Metrological characterization of the longitudinal ultrasonic velocity of cylindrical rock cores

Abstract

The correlation of the use of ultrasonic velocity in rock cores has been a fundamental resource in the analysis of geoscientists and rock engineers. The general rule is that ultrasonic velocities in typically heterogeneous materials, such as composites, rocks, and soils, present greater dispersion than homogeneous ones. This behaviour, although expected due to the more significant internal variation in the composition of its matrix, requires the application of a more cautious measurement protocol, mainly for samples with low parallelism. This work presents a metrological measurement protocol to determine the longitudinal ultrasonic velocities of these materials and their respective measurement uncertainties. The evaluation of the proposed protocol was applied to four samples of cylindrical migmatite rocks, each 76 mm in diameter. The frequency used was centred at 500 kHz. The determination of the transit time in each sample was made from the difference between the first internal reflection of the sample and the direct signal from the receiver. Samples with approximate heights of 51 mm, 62 mm, 77 mm and 101 mm were used. Their faces were divided into four quadrants. Measurements were taken around the geometric centre of the part and in each of the four quadrants. Fivedimensional measurements were performed for each quadrant, and ten transit time measurements were for each of the five defined regions. Due to the low parallelism of the parts, a protocol was adopted where the global uncertainty of each part was increased to that of the part with greater uncertainty. From the results of this research, it was noted that the effect of low parallelism tends to increase the dimensional uncertainty measurements and, consequently, the sample measurement uncertainty values.