Generation mechanism of fracture-induced electromagnetic radiation and directionality characterization in the near field

Abstract

The electromagnetic radiation generated during the fracturing process is generally regarded as vector field that holds an imperative significance in rock engineering and fracture mechanics. With this motive, based on the theory of fracture mechanics and electromagnetics, the nonlinear field at the crack tip and the atomic bond breaking condition are analyzed in this study. Considering the electromagnetic field pattern generated by the oscillating dipole, the generation process of the electromagnetic radiation signal is divided into two stages namely, initiation of the forced oscillation of the dipole and the damped dipole oscillation. Followed by this, the experimental investigations on limestone and shale under the condition of Brazilian tests are performed. In the whole loading process, four sets of three-axis electromagnetic antennas are used for simultaneous measurement in different directions. The experimental and theoretical results exhibit close resemblance. The superposition effect of the electromagnetic field from microcrack initiation to macro-fracture is analyzed further. Based on the obtained results, three indicators are proposed to characterize the directionality of the electromagnetic radiation generated by the rock fracture. It is observed that the changing law of the direction angles is consistent, showing a strong correlation with the symmetrical position. The direction of the magnetic flux density vector generated by rock Brazilian tests tends to be parallel to the crack surface.