Low-Deviation Ultrasonic Detection of Water Content in Rock Using an All-Optical Transceiving Method

The presence of pore water in rock significantly influences the stability of rock masses and even threatens the safety of engineering projects. It is of critical importance to accurately determine the water content in rock. In this paper, an all-optical ultrasound transceiving method is proposed to detect the water content for the first time. Laser irradiation of an optoacoustic composite material serves as the ultrasound source and generates solid waves with a -10 dB bandwidth of 13.7 MHz. A dual-grating optical fiber sensor is packaged for detecting waves with an enhanced sensitivity of 122.23 mV/kPa. The all-optical ultrasound transceiving system is constructed to investigate the water-containing rock sample. Owing to the broadband high-intensity laser excitation and compact high-sensitivity fiber reception, rock transmission waves are fully acquired, including the longitudinal/transverse waves in time/frequency domains. Compared to electrical counterparts, the all-optical transceiving method exhibits a similar response trend as water content increases, yet it shows the lowest absolute deviation of 0.046%. This work demonstrates the feasibility and superiority of optical ultrasound technique over conventional electrical transducers in characterizing physical and mechanical properties of rock samples, as well as engineering microstructures.