National University of Science and Technology MISiS, Moscow, Russia:

V. L. Shkuratnik, Professor, Doctor of Engineering Sciences, ftkp@mail.ru
P. V. Nikolenko, Assistant Professor, Candidate of Engineering Sciences
A. A. Kormnov, Postgraduate student

Under discussion is applicability of ultrasonic correlation logging in estimation of structural damage of roof rocks in underground excavations. This method is implemented as discrete displacement of a probe along a borehole: the probe is composed of ultrasonic noise signal source and two receiving transducer equally spaced from the source; the signals from the transducers are subjected to correlation processing. Based on computer-aided and physical modeling, the method sensitivity in detection of various cracks was estimated. The computeraided modeling used the finite element method in Comsol Multiphysics to solve axially symmetric problem with a central measuring borehole. The physical modeling used specially made gypsum samples with cracks of assigned geometry. The scope of the modeling encompassed different variants of crack inclination relative to a check borehole. It is shown that it is impossible to detect cracks intersecting the measuring borehole at an angle of 30º and smaller. Furthermore, the influence exerted by crack spacing from the borehole walls and by crack length on the correlation characteristics of the noisy probe signals recorded by the receiving transducers was assessed. Being in good correspondence, the results of the computer-aided and physical modeling give evidence on the dependence of the method sensitivity on the dominant wave length of the noisy probe signal. The limit values of crack parameters detectable with the ultrasonic correlation logging are obtained for the actual frequency of 100 kHz of the involved piezo-electric transducers. The determined limits for the range of the method application are connected with the dependence of the method sensitivity on the orientation, length and place of cracks. It is possible to minimize the constraints by optimizing working frequencies of ultrasonic logging and through combination of ultrasonic logging and cross-borehole sounding.
The study was supported by the Ministry of Education and Science of the Russian Federation, under Contract No. 2014/113, Project No. 504.

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