Millimeter Wave Imaging of Cracks in Bulk Coal

Millimeter wave (mmW) propagation parameters including transmission, attenuation, and reflection coefficients of coal were obtained experimentally in the frequency region of W-band (75-110 GHz). In addition, a raster scanning mmW imaging system including reflection and transmission modalities was developed, which forms images according to the difference between the dielectric parameters of cracks and those of the bulk coal samples, using the spectrum module of a W-band vector network analyzer to radiate the mmW signal to the object and obtain the imaging of the coal cracks from the S-parameters. The electromagnetic simulation and imaging results of various types of cracks are analyzed and the different imaging results are compared. The dependence on frequency and modes of operation of the mmW imaging system was systematically studied. With an appropriate imaging algorithm, we were able to employ appropriate imaging modes and choices of frequency for different types of cracks. This paper forms the foundation of further development of the mmW radar imaging system for underground coal seam crack detection and mapping.