A three-axis antenna to measure near-field low-frequency electromagnetic radiation generated from rock fracture

In general, the generation of electromagnetic radiation initiates with the evolution of the crack in a rockmass. Theoretical observations depict that the low-frequency electromagnetic field excited by a dipole on cracks are directional in the near-field range. Therefore, in this paper, to achieve the measurement of low-frequency near-field, an electromagnetic antenna capabale of simultaneously measuring in directions of three axes is designed. After calibration of the standard alternating field, the antenna is used to monitor electromagnetic signals generated during the uniaxial compression and the Brazilian test. For the same fracture, signals differ in directions at the same position, which are actually components in three coordinate directions. And the three-axis antenna is relatively advantageous in response to low-frequency signals compared with the wide-band loop antenna. The outcomes of this research can provide theoretical as well as a practical basis for characterizing electromagnetic radiations associated with rock fractures.