Microwave resonance detection method for hidden crack depth in rock and cementitious mortar

Before the rock in engineering projects such as mining, tunnel excavation, steep road construction, and oil and gas extraction undergoes failure, macroscopic and microscopic cracks will appear. The management or utilization of these cracks is inseparable from the application of crack detection technology. Some of these cracks are located below the rock surface and have a certain depth, which hinders the application of standard techniques. Therefore, based on the microwave resonance vibration model and using a low-power microwave source along with an enhanced transmission voltage amplitude, this paper proposes a microwave resonance detection method for the depth of hidden rock cracks. Experimental verification was conducted on cement mortar material samples and granite material samples with different preset internal crack depths. The propagation and reflection characteristics of cement mortar and granite material samples were analyzed, and the detection range of hidden crack depth was assessed. The results show that using this method, both cement mortar and granite samples have real-time detection capabilities at the central frequency of 3.667 GHz microwave magnetic field. The method has a fast response speed, and the relative error of the measurement results is within ± 5%. The theoretical maximum penetration depth for cement mortar material samples is 83-263 mm, and for granite material samples, it is 8311-83,107 mm. By comparing the actual test results of the cement mortar samples with the theoretical prediction results, it is evident that the microwave resonance crack depth detection device detected a maximum depth of 82 mm, which is consistent with the calculated range. The aforementioned method provides a new means for the detection of cracks below the rock surface.