of reflector depth requires a precise measurement of the shear wave velocity, which was determined from measurements on the surfaces of 45 accessible granite blocks of the two lower block rows of the KC. The whole study was performed on these 45 blocks. The chamber-wide mean value of 3035 m/s ± 90 m/s results in a remaining depth uncertainty of only a few centimeters, arising from variations in stone properties and measurement uncertainties (up to ± 70 mm even for deep reflectors). Images reconstructed using an FT-SAFT enabled the determination of block thickness (i.e., the locations of backwalls), block joints, and the identification of additional internal reflectors at depths of up to approximately 3 m. The 16-channel UST device exhibited an improved signal-to-noise ratio (SNR) and greater penetration depth than the 8-channel UST device, enabling hyperbolic reflectors and anomalies behind individual blocks to be visualized more distinctly. From the reconstructed 2D images of the measurement profiles, a three-dimensional model of the lower two block rows was derived, providing for the first time a systematic view of the internal structure of the granite lining of the two lower block rows of the KC’s walls. The results illustrate the potential of UST in combination with FT-SAFT for the NDT of massive historic stone structures, establishing a basis for subsequent multi-modal NDT analyses and structural evaluations of the Great Pyramid.